UBC Theses and Dissertations

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UBC Theses and Dissertations

Deformation of polycrystalline cobalt Sanderson, Craig Carter 1972

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DEFORMATION OP POLYCRYSTALLINE COBALT by C. C. Sanderson B.A.Sc., U n i v e r s i t y  o f B r i t i s h Columbia, 196  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of METALLURGY  We a c c e p t t h i s t h e s i s as conforming t o the standard r e q u i r e d  from c a n d i d a t e s f o r the  degree o f DOCTOR OF PHILOSOPHY  THE UNIVERSITY OF BRITISH COLUMBIA J u l y , 1972  In  presenting  this  thesis  an a d v a n c e d  degree  the L i b r a r y  s h a l l make i t  I  further  for  scholarly  by h i s of  agree  this  written  at  the U n i v e r s i t y  purposes  for  of  gain  Metallurgy  August 11, 1972  of  Columbia,  British  by  for  Columbia  shall  the  requirements  reference copying  of  I agree and this  that  not  copying  or  for that  study. thesis  t h e Head o f my D e p a r t m e n t  is understood  financial  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, C a n a d a  of  for extensive  may be g r a n t e d It  fulfilment  available  permission.  Department  Date  freely  that permission  representatives. thesis  in p a r t i a l  or  publication  be a l l o w e d w i t h o u t  my  ABSTRACT The  deformation of p o l y c r y s t a l l i n e c o b a l t has  investigated i s stable  been  over the temperature range where the hep  (0 to 0.39  T  phase  ). m  The  structure  procedures has cobalt  of c o b a l t  been d e t a i l e d .  i s a two  retained  various annealing  F o l l o w i n g heat treatment,  phase aggregate of fee and  A maximum of 50-60% r e t a i n e d specimens  following  hep  phases,  fee occurs i n small g r a i n e d  (6 - 10 microns) d e c r e a s i n g to l e s s than fee f o r 60 micron m a t e r i a l .  The  fee phase a l s o decreases w i t h i n c r e a s i n g  15%  amount of purity.  retained  The  v a r i e t y of s u b s t r u c t u r e s a r i s i n g from the r u l t i v a r i a n t t r a n s f o r m a t i o n are The  discussed.  y i e l d stress for cobalt  temperature dependence above and 0.25  T , the  0.2%  differing  below 0.25  T  .  Below  y i e l d s t r e s s i s almost temperature  independent, whereas above 0.25 decreases r a p i d l y . *  exhibits  The  T  the y i e l d m  behaviour belov; 0.25  to the bulk t r a n s f o r m a t i o n of r e t a i n e d  stress T  m  i s related  fee phase w h i l e  the  d e c r e a s i n g s t r e s s l e v e l s observed above 0.2 5 T i n terms of d e c r e a s i n g P e i e r l s s t r e s s on the The  y i e l d stress increases  are e x p l a i n e d m {1122} s l i p p l a n e s .  r a p i d l y as the g r a i n  size  i s reduced.  T h i s e f f e c t i s compared to s i m i l a r behaviour  i n other hep  metals t h a t e x h i b i t a l i m i t e d number of  slip  systems.  iii The d u c t i l i t y o f c o b a l t i s r e l a t e d t o the r e t a i n e d fee phase by equations of the form e = A (10)  °  ,  A larger  f r a c t i o n of r e t a i n e d fee phase g i v e s r i s e t o i n c r e a s e d ductility.  The e l o n g a t i o n t o f r a c t u r e decreases as t e s t  temperature i n c r e a s e s , r e f l e c t i n g obeyance of Considere's Criterion. The observed work hardening r a t e s are h i g h , as are the measured v a l u e s f o r flow s t r e s s . to data o b t a i n e d f o r other metals w h i l e undergoing  The v a l u e s are compared  that transform m a r t e n s i t i c a l l y  deformation.  M e t a l l o g r a p h i c evidence i s presented to s u b s t a n t i a t e the occurence  of non-basal  s l i p i n c o b a l t above 0.25  T. m  Twins having h i g h and low shear v a l u e s occur a t a l l temperatures  where the hep phase i s s t a b l e .  The i n t e n s e  s u r f a c e shear r e s u l t i n g from t r a n s f o r m a t i o n and c o n t i n u i n g ; d i s l o c a t i o n p r o d u c t i o n on v a r i o u s l y o r i e n t e d b a s a l planes is discussed.  iv  TABLE OF CONTENTS PAGE 1.  INTRODUCTION 1.1  1  C o b a l t and the Common HexagonalClose-Packed Metals  1  1.2  Cobalt Single Crystals  8  1.3  The A l l o t r o p i c T r a n s f o r m a t i o n and Structure of Cobalt  12  1.3.1  H i s t o r y of the T r a n s f o r m a t i o n . . . . .  12  1.3.2  Mechanisms  1.4 2.  ,  f o r the  M a r t e n s i t i c Transformation  14  1.3.3  M u l t i v a r i a n t Transformation  17  1.3.4 1.3.5  Retained FCC Phase Thermodynamics o f the Transformation  19 *  20  1.3.6  The H y s t e r e s i s of the Transformation  .  21  Scope o f Present Work...  EXPERIMENTAL PROCEDURE  21 ,  23  2.1  Materials  23  2.2  P r e p a r a t i o n of T e n s i l e Specimens  25  2.2.1  Machining  25  2.2.2  Annealing Procedures....  27  2.2.3  X-Ray A n a l y s i s  27  2.3 2. 4  T e n s i l e and Hardness T e s t s Metallography 2.4.1  O p t i c a l Metallography  32 .  34 34  2.4.2  36  E l e c t r o n Microscopy R e p l i g a s .  EXPERIMENTAL PROGRAM AND RESULTS  37  3.1  37  The S t r u c t u r e 3.1.1  3.1.2  of P o l y c r y s t a l l i n e C o b a l t . .  As Received M a t e r i a l  37  3.1.1.1  Preferred  Orientation....  38  3.1.1.2  Stacking F a u l t Energy and F a u l t A n a l y s i s . . . . . . .  39  Recovery, R e c r y s t a l l i z a t i o n and G r a i n Growth  40  3.1.2.1  Recovery...  43  3.1.2.2  R e c r y s t a l l i z a t i o n and G r a i n Growth.  3.2  .  43  3.1.3  Completeness of T r a n s f o r m a t i o n . . . .  47  3.1.4  Discussion  55  and Summary  T e n s i l e Behaviour o f Cobalt 3.2.1  67 67  Polycrystals Tensile Results 3.2.1.1  True S t r e s s - True S t r a i n Curves  67  3.2.1.2  Y i e l d S t r e s s and U l t i m a t e T e n s i l e Stress  79  3.2.1.3  D u c t i l i t y and F r a c t u r e . . . 107  3.2.1.4  Work Hardening 113  Behaviour 3.2.1.5 3.2.2  Discussion  and Summary... 119  Deformation and the A l l o t r o p i c Transformation 3.2.2.1  Purity  3.2.2.2  Grain  3.2.2.3  Temperature  . 130 13 6  Size...,  139 140  VI.  PAGE  3.2.2.4  Von M i s e s C r i t e r i o n  3.2.2.5  Metallographic Observation  3.2.2.5.1  ,  153  P u r i t y and Size....  155  3.2.2.5.2  Optical Metallography.  156  3.2.2.5.3  Replica Observations..  162  3.2.2.5.4  Summary  Grain  3.2.2.6  146  168  D i s c u s s i o n and , . 171  Summary  3.2.2.6.1  The Y i e l d Stress  171  3.2.2.6.2  Flow  S t r e s s . . . 176  3.2.2.6.3  Elongation i t o F a i l u r e . . . . 176  3.2.2.6.4  Work H a r d e n i n g Behaviour 178  4.  CONCLUSIONS  180  5.  SUGGESTIONS FOR FUTURE WORK  182  6.  APPENDICES  183  6.1  X-Ray A n a l y s i s  183  6.2  Measurment o f T e n s i l e P a r a m e t e r s by an I n t e r s e c t Method  187  7.  REFERENCES  193  vii TABLES PAGE TABLE I  Data Sheet f o r the HexagonalClose-Packed Metals ....  TAELE I I  C r i t i c a l l y Resolved Shear S t r e s s f o r  2,3  V a r i o u s Metals  11  TABLE I I I  M a r t e n s i t i c T r a n s f o r m a t i o n .Studies....  13  TABLE IV  S p e c t r o g r a p h s A n a l y s i s of Cobalt Matrix  24  TABLE V  Summary o f Retained FCC Data  50  TABLE VI  Martensite- T r a n s f o r m a t i o n s i n Hon-Ferrous M a t e r i a l s P o l y c r y s t a l Cobalt 0.2% Y i e l d S t r e s s Data Temperature Dependence o f Flow S t r e s s (Aa/G f o r 100 C temperature change)...  TABLE V I I TABLE V I I I  78 85 94  P  TABLE IX TABLE X  TABLE XI  U l t i m a t e S t r e n g t h Data f o r Cobalt Polycrystals  95  Parameters From an E q u a t i o n of the Form a . . , = cr. + KD ' yield l  100  Summary of True S t r a i n Data f o r P o l y c r y s t a l Cobalt  10 8  TABLE XII  The Two Stage Behaviour o f Flow S t r e s s and Work Hardening Rate as a F u n c t i o n of Temperature... 120  TABLE X I I I  Behaviour of the S t r a i n Induced T r a n s f o r m a t i o n i n C o b a l t Expressed f | an E q u a t i o n of the Form: e = A (10) '* "  c  c  137  TABLE XIV  Summary of Experimental R e s u l t s  172  TABLE XV  T y p i c a l Data From a S t e p - P u l l T e n s i l e Test.  190  viii FIGURES PAGE FIG. 1 FIG. 2  T e n s i l e specimens and important dimensions  26  T y p i c a l r e c o r d o f vacuum a n n e a l i n g treatment  26  FIG. 3  Raw x-ray data f o r 99.7% c o b a l t  30  FIG. 4  Diamond Pyramid Hardness data for cobalt V a r i a t i o n i n g r a i n s i z e f o r 1 hour anneals a t i n d i c a t e d temperatures  45  A n n e a l i n g spectrum i n 99.9% c o b a l t 340X....  48  99.7% c o b a l t , annealed a t 600°C (a) and 80'0°C (b) 1 h r  49  99.9% c o b a l t , annealed a t 900°C f o r 1 h r . 39 y  49  99.998% c o b a l t , annealed a t 600°C (a) and 800°C (b) 1 hr .  49  FIG. 5 FIG. 6 FIG. 7 FIG. 8 FIG. 9  FIG. 10(a) % r e t a i n e d f e e vs g r a i n s i z e (b) % r e t a i n e d f e e vs l / / g r a i n s i z e FIG. 11  99.998% C o b a l t under p o l a r i z e d  51 51 light  900X  54  FIG. 12  A n n e a l i n g Parameters  FIG. 13  Volume changes d u r i n g the m a r t e n s i t i c transformation i n cobalt .. E t c h i n g o f g r a i n boundaries and m a r t e n s i t e p l a t e s . 99.7% c o b a l t . 920X..  FIG. 14  •  FIG. 15(a) 99.9% c o b a l t . 4000X (b) 99.9% c o b a l t . 10,OOOX (c) 99.9% c o b a l t . 10,000X FIG. 16  42  99.9% c o b a l t , 6.5 micron g r a i n s i z e 4000X  56 58 60 60 60 60 62  ix  PAGE FIG.  FIG. FIG.  17  Shear markings f o l l o w i n g t r e a t m e n t . 6500X  18  Annealing twin boundaries 9 9 . 9 % c o b a l t . 5000X  19  heat 62 in 62  Banded s t r u c t u r e a r i s i n g f r o m multivariance in cobalt.  coplaner  (a) 4,000X (b) 10,O00X (c) 10,000X FIG. FIG. FIG.  FIG. FIG. FIG.  FIG.  FIG.  20  99.998% c o b a l t ,  21 22  23 24 25(a)  27  crrain s i z e  2000X  64  True s t r e s s - t r u e s t r a i n curves a t s e l e c t e d temperatures, 99.998% c o b a l t . .  68  I n i t i a l p o r t i o n of true stress t r u e s t r a i n c u r v e s , 99.9% c o b a l t , 6.5 m i c r o n g r a i n s i z e  69  True s t r e s s - s t r a i n curves a t t e m p e r a t u r e s , 99.7% c o b a l t  selected 70  True s t r e s s - s t r a i n c u r v e s a t cobalt  20°C.  Nominal Co, Mg, (b) N o m i n a l Co, Ag,  26  47 m i c r o n  64 64 64  s t r e s s - s t r a i n curves f o r Zn, and T i s t r e s s - s t r a i n curves f o r Cu, and A l  70  71 74  True s t r e s s - t r u e s t r a i n curves f o r m a t e r i a l s undergoing s t r a i n induced martensitic transformation  76  Y i e l d s t r e s s versus t e s t temperature f o r two p u r i t y g r a d e s o f c o b a l t  80  FIG.  28  Y i e l d s t r e s s data  for polycrystal cobalt  82  FIG.  29  Comparison of y i e l d s t r e s s data o b t a i n e d by i n d i v i d u a l t e s t s and interupted s i n g l e s p e c i m e n t e s t i n g . 99.9% c o b a l t , G.5 m i c r o n g r a i n s i z e  83  Y i e l d stress versus t e s t f o r c o b a l t and magnesium  87  FIG.  30  temperature  PAGF Y i e l d s t r e s s versus temperature f o r c o b a l t s i n g l e c r y s t a l s and polycrystals Y i e l d s t r e s s and u l t i m a t e s t r e n g t h f o r 99.9% c o b a l t , 6 . 5 m i c r o n g r a i n  89 data size  91  T y p i c a l data f o r determing the temperature d e p e n d e n c e o f f l o w s t r e s s . 99.7% c o b a l t , 7.0 micron g r a i n s i z e 92 Yield stress versus r e c i p r o c a l root of grain size  square  Y i e l d stress versus r e c i p r o c a l root of grain size  square  98 102  F r a c t u r e s u r f a c e , 99.9% c o b a l t t e s t e d a t 20°C. 6 . 5 m i c r o n g r a i n s i z e . 7 5 0 0 X . . 112 F r a c t u r e s u r f a c e , 99.998% c o b a l t t e s t e d a t 20°C. 47 m i c r o n q r a i n s i z e .  5000X  ....  V a r i a t i o n o f work h a r d e n i n g r a t e s t r a i n f o r 99.9% c o b a l t  with  112 114  The .work h a r d e n i n g b e h a v i o u r o f c o b a l t as a f u n c t i o n o f temperature  117  V a r i a t i o n i n work h a r d e n i n g b e h a v i o u r w i t h i n c r e a s i n g s t r a i n . 99.9% c o b a l t , 6.5 micron g r a i n s i z e  118  T e m p e r a t u r e c h a n g e t e s t s , 99.9% c o b a l t . 6.5 micron g r a i n s i z e 122 T e m p e r a t u r e c h a n g e t e s t , 99.9% c o b a l t . 6.5 micron g r a i n s i z e  123  T e n s i l e s t r a i n induced transformation o f c o b a l t a t room t e m p e r a t u r e  131  Room t e m p e r a t u r e t e n s i l e s t r a i n i n d u c e d transformation f o rcobalt of various grain sizes.. 132 Room t e m p e r a t u r e t e n s i l e s t r a i n i n d u c e d transformation f o r cobalt of various g r a i n s i z e s (semilog) 133 T e n s i l e s t r a i n induced transformation o f c o b a l t a t room t e m p e r a t u r e ( s e m i l o g ) . . . 134  xi PAGE F I G . 47  F I G . 48  F I G . 49  FIG.  FIG.  FIG.  50  51  52  F I G . 53 F I G . 54 FIG. FIG. FIG.  55 56 57  T e n s i l e s t r a i n induced t r a n s f o r m a t i o n for cobalt at various temperatures, (semilog) X - r a y d a t a f o r 99.9% c o b a l t , s t e p p u l l e d a t 20°C and 250°C, 6.5 m i c r o n grain s i z e . . . ,  135  , 142  X - r a y d a t a f o r 99.7% c o b a l t , s t e p p u l l e d a t 20°C and 250°C, 7 m i c r o n grain s i z e . . . ,  143  Volume % s t r a i n induced m a r t e n s i t e p r e s e n t i n 99,9% c o b a l t as a f u n c t i o n of s t r a i n . . . . ,  145  M a r t e n s i t e shear markings i n t r o d u c e d bv a s u r f a c e s c r a t c h i n 99.9% c o b a l t 1900X ,  153  D e f o r m a t i o n o f 99.998% c o b a l t , 850X  157  D e f o r m a t i o n markings i n 99.998% c o b a l t a t f a i l u r e . 1000X  158  G r a i n shape change i n 99.9% c o b a l t 1000X  158  Twinning i n c o b a l t a t 850X  -196°C. 160  Twinning i n c o b a l t a t 850X  350°C. 160  D e f o r m a t i o n o f 99.7% c o b a l t a t 3000X  250°C. 163  F I G . 58  S t r e s s r e l i e f a t a boundary between two r e g i o n s where shear has t a k e n p l a c e on d i f f e r e n t systems. 6500X 164  FIG.  T y p i c a l surface  59  -196°C t e s t .  shear markings i n c o b a l t  6500X  \  164  FIG.  60  Twinning i n c o b a l t a t - 1 9 6 ° C .  3700X....  FIG.  61  Twinning i n c o b a l t a t 250°C. 370OX  166 166  xii  PAGE FIG. FIG. FIG. FIG. FIG. FIG.  62 63 64 65 66 67  Non-basal s l i p i n p o l y c r y s t a l c o b a l t t e s t e d a t 250°C. 7500X  167  Shear markings i n 99.9% c o b a l t at 20°C. 7500X  tested 169  Shear Markings i n 99.9% c o b a l t a t 250°C. 7500X  tested 169  Mechanisms c o n t r o l l i n g y i e l d i n polycrystal cobalt  175  S t e p - p u l l t e n s i l e t e s t . 99.9% c o b a l t , 6.5 micron g r a i n s i z e  188  D e t e r m i n a t i o n of the i n t e r s e c t y i e l d s t r e n g t h from s t e p - p u l l data  189  ACKNOWLEDGEMENTS The author  acknowledges the a d v i c e and a s s i s t a n c e g i v e n '.  h i s r e s e a r c h d i r e c t o r , Dr. N. R. Risebrough.  Thanks a r e  a l s o extended t o other members o f the f a c u l t y and the graduate students f o r h e l p f u l d i s c u s s i o n s .  Financial  a s s i s t a n c e p r o v i d e d by the N a t i o n a l Research C o u n c i l i s g r a t e f u l l y acknowledged.  1.  Introduction  Cobalt  i s a high melting  between n i c k e l cooling,  and  iron  i n the  c o b a l t u n d e r g o e s an  from f a c e - c e n t r e d - c u b i c at approximately  s u c h as 4  ' .  a single  Cobalt  5  periodic t a b l e  allotropic  phase  1 1 2  .  lying  Upon  transformation  (fee) t o h e x a g o n a l - c l o s e - p a c k e d  417°C .  This martensitic  3  proceeds to completion  point t r a n s i t i o n metal  o n l y under very  interface  i s ferromagnetic  and  has  transformation  special  transformation  (hep)  circumstances,  in a single  a Curie Point  crystal  of  1115°C ' . 2  6  The  incomplete  close-packed  phase t o another  possibilities attempt  has  martensitic transformation  for investigation.  the  s t r u c t u r e and  at  t e m p e r a t u r e s where t h e A  literature 1.1  tensile  i s presented  (Table  modes f o r t h e  I).  The  observed  slip  m e t a l s may  t i t a n i u m and  the  metals  table.  t h e most i m p o r t a n t  be  8  and  Metals i n many .  s y s t e m s and  shown i n t h i s  t h e A.S.M. H a n d b o o k  hep  common hep  common h e x a g o n a l m e t a l s as w e l l as  i n f o r m a t i o n are  Zirconium,  phase i s  t h e Common H e x a g o n a l - C l o s e - P a c k e d  drawn f r o m many s o u r c e s ;  The  cobalt polycrystals  in this introduction.  i s u n i q u e among t h e  1  an  summary o f p e r t i n e n t i n f o r m a t i o n a v a i l a b l e i n  Cobalt  Chalmers ,  work,  understanding  hexagonal-close-packed  and  relevant  present  p r o p e r t i e s of  Cobalt  respects  In the  one  interesting  been made t o o b t a i n a d e t a i l e d  of  stable.  y i e l d s many  from  twinning  other  The  being  data  Partridge , 7  Reed-Hill . 9  divided into  b e r y l l i u m are  two  categories.  high melting  are  point  TABLE I  Data  S h e e t f o r t h e Common H e x a g o n a l - C l o s e - P a c k e d  Metals  Metal  Cd  Zn  Mg  Co  Zr  Ti  Be  c/a  1.886  1.856  1.623  1.623  1.592  1.587  1.568  321  420  650  14 9 5  1852  1668  1277  -  -  -  fee hcp4417  bcc hcp4862  bcc hep 4882  bcc hcp + 1260  *  *  *  * 1 2 i1 3  Obs.  Obs.  *  —  -  Obs.  -  *  *  Obs.  Pvramidal {1011} <1120>  Obs.  -  Obs.  -  2nd  2nd  Obs.  Corrugated {1122} <1123  2nd  2nd  Obs.  Obs.  Obs.  -  Obs.  Ratio  Melting P o i n t °C Allotropic Transformation and Temp. °C S l i p Modes Basal {0001} <1120> Prism {1010} <1120>  >  1 h  TABLE I  (Con't)  Twinning  Modes  {1012}  Obs,  Obs,  Obs.  Obs.  {10ll}  Obs.  Obs. *  {101n}  {1013}  4  {1121}  Obs.  {1122}  Obs,  {112n}  {1124}  Estimated Stacking Fault2 Energy ergs/cm  150  * -- P r e d o m i n a n t  slip  mode  2nd  slip  mode  -- S e c o n d a r y  Obs. —  Observed  300  300  20 1  1 1  I  if  9~ 2 1  Obs,  Obs,  Obs.  Obs.  Obs.  Obs.  {1123}  {H24}  300  Obs,  5  180  m e t a l s w i t h c / a r a t i o s l e s s than i d e a l w h i l e z i n c and cadmium, v / i t h h i g h c / a r a t i o s ,  and magnesium w i t h an a l m o s t  i d e a l c / a r a t i o are low m e l t i n g p o i n t m e t a l s .  Cobalt  s t r a d d l e s both groups w i t h a h i g h m e l t i n g p o i n t and a c / a r a t i o s i m i l a r t o magnesium. Attempts have been made t o e x p l a i n the observed m a t i o n modes of the hep m e t a l s based on g e o m e t r i c of t h e i r c / a r a t i o s ' 7  1 0  .  defor-  considerations  T h i s approach appears t o e x p l a i n  the predominant d e f o r m a t i o n mode i n most cases but does not account f o r the i n d i v i d u a l d i v e r s i t y of secondary d e f o r m a t i o n processes. For c / a r a t i o s e q u a l t o o r g r e a t e r  than i d e a l  the predominant d e f o r m a t i o n p r o c e s s f o r e c a s t basal plane i n a close-packed d i r e c t i o n .  i s s l i p on the  Indeed,  s l i p i s the most i m p o r t a n t d e f o r m a t i o n mode i n the z i n c , cadmium, magnesium and c o b a l t . r a t i o s l e s s than i d e a l , p r e f e r r e d mode.  (/8/3)  basal metals  Over a range o f c / a  f i r s t o r d e r p r i s m s l i p becomes  the  Z i r c o n i u m , and t i t a n i t i m w i t h c / a r a t i o s o f  1.592 and 1.587 r e s p e c t i v e l y , s l i p p r e d o m i n a t l y on the {1010} <1120> p r i s m system.  B e r y l l i u m w i t h c / a equal to  1.568 deforms m a i n l y v i a {0001} <112~0> b a s a l s l i p which i s not the expected mode. A l t h o u g h the p r i m a r y d e f o r m a t i o n mode appears t o be a s t r o n g f u n c t i o n of the c / a r a t i o , the secondary d e f o r m a t i o n p r o c e s s e s are more complex and are i n f l u e n c e d s t r o n g l y by v a r i a b l e s such as t e m p e r a t u r e , purity.  s t a c k i n g f a u l t energy and  Although and  prism s l i p  t i t a n i u m , both  been o b s e r v e d .  cobalt  yet prism  slip  slip  basal s l i p  predominates  and  Seeger  1 4  elsewhere.  N  o  specifically  n  b  a  s  but  this  Holt  4  although single  pyramidal  a  l  s  l  i  {H22>  p  slip  was  unable  crystal  for this  an  non  reduce  that  1  kink boundaries  having  9  temperatures,  ease  constraint  Reed-Hill and  titanium  1 5  in zinc .  The  vector.  or accomodation  Other  1 6  ,  9  thus Kink  magnesium , 7  bend p l a n e i s of  dislocations  more complex  kinks, are o f t e n  kink  observed  twins. All  the hexagonal  mode o f t w i n n i n g simple of  at high  { 1 1 2 0 } t i l t b o u n d a r y made up  t h e same B u r g e r s  boundaries,  slip  in orientations  In z i r c o n i u m ,  been o b s e r v e d  z i r c o n i u m , and  o f t e n a simple  near  basal  purpose.  b o u n d a r y f o r m a t i o n has 2  observed  duplicated  t h e need f o r t w i n n i n g a t h i g h t e m p e r a t u r e .  cobalt ' ,  In  slip  been  s p e c i m e n s were s t r e s s e d  i m p o r t a n t d e f o r m a t i o n mode.  postulates  purity  i s basal  not been  to i n i t i a t e  have  also occur.  <1123), has  o b s e r v a t i o n has  systems  i n high  Kink boundary f o r m a t i o n , e s p e c i a l l y is  mode i n z i r c o n i u m  t h e o n l y s l i p mode commonly o b s e r v e d  n ,5 , 11 ii2 ,i 3 _ by  t h e p y r a m i d a l and  Basal s l i p  7  beryllium  i s the primary  shuffles  deformation  metals  involves  the  lowest  i n the p l a n e of twins  When t h e t w i n n i n g becomes l e n t i c u l a r  shear as  i s s m a l l as  {1012}plane.  shear  shear.  i s influenced  by  The  and  observed  the twinning  the twin  the twinning plane  i n c r e a s e i n twin i n t e r f a c e  energy'.  This  requires  f o r {1012} t h e  i t grows b e c a u s e  can d e v i a t e c o n s i d e r a b l y from a large  twin i n the  only  shape  shear ' . 7  9  twin  interface without  When t h e  shear  6 i s l a r g e the twins formed a r e narrow and have e s s e n t i a l l y parallel  boundaries.  In z i r c o n i u m and t i t a n i u m both {1121} and {1122} twins have been observed.  Twins of t h i s form have a much h i g h e r  shear v a l u e than those i n the {1012} p l a n e .  For t i t a n i u m ,  the r a t i o of shear v a l u e s are 1.7/2.3/6.4 f o r {10l2}/ {1122}/ (1121) type twins r e s p e c t i v e l y .  Thus the {1122} and {ll"2l}  twins a r e observed as narrow and s t r a i g h t w h i l e {1012} twins are commonly v e r y wide and l e n t i c u l a r i n shape. Magnesium w i t h a c/a r a t i o s i m i l a r t o c o b a l t , twins on the {1011} and {1013} p l a nes as w e l l as on the {1012} p l a n e . Twinning has a l s o been observed on higher order planes of the form {101n}.  The complexity a r i s i n g i n {1013} and h i g h e r  order twinning has l e d i n v e s t i g a t o r s t o propose  t h a t a double-  twinning mechanism i n v o l v i n g r e t w i n n i n g of a primary twin i s r e q u i r e d t o form these twins. {1012} twins have been observed i n c o b a l t by a number of i n v e s t i g a t o r s  4  '  5  1 ,l 2  .  Davis  11  observed { 1121} z i g - z a g  twinning i n c o b a l t , s i m i l a r t o those observed i n t i t a n i u m by Rosi  1 5  .  Seeger observed both { 1122} and { 1124} twinning i n  single crystal cobalt observed i n t i t a n i u m .  k  ;  these twin planes have a l s o been  Holt * 1  i n a comprehensive  single  crystal  study observed { 1011} twins which have a l s o been observed i n magnesium. The p r e c e d i n g should not be construed as a complete c o m p i l a t i o n of the s l i p and twinning modes noted i n the l i t e r a t u r e f o r the hexagonal metals. presented t o a l l o w a comparison  The data has been  t o be made between c o b a l t  and the other common hexagonal metals w i t h r e s p e c t t o t h e i r  7 normal deformation that for  no p r o m i n e n t cobalt  processes. secondary  The i m p o r t a n t  slip  but a m u l t i p l i c i t y  t h e o t h e r common h e x a g o n a l  system  observation i s  h a s been  observed  o f t w i n n i n g modes e x i s t .  metals  several  slip  For  systems a r e  observed. The  lack of obvious  may a r i s e  from  two s o u r c e s .  common h e x a g o n a l As  noted  plane any  secondary  metal  i n Table  i n cobalt  First,  energy  although  systems i n c o b a l t  cobalt  i s the only  t h a t has a l o w s t a c k i n g f a u l t  I, the s t a c k i n g f a u l t  i s at least  a factor  o f t h e o t h e r common h e x a g o n a l  fault  slip  energy  of five  metals.  h a s b e e n m e a s u r e d by s e v e r a l  there are small differences  energy.  on t h e b a s a l  less  than f o r  The s t a c k i n g t e c h n i q u e s and  i n the values  obtained,  2 it  i s unanimous t h a t  t h e v a l u e i s b e l o w 30 erg/cm  1 9 . 2 0 .21 ^  2 The  value usually  i s 20 e r g s / c m  quoted  t o 150 t o  compared  2 300  ergs/cm  low  stacking fault  of  f o r t h e o t h e r common h e x a g o n a l  dislocations  a cross-slip  Therefore, secondary To  i n cobalt  ensures  i n the basal plane are  For d i s l o c a t i o n s by  energy  t o move f r o m process  planes  that  '  2 3 1 2 4  d i s l o c a t i o n s must  on t h e s e p l a n e s  obtain appreciable dislocations  2 2  to other  o f many d i s l o c a t i o n s  to provide s l i p  The  the majority  extended  the basal plane  the p a r t i a l  the p r o b a b i l i t y  metals.  . planes  constrict.  a r r i v i n g on i s small.  on t h e s e c o n d a r y  planes,  t h e y must e i t h e r n u c l e a t e o r be grown i n , on t h e s e p l a n e s . T h i s i n t r o d u c e s t h e second problem r e g a r d i n g secondary s l i p in cobalt. Upon c o o l i n g martensitically  from  high temperature,  t o hep c o b a l t .  fee cobalt  transforms  One o f t h e c l o s e - p a c k e d  {111}  planes in  i n t h e f a c e - c e n t r e d p h a s e becomes t h e b a s a l p l a n e  the hexagonal  remains  lattice.  relatively  T h i s i s the o n l y plane  unchanged d u r i n g t r a n s f o r m a t i o n .  disappearance  or a b s o r p t i o n of d i s l o c a t i o n s  p l a n e s o r any  other plane  is  not understood  several  locations  basal  to y i e l d  2 2  Cobalt Single Several  alloy  crystals such  investigations cobalt  Alstetter  and  work by  Planck  Stage  A and The  co-workers  latter  for  '  2 8  be  1 2  '  1 3  ,  non-basal  disslip  2  9  '  2 4  3  0  '  2 5  1  3 1  group  ' ,  will  review  single  a group  have l i m i t e d  2 6  .  of  their behaviour  Christian  have t e s t e d  2 7  ,  crystals  be d i s c u s s e d i n t h e  next  o f t h e t r a n s f o r m a t i o n and  crystals  ' resolved  2 4  literature.  outlined.  with p u r i t y  99.998% p u r i t y  i n the  mechanism o f t h e t r a n s f o r m a t i o n .  B are observed  critical!  cobalt-nickel  to the deformation  1  '  and  Holt'*, and  Institute  to other hexagonal  Stage  significantly  1 1  5  topics w i l l  similar  of c o b a l t  c r y s t a l s ' *'  Hexagonal c o b a l t curves  in cobalt  films.  are a v a i l a b l e  s e c t i o n where a c o m p l e t e related  observed  by  amounts o f d e f o r m a t i o n by  predominantly  the  been o b s e r v e d  Sufficient  in thin  to d i s c o v e r the d e t a i l e d The  *.  as D a v i s  single  -Clll>  other  Crystals  w o r k e r s a t t h e Max  of  2 1  investigations  single  Researchers  '  2 3  finite  have n o t been o b s e r v e d  1.2  I t has  the d i s l o c a t i o n s '  on  The  i n the f a c e - c e n t r e d - c u b i c phase  at present.  authors, that  are p r i m a r i l y  that  1 2  '  1  exhibit  metals.  tensile  In c o b a l t  but a t h i r d  stage  crystals,  i s not.  shear  stress  of c o b a l t  varies  '  rising  from  psi  2 k  '  1 5 f  t o 2800 p s i f o r 99.1%  1400  material  1  \  T h i s parameter  i s a l s o temperature dependent r i s i n g  1400 p s i a t room temperature to 2400 p s i a t Seeger  1 4  from  -196°C  p o s t u l a t e d t h a t the h i g h shear s t r e s s was due  to  e i t h e r r e t a i n e d c u b i c phase or a v e r y h i g h b a s a l d i s l o c a t i o n density. Hep c o b a l t e x h i b i t s a l o n g i n i t i a l r e g i o n (Stage A) which may extend t o s e v e r a l hundred p e r c e n t  strain.  The  work h a r d e n i n g r a t e i n Stage A (0 ) i s t e m p e r a t u r e dependent, r i s i n g from 2000 p s i t o 2500 p s i as the t e m p e r a t u r e i s reduced from room temperature t o -196°C  1 2 1 1k  ' .  also  0  5  increases with increasing impurity content. D u r i n g t e s t s o f hexagonal c o b a l t c r y s t a l s , the  slip  l i n e s p a c i n g d e c r e a s e s w i t h i n c r e a s i n g d e f o r m a t i o n up to 20% s t r a i n .  Throughout the remainder o f Stage A , the  h e i g h t i n c r e a s e s w h i l e s l i p l i n e s p a c i n g remains I i* i 2 4 i 2 5 _  step  constant  T h i e r i n g e r determined t h a t the d i s l o c a t i o n  d e n s i t y i n 99.998% c o b a l t l i e s between 0.4 and 1.2 X 1 0 2 4 per cm w i t h a s t a c k i n g f a u l t d e n s i t y o f 2 t o 6 X 10 per cm. 9  He observed t h a t the d i s l o c a t i o n d e n s i t y i n c r e a s e s l i n e a r l y w i t h s t r e s s and s t r a i n i n Stage A . Thieringer  2 4 1 2 5  approximate  Boser  b o t h r e p o r t e d t h a t no i n c r e a s e  5  and  i n non-basal  d i s l o c a t i o n s occured during deformation. The number o f a c t i v e s l i p l i n e s and the work h a r d e n i n g r a t e i n c r e a s e upon e n t e r i n g Stage B .  Thieringer  found t h a t the s t r e s s a t which Stage B b e g i n s i s  2 , 4 1 2 5  independent  of p u r i t y and o c c u r s a t 2300 p s i a t room t e m p e r a t u r e .  The  onset of Stage B i n hexagonal c o b a l t has been e x p l a i n e d b y : the o p e r a t i o n of a second s l i p s y s t e m , 1 2  a strong  increase  i n the frequency of t w i n n i n g * ' , the a g g l o m e r a t i o n o f 2 1  2 5  i m p u r i t i e s , and the p r o d u c t i o n o f excess v a c a n c i e s ' * . 5  The c r i t i c a l l y r e s o l v e d shear s t r e s s and work h a r d e n i n g r a t e s are much h i g h e r f o r fee c o b a l t c r y s t a l s than f o r hep crystals'*.  Stage I o f the f a c e - c e n t e r e d - c u b i c  t e n s i l e curve  i s not d e t e c t e d due t o the h i g h t e s t temperatures  required  t o a t t a i n the c u b i c phase w h i l e Stage I I and Stage I I I p o r t i o n of the c u r v e are o b s e r v e d .  The shape of the t e n s i l e  are s i m i l a r t o those f o r o t h e r fee Holt tested  curves  metals.  specimens w h i l e c y c l i n g t h r o u g h the  f o r m a t i o n temperature  range.  The temperature  trans-  was changed  i n s t e p s i n some c a s e s and c o n t i n u o u s l y i n o t h e r s .  These  t e s t s showed t h a t the f l o w s t r e s s i s not a s t r o n g f u n c t i o n of the c r y s t a l s t r u c t u r e but depends o n l y upon the e x i s t i n g defect  structure.  The work h a r d e n i n g r a t e , on the  other  hand, i s s t r o n g l y dependent upon the c r y s t a l s t r u c t u r e , much h i g h e r i n the fee  being  phase.  A comparison of c r i t i c a l l y r e s o l v e d shear s t r e s s f o r a number of c r y s t a l s i s p r e s e n t e d  i n Table I I .  The s t r e s s  f o r c o b a l t i s c o n s i d e r a b l y h i g h e r than f o r m e t a l s w i t h similar structure.  The hep m o d i f i c a t i o n has a c / a  ratio  s i m i l a r t o magnesium and deforms p r e d o m i n a n t l y on the b a s a l p l a n e as does magnesium, y e t i t has a c r i t i c a l l y r e s o l v e d shear s t r e s s more r e p r e s e n t a t i v e  of the m e t a l s t i t a n i u m  and z i r c o n i u m which deform on the p r i s m system.  In  fact,  the c r i t i c a l l y r e s o l v e d shear s t r e s s f o r c o b a l t on the  basal  p l a n e i s l a r g e r than the v a l u e r e q u i r e d t o y i e l d p r i s m s l i p  TABLE I I  C r i t i c a l l y Resolved  METAL  Shear S t r e s s f o r V a r i o u s  SLIP SYSTEM  C.R.S.S.  (psi)  Metals  TEST T / M . P t .  PURITY  Cd  hep  Basal  82  0.51  99.996  Zn  hep  Basal  26  0.43  99.999  Mg  hep  Basal  63  0.33  99.996  Co  hep  Basal  1400  0.17  99.998  Zr  hep  Prism  900  0.14  -  Ti  hep  Prism Basal  1980 16000  0.16 0.16  99.99  Be  hep  Basal  5700  0.19  -  Co  fee  <111><110>  2680  0.40  99.998  Al  fee  (111><110>  14 8  0. 32  99. 93  Ag  fee  {111><110>  54  0. 24  99.99  Au  fee  <111><110>  132  0.23  -  Cu  fee  <111><110>  92  0.22  99.999  Ni  fee  <111><110>  820  0.17  -  Fe  bcc  {110} <110> {112} {123}  4000  0.17  99.6  Mo  bcc  7000  0.10  -  Data drawn from: Holt  4  Ahktar  3  Deiter  3  2  3  ,  Reed-Hill  3  4  3  5  in  zirconium.  e n e r g y and  This data  i m p l i e s t h a t the  the m a r t e n s i t i c t r a n s f o r m a t i o n  a unique  situation  that at present  1.3  The  Allotropic  Transformation  The  relationship  is  complicated  occurs that hep  due  a t a low  outline  the  resulting 1.3.1  c o b a l t cjoes n o t  This  '  3 7  .  transformation  a v a i l a b l e data  The  Cobalt  cobalt  on  The the  that  literature  completely  Thus, p o l y c r y s t a l  original  the  shows  to  cobalt,  temperature,  at  contains  following section transformation  the  will  and  the  Transformation  discovery  that cobalt existed i n  m o d i f i c a t i o n s i s c r e d i t e d to H u l l  p a r a m e t e r s and  the  Until  1942,  t h e r e was  high  temperature  allotropic  high  temperature x-ray  showed c o n c l u s i v e l y t h a t a More r e c e n t  temperature  t o the  III. regarding  transformation Edwards  second  5 5  paramagnetic  3 9  and  '  k 9  a  .  second The  others  transformation  observed  of  temperatures.  investigators attribute  transformation  ferromagnetic  i n Table  controversy  work by  1921.  in  3 8  two  the d e t e r m i n a t i o n  transformation  e a r l y work i s summarized  occur.  understood.  polycrystal  transform  e a r l y cobalt studies d e a l t with  lattice  produce  S t r u c t u r e of  and  fault  structures in cobalt.  allotropic The  3 6  modifications.  H i s t o r y of The  in cobalt  to the m a r t e n s i t i c t r a n s f o r m a t i o n  p h a s e upon c o o l i n g  allotropic  and  homologous t e m p e r a t u r e .  polycrystal  stacking  i s not w e l l  between s i n g l e  t e m p e r a t u r e s below the two  low  the  did  5 6  detailed '  5 7  ,  not  high  t o t h e chancre f r o m  s t a t e , i . e . the  Curie  the Point . 6  13  1.3.2  Mechanisms f o r the M a r t e n s i t i c T r a n s f o r m a t i o n The mechanism whereby the h i g h temperature fee  transformed  lattice  i n t o the low temperature hep phase was o r i g i n a l l y  e n v i s i o n e d by v a r i o u s r e s e a r c h e r s  5 8  '  as b e i n g a c c o m p l i s h e d  5 9  by the passage o f S c h o c k l e y p a r t i a l d i s l o c a t i o n s over second p l a n e i n the fee l a t t i c e t o y i e l d the hep All  theories  r e g a r d i n g the t r a n s f o r m a t i o n  every  lattice.  have assumed  type of d i s l o c a t i o n m o t i o n , they d i f f e r o n l y i n  this  proposals  as t o how the p a r t i a l d i s l o c a t i o n s a r i s e and how they o p e r a t e t o t r a n s f o r m a b u l k of m a t e r i a l from one phase t o the  other. Christian  f i r s t advanced a h y p o t h e s i s  5 9  involving  r e f l e c t i o n of p a r t i a l d i s l o c a t i o n s at a free r i s e t o a hep l a t t i c e .  6 0  He assumes t h a t p e r f e c t dissociate  i n the  later  The p o l e mechanism p o s t u l a t e d  i s a more s u c c e s s f u l attempt a t  6 1  to give  T h i s mechanism was thought by  workers t o be i m p r o b a b l e . Seeger '  surface  explanation.  d i s l o c a t i o n s o f the form a/2  (111) p l a n e .  by  [101]  I f the d i s l o c a t i o n i s p i n n e d  by a s e s s i l e  " p o l e " d i s l o c a t i o n w i t h a screw component  equal  t o t w i c e the  (111)  will  interplaner  s p a c i n g the hep l a t t i c e  be g e n e r a t e d from the fee l a t t i c e . found problems w i t h t h i s mechanism  Following 3  1  1  6  2  -  7  1  investigators  r e l a t e d to  number o f these h i g h energy p o l e d i s l o c a t i o n s r e q u i r e d g i v e r i s e to b u l k t r a n s f o r m a t i o n . s i n g l e c r y s t a l w h i s k e r s are Bollman , 2 2  the to  T h i s i s c r i t i c a l when  considered .  i n 1961, p o s t u l a t e d  6 2  t h a t the  transformation  proceded by a n u c l e a t i o n mechanism based on the of s t a c k i n g f a u l t s on v a r i o u s {111} p l a n e s .  intersection  To accomodate  15  s t r e s s v/hen one stacking  f a u l t was  i t g r e w and theory  nucleated.  Due  t h e mechanism r e p e a t e d .  Transformation  i n the parent  The  m u s t be u n d e r way  before  new  problem w i t h  on more t h a n one  crystal,  a  t o f r e e energy c o n s i d e r a t i o n s  i s t h a t i t does not a l l o w f o r m a t i o n o f a  crystal. planes  s t a c k i n g f a u l t i m p i n g e d on a n o t h e r ,  this  single  of the  {111}  ( i . e . m u l t i v a r i a n t transformation)  s u i t a b l e conditions are  available  to allow further transformation. Altstetter c r y s t a l s and  and  coworkers  polycrystals.  studied cobalt  2 6 - 3 1  T h e i r f o r m u l a t i o n of  t r a n s f o r m a t i o n appears promising  single the  i n t h a t the mechanism  p r o p o s e e x p l a i n s many o b s e r v a t i o n s made by o t h e r s " 2  D e l a m o t t e and A l t s t e t t e r on w o r k by V e n a b l e s  7 2  and  that p a r t i a l dislocation (111)  plane  and  present  3 1  12 5 1 3 6  they *  a nucleation theory  other workers ' *. 7 3  3 7  '  6 5  ~  7 X  based  They p r o p o s e  7 1  l o o p s a r e n u c l e a t e d on  every  second  c o n s i d e r the f r e e energy f o r f o r m a t i o n  of  a l o o p o f r a d i u s r as f o l l o w s : AF =  2Trr  (Gb /4Tr) l n ( 2 r / r 2  *—  )  -  9_>  y  i G = o = c = Ag t  irr (Tb) 2  »  y  - ^r cAg  1  i i  .....1)  2  \  y  t,  i i i  shear modulus shear s t r e s s twice the ^ayer^spacing = e n e r g y p e r cm for transformation  b = Burgers  vector  Where: i)  i s the e l a s t i c  energy  ( d i s l o c a t i o n loop  energy) r e q u i r e d f o r d i s l o c a t i o n f o r m a t i o n w i t h v e c t o r b. G,  at the  low v a l u e .  For r e a l i s t i c  n u c l e a t i o n r a t e s the  T h i s p r o b l e m has  Burgers  shear  i n t e r f a c e b e t w e e n p h a s e s m u s t h a v e an  line  modulus,  anomalously  b e e n t r e a t e d by o t h e r  workers . 7 5  .  ii)  i s the r e d u c t i o n  i n f r e e energy due  p a r t i a l d i s l o c a t i o n s sweeping over the under the  i n f l u e n c e of s t r e s s T .  internal constraints  to  fcc-hcp i n t e r f a c e  T can a r i s e from both  as w e l l as from e x t e r n a l l y  applied  shear s t r e s s . iii)  i s the  f r e e energy a v a i l a b l e due  t r a n s f o r m a t i o n of a volume of m a t e r i a l the more s t a b l e phase, spacing.  g  i s the  t  t r a n s f o r m i n g one  from one  c i s equal to twice the  to phase to layer  f r e e energy per c u b i c c e n t i m e t e r f o r  phase i n t o the  other.  T h i s mechanism operates f o r the  initial  fee to  t r a n s f o r m a t i o n when n u c l e i of Shockley p a r t i a l  hep  dislocations  form from p e r f e c t a/2[l01] type d i s l o c a t i o n s t h a t have been drawn from sub-boundaries a t low deformation. 1/2 As  the  I t may  be expressed  a [ 1 0 1 ] * 1/6  a  [112]  stacking  be n u c l e a t e d .  faults  + 1/6  a  [211]  2)  (loops of p a r t i a l  dislocation)  successive  a <121>  intrinsic  loop to n u c l e a t e . stacking  <121> due  variant  In other words, once an  f a u l t e x i s t s , formation of  p a r t i a l d i s l o c a t i o n s w i l l be c o n t r o l l e d by of shear s t r e s s .  a  p l a n e s , the c o n s t r a i n t s  to t r a n s f o r m a t i o n w i l l cause a d i f f e r e n t c o p l a n a r 1/6  increases  As c e r t a i n numbers of a g i v e n 1/6  loop are n u c l e a t e d on  by  as:  temperature i s decreased the d r i v i n g f o r c e  u n t i l further can  s t r e s s , or produced  the  subsequent l o c a l value  1.3.3  Multivariant  Transformation  A multivariant transformation on more t h a n one o f t h e p l a n e s in  this  case,  formation (111)  proceeds  type The  plane,  dislocations  i s termed c o p l a n a r  i . e . operation  planes  i f sufficient  The pointed  shear  multivariance  strain  transforming  authors. during loops  surface  {111}  on a l l  and no e x t e r n a l  .  Bulk t r a n s f o r m a t i o n  occur  little  shear d i r e c t i o n  by  average of the  Thus,  i t i s not by some  external applied stresses are involved the n u c l e a t i o n of p a r t i a l  f o r 1/6  minimize the e f f e c t  a  [112]  Delamotte  3 1  observed  dislocation  shear d i r e c t i o n s t h a t  of the external s t r e s s . surface  maximum c a l c u l a t e d f o r t h i s  In p o l y c r y s t a l m a t e r i a l , nucleated  type  temperature fee  s h e a r m a r k i n g s were o v e r l o o k e d  transformation,  theoretical  7 6  {111}- p l a n e c h a n g e s o f t e n .  If high  technique,  [121]  a  shear markings has been  i s p o s s i b l e with  i f the operative  that  will  1/6  a r e imposed.  out i n the l i t e r a t u r e  surprising  given  to coplanar  can occur  n u c l e i are present  absence of s t r o n g  coplanar  on a  The a n a l y s i s i s e q u a l l y  i n the high  p h a s e and t h e r e f o r e t r a n s f o r m a t i o n  constraints  If trans-  multivariance.  rise  of d i f f e r e n t  -(111) p l a n e .  i n a given  f o r a l l {111}  planes  plane.  mechanism o u t l i n e d above g i v e s  multivariance  valid  type  i n more t h a n one d i r e c t i o n this  place  available f o r transformation;  (111)  more t h a n one  i s one t h a t t a k e s  By  will  this  shear c l o s e t o the transformation  the operative  2 8  .  partials  a r e i n f l u e n c e d by t h e c o n s t r a i n t s a r i s i n g  out of  t h e r m a l a n i s o t r o p y and volume changes due t o the formation.  trans-  Thus, f o r p o l y c r y s t a l l i n e m a t e r i a l v a r i o u s  shear v a l u e s a r e observed depending on g r a i n s i z e and o t h e r factors. U u l t i v a r i a n t transformations  i n cobalt polycrystals  have been observed by numerous i n v e s t i g a t o r s  2 8 1 7 1 1 7 6 1 7 7  T h i s phenomenon! has a l s o been observed i n a l l o y where a s i m i l a r phase t r a n s f o r m a t i o n  occurs  7 8  '  .  systems 7 9  .  The absence of a m u l t i v a r i a n t t r a n s f o r m a t i o n  i n the  p r o d u c t i o n of c o b a l t s i n g l e c r y s t a l s has been e x p l a i n e d i n v a r i o u s ways.  Seeger ' 6 0  6 1  p o s t u l a t e d t h a t the  {111}  p l a n e h a v i n g the g r e a t e s t a r e a i n the s i n g l e c r v s t a l i s the one t h a t o p e r a t e s d u r i n g the t r a n s f o r m a t i o n ; investigators  found t h i s was not the c a s e .  put forward by A l t s t e t t e r and A d a m s for s i n g l e c r y s t a l s ,  30  later  The e x p l a n a t i o n  and v e r i f i e d by them  i s t h a t the u n i d i r e c t i o n a l c o o l i n g  during c r y s t a l s o l i d i f i c a t i o n  i s the d e c i d i n g f a c t o r .  {111} p l a n e t h a t has the g r e a t e s t area normal t o the  The cooling  d i r e c t i o n i n v a r i a b l e g i v e s r i s e to the t r a n s f o r m a t i o n . a l s o d i s c o v e r e d t h a t upon r e h e a t i n g through the t r a n s f o r m a t i o n occur.  These a r e e i t h e r  the s i n g l e c r y s t a l  two p o s s i b i l e o r i e n t a t i o n s may  the o r i g i n a l fee o r i e n t a t i o n o r  t w i n as has been v e r i f i e d by A d a m s After  They  30  its  and observed by Holt * .  the i n i t i a l c o o l i n g t r a n s f o r m a t i o n  1  for a single  c r y s t a l or a p o l y c r y s t a l , the next a n n e a l i n g c y c l e has an important e f f e c t upon c o o l i n g .  on the t r a n s f o r m a t i o n t h a t w i l l then o c c u r  I f the a n n e a l i n g t r e a t m e n t i s c a r r i e d out  below 600°C, the o p e r a t i v e h a b i t p l a n e s d u r i n a the  heating  transformation  a r e t h e same h a b i t p l a n e s  cooling  Thus, a s i n g l e  and If  '  7 1  3 0  .  a polycrysta}. maintains a specimen  i s annealed  600°C f o r v e r y  crystal  a t higher  a multivariant transformation  within  a given  the production  to  temperature  Retained Except  very  annealing  will  Further  further refine  The above o b s e r v a t i o n s  case  l a r g e r than  of single  c r y s t a l s (or  c o b a l t a t room t e m p e r a t u r e  8 0  specimens  force tending  t o complete the  factors that a f f e c t  t h e amount o f  takes  low  temperature  and  a n n i h i l a t i o n may o c c u r ,  they  will  7 6  .  place at a  relatively  ( 0 . 3 9 T ) where some d i s l o c a t i o n m  sources  rearrangement  b u t major r e d i s t r i b u t i o n  As t h e u n t r a n s f o r m e d  remain f e e e i t h e r  dislocation  and g r a i n  involved.  transformation  place  arises  and t h e s m a l l  fee are the defect s t r u c t u r e , p u r i t y  take  isa  The r e t e n t i o n o f f e e i n  1 micron or i n bulk  Other  o f t h e specimens The  a r e due  FCC P h a s e  thermodynamic d r i v i n g  size  annealing  the s t r u c t u r e  from t h e m u l t i v a r i a n c e o f t h e t r a n s f o r m a t i o n  retained  exhibit  and t h e t r a n s f o r m a t i o n .  i n the s p e c i a l  transformation.  or at  each r e g i o n o f c r y s t a l  upon c o o l i n g .  o f f e e and hep p h a s e s .  particles  crystal  and m o b i l i t y o f d i s l o c a t i o n s a s r e l a t e d  fine p a r t i c l e s )  mixture  will  fee grain.  to  1.3.4  type  remains a s i n g l e  temperatures,  long p e r i o d s o f time,  of this  during  t h e same d e g r e e o f m u l t i v a r i a n c e .  t h a t was o f one o r i e n t a t i o n b e f o r e  treatments  that operate  areas  decrease  cannot  i n size  because o f a l a c k o f s u i t a b l e  o r b e c a u s e t h e surroundi.ng  transformed;  m a t e r i a l cannot accommodate f u r t h e r volume changes. r e s e a r c h on p o w d e r s  5 5 - 5 7  polycrystalline c o b a l t  , thin f i l m s  8 1  "  8 3  t h a t any  type of deformation  towards c o m p l e t i o n 1.3.5  8 3 - 8 6  studied  3 7  , whiskers  and  6 2 - 6 k  I t has a l s o been f o r c e s the  transformation  .  ;  Thermodynamics of the The  '  , measured v a l u e s f o r r e t a i n e d  fee phase vary from zero to over 50%. observed  3 6  During  Transformation  thermodynamics of the t r a n s f o r m a t i o n have been 3 0  and  approximately  the enthalpy 100  f o r the t r a n s f o r m a t i o n i s  c a l o r i e s per mole.  Adams  determined  30  t h a t about 15% of the enthalpy change i s a s s o c i a t e d w i t h defect production.  T h i s i s s u f f i c i e n t energy to produce  s t a c k i n g f a u l t s on every 10th plane, a d i s l o c a t i o n d e n s i t y 1 1 2 of 10 0.04%.  /cm  , or an i n c r e a s e i n vacancy c o n c e n t r a t i o n of  T h e r e f o r e , as c o b a l t i s transformed,  s t r u c t u r e may Yegolayev ' 8 5  increase perceptibly. and H o u s k a  8 6  76  the d e f e c t  As p o i n t e d out  by  the f i n a l d e f e c t s t r u c t u r e  depends upon the temperature to which the c o b a l t specimen is cycled.  D e f e c t s are produced d u r i n g each t r a n s f o r m a t i o n  c y c l e but a n n i h i l a t i o n a l s o takes p l a c e a t the temperature. balance  high  As the number of c y c l e s i n c r e a s e s , e i t h e r a  i s reached  produced by one  where a l a r g e p r o p o r t i o n of the d e f e c t s  c y c l e i s a n n i h i l a t e d a t the h i g h  or r e c r y s t a l l i z a t i o n begins.  The  temperature  r e s u l t s of any g i v e n s e t  of c y c l i n g experiments depend upon the specimens used the temperatures i n v o l v e d .  and  1.3.6  The H y s t e r e s i s o f the  Transformation  Data f o r the t r a n s f o r m a t i o n , 2 8  hysteresis  indicates that  i s s m a l l e r f o r s i n g l e c r y s t a l s than  polycrystals .  for  F o r b o t h t y p e s o f m a t e r i a l the  3 0  hysteresis  i n c r e a s e s w i t h c y c l i n g t h r o u g h the t r a n s f o r m a t i o n , e v e n t u a l l y reaches a s t a b l e v a l u e .  Adams  30  the  but  determined  M s - A s t o be 13°C f o r s i n g l e c r y s t a l s , and 30°C f o r polycrystals.  Other a u t h o r s  3 6  c o n s i d e r a b l y l a r g e r for other  '  8 5 1 8 6  have found v a l u e s  forms o f c o b a l t .  Parr  6 3  ,  w o r k i n g w i t h c o b a l t w h i s k e r s , observed a h y s t e r e s i s o f l e s s than 5 ° C .  The h y s t e r e s i s  p a r t i c l e s and t h i n f i l m s .  i s very large for small  Petrov  8 0  found t h a t 500 Angstrom  a e r s o l p a r t i c l e s o f c o b a l t d i d not t r a n s f o r m t o hep a t any temperature i n the absence of d e f o r m a t i o n . upon h e a t i n g , 500°C.  the t r a n s f o r m a t i o n  Votava  3 6  '  However,  to fee o c c u r e d a t  w o r k i n g w i t h t h i n f i l m s i n the  3 7  about electron  m i c r o s c o p e found t h a t the h y s t e r e s i s was i n c r e a s e d r a d i c a l l y by one c y c l e through the t r a n s f o r m a t i o n . postulated (M  t h a t the observed h y s t e r e s i s  e q u a l s 100°C, A  g  He  o f 450°C  e q u a l s 550°C) was due to l o s s of  n u c l e i f o r t r a n s f o r m a t i o n and l a c k of c o n s t r a i n t thin  films.  1.4  Scope of P r e s e n t Work  i n the  The o b j e c t of the p r e s e n t study i s t o p r o v i d e a d e t a i l e d p r o f i l e of the t e n s i l e p r o p e r t i e s  of c o b a l t  p o l y c r y s t a l s over the temperature range where the phase i s s t a b l e .  hexagonal  A major p o r t i o n of the work i s r e l a t e d  to  the e f f e c t of the i n c o m p l e t e a l l o t r o p i c phase t r a n s f o r m a t i o n on the d e f o r m a t i o n b e h a v i o u r of It  cobalt.  i s proposed t h a t by m o n i t o r i n g the completeness of  the t r a n s f o r m a t i o n a t a l l s t a g e s of d e f o r m a t i o n ,  clarification  of much anomalous d a t a p r e s e n t i n the l i t e r a t u r e may r e s u l t . The d e f o r m a t i o n of c o b a l t i s examined w h i l e v a r y i n g annealing procedures, t e s t temperature.  purity,  completness of t r a n s f o r m a t i o n and  The e x p e r i m e n t a l program b e g i n s w i t h  e v a l u a t i o n of the s t r u c t u r e of specimens b e f o r e  testing.  A l l f u r t h e r e x p e r i m e n t a l r e s u l s are r e l a t e d t o the observed i n t h i s prepared m a t e r i a l .  structures  23  2.  Experimental  2.1  Materials  The l a c k the  o f a g r e e m e n t between  literature  purity  Procedure  has o f t e n  of the c o b a l t  production  8 1  .  remelted  The m a j o r i t y  form.  8 7  '  8 8  .  The method  bulk m a t e r i a l  p r o d u c e d by powder  electrolytic  i n Table  IV.  Laboratories,  Gordon Mines L i m i t e d ,  The m a j o r d i f f e r e n c e  differ  Saskatchewan,  i s the n i c k e l content.  i r o n a r e the major  elements t h a t  obtained  and S h e r r i t t  grades of  In a l l c a s e s , n i c k e l , Zinc,  lead,  and  have been shown t o have d e l e t e r i o u s of cobalt  from  Alberta.  the three  impurities.  on t h e t e n s i l e p r o p e r t i e s  in this  present i s  a n a l y s e s were  between  are well  below  silicon other  effects critical  levels . 82  All  three  worked  rod.  was  follows:  as  grades of c o b a l t  were  of  9 9 . 7 % , 9 9 . 9 % , and 99.998%  Colnbrook, England  Fort  also  materials  on t h e i m p u r i t i e s  Cobalt  or i n  The p r o p e r t i e s  l e v e l s o f p u r i t y have been i n v e s t i g a t e d  a detailed report  and  e f f e c t s on  have  metallurgy  cobalt;  cobalt  cobalt  literature  powders  source of m a t e r i a l .  The n o m i n a l p u r i t y v a l u e s a r e  Koch L i g h t  i n the  cobalt  study.  presented  of  e i t h e r as d e p o s i t e d  years,  somewhat f r o m t h e r e f i n e d Three  of the data  cobalt,  In r e c e n t  become an i m p o r t a n t the  tested  with e l e c t r o l y t i c  from  been a t t r i b u t e d t o d i f f e r e n c e s i n  has a l s o been shown t o have i m p o r t a n t  properties deals  r e s u l t s gathered  obtained  The d i a m e t e r o f t h e "as r e c e i v e d "  as c o l d material  TABLE IV  Element  Spectrographic Analysis of Cobalt  99.7%  +  99.9%  99.998%  +  +  Matrix  99.998%*  %  %  %  P.P.M.  Ni  0.1  0.02  <0.005  N.D.  Si  0.05  N.D.  <0.005  7  Fe  0.005  <0.01  -  3  Al  0.005  -  -  -  i)  E l e m e n t s q u o t e d a t l e v e l s l e s s t h a n 100 P.P.M. (<0.01%) i n a l l s p e c i m e n s by S h e r r i t t G o r d o n — A s , Cd, L i , T e , Zn.  ii)  E l e m e n t s d e t e c t e d a t l e v e l s o f 1 P.P.M. o r l e s s by Kock L i g h t L a b o r a t o r e s i n t h e 99.998% m a t e r i a l -- Ag, C a , Cu, Mg.  iii)  The f o l l o w i n g e l e m e n t s were s p e c i f i c a l l y b u t n o t d e t e c t e d (N.D.)  sought  — By S h e r i t t G o r d o n Ag, B, Ba, Be, B i , C a , C r , Cu, Ge, Kg, Mg, Mn, M o , P b , Sb, Sn, T i , V, Z r . -- By Kock L i g h t A l , A s , Au, B, Ba, Be, Cd, C r , C s , Ga, Ge, Hg, I n , I r , K, L e , Mn, Mo, Na, Nb, Os, Pb; P t , Rb, Re, Rh, Ru, Sb, Se, Sn, S r , T a , T e , T i , T l , V, Yl, Zn, Z r .  A n a l y s i s by r e s e a r c h and d e v e l o p m e n t d i v i s i o n , S h e r i t t G o r d o n M i n e s L i m i t e d , For!t S a s k a t c h e w a n , A l b e r t a , C a n a d a . Maximum s e n s i t i v i t y q u o t e d a s 5 0 P.P.M. :  A n a l y s i s by Kock L i g h t L a b o r a t o r i e s , C o l n b r o o k , England. Maximum s e n s i t i v i t y q u o t e d a s +50% o f t h e amount p r e s e n t .  25 Nominal P u r i t y  Diameter  Q. "6.  99.7  3 mm (0.125")  99.9  6.25 mm (0.250")  99.998  5 mm. (0.200")  The v e r y h i g h p u r i t y m a t e r i a l  (99.998%) was s u p p l i e d  by Kock L i g h t ; the two lower p u r i t y grades  (99.7% and 99.9%)  were o b t a i n e d from A . D. MacKay I n c . , New Y o r k , U . S. A . 2.2 2.2.1  P r e p a r a t i o n o f T e n s i l e Specimens Machining The i m p o r t a n t d i m e n s i o n s of the t e n s i l e  specimens  which were produced on a s m a l l l a t h e are shown i n F i g u r e 1. From the 5 mm and 6.2 5 mm m a t e r i a l , double  buttonhead  specimens were machined t o m i n i m i z e problems r e l a t e d gripping constraints during t e n s i l e t e s t s . material,  to  For the 3 mm  a d o u b l e buttonhead t y p e o f specimen would have  g i v e n an unworkably s m a l l specimen d i a m e t e r . m a t e r i a l was t h e r e f o r e specimens.  The 3 mm  machined i n t o s i n g l e buttonhead  In a l l cases the gauge l e n g t h was m a i n t a i n e d  a t t e n times the reduced specimen d i a m e t e r .  A l l specimens  were machined 0.1 mm (0.004") o v e r s i z e t o a l l o w f o r subsequent e l e c t r o p o l i s h i n g . T e n s i l e t e s t i n g g r i p s f o r the I n s t r o n were machined from tool s t e e l  ( A t l a s Keewatin) and heat t r e a t e d t o a hardness  of 54-56 R .  The p u l l rods and s l e e v e s f o r the. s p l i t g r i p s  were made from 316 s t a i n l e s s  steel.  ro  LO  o  CN  Purity 99.9%  --25 m m  CM  u  99.998% 20 99.7%  U-15-*) a l l .dimensions i n mm. Fig.  1  T e n s i l e specimens and i m p o r t a n t d i m e n s i o n s  0  Fig.  2  10 60 70 80 Time a f t e r e n t e r i n g furnace T y p i c a l r e c o r d of vacuum a n n e a l i n g t r e a t m e n t  2.2.2  A n n e a l i n g Procedures A t y p i c a l heat treatment p r o f i l e i s shown i n F i g u r e  2.  Treatments were c a r r i e d out a t temperatures up to 1000°C w i t h temperature monitered v i a a chromel-alumel  thermocouple  p l a c e d d i r e c t l y among the specimens undergoing heat treatment. Heat up r a t e s were r a p i d , f o u r minutes t o reach 600°C ambient  from  temperature, i n c r e a s i n g t o a maximum of e i g h t t o  ten minutes t o a t t a i n 1000°C from ambient.  The r a t e of  furnace c o o l i n g through the t r a n s f o r m a t i o n temperature range was  between four and s i x degrees c e n t i g r a d e per -5  minute. mm  Vacuum was maintained between 0.4 and 1.0 X 10  of Hg throughout the a n n e a l i n g procedures. The specimens r e t a i n e d an e x c e l l e n t s u r f a c e through the  heat treatments and s u r f a c e markings due to the m a r t e n s i t i c t r a n s f o r m a t i o n c o u l d only be a s c e r t a i n e d by r e p l i c a t e c h n i q u e s . 2.2.3  X-Ray A n a l y s i s Q u a n t i t a t i v e x-ray a n a l y s i s was  adopted f o r d e t e r m i n i n g  the p r o p o r t i o n s of the two a l l o t r o p i c m o d i f i c a t i o n s of c o b a l t present i n a l l specimens.  The a n a l y s i s was  c a r r i e d out a f t e r  heat treatment and f o l l o w i n g deformation procedures. The method adopted was by Sage and G u i l l a r d many authors  6 5- 7 1  182  '  8 h  »  9 0  ,  8 5  f i r s t put i n q u a n t i t a t i v e form  i n 1 9 4 9 , and has been u t i l i z e d  .  standard which s i m p l i f i e s  The method does not r e q u i r e a analysis..  Information r e g a r d i n g the presence of the two i s o b t a i n e d by comparing  by  phases  the d i f f r a c t e d i n t e n s i t y of the  (1011)  l i n e i n the hep phase t o t h a t of the phase.  (200) l i n e i n the fee  When the m u l t i p l i c i t y f a c t o r and other v a r i a b l e s are  taken i n t o account, the p r o p o r t i o n o f fee phase p r e s e n t be determined x =  may  from the formula:  21, fee fee  hep  ..... 3)  x = p r o p o r t i o n of metastable fee phase I, = I n t e n s i t y of the rcc  (200) fee l i n e  I  (loll)  n C  p = I n t e n s i t y of the  hep  line  The d e r i v a t i o n of t h i s formula and the r e q u i r e d i s presented i n Appendix A l l x-ray work was  1. c a r r i e d out w i t h a P h i l l i p s PW  d i f f r a c t o m e t e r u t i l i z i n g manganese f i l t e r e d The r a t e a t which the d i f f r a c t o m e t e r was from 1/4  t o 2 degrees  20 per  o c c u r s a t approximately 60.3  iron  rotated  1011/60  radiation. varied  minute.  Using i r o n r a d i a t i o n , the  peak approximately 66.7  analysis  (1011) hep  degrees  degrees  20 . 8 h  i n t e n s i t y peak  20, and the  (200) f e e  To a l l o w i n t e n s i t y  c a l c u l a t i o n s d i r e c t l y from the x-ray c h a r t , a slow r a t e of d i f f r a c t o m e t e r r o t a t i o n was  used  (one q u a r t e r degree  per  minute) t o p r o v i d e a c l e a r i n t e n s i t y t r a v e r s e from 59 t o 69 degrees  20.  The areas under the r e s p e c t i v e peaks were  then measured and was  s u b s t i t u t e d i n t o formula 3.  T h i s method  t e d i o u s and due t o i n h e r e n t s c a t t e r i n t h i s type of  measurement a s t a t i s t i c a l l y more r e l i a b l e method undertaken  was  f o r the m a j o r i t y of the x-ray a n a l y s i s .  The method may be o u t l i n e d as  follows:  The x - r a y equipment a l l o w e d i n t e g r a t i o n over a g i v e n p e r i o d o f t i m e , or f o r a p r e d e t e r m i n e d number of p u l s e s . The two i n t e n s i t y peaks i n q u e s t i o n c o u l d be c o m p l e t e l y t r a v e r s e d w i t h i n 4 degree r a n g e s ,  59 t o 63 degrees f o r  (1011) hep peak and 65 t o 69 degrees f o r the fee The background x - r a y count t o be s u b t r a c t e d  the  peak.  from the  total  i n t e g r a t e d i n t e n s i t i e s o b t a i n e d by c o u n t i n g a l l p u l s e s over the 4 degrees 20 above, was determined by s c a n n i n g 2 degrees 20 on b o t h s i d e s o f the peak i n q u e s t i o n .  The r e s p e c t i v e  a n g l e s 20 are g i v e n below and a t y p i c a l x - r a y c h a r t i s shown i n F i g u r e 3. Degrees 20  Description  57-59  1/2 o f background f o r hep peak  59-63  T o t a l hep peak  63-65  1/2 o f background f o r hep and fee (B.G.  ( B . G . #1)  (1011) peaks  #2)  65-69  T o t a l fee peak  (200)  69-71  1/2 o f background f o r fee peak  ( B . G . #3)  Therefore: I n t e n s i t y o f hep peak  d  I n t e n s i t y o f fee peak  (I  h  c  f c c  )  ) = T o t a l hep -  ( B . G . #1 + B . G .  = T o t a l fee -  ( B . G . #2 + B . G .  In p r a c t i c e , i t was found t h a t the d i f f e r e n c e s between B . G . #1, B.G.  #2 and B . G . #3 were v e r y s m a l l and s c a n n i n g B . G . #1, and  B.G.  #3 was not j u s t i f i e d because the measurements  improve the a c c u r a c y of the a n a l y s i s .  d i d not  For t h i s reason,  the  net i n t e g r a t e d i n t e n s i t y of the two peaks was determined by s u b t r a c t i n g t w i c e the " B . G . #2" i n t e n s i t y from each peak.  Degrees 2 0 Fig.  3  Raw x - r a y data f o r 99.7% c o b a l t  o  31 A minimum o f f i v e i n t e n s i t y i n t e g r a t i o n s were c a r r i e d out on each specimen.  Each i n t e g r a t i o n was taken from a  d i f f e r e n t area on the specimen and the r e s u l t s from the f i v e scanning procedures were then added and s u b s t i t u t e d i n equation 3. The data obtained u s i n g t h i s method was r e p r o d u c i b l e , but the s c a t t e r i n r e s u l t s was always l a r g e .  T h i s problem 9 1  w i t h q u a n t i t a t i v e x-ray a n a l y s i s i s d i s c u s s e d by Giamei attempted  t o reduce  who  the s c a t t e r by scanning a number o f  peaks and s o l v i n g the data v i a computer techniques.  The t e c h -  nique i s not a p p l i c a b l e where a l a r g e number of specimens are t o be analyzed because o f the equipment time i n v o l v e d . The  l a r g e number o f specimens analyzed and the number  of scanning procedures observed ,  .  9  c a r r i e d out ensure  t h a t any trends ,  a r e i n f a c t r e a l and not a consequence o f the 1  analysis When data r e g a r d i n g the p r o p o r t i o n o f the two phases present i s quoted,  the v a l u e g i v e n w i l l be the average  v a l u e f o r a l l specimens t h a t have the same p u r i t y and have undergone the same treatment. There a r e s e v e r a l f a c t s t h a t should be noted when c o n s i d e r i n g x-ray d a t a .  Diffraction i s essentially a  s u r f a c e measurment, w i t h the m a j o r i t y o f the d i f f r a c t e d x-rays coming from the outer 25 microns  of m a t e r i a l .  When  the g r a i n s i z e i s o f this, o r d e r , the x-ray r e s u l t s a r e due almost e x c l u s i v e l y t o the s u r f a c e g r a i n s .  The s u r f a c e g r a i n s  e x i s t under d i f f e r e n t c o n s t r a i n t than the i n t e r i o r g r a i n s and t h i s becomes important when the m a r t e n s i t i c t r a n s f o r m a t i o n  i n cobalt i s considered.  I f the surface grains are less  c o n s t r a i n e d , t h e t r a n s f o r m a t i o n w i l l proceed completion  f u r t h e r towards  i n these g r a i n s ; thus the x-ray r e s u l t s  will  g i v e a high v a l u e f o r the completeness o f the t r a n s f o r m a t i o n . T h i s a n a l y s i s leads t o the c o n c l u s i o n t h a t t h e measured amount o f t r a n s f o r m a t i o n i s a maximum, and the i n t e r i o r of  the specimen may c o n t a i n more metastable  face-centered  phase than the x-ray data r e v e a l s . Recognizing  the above c o n s i d e r a t i o n s , the x-ray  data  i s more a c c u r a t e where a s m a l l g r a i n s i z e i s i n v o l v e d because the d i f f r a c t i o n w i l l  take p l a c e from  g r a i n s as w e l l as s u r f a c e g r a i n s .  interior  In any case, the a n a l y s i s  adopted ensures t h a t the measured amount o f t r a n s f o r m a t i o n can be c o n s i d e r e d a maximum v a l u e . 2.3  T e n s i l e and Hardness T e s t s All  t e n s i l e t e s t s were c a r r i e d out on a f l o o r model  I n s t r o n machine u s i n g cross-head speeds between 0.2 in./min. _3 and 2 X 10 in./min. The m a j o r i t y o f t e s t s were c a r r i e d _2  out a t 2 X 10 in./min. corresponding t o a s t r a i n r a t e o f -2 -2 2 X 10 per min. f o r the l a r g e s t specimens and 3.3 X 10 per min. f o r the s m a l l e s t .  ;  T e s t i n g media f o r the temperature range i n v e s t i g a t e d were as f o l l o w s :  (  Media liquid  Temperature  -196°C  nitrogen  petroleum  -140°C t o -100°C  ether  ethanol  -100°C t o 0°C  water  0°C t o 100°C  silicone o i l  100°C t o 250°C  salt  The  bath  (draw temper  temperature  specimen.  The t e s t  250°C t o 400°C  275)  of the testing  a copper-constantan thermocouple  while  temperature  b a t h s was m e a s u r e d  immersed c l o s e  basis  stress  and t r u e  At testing  assuming  uniform deformation  low t e m p e r a t u r e s , resumed  i n less  the baths could  than a minute  handling d i f f i c u l t i e s  extended  b a t h s were b e i n g c h a n g e d  the t e n s i l e  hardness  t e s t s were c a r r i e d  t o a l l o w comparisons  the l i t e r a t u r e .  be c h a n g e d a n d temperatures control  before continuing a that  specimens  the temperature were m a i n t a i n e d  load.  T e s t e r w i t h t h e 10 Kg. a p p l i e d  in  throughout  while a t high  During t h e time  t e n s i o n by a s m a l l c y c l i c  produced  values  and t e m p e r a t u r e  t h e time r e q u i r e d  t o 5 t o 8 minutes.  All  These  on t h e  length.  the p h y s i c a l problems  system.  s t r a i n d a t a were c a l c u l a t e d  o f i n s t a n t a n e o u s a r e a and l e n g t h .  t h e gauge  in  + 1°C  t e s t i n g was underway.  were c a l c u l a t e d  test  with  to the  was m a i n t a i n e d w i t h i n  T e n s i l e d a t a a r e p r e s e n t e d i n t h e F.P.S. True  Range  load.  o u t on a V i c k e r s  Hardness  The D.P.H. d a t a was  t o be made w i t h d a t a  available  2.4  Metallography The metallography of c o b a l t i s d i f f i c u l t  because  of  the complex s t r u c t u r e s a r i s i n g due to the incomplete m a r t e n s i t i c t r a n s f o r m a t i o n i n the m a t e r i a l  9 2  '  9 3  .  The  g r a i n s i z e , i n t e r n a l s t r u c t u r e , and deformation mechanisms were examined o p t i c a l l y and v i a r e p l i c a t e c h n i q u e s . 2.4.1  O p t i c a l Metallography The incomplete m a r t e n s i t i c t r a n s f o r m a t i o n i n c o b a l t  can be f o r c e d towards completion by d e f o r m a t i o n .  This  s t r a i n induced t r a n s f o r m a t i o n was measured a t d i s t a n c e s g r e a t e r than 50 microns from a s c r a t c h . at  l e a s t 100 microns were removed from a l l machined or  ground was  For t h i s reason,  s u r f a c e s by e l e c t r o p o l i s h i n g b e f o r e any metallography  attempted. A number of the c i r c u l a r t e n s i l e specimens were  ground  f l a t and then e l e c t r o p o l i s h e d to o b t a i n a l a r g e enough  f l a t area f o r g r a i n s i z e d e t e r m i n a t i o n s . s e c t i o n of the t e n s i l e specimens was  The c i r c u l a r c r o s s  a b a r r i e r t o good  metallography. The most s u c c e s s f u l e l e c t r o p o l i s h i n g s o l u t i o n found t o be 15% p e r c h l o r i c a c i d i n a c e t i c a c i d . specimens were suspended beaker  The  i n a water c o o l e d s t a i n l e s s  i n which the p o l i s h i n g s o l u t i o n was  continuously.  was  P o l i s h i n g was  stirred  c a r r i e d out a t 20 v o l t s , w i t h  the specimen  r o t a t i n g i n the s t i r r e d  specimen was  s l o w l y i n v e r t e d every 30 seconds  These p r e c a u t i o n s produced  steel  solution.  The to avoid taper.  specimens w i t h + 0.01  mm.  maximum  t a p e r a l o n g the gauge l e n g t h .  T h i s procedure y i e l d s  a s u r f a c e w i t h an apparent m i r r o r f i n i s h .  The  surface  shows l i t t l e e v i d e n c e of p r e f e r e n t i a l a t t a c k a t g r a i n b o u n d a r i e s o r o t h e r h i g h energy  sites.  The m e t a l l o g r a p h i c f e a t u r e s o f the specimens were determined by u t i l i z i n g a second a n o d i c e t c h . used was 5% c o n c e n t r a t e d water.  The e t c h a n t  hydrochloric acid in d i s t i l l e d  The specimen was p l a c e d i n the e t c h i n g b a t h and g i v e n  a v e r y s h o r t p u l s e o f c u r r e n t a t l e s s than one v o l t and then examined.  The r e s u l t i n g s t r u c t u r e  to the s i z e o f the p u l s e of c u r r e n t .  i s very  sensitive  Overetching occurs  v e r y e a s i l y and extreme c a u t i o n must be e x e r c i s e d i f a reproducible surface As an a d j u n c t  is desired.  t o the x - r a y a n a l y s i s of the p r o p o r t i o n  of the two phases p r e s e n t , a p o l a r i z e d l i g h t was employed.  technique  S e v e r a l problems a r o s e when m e t a l l o g r a p h y  under p o l a r i z e d l i g h t was a t t e m p t e d .  A p o l i s h e d o r ground  s u r f a c e was o f no use because o f the s t r e s s induced t r a n s f o r m a t i o n and an e l e c t r o p o l i s h e d s u r f a c e d i d not g i v e a d e f i n i t i v e r e s u l t due t o the presence of a t h i n o x i d e layer.  O b s e r v a t i o n c o u l d o n l y be made f o l l o w i n g  the  second e t c h i n g procedure and t h i s caused problems r e l a t i n g t o the s c a t t e r i n g of l i g h t from g r a i n b o u n d a r i e s and fine structure.  The p e r i o d i c n a t u r e o f the  surface  transformation  shears g i v e r i s e t o a f i n e s t r u c t u r e which can e t c h  to  y i e l d an a n i s o t r o p i c e f f e c t a l t h o u g h s m a l l l a m e l l a r volumes of the fee phase may s t i l l e x i s t .  For these reasons  p r e a t i o n o f p o l a r i z e d l i g h t photomicrographs was cautiously.  inter-  attempted  36  P o l a r i z e d l i g h t metallography proved to be most successful with large grained m a t e r i a l . of i s o t r o p i c m a t e r i a l  When the p r o p o r t i o n  (fee) measured by t h i s technique  was  compared to the r e s u l t s of the x-ray a n a l y s i s , a c c e p t a b l e agreement was 2.4.2  found.  E l e c t r o n Microscope R e p l i c a s R e p l i c a s were produced  by soaking c e l l u l o s e a c e t a t e  sheet i n acetone and then p r e s s i n g the sheet to the s u r f a c e to be analyzed. specimen  The a c e t a t e sheet was  removed from the  and shadowed w i t h chromium and coated w i t h carbon.  The a c e t a t e was  d i s s o l v e d away i n acetone, l e a v i n g the  r e p l i c a to be mounted i n 150 mesh copper g r i d s . r e p l i c a s were examined i n a H i t a c h i HU11A microscope  a t 50  All  electron  KV.  The r e p l i c a procedure was  c a r r i e d out on specimens  t e s t e d between -196°C and 250°C.  To ensure  protection  of the s u r f a c e a t a l l times w h i l e t e s t i n g , t h i c k grease was  spread over the specimen  surface.  The  silicone grease  c o u l d be d i s s o l v e d away i n t r i c h l o r e t h a n e when a r e p l i c a was  desired.  The upper  r e p l i c a procedure was  temperature  limit  (250°C) f o r the  d i c t a t e d by the breakdown of the  p r o t e c t i v e grease, which allowed oxide t o form on the c o b a l t surface.  R e p l i c a s were produced  from annealed  as w e l l as from deformed specimens.  structures  The procedure was  c a r r i e d out on the f r a c t u r e s u r f a c e s of a number of  also  specimens.  3.  Experimental Program and R e s u l t s  3.1  The S t r u c t u r e of P o l y c r y s t a l l i n e C o b a l t T h i s s e c t i o n o u t l i n e s the methods used t o determine  the s t r u c t u r e of the specimens on which a l l t e n s i l e procedures were c a r r i e d out.  The data produced d u r i n g  t h i s work are combined w i t h i n f o r m a t i o n taken from the l i t e r a t u r e i n order to examine the complex s t r u c t u r e s observed.  I t i s then p o s s i b l e t o p r e d i c t the s t r u c t u r e s  that w i l l e x i s t i n cobalt p o l y c r y s t a l s a f t e r a given set of  a n n e a l i n g procedures.  3.1.1  As Received M a t e r i a l From the c o l d worked c o b a l t rod, random samples from  each p u r i t y l o t were prepared f o r t e s t i n g .  Quantitative  x-ray a n a l y s i s showed t h a t the m a t e r i a l was  almost  hep phase.  I n t e g r a t i n g the measured peaks and  i n equation 3 gave a fee content of l e s s than %  f  c  c  =  2 l  fcc fee  •  5%. 3)  hep  with specimens from each p u r i t y grade.  material,  substituting  (100)  T e n s i l e t e s t s were c a r r i e d out a t room  d u c t i l i t y was  100%  found to be l e s s than 3%.  temperature  In a l l cases the In the h i g h p u r i t y  (99.998%) the specimens f r a c t u r e d a t y i e l d .  3.1.1.1 The  Preferred texture  Orientation  i n c o l d worked c o b a l t has  by s e v e r a l a u t h o r s  '  6 6  8 2  '  1 8 9  9 4  .  Wilcox  e l e c t r o d e p o s i t e d c o b a l t sheet and et a l worked w i t h bars and  8 9  been i n v e s t i g a t e d  investigated  sponge m a t e r i a l .  8 2  rods of commercial grade c o b a l t .  Wilcox observed t h a t e l e c t r o d e p o s i t e d c o b a l t has deposited  Beckers  an  as  {1010} t e x t u r e which i s d i f f i c u l t to a n n i h i l a t e .  A f t e r annealing  above the t r a n s f o r m a t i o n  temperature  i n t r o d u c i n g 20%  c o l d work he observed a {0001} <1120>  and rolling  t e x t u r e w i t h the b a s a l planes r o t a t e d 20 to 25 degrees i n the r o l l i n g d i r e c t i o n from the r o l l i n g plane normal. observed a s i m i l a r r e s u l t f o r hot r o l l e d  slabs.  Beckers  For  extruded rods of c o b a l t , a p r e f e r r e d o r i e n t a t i o n w i t h a high d e n s i t y of  {0001} planes p e r p e n d i c u l a r  to  the  8 2  e x t r u s i o n a x i s i s obtained  .  i n s e v e r l y worked m a t e r i a l was annealing 3.1.1.2 The  The  preferred orientation  found to disappear when  treatments were c a r r i e d out above 5 0 0 ° C . 8 2  Stacking low  F a u l t Energy and  Fault Analysis  s t a c k i n g f a u l t energy of c o b a l t and  i n t r o d u c t i o n of many s t a c k i n g f a u l t s both by and  by t r a n s f o r m a t i o n  the  deformation  have l e d many i n v e s t i g a t o r s to study  the f a u l t i n g d e n s i t i e s i n c o b a l t .  Various  experimental  methods have been chosen w i t h most work performed v i a x-ray t e c h n i q u e s  7  6 18 5  magnetic resonance  '  8 6 19 5  '  9 6  (NMR)  97  .  Measurements of frequencies  and  nuclear  direct  observation  i n the e l e c t r o n microscope have a l s o been c a r r i e d o u t  2 1  *'  3 6  '  9 8  A of  stacking  layers  fault  i s an e r r o r  in a crystal lattice.  g r o w t h o f c r y s t a l s and a l s o  i n t h e o r i g i n a l sequence Faults  from  arise  during  deformation.  The d i s t i n c t i o n  between t h e two t y p e s o f f a u l t s s h o u l d be made c l e a r . The  i d e a l hep s t r u c t u r e  c a n be d e s c r i b e d  sequence o f c l o s e - p a c k e d p l a n e s . regular  sequence w i t h t h e r e s t r i c t i o n  must be d i f f e r e n t . the a d d i t i o n fact  that  fault  A fault  occurs.  layers.  packed  second  layer  The f a u l t  stacking hep  •  that  adjacent  i n this  layers  i s governed  by t h e  i s i d e n t i c a l e x c e p t when a i s unlike  fault,  t h e p r e c e d i n g two  i t i s presumed  that  sequence e x i s t s b e f o r e d e f o r m a t i o n  a  takes  placi  hep • ,  ..  1  planes  layer  For a deformation  perfect  i s an e r r o r  T h u s , t h e g r o w t h o f a n hep c r y s t a l by  of close  every  a s a n ABABABAB  ABABABABCBCBCBCBC  Growth  Fault  fee hep I  hep  '  H  .  1  ABABABABCACACACACA L  Deformation  —i—'  Fault  fee Either two  out-of-phase  fault  can also  A planes two  type o f f a u l t  c a n be f o r m e d by t h e g r o w t h o f  hep l a t t i c e s  together.  be f o r m e d by p a r t i a l  i n t o C p l a n e s , and B p l a n e s  fault  slip  The d e f o r m a t i o n which  into A planes.  The  t y p e s may be d i f f e r e n t i a t e d b e c a u s e a g r o w t h  fault  contains three planes of fee stacking  fault  four  layers.  differentiation faults  converts  The a n a l y s i s  and a d e f o r m a t i o n  of x-ray data  to yield  between g r o w t h and d e f o r m a t i o n  stacking  h a s b e e n c a r r i e d o u t by A n a n t h r a m a n and C h r i s t i a n  9 9  .  A n a l y s i s v i a NMR has been p u b l i s h e d by Toth and c o w o r k e r s . 97  The r e s u l t s o f these a n a l y s e s may be summarized as f o l l o w s : i)  In both the annealed and the deformed  the f a u l t d e n s i t y i s h i g h w i t h f a u l t s observed fee and the hep phases.  state  i n both the  A h i g h l y deformed specimen may  have a t o t a l f a u l t d e n s i t y as h i g h as one f a u l t e d plane i n every t e n .  In annealed m a t e r i a l t h i s v a l u e may drop t o one  plane i n t h r e e hundred. ii)  A l l observed  i n t r i n s i c type.  s t a c k i n g f a u l t s a r e o f the  More complex e x t r i n s i c f a u l t s a r e a l s o  p o s s i b l e but they have not been iii) present.  observed ' . 9 7  9 8  Both growth and deformation f a u l t s a r e  The d e n s i t y of growth f a u l t s i s not s t r o n g l y  a f f e c t e d by a n n e a l i n g procedures but i s s u b s t a n t i a l l y decreased by deformation and i s i n c r e a s e d by c y c l i n g the t r a n s f o r m a t i o n .  Deformation  through  f a u l t s , as t h e i r name  i m p l i e s , i n c r e a s e w i t h the amount o f deformation i n t r o d u c e d . The d e n s i t y o f these f a u l t s i s reduced procedures, even a t temperatures  s h a r p l y by a n n e a l i n g  below the t r a n s f o r m a t i o n  temperature. 3.1.2  Recovery,  R e c r y s t a l l i z a t i o n and G r a i n Growth  The a n n e a l i n g spectrum by s e v e r a l authors  81 1 00 - 1 03  important parameters  .  o f c o b a l t has been i n v e s t i g a t e d  Some s t u d i e s d e t a i l e d the  a f f e c t i n g the a n n e a l i n g behaviour  such  as p r i o r treatment, p u r i t y , type o f specimen, e t c , w h i l e o t h e r s d i d not.  Thus, the data i n many cases d i f f e r  significantly.  The l a c k of agreement a r i s i n g from d i f f e r e n c e s i n p u r i t y has been o u t l i n e d by M o r r a l  8 7  and  Winterhager . 88  The most o f t e n quoted p r o p e r t y i s diamond pyramid hardness  (DPH), and f o r t h i s reason a r e c o r d of DPH v e r s u s  a n n e a l i n g procedures was  o b t a i n e d d u r i n g t h i s study t o  a l l o w comparisons to be made. (DPH)  The diamond pyramid  f o r the a s - r e c e i v e d m a t e r i a l was Purity  DPH  99.7% Co  285  99.9% Co  279  99.998% Co  260  hardness  as f o l l o w s :  F i g u r e 4 g i v e s the d,ata gathered f o r the t h r e e p u r i t y l e v e l s i n v e s t i g a t e d and comparable literature.  data drawn from the  In a l l cases, data are shown f o r room  temperature hardness a f t e r a one hour anneal a t the indicated  temperature.  The g e n e r a l c o n c l u s i o n s drawn from t h i s data  may  be r e p o r t e d as f o l l o w s : i)  The room temperature hardness i s not  a f f e c t e d by a n n e a l i n g below 220°C high p u r i t y m a t e r i a l .  (0.28T ), even f o r m  As the i m p u r i t y l e v e l i n c r e a s e s the  temperature a t which any major d i s l o c a t i o n occurs i n c r e a s e s .  rearrangement  Thus, i n the temperature range  to 350°C p o l y g o n i z a t i o n a  n  220°C  ^ r e c o v e r y occur depending  upon p u r i t y . ii)  Over the temperature range t h a t  includes  the r e g i o n where the m a r t e n s i t i c t r a n s f o r m a t i o n takes p l a c e , r e c o v e r y and r e c r y s t a l l i z a t i o n occur.  I t would  • O A  160  200  400  Temperature, 1 h r . r  icr ,  99.7% C o b a l t 99.9% C o b a l t 99.998% C o b a l t P r e s e n t work  600  800  a n n e a l s , °C  Diamond Pyramid Hardness d a t a f o r  cobalt.  1000  appear t h a t r e c y r s t a l l i z a t i o n a t a temperature c o b a l t i s s t a b l e , may iii)  where hep  be p o s s i b l e f o r very pure m a t e r i a l .  G r a i n growth predominates a t  temperatures  above 600°C. 3.1.2.1  Recovery  Although quoted  the recovery of hardness  v a l u e s has been  as low as 220°C by F e l l e r - K n e i p m i e r  i n the bulk flow s t r e s s a t room temperature observed  f o r a n n e a l i n g treatments  The work by S h a r p  1 0 2  no change  10 1  has been  below 350°C  (0.35T ). m  showed t h a t f o l l o w i n g deformation  at -196°C some r e c o v e r y of e l e c t r i c a l r e s i s t i v i t y  occured  i n c o b a l t a t 0.06T and Q.13T , but found no r e c o v e r y m m of flow s t r e s s f o r s h o r t anneals a t 0.38T . They m observed some flow s t r e s s r e c o v e r y w i t h long term anneals at 0.38 and 0.39T . m. A s e t of t e s t s s i m i l a r to t h a t by Sharp  1 0 2  et a l  were c a r r i e d out i n the p r e s e n t study t o s u b s t a n t i a t e t h e i r r e s u l t s f o r the c o b a l t used  i n the p r e s e n t  No r e d u c t i o n i n bulk flow s t r e s s was treatments 0.37T  m  below 0.35T . m  investigation.  obtained f o r  Annealing f o r s e v e r a l davs a t J  allowed r e c o v e r y of o n l y 2% of the flow s t r e s s i n  the p u r e s t m a t e r i a l . 3.1.2.2 R e c r y s t a l l i z a t i o n and G r a i n Growth The r e c r y s t a l l i z a t i o n temperature l i e s between 0.4T  m  and  to 610°C f o r c o b a l t .  0.5T  x m  range f o r most metals  , a temperature  range from 430°C  The a n n e a l i n g treatment r e q u i r e d f o r  r e c r y s t a l l i z a t i o n i s modified c o l d work present,  and  by the i n i t i a l g r a i n s i z e ,  the p u r i t y of the specimens  I n c r e a s i n g p u r i t y tends to lower the  considered.  recrystallization  temperature as does i n c r e a s i n g amounts of c o l d work and smaller g r a i n  sizes.  B i b r i n g and and  Sebileau ~ 6 5  7 1  worked w i t h  99.5%  cobalt  p o s t u l a t e d t h a t r e c r y s t a l l i z a t i o n occured at temperatures  below the t r a n s f o r m a t i o n metallographic  temperature.  evidence and with  They presented  the low p u r i t y m a t e r i a l  they employed t h i s r e s u l t i s d o u b t f u l . study, B e c k e r s  8 2  hour a t 450°C and  one  h a l f hour a t 500°C f o r 99.7%  work, no change was  started after  r e a r y s t a l l i z a t i o n was cobalt.  visible after  In the  present  observed i n the s u r f a c e s t r u c t u r e of  specimens f o r annealing r e c r y s t a l l i z a t i o n was The  In a more r e c e n t  determined t h a t r e c o v e r y  one  no  procedures below 450°C.  v i s i b l e a f t e r s h o r t time  At  500°C,  anneals.  r a t e a t which g r a i n growth proceeds i s i n f l u e n c e d  by the p u r i t y of the c o b a l t t r e a t e d .  As the p u r i t y  i n c r e a s e s , the number of o b s t a c l e s r e t a r d i n g g r a i n coalescence decreases,  and  r e d u c t i o n of high angle boundaries  thus g r a i n growth proceeds more q u i c k l y .  r e s u l t i s shown c l e a r l y i n F i g u r e present  study are p l o t t e d -  5 where the data  This from  the  S e v e r a l data p o i n t s drawn from  the l i t e r a t u r e have been i n c l u d e d f o r comparison. The method used f o r determining forward but t e d i o u s . graphed and  Metallographic  g r a i n s i z e was  straight  specimens were photo-  the number of g r a i n s present were counted  directly.  60  U  50  c o H u 40 •H  A  99.998%  0  99.9%  cobalt  B  99.7%  cobalt  /\  Beckers- -  o  Muller  •  cobalt  8  Fraser  2  8 1  c •H d) N •H 30 CQ C fd  a  20  10  ± 400  300  500  600  700  A n n e a l i n g Temperature Fig.  5  Variation  i n grain  size  f o r 1 hour  800  900  1000  °C.  anneals at i n d i c a t e d  temperatures,  4k  The  magnification  used f o r a n a l y s i s was v a r i e d t o o b t a i n  as many g r a i n s as p o s s i b l e i n the f i e l d o f view w h i l e r e t a i n i n g reasonable g r a i n d e f i n i t i o n .  The smaller  grain  s i z e s were a l s o checked by a g r a i n count taken from r e p l i c a s examined i n the e l e c t r o n microscope. When g r a i n s i z e s i n p o l y c r y s t a l l i n e c o b a l t a r e c i t e d , it  should be noted t h a t the v a l u e s given  temperature f e e phase.  r e f e r t o the high  D i f f i c u l t y a r i s e s i n measuring  the f e e g r a i n s i z e because of the d i s t r i b u t i o n o f hep martensitic The  p l a t e s i n the f e e g r a i n s .  r e l a t i o n s h i p between the g r a i n s i z e and the  f i n e n e s s of the hep s t r u c t u r e i s complex. are p a r t i a l l y f e e a f t e r annealing  A l l specimens  regardless  o f the g r a i n  s i z e , but the g r a i n s i z e a f f e c t s the amount of r e t a i n e d fee phase i n two d i s t i n c t ways. size increases retained  above a c e r t a i n small v a l u e the amount o f  f e e decreases r a p i d l y .  the amount of r e t a i n e d 10%.  F i r s t , as the f e e g r a i n  For large grain s i z e s ,  f e e remains c o n s t a n t a t approximately  Secondly, as the g r a i n s i z e i n c r e a s e s  of the t r a n s f o r m a t i o n the r e t a i n e d  the m u l t i v a r i a n c e  changes, t h a t i s , the manner' i n which  f e e i s d i s t r i b u t e d throughout a g r a i n changes.  T h i s r e s u l t occurs because the d i s t a n c e  over which each  m a r t e n s i t e p l a t e may propogate b e f o r e being changes w i t h g r a i n s i z e .  This distance  obstructed  can be f u r t h e r  d i v i d e d as t o " i n a d i r e c t i o n p a r a l l e l t o " and " i n a d i r e c t i o n perpedicular grow.  t o " the m a r t e n s i t i c  p l a t e s as they  The  main p o i n t to be drawn from what has  been s a i d  above i s t h a t the measured g r a i n s i z e i s not an i n d i c a t i o n of the  s i z e of r e g i o n s  crystal structure.  of c r y s t a l l a t t i c e having the same  The  measured v a l u e should be  considered  a measure of the coarsness of the fee s t r u c t u r e e x i s t i n g before transformation On  takes  place.  the f o l l o w i n g pages  (Figure 6 - 9 )  the  annealed  s t r u c t u r e s of the c o b a l t used i n t h i s study are The  g r a i n s t r u c t u r e i s not an e q u i l i b r i u m  the boundaries present are o f t e n s t r a i g h t and d i r e c t i o n a t r i g h t or acute angles.  presented. structure;  change  A l a r g e amount of  i n t e r n a l s t r u c t u r e i s v i s i b l e which causes d i f f i c u l t y i n determining i n d i v i d u a l g r a i n s . An a n a l y s i s of t h i s i n t e r n a l s t r u c t u r e i s presented f o l l o w i n g d i s c u s s i o n of the completeness of the 3.1.3  transformation.  Completeness of Transformation X-ray procedures were performed on v a r i o u s  i n d i v i d u a l specimens.  The  s c a t t e r i n data from one  of a specimen to another was  found to be w i t h i n  with occasional  erratic results.  were always low  and  The  area  +5%  erratic results  a t t r i b u t e d to improper h a n d l i n g .  s c a t t e r i n r e s u l t s from one to be g r e a t e r  areas of  specimen to another was  than t h a t w i t h i n a given  The found  specimen.  I t became apparent t h a t not only the v a r i a t i o n s i n as r e c e i v e d m a t e r i a l but a l s o the  specimen  procedures i n v o l v i n g machining, could the amount of r e t a i n e d  preparation  influence  fee phase. (Table  V).  radically  48  Fig.  6  A n n e a l i n g spectrum i n 99.9% c o b a l t .  340X  49  Fig.  Fig.  7  8  99.7%  99.9%  cobalt,  cobalt,  annealed  annealed  a t 600°C  (a) and  a t 900°r f o r 1 h r .  i  ,  m  (a) - 9y 9  99.998% c o b a l t ,  annealed  (b) 1 h r .  39u.  "r  df*  ' S"« *  Fig.  800°C  -  .  V .  (b) - 47y a t 600°C  (a) and  800°C  (b) 1 h r .  TABLE V  Summary o f Retained FCC Data  Purity Anneal 99. 7%  99.9% A  99.9% B  99.9% C  99.9% Total  450°C - 1 Hr  49.5  500°C - 1 Hr  59.5  55.1  56.8  550°C - 1 Hr  50.7  43.1  46.0  600°C - 1 Hr  59. 4  650°C - 1 Hr 700°C - 1 Hr  49.5  41.1  38.7 46. 5  750°C - 1 Hr 800°C - 1 Hr  49.6  45.3  42.7 30.4  37.2  41.6  25.3  34.0 27.3  27.3  900°C - 1 Hr  17.0  17.0  1000°C  14.1  14.1  A, B, C, r e p r e s e n t  31. 7  31.8  800°C - 1 Hr  L Hr  40. 8  33.5 41.1  31.8 41. 8  99. 998%  30. 9  d i f f e r e n t l o t s of "as r e c e i v e d " m a t e r i a l  The d i f f e r e n c e s between v a r i o u s batches of m a t e r i a l could not be r e c t i f i e d  " a f t e r the f a c t " but the i n f l u e n c e  of specimen p r e p a r a t i o n was removed by e l e c t r o p o l i s h i n g a minimum of 100 microns from the s u r f a c e of a l l specimens before any heat treatment.  In t h i s way, anomalous r e s u l t s  were avoided. The r e s u l t s of the x-ray a n a l y s i s on annealed specimens are g i v e n i n F i g u r e 10.  The s c a t t e r i n the r e s u l t s i s  l a r g e , e s p e c i a l l y when i t i s c o n s i d e r e d  t h a t each p o i n t  60  50  •  99.7%  cobalt  G  99.9%  cobalt  A  99.998%  I u u  A  40  •  cobalt  99.9% c o b a l t and 5 5 0 ° C .  •  annealed  a t 500°C  T3 (D C  •H 30 4->  QJ PI  20  10  10 Fig. 60  10(a)  J-  50  u u  20 Grain % retained  g  99.7%  cobalt  0  99.9%  cobalt  A  99.998%  O  99.6% c o b a l t  _L 40  30 Size (u)  fee vs g r a i n  50  60  size  cobalt -  Muller  8 3  o  40  m  c 30  A  -  •H  *  4-> 0)  <*> 20  io - o 0  0  o ±  J-  0.1 1//Grain  Fig. Fiq.  0.2 S i z e (y  _ . ) 7  ±  4.  0.3  10(b) % r e t a i n e d f e e v s 1 / / G r a i n Si.ze 10 P r o g r e s s o f the t r a n s f o r m a t i o n as a f u n c t i o n  .0.4 , of q r a m  . size  represents  b e t w e e n 1 0 and  analyses).  Nevertheless,  published The  elsewhere  8 2  retained  size,  decreasing  Also,  as  the  fee  purity  i s an  A  towards  small  anneal.  The  t h a t may  take place  material.  obstacles  nuclei  are  p r e s e n t but For  l e s s dense;  and  t h i s low  the  grain  t o hep.  material,  region  of  c r y s t a l before  plates  or  grain  gives  rise  grain  the  The  of  size  grain  increases.  transformation  i n terms of  low  temperature defects  i s more l i m i t e d grained  transformation n u c l e i may  defect  lattice  temperature  are  available  operate free of  other  strong  martensitic  have a s i t u a t i o n where many growth of  s i z e s the  the  nuclei  defect  is  strictly  structure  a v a i l a b l e to transform  is the  I f a n u c l e i b e g i n s t o grow i n i t may  propogate through a  interference  boundaries c o n s t r a i n  c l e a r from the  function  a n n i h i l a t i o n of  fewer n u c l e i are  grained  results  clear.  s i z e r e s u l t s from a  T h u s , we  large  from f e e  the  g r a i n boundaries or  small.  is  as  explained  through which the  s u c h as  of  treatments y i e l d i n g large  are  large  be  at  plates  limited.  important  Many n u c l e i f o r t h e  regions  typical  2 0 0 x-ray  completion.  grain  mobility  than f o r annealing  -  r e s u l t s are  i s increased,  T h e s e r e s u l t s may structure.  (50  .  very quickly  proceeds f u r t h e r  lattice  they are  important q u a n t i t a t i v e  amount o f  but  4 0 specimens  x-ray data  t o a more c o m p l e t e  that  from o t h e r further  this latter  transformation.  the  larger  multivariant  growth.  It  situation  The  i n c r e a s i n g d i f f i c u l t y i n completing the t r a n s -  formation as the i m p u r i t y content i n c r e a s e s i s due t o s u b s t i t u t i o n a l atoms toughening movement.  the l a t t i c e f o r d i s l o c a t i o n  The l o c k i n g o f s t a c k i n g f a u l t s i n t h i s manner,  making i t d i f f i c u l t f o r them t o move and t r a n s f o r m the l a t t i c e , has been observed by A l t s t e t t e r e t a l  2  8  -  3  1  .  The p r e s e n t o b s e r v a t i o n s agree w i t h data taken the l i t e r a t u r e .  Beckers  8 2  from  observed maximum r e t a i n e d f e e  (50-60%) by a n n e a l i n g a t 500-600°C and observed a drop t o approximately 30% f e e f o r 800°C anneals.  G r a i n s i z e s were  not quoted but they may be assumed t o be i n the same g e n e r a l range as i n the p r e s e n t study.  Fraser  8 1  , also  found  the maximum r e t a i n e d f a c e - c e n t r e d phase t o occur i n t h i s a n n e a l i n g range.  Muller  8 3  annealed  sheet c o b a l t of  v a r i o u s p u r i t i e s , between 800 and 1300°C and determined t h a t from 300 micron to 30,000 micron g r a i n s i z e , the r e t a i n e d f e e phase amounted t o approximately 10%. F o r h i s f i n e s t g r a i n s i z e of 35 microns, he measured 4 0% r e t a i n e d f e e .  approximately  Other data from the l i t e r a t u r e i s a v a i l a b l e  but i t i s based on powder s p e c i m e n s  8k  '  5 5  ~  5 7  .  As mentioned e a r l i e r , p o l a r i z e d l i g h t was i n v e s t i g a t e d as a secondary  t o o l f o r determining the d i s t r i b u t i o n of the  two phases i n c o b a l t .  A photomicrograph  a l a r g e g r a i n e d specimen i n F i g u r e 11.  i s reproduced f o r The m a t e r i a l i s  99.998% c o b a l t w i t h an average g r a i n s i z e of approximately 47 microns.  F i g u r e 11 shows the complexity o f the t r a n s -  formation w i t h i n a g r a i n i n a very s t r i k i n g manner.  The  c e n t r a l g r a i n has been s t r o n g l y c o n s t r a i n e d d u r i n g the transformation.  I t was o r i g i n a l l y a twinned  fee g r a i n and  Fig.  11  99.9981 C o b a l t  under  polarized  light.  900X  the m u l t i v a r i a n t  transformation  which the t r a n s f o r m a t i o n  has  shows more than 4 planes  proceeded.  The  dark areas i n  t h i s g r a i n remain dark throughout the r o t a t i o n of p o l a r i z e r and  are r e t a i n e d  t h i s specimen y i e l d s 31%  fee areas.  retained  on  the  X-ray a n a l y s i s of  fee phase a t room  temperature which approximates the v a l u e taken from  the  p o l a r i z e d l i g h t photomicrograph. 3.1.4  Discussion The  and  Summary  f o l l o w i n g d i s c u s s i o n o u t l i n e s the  w i l l occur i n p o l y c r y s t a l c o b a l t f o l l o w i n g annealing The  that  standard  procedures. microstructures  observed may  c l a s s i f i e d on the b a s i s of annealing three  structures  types of treatment d i s c u s s e d a)  temperature  annealing  be  arbitrarily  temperature.  The  are:  j u s t above the  transformation  (approximately 450°C)  b)  annealing  phase i s r e t a i n e d c)  (500  annealing  i n the range where maximum fee - 600°C). above 600°C where g r a i n growth  predominates. The annealing  s t r u c t u r e s produced i n c o b a l t are r e l a t e d to treatment as o u t l i n e d i n F i g u r e  cases, the s t a r t i n g m a t e r i a l  12.  an  In a l l  i s s e v e r e l y c o l d worked  and  of p h y s i c a l dimensions l a r g e compared to the fee g r a i n  size.  t  A  CD  u  -p ro  u  CD  P,  e CU EH  time  T = annealing t  temperature  = time a t a n n e a l i n g  temperature  = time i n fee phase A  = the temperature a t which the fee phase begins t o form on h e a t i n g  M  = the temperature a t which the hep phase begins to form on c o o l i n g = the h i g h e s t temperature a t which the hep can be produced by deformation  A^ = the lowest temperature a t which fee phase be produced by deformation M, - A, - 417°C d d  F i a . 12  3  A n n e a l i n g Parameters  phase can  a) temperature  Annealing  j u s t above the  transformation  (A ) g  Upon h e a t i n g through the t r a n s f o r m a t i o n , the specimen undergoes recovery and p o l y g o n i z a t i o n . s t r u c t u r e transforms phase.  The  completely  Simultaneously,  the  to the high temperature fee  transformation i s nucleated at stacking f a u l t s  i n the hep phase or a t r e t a i n e d r e g i o n s of f e e  2 7  '  7 8  .  From  many n u c l e i , fee g r a i n s w i t h a dense d e f e c t s t r u c t u r e are formed.  The  t r a n s f o r m a t i o n upon h e a t i n g i s accompanied  by an i n c r e a s e i n volume. p o s s i b l e i s 3.6 The  X io  - 3 2 8  "  3 1  The .  t h e o r e t i c a l maximum i n c r e a s e (see F i g u r e  13).  f a u l t d e n s i t y i n the room temperature product  upon " t " .  depends  I f t , i s v e r y s m a l l , l e s s than one minute, the  number of deformation  f a u l t s w i l l be reduced  and  of growth f a u l t s w i l l be higher than o r i g i n a l l y If t , i s l a r g e , the number of deformation  the number present.  faults w i l l  zero, but the growth f a u l t d e n s i t y w i l l remain h i g h . i s g r e a t e r than s e v e r a l minutes r e c r y s t a l l i z a t i o n at g r a i n boundaries.  If t,  begins  A p a r t of the d r i v i n g f o r c e appears  to be a s u p e r s a t u r a t i o n of v a c a n c i e s g i v i n g r i s e recrystallization n u c l e i  1 0 1  .  A one  produces a p a r t i a l l y r e c r y s t a l l i z e d Long time anneals  approach  to  hour anneal a t 450°C s t r u c t u r e (Figure 6 ) .  a t t h i s temperature do not y i e l d a f u l l y  recrystallized structure. Upon c o o l i n g , the volumes of c r y s t a l l a t t i c e where r e c r y s t a l l i z a t i o n has not occured,  transform to the  phase on the same l a t t i c e planes t h a t operated  hep  during  the  53  Heating Transformation hep fee p o s i t i v e volume change - 0.3% -3  Transformation h a b i t plane (0001) hep  4.2 X 10 expansion perpendicular to basal plane -3  0.3 X 10 contraction p a r a l l e l to basal plane  0.3 X 10 -3  Shear V a l u e s 19°28 S c 0.3 56 1  (111)  fee  4.2 X 10  Cooling Transformation fee -* hep d e c r e a s e i n volume -0.3% -3  4.2 X 10 contraction p e r p e n d i c u l a r to b a s a l p l a n e -3  0.3 X 10 expansion p a r a l l e l to b a s a l p l a n e  Fig.  13  Volume changes d u r i n g the m a r t e n s i t i c in cobalt.  transformation  heating c y c l e  1 1 1 27  '  7 1  .  The t r a n s f o r m a t i o n proceeds  in this  manner because h i g h d e n s i t i e s of d i s l o c a t i o n s are not a v a i l a b l e on planes other than the  (111) v a r i a n t c o r r e s p o n d i n g t o  (0001) hep above the t r a n s f o r m a t i o n temperature. In the areas t h a t have undergone r e c r y s t a l l i z a t i o n , d i s l o c a t i o n d e n s i t i e s are s i m i l a r on a l l {111}  planes.  The planes t h a t w i l l operate t o form the hep phase upon c o o l i n g w i l l be determined on these new  fee g r a i n s .  by the c o n s t r a i n t s  imposed  The c o n s t r a i n t s a r i s e from the  decrease i n volume accompanying the t r a n s f o r m a t i o n , and the thermal a n i s o t r o p y of the hep  lattice.  The amount of r e t a i n e d fee r e s u l t i n g from t h i s recrystallized  s t r u c t u r e a t room temperature  a wide range, but i s i n v a r i a b l e h i g h b)  A n n e a l i n g i n the range  v a r i e s over  (30% - 65%). 500  - 600°C  A treatment of t h i s nature g i v e s r i s e t o recrystallization.  deformation f a u l t d e n s i t y w i l l be  may  The growth f a u l t d e n s i t y  w i l l be h i g h i n the room temperature  The  complete  A s m a l l amount of g r a i n growth  occur a t the h i g h e r temperature.  partially  product w h i l e the  low.  s i z e of g r a i n s i n any i n d i v i d u a l specimen v a r i e s  over a wide range and cannot be c o n s i d e r e d an structure.  equilibrium  Many s t r a i g h t and acute angle boundaries  present as w e l l as a v a r i e t y of s u b s t r u c t u r e .  The  are  visible  s u b s t r u c t u r e v a r i e s a c c o r d i n g t o the e t c h i n g p r o c e d u r e s . 9 3  F i g u r e s 9(a) and  14 show the s t r u c t u r e o b t a i n e d by a t t a c k  F i g . 14 Etching of g r a i n b o u n d a r i e s and m a r t e n s i t e p l a t e s . 99.7% c o b a l t . 920X  F i g . 15(a)  99.9% 4000X  Fig.  F i g . 15(c)  99.9% c o b a l t . 10,000X  15(b)  99.9% c o b a l t . 10,000X  cobalt,  on both the g r a i n boundaries and m a r t e n s i t e I t should be noted t h a t p l a t e i n t e r g r o w t h  p l a t e boundarie  or overgrowth i n  d i f f e r e n t d i r e c t i o n s a t d i f f e r e n t depths i n t o the g r a i n s i s evident.  As o u t l i n e d by F e l l e r  9 2  of c e r t a i n l a t t i c e planes may occur phenomena i s shown i n F i g u r e 15.  , preferential i n cobalt.  etching  This  The e t c h i n g procedure  c l e a r l y d e l i n e a t e s r e g i o n s of l a t t i c e o f d i f f e r i n g orientation. The  average g r a i n s i z e obtained  i n this  annealing  range i s l e s s than 10 microns f o r a l l p u r i t y l e v e l s investigated.  Two t y p i c a l views f o r 99.9% Co are shown  i n F i g u r e 16.  Surface t i l t i n g due t o t r a n s f o r m a t i o n i s  o f t e n severe F i g u r e 17.  i n t h i s f i n e g r a i n e d m a t e r i a l as shown i n T h i s s u r f a c e was e l e c t r o p o l i s h e d b e f o r e  treatment but no f u r t h e r e t c h i n g was performed.  heat  The shear  markings due t o the m a r t e n s i t i c t r a n s f o r m a t i o n a r e obvious. O p t i c a l metallography i s d i s c o u r a g i n g thus r e p l i c a work i s presented  f o r c o b a l t and  t o c o n f i r m the d e t a i l e d  appearance o f the s u r f a c e i n small g r a i n e d  specimens.  A  range of s t r u c t u r e s i s observed on a s i n g l e s u r f a c e o f a grain.  The most common f e a t u r e s are as f o l l o w s : i)  A few g r a i n s have the appearance o f a  s i n g l e c r y s t a l upon viewing grain.  a s i n g l e plane through the  These g r a i n s l i k e l y c o n t a i n some m u l t i v a r i e n t  p l a t e growth a t c o n s t r a i n e d g r a i n boundary r e g i o n s : (Figure 16 (a) ) .  62  F i g . 16  99.9% c o b a l t ,  6.5 micron g r a i n s i z e ,  F i g . 17  Shear markings following heat treatment. 6500X  F i g . 18  4000X  A n n e a l i n g twin boundaries i n 99.9% c o b a l t . 5000X  63  ii)  Many g r a i n s e x h i b i t d i f f e r e n t o r i e n t a t i o n  i n areas d e l i n e a t e d by a n n e a l i n g twin boundaries fee phase;  (Figures 15 and 18). iii)  Some g r a i n s appear t o have random areas  of d i f f e r e n t o r i e n t a t i o n throughout. i s shown c l e a r l y under p o l a r i z e d l i g h t iv) grains;  i n the  T h i s type o f s u b s t r u c t u r e (Figure 11).  A banded s t r u c t u r e i s d e t e c t e d  i n some  (Figure 19). These bands a r e i n t e r p r e t e d as hep  r e g i o n s d i f f e r i n g o n l y i n o p e r a t i o n a l shear d i r e c t i o n i n a given  (111) type v a r i e n t d u r i n g t r a n s f o r m a t i o n .  s i m i l a r r e s u l t has been noted  f o r a martensitic transformation  i n the copper-germanium s y s t e m i n d i s c u s s i o n s by N e l s o n . 2 8  techniques  A  79  and p o s t u l a t e d f o r c o b a l t  The bands r e s o l v e d v i a r e p l i c a  a r e 0.1 t o 1.0 microns t h i c k and completely  traverse a grain.  Bands o f t h i s t h i c k n e s s r e q u i r e o p e r a t i o n  of s i m i l a r p a r t i a l d i s l o c a t i o n s on 150 t o 1500 close-packed planes.  The f a c t t h a t t h i s banded s t r u c t u r e i s not always  observed  i s e x p l a i n e d by the f i n e n e s s w i t h which the  t r a n s f o r m a t i o n may operate with r e s p e c t t o shear  directions.  If the shear d i r e c t i o n changes every few close-packed planes,  shear markings a r e too f i n e t o d i s c e r n by r e p l i c a  techniques.  Bands a r e observed  o n l y when the c o n s t r a i n t s  on a volume of c r y s t a l y i e l d a s t r o n g p r e f e r e n c e f o r a g i v e n shear d i r e c t i o n .  T h i s a n a l y s i s e x p l a i n s why f o r some  years t h e r e was a d i s p u t e over the m a r t e n s i t i c nature of the transformation i n cobalt. microscope, the shear  Without the a i d of the e l e c t r o n  is difficult  to d i s c e r n .  Fig. Figs.  Fig.  19(a)  4000X  19(a) - 1 9 ( c )  19(c)  10,000X  F i g . 19(b)  10,000X  Banded s t r u c t u r e a r i s i n g f r o m multivariance i n cobalt.  F i g . 20  coplaner  99.998% c o b a l t , 47 micron g r a i n s i z e , 2000X  65  For  annealing  treatments with  the  retained  all  s p e c i m e n s a r e 40 -  not  occur as i n d i v i d u a l f e e g r a i n s ,  fee  lattice  f e e p h a s e i s a maximum.  For  barely grain  l e s s complex.  varient  for  f i n e grained  decreases.  approximately  fee.  rise  to grain  grain  growth i s  size increases, the  a t room t e m p e r a t u r e  structure for grain  noted  becomes  i n fine  s i z e s over  grained  approximately  i n large  grains.  Multivariance  of d i f f e r e n t o r i e n t a t i o n are coarser material:  the grain  s i z e s over  grains.  L a r g e v o l u m e s o f c r y s t a l t r a n s f o r m on a  the plates  fee  observed  The banded  but  As  f o r 99.7% c o b a l t ,  i s not observed  (111)  single  phase does  b u t as r e g i o n s o f  individual  As t h e f e e g r a i n  substructure  15 m i c r o n s .  temperature,  99.998% c o b a l t , g r a i n g r o w t h i s p r o c e e d i n g  underway.  material  The r e t a i n e d  above 600°C g i v e s  cycle  a t 600°C w h i l e  rapidly  A t room  above 600°C  Annealing  annealing  growth.  60% f e e .  distributed within  c) An  T between 500 and 600°C,  (Figure  size increases  F o r an f e e g r a i n 3 0%  of the l a t t i c e  300 m i c r o n s ,  For the l i m i t i n g  occurs than  20).  t h e amount o f r e t a i n e d s i z e o f 50 m i c r o n s , remains fee.  For grain  t h i s v a l u e d r o p s t o 10% r e t a i n e d case o f a s i n g l e c r y s t a l the  transformation  proceeds t o completion.  The further  structure  of the cobalt  specimens produced f o r  t e s t i n g may be summarized a s f o l l o w s :  i) furnace per  A l l s p e c i m e n s were r e c r y s t a l l i z e d  cooled  minute.  Their  p u r i t y ranged  The  grain  than  5 to over  50  decreased  from over  50%  iii) fee  and  hep  different  6°C  t o room t e m p e r a t u r e a t a p p r o x i m a t e l y  ii) less  cobalt,  99.7% t o 99.998%.  from  s i z e f o r the  material  microns, w h i l e the to  relationships.  That  hep  strict  i s , a l l hep  fee  were a m i x t u r e  Most c r y s t a l s had  o r i e n t a t i o n with very  retained  regions  orientation  regions  were s t a c k s  i n t e r s e c t a t a n g l e s between  planes  grain.  parent  iv) The  The  small  that  anisotropy formation  and  transformation  the  not  an  the  equilibrium  The  twin b o u n d a r i e s i n the p l a t e s on  multivariance, i n a <112> banded  i n the  substructure  was  within  fee  always present.  i n f i n e grained  (111)  of {111}  one.  complete i t  was  thermal i n the  trans-  phase. grains  was  delineated  grains.  more t h a n one  d i r e c t i o n i n a given  structure  from  fee  transformation parent  to  a v a i l a b l e , but  volume change i n v o l v e d  s h e a r p r o d u c e d by  martensitic  was  constraints arising  retained material v)  The  s t r u c t u r e was  thermodynamic d r i v i n g f o r c e a t t e m p t i n g  the m a r t e n s i t i c so  fee  of  of  close-packed planes that i n the  from  30%.  l e s s than  A l l polycrystal grains  phases.  varied  (111)  The  varied. annealing  growth  type planes,  Repeated  termed  transformation  plane produced  material.  of  a  3.2  T e n s i l e Behaviour of C o b a l t P o l v c r v s t a l s  3.2.1  Tensile Results  3.2.1.1  True S t r e s s - True S t r a i n Curves  True s t r e s s - t r u e s t r a i n curves f o r c o b a l t a t s e l e c t e d t e s t temperatures are shown i n F i g u r e s 21-23.  The behaviour  as a f u n c t i o n of g r a i n s^ze i s shown i n F i g u r e 24. important o b s e r v a t i o n s may i) pure metal.  Several  be drawn from t h i s d a t a ;  the measured s t r e s s l e v e l s appear h i g h f o r a The u l t i m a t e t e n s i l e s t r e n g t h a t room 3  temperature exceeds 150 X 10 ii)  p s i f o r f i n e grained m a t e r i a l .  the temperature dependence of the flow  stress i s large.  As shown i n F i g u r e s 21-23  the flow s t r e s s  a t y i e l d i n c r e a s e s by g r e a t e r than a f a c t o r of 2 f o r a temperature change from 400°C to. -196°C.  The  ultimate  s t r e n g t h decreases by a f a c t o r o f 5 over the same temperature range  (Figure 21). iii)  the y i e l d s t r e s s decreases by a f a c t o r of 2  as the g r a i n s i z e i s i n c r e a s e d from 6.5 microns t o 60 microns  (Figure 24). iv)  the t e n s i l e curves e x h i b i t an  i n the i n i t i a l p o r t i o n of the c u r v e . shown c l e a r l y i n F i g u r e 22 f o r 99.9%  inflection  T h i s anomaly i s cobalt.  The s t r e n g t h of c o b a l t i s compared t o o t h e r common hep metals i n F i g u r e 25 (a).  The t e n s i l e behaviour f o r hep  metals having c/a r a t i o s h i g h e r (Zn), s i m i l a r lower  (Ti) than c o b a l t are i n c l u d e d .  (Mg), and  The d a t a i s presented  400°C  Ol  0 F i g . 21  1  1  2  4  I  1  I  I  6 8 10 12 True S t r a i n (%) True s t r e s s - t r u e s t r a i n c u r v e s a t s e l e c t e d t e m p e r a t u r e s ,  L 14 99.998% c o b a l t .  69  70 250  Grain  Size -196°C  (fee)  8 10 12 14 True S t r a i n (%) Fig.  23  True s t r e s s - s t r a i n c u r v e s a t s e l e c t e d 99.7% c o b a l t .  0 Fig.  16  24  14 8 10 12 True S t r a i n (%) True s t r e s s - t r u e s t r a i n c u r v e s a t 2  4  6  18  20  22  temperatures,  16 20°C  18  20  cobalt.  22  f o r room temperature p r o p e r t i e s normalized t o reduce the a f f e c t s of shear modulus and m e l t i n g p o i n t . c o b a l t undergoes  The f a c t  that  a martensitic transformation during  deformation whereas the other metals l i s t e d cio not, makes comparisons  on the b a s i s of c r y s t a l s t r u c t u r e  tenuous.  In T a b l e I I the c r i t i c a l l y r e s o l v e d shear f o r a v a r i e t y of metals i s l i s t e d . the common hep metals may b a s i s of flow s t r e s s . demonstrates  One  stress  I t would appear  be d i v i d e d i n t o two groups on the group  (Cd, Zn, Mg)  which  a low c r i t i c a l l y r e s o l v e d shear s t r e s s ,  t o the m a j o r i t y of fee metals, arid a second group of Co,  that  similar  comprised  Zr, T i , and Be which e x h i b i t much h i g h e r v a l u e s of  r e s o l v e d shear s t r e s s .  The importance t h a t must be  attached t o the presence of g r a i n boundaries i n the hep metals i s r e v e a l e d c l e a r l y when the s i n g l e c r y s t a l data i n T a b l e I I i s compared to the p o l y c r y s t a l data shown i n F i g u r e 25 (a). S i n g l e c r y s t a l s of T i are much s t r o n g e r than s i n g l e c r y s t a l s of the other metals shown i n F i g u r e 25 (a) y e t i n p o l y c r y s t a l l i n e form T i cannot be c o n s i d e r e d a s t r o n g hep metal when compared to Zn, Co, or Mg. c o n t r a d i c t i o n occurs because T i may  overcome more e a s i l y  than the other metals, the requirement t h a t be r e t a i n e d a t g r a i n boundaries.  T h i s apparent  coherency  A t room temperature,  t i t a n i u m s l i p s on the b a s a l , prism and pyramidal  systems.  Although both Mg and Zn deform a t v e r y low s t r e s s e s as s i n g l e c r y s t a l s , the c o n s t r a i n t imposed  by the i n t r o d u c t i o n  of g r a i n boundaries r a d i c a l l y required  increases  to continue deformation.  grain boundaries i s maintained the  corrugated  slip  The  normalized  stress level  'at  planes  the stress l e v e l s  I n Zn, c o h e r e n c y a t  by d i s l o c a t i o n m o t i o n on  i n addition to the basal required  to maintain  g r a i n b o u n d a r i e s a p p e a r s t o be h i g h e r  For  Mg, t h e o p e r a t i o n  of sufficient  coherency  f o r Zn t h a n T i .  slip  s a t i s f y Von M i s e ' s C r i t e r i o n r e q u i r e s  systems t o  very  high  stress  l e v e l s a s r e f l e c t e d by t h e s t e e p c u r v e a s s o c i a t e d t h i s metal. that  Co w i t h t h e same c / a r a t i o  The r e s u l t s f o r c o b a l t The  fall  the observed  o f hep s i n g l e c r y s t a l s may n o t be g e n e r a l i z e d  in  individually. of  that  In F i g u r e  Cobalt  i s considerably  stronger  basis.  is  larger f o r cobalt.  For true The  Secondly,  grained  may be made.  t h a n Ag, Cu, o r A l on a  the a f f e c t of grain  anomaly i s n o t a s p r o n o u n c e d  f o r 99.99 8%  a  effect i n c o b a l t  stress values  large  stress-  Figures  cobalt.  22-24.  Although  c u r v e s a r e a v a i l a b l e , no m e n t i o n h a s b e e n made o f 8 1 - 8 3  .  This  i s n o t s u r p r i s i n g , as t h e  anomaly i s n o t l a r g e and becomes c l e a r o n l y true  refinement  c o b a l t an anomaly i n t h e t r u e  s t r a i n c u r v e s o c c u r s a t low s t r a i n v a l u e s ,  published this  small  systems  f o r Co and a g r o u p  Two o b s e r v a t i o n s  normalized clearly  slip  e a c h m e t a l must be i n v e s t i g a t e d  25(b) c u r v e s  f e e m e t a l s a r e shown.  behaviour  to include  The l i m i t e d numbers o f o p e r a t i v e  hep m e t a l s r e q u i r e  f e e and  between t h o s e f o r Zn and Mg.  d a t a o u t l i n e d a b o v e shows t h a t  polycrystals.  with  a s Mg i s u n i q u e i n  i t i s a m i x t u r e o f two c l o s e p a c k e d p h a s e s ,  hep.  planes.  scale.  f o r many v a l u e s  of s t r a i n  after calculating and p l o t t i n g t o  74  0  10  True S t r a i n Fig.  25(b)  20  30  (%)  Nominal s t r e s s - s t r a i n c u r v e s f o r Co, A g , C u , and A l .  The anomaly i s not obvious f o r a l l specimens, pronounced  but i s most  f o r specimens having a f i n e g r a i n s i z e .  grain size i s equivalent to a large i n i t i a l  A small  f r a c t i o n of  r e t a i n e d f e e , i n other words a l a r g e volume of m a t e r i a l available for martensitic transformation. P u b l i s h e d t r u e s t r e s s - t r u e s t r a i n curves f o r m a t e r i a l s known t o t r a n s f o r m m a r t e n s i t i c a l l y d u r i n g deformation are presented i n F i g u r e 26. steel  1  0 5  Included are data f o r 303  , H a d f i e l d ' s Manganese S t e e l  and c o b a l t .  1 05  ,  stainless  equi-atomic N i - T i  0 6  ,  /  The r e t a i n e d h i g h temperature  phase i n 18-8  s t e e l and H a d f i e l d ' s Manganese s t e e l i s f e e .  stainless  Some hep  m a r t e n s i t e i s generated by p l a s t i c deformation i n the stainless.  1  18-8  T h i s hep m a r t e n s i t e i s b e l i e v e d t o be a t r a n -  s i t i o n phase and most of the end product m a r t e n s i t e produced i s bcci o . 5  T h i s s t r e s s induced m a r t e n s i t e g i v e s r i s e t o  the low i n i t i a l work hardening r a t e i n the 18-8 s t e e l a t l i q u i d n i t r o g e n temperatures e x p l a n a t i o n i s proposed equi-atomic N i - T i shown. 18-8  1  05  .  A  stainless  similar  |:or the anomalous behaviour i n the The l a r g e anomaly observed f o r  s t a i n l e s s and equi-atomic Ni-^Ti are not e v i d e n t f o r  H a d f i e l d ' s Manganese S t e e l .  Although a l l four m a t e r i a l s  transform to m a r t e n s i t e w h i l e undergoing deformation, i t i s probable t h a t no i n i t i a l H a d f i e l d ' s s t e e l because  low s t r a i n hardening o c c u r s i n  no s i g n i f i c a n t q u a n t i t y of low  energy m a r t e n s i t e i s formed.  Of a l l m a r t e n s i t i c t r a n s f o r m a t i o n s  the t r a n s f o r m a t i o n from fee to hep i s the lowest energy  form,  r e q u i r i n g o n l y a simple shear which need not be accompanied  76  350  18-8  stainless  steel Hadfield's steel  300 •H LQ X  250  IC W d)  200  References Co  Ni-Ti  -P <L>  - present -  Marcinkowski  18-8 s t a i n l e s s and Hadfield's steel Raghaven  150  1 0  EH  work  6  100 50 1 10  1 20  JL  30  40  True S t r a i n Fig.  26  1 50  1 60  70  1 80  (%)  True s t r e s s - t r u e s t r a i n curves f o r m a t e r i a l s undergoing s t r a i n induced m a r t e n s i t i c transformation  1 0 5  by f u r t h e r deformation t o form the second phase Thus, w h i l e 18-8  (Table V I ) .  s t a i n l e s s s t e e l forms a low energy form  of hep m a r t e n s i t e and N i - T i does l i k e w i s e ,  the H a d f i e l d ' s  s t e e l probably forms a more complex m a r t e n s i t e d i r e c t l y . C o b a l t , l i k e the 18-8  s t a i n l e s s s t e e l and the N i - T i  formij a simple hexagonal s t r u c t u r e  from the fee phase.  The anomalies are l a r g e r f o r the 18-8  stainless  and N i - T i because these m a t e r i a l s are i n i t i a l l y retained  f a c e - c e n t e r e d phase w h i l e c o b a l t  of both phases.  steel 100%  i s a mixture  TABLE V I  Martensite  Transformations  i n Non-Ferrous M a t e r i a l s  M a t e r i a l and Composition  Structural Change  Additional Deformation  I n - T l (^20 a t . % T l ) Au-Cu (o,50 a t . % Cu) Au-Mn (^50 a t . % Mn) Au-Cd (^50 a t . % Cd) Au-Cd ("V4 7.5 a t . % Cd) U-Mo (5-10 a t . % Mo) Cu-Zn (^40 wt. % Zn)  fee -- f c t fee — ortho. bee -- b e t bee bet bee — o r t h o . — ortho. bee — bee -- o r t h o . bee -- f e e bee — of eret h o . bee — bee — t e t . bee — hep bee hep bee — hep —- - o r t h o . tet. rhomb. — b e t fee hep  T, T, T T, T, T, T, T, T, F, F, T, T, T, D, p X  II  II  Cu-Al  (^12 wt. % A l )  fl  II  II  II  Li T i , Zr T i - M n (^5wt. % Mn) U-Cr (^1 a t . % Cr) Hg Co  —  S P M, S M, S F E E c, E, P E E, P E, p D, 7 7 p  Notation: T  Twinning  X  No a d d i t i o n a l required  E  Thin f o i l made  M  Parent  P  Product  S  Single interface transformation observed  deformation  ?  Unknown o r i n f o r m a t i o n uncertain  C  Transformation  D  Common modes  in  deformation  thin  foil  observations  structure structure  ordered ordered  3.2.1.2 To yield  Y i e l d Stress  and U l t i m a t e  Stress  d e t e r m i n e and i s o l a t e t h e e f f e c t o f p u r i t y on t h e  strength  of cobalt,  carried  o u t on m a t e r i a l  similar  grain  of  Tensile  retained  sizes.  of  5,  number o f t e s t s were  99.7% and 99.998% p u r i t y w i t h  Some v a r i a t i o n i n t h e i n i t i a l  face-centered  From F i g u r e  a large  amount  p h a s e was u n a v o i d a b l e .  i t was o b s e r v e d  that  a n n e a l e d one h o u r a t 700°C h a s a g r a i n  99.7% c o b a l t  s i z e of approximately  10 m i c r o n s , a s d o e s 99.998% c o b a l t a n n e a l e d one h o u r a t 600°C. for  The i n i t i a l  amount o f r e t a i n e d  t h e low p u r i t y m a t e r i a l  f e e phase i s  and 4 0.8% f o r t h e h i g h  46.5%  purity  material. As on  t h e 0.2% o f f s e t y i e l d  Increasing  micron grain  size.  temperature range from  of  i n Figure  i n cobalt  i n strength  content a t the l e v e l s  between t h e  remains constant throughout t h e  -196°C t o +400°C.  27, i t i s c l e a r t h a t  polycrystal cobalt  i s not large.  4000 p s i f o r m a t e r i a l w i t h a 10  The d i f f e r e n c e  grades of m a t e r i a l  presented  stress  i s not a strong  From t h e d a t a the y i e l d  function  of  a strong  fashion  directly,  the impurity  cause l a r g e v a r i a t i o n s i n y i e l d annealing is  treatments.  a strong  function  strength impurity  investigated.  A l t h o u g h p u r i t y does n o t a f f e c t t h e y i e l d in  purity  99.7% t o 99.998% d e c r e a s e s t h e y i e l d  p u r i t y from  s t r e s s by a p p r o x i m a t e l y  two  27, the e f f e c t o f i n c r e a s i n g  shown i n F i g u r e  strength  strength  differences  for identical  In o t h e r words, t h e y i e l d of grain  size.  T e n s i l e data  strength f o r cobalt  has commonly been t a b u l a t e d w i t h r e f e r e n c e t o a n n e a l i n g temperature  81  <  8 2  '  8 8  .  The d i s c r e p a n c i e s i n t h i s data would  be reduced i f the t e n s i l e parameters were normalized t o account f o r g r a i n s i z e .  F o r example, 99.7% c o b a l t and  99.998% c o b a l t having undergone i d e n t i c a l  (one hour)  a n n e a l i n g treatments a t 800°C, d i f f e r i n y i e l d by approximately 30,000 p s i .  strength  (See F i g u r e 28).  The v a r i a t i o n i n s t r e s s w i t h t e s t temperature i n F i g u r e s 28 and 29. dependence of the y i e l d  i s shown  For a l l m a t e r i a l s , the temperature s t r e s s has two d i s t i n c t  regions.  At low temperatures, the y i e l d s t r e s s decreases v e r y slowly w i t h i n c r e a s i n g temperature. the y i e l d  s t r e s s drops r a p i d l y w i t h temperature;  i s not as pronounced Due  A t h i g h e r temperatures,  f o r large grained material.  the e f f e c t (Figure 28).  t o the s c a t t e r i n r e s u l t s between i n d i v i d u a l  specimens,  test  e s p e c i a l l y r e g a r d i n g the y i e l d s t r e s s , i t was  d i f f i c u l t t o a c c u r a t e l y d e f i n e the two r e g i o n s of temperature dependence.  In an attempt t o a l l e v i a t e t h i s problem a f u r t h e r  group of specimens were t e s t e d . S i n g l e specimens were step p u l l e d over the complete temperature range from -196°C t o 400°C.  Some specimens were  i n i t i a l l y y i e l d e d a t low temperatures and o t h e r s a t h i g h temperatures. , The specimens were then r e t e s t e d every 20 or 30 °C f o r s m a l l increments of s t r a i n .  An i n t e r s e c t  method was used t o s u b t r a c t out the work hardening t h a t had taken p l a c e up t o the g i v e n s t e p - p u l l being analyzed t o a r r i v e a t the y i e l d  s t r e s s f o r the temperature  i n question.  Test Temperature (°C) Fig.  28  Y i e l d Stress data for p o l y c r y s t a l c o b a l t . 00  120 U  •H M  C  99.9% c o b a l t - 42.7% f e e  100  •H  99.9% c o b a l t -  step-pull results  X LO CO  0) -p  CO  0) EH  CO CO QJ  u  +>  CC Tj iH  0)  -H  >H 0\°  -200 Fig,  29  -100  100 200 T e s t Temperature (°C)  300  400  Comparison of y i e l d s t r e s s d a t a o b t a i n e d by i n d i v i d u a l t e s t s and s i n g l e specimen t e s t i n g . 99.9-%. c o b a l t , 6 . 5 m i c r o n g r a i n s i z e  interupted CO  The data p r o v i d e d by t h i s type of t e s t must be e v a l u a t e d carefully.  In a d d i t i o n t o p h y s i c a l measurement problems,  there a r e other areas f o r concern.  The s t r e s s r e l a x a t i o n  t h a t i s attendant t o the m a r t e n s i t i c t r a n s f o r m a t i o n o f c o b a l t can cause anomalies  upon r e y i e l d i n g specimens of c o b a l t  At higher temperatures,  dynamic r e c o v e r y may occur w h i l e  t e s t i n g and r e c o v e r y procedes temperature  1 0 7 - 1 0 9  d u r i n g the l a p s e s w h i l e the  of the t e s t i n g environment i s being a d j u s t e d .  A l a r g e c o r r e c t i o n must be a p p l i e d t o the raw t e n s i l e data to a d j u s t f o r the v a r i a t i o n o f work hardening r a t e w i t h s t r a i n and temperature.  The c a l c u l a t i o n s c a r r i e d out f o r  t e s t s of t h i s type a r e o u t l i n e d i n Appendix 2. F i g u r e 29 i s a p l o t of s t e p - p u l l t e s t data f o r 99.9% c o b a l t annealed  one hour a t 600°C.  The data gathered  many i n d i v i d u a l t e s t s i s i n c l u d e d f o r comparison.  from  The  agreement between the two s e t s of data i s encouraging. F u r t h e r t e s t s were c a r r i e d out over v a r i o u s ranges w i t h a v a r i e t y o f specimens. agreement w i t h the r e s u l t s determined  temperature  In a l l cases, a c c e p t a b l e by many t e s t s were  found. F i g u r e 28 p r e s e n t s the v a r i a t i o n i n y i e l d s t r e s s v/ith t e s t temperature  f o r a l l material tested.  i n f o r m a t i o n t o be drawn from t h i s graph  The p e r t i n e n t  i s reproduced i n  Table V I I .  Two important o b s e r v a t i o n s may be made.  temperature  dependence of y i e l d s t r e s s has two d i s t i n c t  r e g i o n s and y i e l d  The  s t r e s s i s a s t r o n g f u n c t i o n of g r a i n  TABLE V I I  P o l y c r y s t a l C o b a l t - 0.2% Y i e l d S t r e s s Data  0.2% Y i e l d S t r e s s (True S t r e s s X G / G / True Stress) Q  Grain S i z e (u)  Puritv (%)  -196°C  20°C  250°C  400°C  Temp, f o r Slope Change °C " T / T . m  Slope a t High Temp, Slope a t Low Temp.  6. 5  99.9  104.0/101. 2  98.0/88.3  78.0/62.5  50.0/38.9  160  0.25  7.2  7. 0  99.7  99.5/ 96. 8  97.0/87.4  72.0/61.7  53.0/41.2  128  0.22  14.8  9.0  99.998  '86 .5/ 84. 2  81.5/73.4  68.0/54.5  48.5/37.7  167  0.25  8.0  10. 3  99.7  89.0/ 86. 6  86.0/77.4  75.0/60.0  53.0/41.2  195  0.27  9.0  14. 5  99.9  88.0/ 85. 6  82.5/74.3  73.0/58.5  50.0/38.9  225  0.28  6.0  17. 5  99.7  81.5/ 79. 3  79.0/71.1  72.5/58.1  53.0/41.2  220  0.28  8.9  23. 5  99.998  78.0/ 75. 9  71.5/64.4  56.0/44.9  36.0/28.0  170  0.25  7.2  24. 0  99.9  75.0/ 72. 7  66.0/58.4  55.0/44.1  39.0/30.3  225  0.28  2.3  47. 0  99.998  48.5/ 47. 2  47.0/42.3  45.0/35.5  33.0/25.0  288  0.32  11.5  <6. 5  99.9  95.5/ 92. g*  100.4/90.4*  81.4/65.2*  62.0/48.2*  60. 0  99.9  —  49.2/44.3*  —  —  -  -  -  —  —  —  * S i n g l e T e n s i l e Test  00  The y i e l d s t r e s s behaviour of c o b a l t temperature  i s unlike  as a f u n c t i o n  t h a t f o r most common m e t a l s .  drops s t e e p l y w i t h i n c r e a s i n g  temperatures a t low  and becomes r e l a t i v e l y temperature temperatures. magnesium For  32  m  temperatures  independent a t h i g h e r  and s i n g l e c r y s t a l cobalt *. 1  65 micron magnesium, the y i e l d  drops q u i c k l y u n t i l 0.43 T  stress  S i m i l a r behaviour i s observed i n p o l y c r y s t a l  slowly w i t h temperature up to 0 . 2 1  0.43  In  3 3  copper, z i n c , and molybdenum f o r example, the y i e l d  of  the v i e l d  T  m  T  (125°C)  m  s t r e s s decreases  (-80°C)  and  i s reached.  then Beyond  s t r e s s c o n t i n u e s to decrease a t a lower  r a t e once a g a i n .  (Figure  30).  To a l l o w comparisons  between p o l y c r y s t a l c o b a l t and p o l y c r y s t a l magnesium, the data f o r both metals are p l o t t e d with the o r d i n a n t normalized f o r shear modulus and the homologous p l o t t e d as a b e i s s a .  temperature  From t h i s graph, i t i s c l e a r t h a t  g e n e r a l shape of the curves are s i m i l a r .  Also,  a t which both metals change behaviour, and the dependence of s t r e s s are  the  the  temperature  temperature  comparable.  A comparison between the y i e l d behaviour of p o l y c r y s t a l and s i n g l e c r y s t a l c o b a l t  i s presented i n F i g u r e 3 1 .  resolved  shear s t r e s s  than 1/2  the measured vajue f o r t e n s i l e y i e l d  factor i s considered. are p l o t t e d as  2  g./^  G  The  (T) i n a p o l y c r y s t a l i s somewhat l e s s i f the Schmid  For t h i s reason, the p o l y c r y s t a l data t  o  a l l o w comparison w i t h the s i n g l e  8.0  7.0 7 LI c o b a l t References  6.0  Cobalt  - p r e s e n t work  Magnesium  -  Ahktar  3 2  5.0  CO IC  0)  u  lu  -p CO CG  65 y  magnesium  4.0  rH Tj  0! O  •H >•  o\° CN •  O  u  m 0)  47 y  3. 0 \-  cobalt  x;  CO  2.0  1.0  0.1  0.2  0.3  Homologous T e m p e r a t u r e Fig.  30  Y i e l d s t r e s s versus t e s t temperature  0.4  0.5  (T°K/T^K)  for cobalt  and  magnesium.  CO  c r y s t a l data.  Although t h i s c o r r e c t i o n was  made, i t i s  still  necessary to p l o t the s i n g l e c r y s t a l data on a s c a l e a f a c t o r of 10 l a r g e r than p o l y c r y s t a l r e s u l t s f o r the trends to v i s i b l y examined. of the m e l t i n g temperature.  The  abcissa  p o i n t , and The  be  i s shown both as a f r a c t i o n  as a f r a c t i o n of the  l a t t e r scale i s included  transformation  to p r o v i d e a  measure of the m e t a s t a b i l i t y of the fee phase p r e s e n t i n the c o b a l t p o l y c r y s t a l s .  The  data f o r fee c o b a l t  single  c r y s t a l s i s a l s o presented to show the sharp d i f f e r e n c e s i n behaviour between the two Several The  conclusions  may  phases as s i n g l e c r y s t a l s . be drawn from F i g u r e  31.  p o l y c r y s t a l l i n e behaviour i s s i m i l a r to t h a t observed  in single crystals.  Although a b s o l u t e  v a l u e s f o r the  vary, the f a c t t h a t both types of m a t e r i a l d i s t t n e t types of temperature dependence i s  exhibit  curves  two  informative.  A second important o b s e r v a t i o n  i s t h a t the  critical  temperature a t which the y i e l d  s t r e s s changes behaviour,  i n c r e a s e s w i t h i n c r e a s i n g g r a i n s i z e i n a manner t h a t y i e l d s upon e x t r a p o l a t i o n a v a l u e c l o s e to t h a t obtained f o r s i n g l e c r y s t a l s f o r very l a r g e grained  polycrystals.  The  28 i s a l s o shown  l i n e representing  in Figure  31.  t h i s trend  i n Figure  T h i s l i n e , determined from p o l y c r y s t a l data,  p r e d i c t s a change i n y i e l d behaviour a t 0.36T t h a t y i e l d s a t 2 to 3,000 p s i .  Hep  T .  for cobalt  s i n g l e c r y s t a l s of  i n f a c t y i e l d at t h i s s t r e s s l e v e l and behaviour at approximately 0.35  m  do change y i e l d  cobalt,  o  0,1  0.2  F i g . 31  0.3  0.4 0.5 0.6 0.7 0.8 0.9 F r a c t i o n of the Transformation Temperature  Y i e l d s t r e s s versus temperature  for cobalt  1.0 (T°K/M, ° K )  s i n g l e c r y s t a l s and  polycrystals 00 VO  A t t h i s j u n c t u r e two  important f a c t s are c l e a r .  y i e l d s t r e s s of c o b a l t appears processes.  One  temperatures, temperature  t o be c o n t r o l l e d by  The  two  p r o c e s s , which i s almost athermal a t low  and a second  process which i s s t r o n g l y  dependent a t higher temperatures.  Secondly,  the change from one process t o the other occurs a t h i g h e r temperatures  as the g r a i n s i z e i n c r e a s e s .  A d i s c u s s i o n of the processes t h a t may the temperature  be  determining  dependence w i l l be d e f e r r e d u n t i l  further  aspects of t e n s i l e behaviour of c o b a l t have been p r e s e n t e d . The two r e g i o n s of temperature  dependence noted f o r  c o b a l t a t y i e l d , disappear as s t r a i n i n c r e a s e s . temperature  dependence of the flow s t r e s s a t y i e l d and  i s shown i n F i g u r e 32. grained  The failure  The data i s presented f o r a s m a l l  (6.5 micron) group of specimens of 99.9%  purity.  F i g u r e 33 shows a t y p i c a l s e t of data p o i n t s from which Table V I I I and  IX were c o n s t r u c t e d .  S i m i l a r curves to t h a t  shown i n F i g u r e 33 were obtained f o r a l l m a t e r i a l t e s t e d . In attempting  to compare the flow s t r e s s  w i t h t h a t of magnesium a problem a r i s e s . d u c t i l i t y of pure magnesium,  The  behaviour limited  ( l e s s than 2% f o r 65  micron  m a t e r i a l ) , does not a l l o w f o r comparison of the flow 3 2  s t r e s s a t the h i g h v a l u e s of s t r a i n t h a t can occur i n c o b a l t . The two m a t e r i a l s do e x h i b i t p a r a l l e l behaviour over  the  range of s t r a i n where comparisons can be made. The disappearance  of the e s s e n t i a l l y athermal  of flow s t r e s s a t low temperatures  behaviour  as s t r a i n i n c r e a s e s  imply t h a t the mechanism c o n t r o l l i n g the i n i t i a l  may  deformation  T6  #  U.T . S.  25  -200  -100  0  100  200  T e s t Temperature Fig.  33  300  400  500  (°C)  T y p i c a l data f o r d e t e r m i n i n g the temperature dependence o f stress. 99.7% c o b a l t , 7.0 m i c r o n g r a i n s i z e .  flow  vo  i n c o b a l t a t low  temperatures, does not c o n t r o l the s t r e s s  l e v e l s at l a r g e v a l u e s of The  strain.  temperature dependence of the flow s t r e s s i s g i v e n  i n Table V I I I as a f r a c t i o n of the shear modulus f o r a change i n temperature of 100°C.  I t i s c l e a r that for a l l  m a t e r i a l s t e s t e d , the temperature dependence of y i e l d  at  low temperature i s about an order of magnitude l e s s than a t high temperature. e l i m i n a t e t h i s two  The  amount of s t r a i n r e q u i r e d t o  stage c h a r a c t e r i s t i c decreases as  g r a i n s i z e becomes l a r g e r . two  s t r a i n f o r 7 micron m a t e r i a l , y e t f o r  micron m a t e r i a l of the same p u r i t y , no d i f f e r e n c e  i s observed a t 5% s t r a i n . The  (Table V I I I ) .  a b s o l u t e e f f e c t of p u r i t y on the flow s t r e s s i s  small a t a l l v a l u e s of s t r a i n . was  cobalt,  r e g i o n s of temperature dependence are observed a t  g r e a t e r than 10% 17.5  For example, i n 99.7%  the  presented  at y i e l d .  In F i g u r e 29,  This d i f f e r e n c e i n stress l e v e l  remains small a t a l l v a l u e s of s t r a i n . the i m p u r i t y l e v e l does not a f f e c t y i e l d manner may strain  now  the d i f f e r e n c e  Thus, the f a c t t h a t i n a strong  be expanded to i n c l u d e the complete s t r e s s -  curve.  As o u t l i n e d e a r l i e r , the u l t i m a t e s t r e n g t h of c o b a l t i s very h i g h f o r a pure metal. a g e n e r a l way  T h i s f a c t was  p r e v i o u s l y i n F i g u r e 25.  shown i n  From Table  IX i t may  be seen t h a t p o l y c r y s t a l c o b a l t f a i l s a t s t r e s s l e v e l s approaching G/50  a t -196°C and  G/70  a t room temperature.  The h i g h e s t s t r e s s l e v e l s u s t a i n e d by pure p o l y c r y s t a l magnesium i s approximately at c o n s i d e r a b l y lower  G/100 .  levels  32  3 3  .  Other pure metals  fail  TABLE V I I I Purity Grain Sizes  Temperature Dependence o f Flov? S t r e s s  7. 0 7. 6  25. 8 Low temp, slope where required 50. 0 Aa/G  Temp. of change T/T ' m  106. 0  99.7% 10,.3 17 .5 ,  99.9% 14,.5  24 .0  9 .0  99.998% 23 .5 47 .0  (%)  10,.6  19,.7  19,.7  36 .3  12, .1  15,.2  7,.6  0.2  34,.0  38,.0  59..0  68,.0  58,.0  50,.0  58,.0  39,.0  2. 0  62,.0  -  107. .0  118, .0  -  -  -  -  5. 0  -  -  '-  -  -  -  -  -•  10. 0  -  -  -  -  -  -  -  -  U.T'.S.  0.22  0,.27  0..27  0..25  0.,28  0..26  0.,25  0.,25  0.26  0,.28  0,.28  0..28  0,.29  0..28  0..26  -  109. .0  94..0  High temp. 116. 0 s l o p e , or single 117. 0 slope where required 140. 0  103. .0  100. .0  144. 0  Strain  12,.1  112. 0  Aa/G  6,.5  (Ao_ f o r 100°C temperature change) G  0..32  -  153. , 0  118. .0  82..0  97.,0  109. ,0  88.,0  0.2  153. ,0  140. ,0  120. .0  128. ,0  102.,0  90.,0  2. 0  115. .0  111. . 0 155. ,0  145. .0  131. .0  152.,0  140. ,0  92..0  5. 0  143. .0  120. .0  157. ,0  149. ,0  143. .0  -  -  -  168. .0  175. ,0  250. 0  220. ,0  190. ,0  10. 0 U.T • S.  VO  4*  TABLE IX  Grain S i z e (y)  U l t i m a t e Strength Data f o r Cobalt  Purity (%)"  Polycrystals  Ultimate t e n s i l e stress in ksi. -196°C  20°C  (true s t r e s s X G /G) / (True S t r e s s ) ° 250°C  400°C  6.5  99.9  247/241  184/165  115/ 92  66/ 51  7.0  99.7  211/205  176/158  136/109  111/ 86  9.0  99.998  158/154  125/112  90/ 72  65/ 50  10.3  99.7  222/216  183/164  138/111  110/ 85  14.5  99.9  216/210  163/146  103/ 83  64/ 49  17. 5  99.7  226/220  183/164  135/108  105/ 81  23. 5  99.998  142/138  125/112  78/ 62  57/ 44  24.0  99.9  192/147  145/130  94/ 75  60/ 47  47.0  99.998  116/113  95/ 85  70/ 56  55/ 43  Note:  99.998% c o b a l t e x h i b i t s much l e s s d u c t i l i t y than the lower p u r i t y grades.  The grain  strong v a r i a t i o n  size  stress  in yield  i s shown v e r y c l e a r l y  f o r 47 m i c r o n m a t e r i a l  for material having a grain Grain boundaries  as b a r r i e r s  to s l i p ,  and  i n F i g u r e 31.  i s less  t h a n one  s i z e of 7  microns.  affect  s t r e n g t h o f m e t a l s v i a two  strength with  the  strain  routes.  within  individual  ups  form  grains.  they  head o f t h e p i l e  and  The m e a s u r e d  polycrystalline  During the i n i t i a l  further  up,  up  at grain  dislocation  stresses  stages of  boundaries. dislocation  production.  increase u n t i l  s t r a i n hardening  from  slip  this  avenue w i l l  i n adjoining  a larger  the e f f e c t  effect  i s not l a r g e .  i s observed  grain. depend  grains. modes  In the  hep  to the l i m i t e d  slip  available.  In t h e o u t l i n e a b o v e , p i l e  ups  g r a i n b o u n d a r i e s were p o s t u l a t e d . t o assume r e g u l a r  the important p o i n t t h e end  of d i s l o c a t i o n s I t i s not  arrays of d i s l o c a t i o n s  exact d i s t r i b u t i o n  around  due  the  dislocation  I n t h e c a s e o f f e e o r bec m e t a l s , where ample s l i p have b e e n o b s e r v e d ,  At  a c r o s s the boundary i n another  upon the ease of i n i t i a t i n g  An  act  coherency  i n c r e a s e t h e b a c k s t r e s s on  movement i s i n i t i a t e d  systems  that  complex modes o f d e f o r m a t i o n must t a k e p l a c e  sources i n h i b i t i n g  metals  yield  half  hardening  secondly to maintain  deformation, d i s l o c a t i o n s p i l e As p i l e  The  Grain boundaries  between g r a i n s d u r i n g d e f o r m a t i o n o f a aggregate,  changing  necessary  in this  of the d i s l o c a t i o n s .is not  i s that  of a s l i p  at  manner. important;  there i s a strong stress  band.  When t h e s t r e s s  field  field  i n i t i a t e s s l i p i n an a d j o i n i n g the  initial  g r a i n , the back s t r e s s on  sources i s reduced and the hardening due t o  the back s t r e s s e s  increases  less rapidly.  The  effectiveness  of g r a i n boundaries as d i s l o c a t i o n b a r r i e r s i s most important during  the i n i t i a l p o r t i o n of the s t r e s s - s t r a i n curve w h i l e  the d i s l o c a t i o n p i l e ups are forming.  Thus, the e f f e c t  of g r a i n boundaries on the t e n s i l e parameters  i s more  marked i n the i n i t i a l p o r t i o n of the t e n s i l e curve. For may  a number of metals, g r a i n s i z e and y i e l d  stress  be r e l a t e d through an equation of the form: a  Q  = a a  ±  o  + KD  4)  _ 1 / 2  = yield J  stress  = f r i c t i o n s t r e s s opposing motion of d i s l o c a t i o n s K = measure of the i n t e n s i t y o f d i s l o c a t i o n p i l e ups a t b a r r i e r s D = g r a i n diameter The d e r i v a t i o n of t h i s r e l a t i o n s h i p i s g i v e n i n most standard t e x t s * . 31  determines K.  The slope of a p l o t of a  against  a . i s the s t r e s s r e a u i r e d 1  the r e s i s t a n c e s  2  to f o r c e a d i s l o c a t i o n  a r i s i n g from i m p u r i t i e s ,  A p l o t of the y i e l d shown i n F i g u r e  34.  sub-  force, etc.  i s temperature and composition dependent, but  of the e x t e r n a l l y a p p l i e d  is  -  w i t h the square r o o t of the shear  g r a i n boundaries, the P e i e r l s - N a b a r r o it  D "^  o K i s assumed to be almost independent of  temperature, v a r y i n g modulus,  versus  Therefore independent  stress. strength  versus 1//D  Data f o r z i n c a l l o y s  are a l s o shown f o r comparison.  Two  for cobalt 1 1 0  and  v a l u e s f o r 98.6%  copper  3 3  cobalt  98  References 120  •  99.7% c o b a l t  O  99.9% c o b a l t  A  99.998% c o b a l t  Zn a l l o y s - Risebrough 1 1 o Cu - McLean s 3  Q 98.6% c o b a l t - M u l l e r Present work 6 3  100  «! 80 v. in  cu u -p  60  o 40  20  0.1  0. 2  (Grain s i z e ) Fiq,  34  0. 4  0.3 1 / / 2  0. 5  (micron)" ^ 1  Y i e l d s t r e s s versus r e c i p r o c a l square r o o t of g r a i n s i z e .  drawn from the r e c e n t work by M i i l l e r  8 3  a r e i n c l u d e d as they  d e a l w i t h l a r g e g r a i n e d c o b a l t o f low p u r i t y . for  The v a l u e s  and K, f o r t h i s v a r i e t y of m a t e r i a l a r e reproduced i n  T a b l e X.  and K a r e q u i t e l a r g e f o r c o b a l t when compared  t o v a l u e s f o r other metals. A l a r g e v a l u e f o r a^, i s not unexpected.  The  h e t e r o g e n e i t y o f the c o b a l t l a t t i c e a f t e r c o o l i n g the t r a n s f o r m a t i o n i s w e l l documented.  The l a r g e amount of  l a t t i c e d e b r i s present has been o u t l i n e d by and o t h e r s  3 6  '  7 6  .  through  Yegoleyev ' 8 5  8 6  Even i n s i n g l e c r y s t a l c o b a l t , the  d i s l o c a t i o n d e n s i t y i s found t o be much higher than i n w e l l annealed  c r y s t a l s of other  metals . 2 5  The extremely h i g h v a l u e s f o r K r e q u i r e e x p l a n a t i o n . I t i s not reasonable t o expect the b a r r i e r s i n c o b a l t t o be 10 times as d i f f i c u l t t o overcome as those i n copper, or a f a c t o r of 5 stronger than those i n z i n c . An e x p r e s s i o n p r o v i d i n g the important  parameters  i n c l u d e d i n K may be w r i t t e n as f o l l o w s : K<x(Gba )  5)  1/2  c  G = shear modulus b = Burger's v e c t o r a = c r i t i c a l s t r e s s a t the head o f a p i l e up r e q u i r e d t o i n i t i a t e s l i p i n a neighbouring grain. c  C l e a r l y , i f metals w i t h v e r y d i f f e r e n t shear  moduli  are t o be compared, the data should be normalized f o r shear modulus.  The r a t i o  1/2 (G„ /G_, ) i s about 0.66. Zn' Co  I f we normalize the K v a l u e f o r c o b a l t by t h i s f a c t o r we f i n d t h a t K f o r c o b a l t a t room temperature  i s 120,000 p s i / p  1/2  TABLE X  Parameters From an Ecruation of the Form:  Test Temperature T/T °C ' m  c.  I psi  a. l  °yield  +  KD-V2  K • / 1/2 psi/u  Comments  -196  0.04  22,000  212,000  C o b a l t - 99.7 - 99.998%  20  0.17  19,500  182,000  C o b a l t - 99.7 - 99.998%  20  0.17  24,000  230,000  C o b a l t - 98.6%  100  0.21  18,500  182,000  C o b a l t - 99.7 - 99.998%  250  0.30  16,000  126,000  C o b a l t - 99.7 - 99.998%  400  0.39  13,500  81,000  C o b a l t - 99.7 - 99.998%  -100  0.25  11,000  44,000  Zinc A l l o y s  20  0.22  3, 000  26,000  Copper  1 1 0  - Zn, C r , T i , ZnO, N i  3 3  o o  101  compared to 44,000 p s i / y  for zinc a l l o y s .  /  The d i f f e r e n c e  i n Burgers v e c t o r between the two m a t e r i a l s should not important d i f f e r e n c e s .  I f cr  yield  i s a s t r o n g f u n c t i o n of  temperature i n c o b a l t , K v a l u e s must be compared a t s i m i l a r homologous temperatures.  From F i g u r e 3 5 , i t may  t h a t K f o r c o b a l t does vary w i t h temperature. and  seen  Between -196°C  100°C the v a r i a t i o n i s not l a r g e , but K drops c o n s i d e r a b l y  above t h i s temperature.  I f K v a l u e s a t 250°C  (0.3 T )  are compared to v a l u e s f o r the z i n c a l l o y s a t we  be  (0.25 T )  f i n d t h a t the K v a l u e f o r c o b a l t i s 1.8 times t h a t f o r  zinc a l l o y s .  In F i g u r e 34 a d o t t e d l i n e has been i n s e r t e d  t o r e p r e s e n t a curve f o r c o b a l t normalized temperatures alloys.  to a l l o w comparison to the curve f o r z i n c  The c o i n c i d e n c e of CK v a l u e s f o r the two  should be judged alloys  1  f o r G and homologous  1 1  f o r t u i t o u s as o\ f o r d i l u t e  i s approximately  curves  zinc-aluminum  0, whereas the z i n c a l l o y s t h a t ,  g i v e r i s e to the l i n e p l o t t e d i n F i g u r e 34 e x h i b i t a strong c o n t r i b u t i o n to  from p r e c i p i t a t e  hardening.  The o n l y parameter which has not been analysed i s D. I f the g r a i n s i z e D has been measured i n c o r r e c t l y , measured v a l u e s of K are a l s o i n e r r o r . used to assemble F i g u r e s 34 and w i t h the fee g r a i n s i z e .  the  The D v a l u e s  35 are the v a l u e s a s s o c i a t e d  In s e c t i o n 3.1 i t was  shown t h a t  the fee g r a i n s i z e i s o n l y a measure of the coarsness the h i g h temperature phase. t h a t may  The r e g i o n s of c r y s t a l  of  lattice  be c o n s i d e r e d i n d i v i d u a l g r a i n s depends upon the  m u l t i v a r i a n e e of the t r a n s f o r m a t i o n .  The boundaries  between  102  20°C l i n e from F i g . 34  120  -  •  99.7% c o b a l t  O  99.9% c o b a l t  A  99.998%  cobalt -196°C  100 100°C •rH  CO  X  80  COCO  250°C  0)  >H -P CC T3  60  rH  •r-i  >• O  400°G 40  20  0.2  0.3 -1/2  0. 4 -1/2  0.5  0  0.1  Fig.  (Grain s i z e ) (miron) Y i e l d s t r e s s versus r e c i p r o c a l sauare r o o t of grain size.  35  multivariant are c l e a r l y Although,  areas are not t r u e g r a i n boundaries, but stronger barriers  i t i s physically  than  subgrain boundaries.  impossible to ascertain  s i z e of the v a r i o u s transformed regions films of  i n the e l e c t r o n microscope)  lattice  must be  they  the  (except i n t h i n  these v a r i o u s regions  s m a l l e r than the f e e g r a i n  size.  In  v i e w o f t h e above o b s e r v a t i o n s , t h e s l o p e s d e t e r m i n e d Figure  34 and  35  s h o u l d be  lower  t h e d i s t a n c e between b o u n d a r i e s barriers the fee  t h a t a c t as  i s smaller than the diameter  to that  for zinc,  i n F i g u r e 34 may  zinc  a p o i n t on  at constant stress  Move d a t a p o i n t A,  to y i e l d  t o p o s i t i o n A'  to r e f l e c t  the.error  s i z e determined Upon c o m p a r i n g 47 m i c r o n ,  result size is  lattice.  Then move t o A"  For  99.998%  t h i s v a l u e w i t h F i g u r e 11,  The  a  "Grain"  impossible, i t i s clear than  having that  the  5 microns.  10  light,  an a c c u r a t e  similar  microns.  photomicrograph,  99.998% c o b a l t u n d e r p o l a r i z e d Although  and  w h i c h i s assumed  i n t h i s manner i s a p p r o x i m a t e l y  less  example:  c o b a l t of  i n measurement o f D.  f o r regions of l a t t i c e  nor  curve  to represent normalizing f o r G  i s not unreasonable.  15 m i c r o n s  the  cobalt  a measure of D f o r  r e p r e s e n t i n g 47 m i c r o n  homologous t e m p e r a t u r e .  of  of  the normalized  be p r o j e c t e d b a c k o n t o  the m i s o r i e n t e d regions of c o b a l t  purity  dislocation  i t i s assumed t h a t a n o r m a l i z e d K f o r c o b a l t i s  similar  for  shown b e c a u s e  grains.  If  curve  i n cobalt  than as  in  the "average"  orientation  "average"  i s not  over  Although the complex m i c r o s t r u c t u r e i n c o b a l t makes an a c c u r a t e e v a l u a t i o n of K d i f f i c u l t ,  i t i s clear that K  i s l a r g e as i n other metals t h a t l a c k a m u l t i p l i c i t y of slip  systems a t room  temperature.  The measured v a r i a t i o n i n K, is interesting.  (Table X ) .  and a^, w i t h  temperature  C l e a r l y both parameters  l i t t l e between -196°C and 100°C, K drops o n l y 15%, between 250°C and 400°C, a temperature h a l f as l a r g e , K decreases 35%.  differ  yet  change o n l y one  S i m i l a r l y o\ decreases  more r a p i d l y a t higher temperatures.  These o b s e r v a t i o n s  p a r a l l e l the e a r l i e r o b s e r v a t i o n s r e g a r d i n g t e n s i l e behaviour above and below 0.25 o\  T . m  i s a measure of the f o r c e necessary t o d r i v e a  d i s l o c a t i o n a g a i n s t the r e s i s t a n c e of i m p u r i t i e s ,  precipitate  p a r t i c l e s , s u b g r a i n boundaries, and the P e i e r l s - N a b a r r o force.  Of a l l these terms, the P e i e r l s f o r c e i s s t r o n g l y  temperature  dependent whereas the o t h e r s are not.  sharp change i n i n the P e i e r l s  w i t h temperature may  Thus a  r e f l e c t the change  force.  The P e i e r l s f o r c e , or the f o r c e r e q u i r e d to d r i v e a d i s l o c a t i o n over a s l i p plane i s s m a l l on c l o s e s l i p planes.  packed  Thus, i t i s not l a r g e i n the fee m e t a l s .  In the bee metals, where the s l i p plane i s not c l o s e the P e i e r l s f o r c e i s l a r g e .  packed,  In hep metals, the P e i e r l s  force  r e q u i r e d to d r i v e d i s l o c a t i o n s over the b a s a l plane i s g e n e r a l l y c o n s i d e r e d t o be s m a l l , but t o o b t a i n "corrugated s l i p " on the {1122} planes may force.  r e q u i r e a very h i g h P e i e r l s  Thus, one p o s s i b i l i t y f o r the sharp drop i n a. above  105 0.25 T i n c o b a l t i s t h a t t h e P e i e r l s m  stress f o r d i s -  l o c a t i o n m o t i o n o n t h e {1122} <1123> s l i p rate controlling. at  system  becomes  The f a c t t h a t C K d o e s n o t d r o p a s q u i c k l y  l o w t e m p e r a t u r e s may r e f l e c t t h e f a c t t h a t some o t h e r  mechanism o p e r a t e s , r e d u c i n g t h e need f o r c o r r u g a t e d slip.  The m o t i o n o f t r a n s f o r m a t i o n d i s l o c a t i o n s o n  various  {111} p l a n e s i n t h e f e e p o r t i o n s o f t h e l a t t i c e  w o u l d be o n e e x a m p l e o f a n a t h e r m a l p r o c e s s w h i c h  could  r e d u c e t h e n e c e s s i t y f o r movement o f d i s l o c a t i o n s o n t h e corrugated s l i p plane. The of  two f a c t o r s t h a t may y i e l d  K are G  1 / / 2  a n d a^ ^ . 1  decreases about  2  the temperature  B e t w e e n -196°C a n d 100°C,  dependence G  1 / / 2  5%, t h u s two t h i r d s o f t h e d r o p i n K  b e t w e e n -196°C a n d 100°C m u s t be d u e t o a r e d u c t i o n i n t h e critical  stress  (a ) r e q u i r e d t o i n i t i a t e  neighbouring g r a m .  To p r o p o s e  slip  ina  1/2 that o ' d e c r e a s e s 10% c  a s t e m p e r a t u r e i n c r e a s e s f r o m 0.04 T t o o v e r 0.2 T i s m m reasonable. On t h e o t h e r h a n d , t h e c h a n g e i n K b e t w e e n 250°C a n d 400°C i s 3 5 % a n d o n l y 2% o f t h e v a r i a t i o n  may  1/2 1/2 be a t t r i b u t e d t o a c h a n g e i n G ' ; t h u s , p ' must d e c r e a s e by o v e r 30%. the  Th,is i s e q u i v a l e n t t o s a y i n g t h a t  c r i t i c a l s t r e s s a t t h e h e a d o f a p i l e up r e q u i r e d t o  initiate  slip  i n a n e i g h b o u r i n g g r a i n , d e c r e a s e s by over  50% b e t w e e n 0.30 T^ a n d 0.38 T .  C l e a r l y some  strongly  temperature dependent d i s l o c a t i o n p r o c e s s i s o p e r a t i v e at  these temperatures.  106  Presentation  of the y i e l d  s t r e s s as a f u n c t i o n of t h e  r e c i p r o c a l s q u a r e r o o t o f g r a i n s i z e and t h e e n s u i n g d i s c u s s i o n has uncovered s e v e r a l i m p o r t a n t regarding  yield  i n polycrystal cobalt.  features  The y i e l d  strength  i n c o b a l t i s a f f e c t e d i n a s t r o n g manner b y t h e g r a i n boundaries present.  As b a r r i e r s t o d i s l o c a t i o n m o t i o n , t h e  grain boundaries i n cobalt provide similar  t o t h a t found i n other  exhibit a m u l t i p l i c i t y of s l i p The  a strengthening  effect  h e p m e t a l s t h a t do n o t s y s t e m s a t room t e m p e r a t u r e .  s t r e s s l e v e l s m e a s u r e d i n c o b a l t a r e much h i g h e r  than  f o r z i n c , cadmium, o r m a g n e s i u m b e c a u s e o f d i f f e r e n c e s i n s h e a r m o d u l u s and m e l t i n g s t r e s s , a^, i n c o b a l t i s v e r y normalizing  for  The  high.  The f r i c t i o n a l  In fact,  after  G and homologous t e m p e r a t u r e , pure  cobalt exhibits containing  temperature.  values  obtained  f o r zinc alloys  a high density of p r e c i p i t a t e p a r t i c l e s .  v a r i a t i o n i n a. a n d K a b o v e a n d b e l o w 0.25 T i m  i m p l i e s t h a t some t e m p e r a t u r e d e p e n d e n t d i s l o c a t i o n m e c h a n i s m becomes i m p o r t a n t o n l y a b o v e 0.25 T . temperatures  i s l a r g e b u t does n o t v a r y  with  temperature.  At high  with  temperature.  Similar observations  One p r o p o s a l  temperature,  consistent with  A t lov;  strongly drops q u i c k l y  apply  t o K.  the observed r e s u l t s  i s t h e r e l a t i o n s h i p b e t w e e n P e i e r l s s t r e s s o n t h e {1122} corrugated  p l a n e i n hep c p b a l t a n d t h e s t r e s s i n d u c e d  transformation  that occurs i n cobalt.  r e l a t i o n s h i p m u s t be d e f e r r e d martensitic transformation  until  martensitic  A discussion of this  the data regarding the  has been p r e s e n t e d .  107 3.2.1.3  D u c t i l i t y and  The defined nil  Fracture  d u c t i l i t y of c o b a l t  (% e l o n g a t i o n ) i s not  i n the l i t e r a t u r e w i t h measured v a l u e s from almost  to as high as 25% being q u o t e d .  The  8 3  low  measured i n work c a r r i e d out p r i o r to 1940 due  to i m p u r i t i e s i n the c o b a l t .  sulphur, and  well  ductilities  were undoubtedly  I f the c o n c e n t r a t i o n s  z i n c , or l e a d exceed very low  l e v e l s , (20,  of  100,  20 p a r t s per m i l l i o n r e s p e c t i v e l y ) c o b a l t behaves  i n a b r i t t l e manner . 82  Although the b a s i c i m p u r i t y  have been uncovered i n r e c e n t years, are s t i l l  effects  low d u c t i l i t y r e a d i n g s  i n evidence f o r v e r y pure p o l y c r y s t a l c o b a l t . 8 3  Sulphur, z i n c , and  l e a d are w e l l below the l e v e l s  where they cause b r i t t l e behaviour i n a l l grades of used f o r the p r e s e n t study.  Although hundreds of  specimens  were t e s t e d to f a i l u r e , no c l e a r p i c t u r e r e g a r d i n g d u c t i l i t y of c o b a l t emerged.  The  v a l u e s f o r the  cobalt  the three  grades of c o b a l t i n v e s t i g a t e d are presented i n Table XI. The  s c a t t e r i n r e s u l t s was  shown i n the t a b l e .  d u c t i l i t y of 99.7%  data has  various  The  been  are  prevalent  published.  c o b a l t i s high  s i z e s t e s t e d , averaging 19.2% p u l l e d to f a i l u r e .  mean v a l u e s  A large scatter i s also  i n other s t u d i e s where raw The  always l a r g e and  for a l l grain  s t r a i n f o r the  55  specimens  differences in d u c t i l i t y for  the  g r a i n s i z e s are too small to be a b l e to propose,  v/ith a u t h o r i t y , any  trend,  although there  lower d u c t i l i t y as the t e s t temperature i s  i s a trend increased.  towards  TABLE XI  Summary of True S t r a i n Data F o r P o l y c r y s t a l  Purity  Annealing Temp. (1 Hr.)  Grain S i z e • (y)  Cobalt  Strain  (%) a t  Failure  Average  -196°C  + 20°C  250°C  400°C  99.7%  600 - 800°C  7 - 17.5  19.2  21.6  19.7  17.3  15.4  99.9%  500°C  <6.5  23.9  25.0*  20.5  29.9*  24.8*  550°C  <6.5  22.9  11.8*  22.2*  33.2*  .23.6*  600°C  6.5  20.8  24.7  21.8  18.9  16.9  650°C  10.0  21.7  14.6*  26.4*  14.5*  30.5*  700°C  14.5  10.8  14.4  11.7  9.0  7.2  800°C  24.0  5.6  6.3  4.2  2.8  1000°C  60.0  5.9  9-47  4.5  99.998%  600 - 800°C  8 .3  -  5.9 4.8  4.1  -  3.4  9.5  * S i n g l e Specimen Tested  to  1  CO  109 For  99.9% cobalt, the s c a t t e r i n r e s u l t s i s large, b u t  the trends very  i n d u c t i l i t y a r e more pronounced.  Except f o r  small g r a i n e d m a t e r i a l , which has over 2 0 % d u c t i l i t y  at a l l temperatures, the d u c t i l i t y decreases as t h e t e s t temperature i n c r e a s e s .  The d u c t i l i t y a l s o drops r a p i d l y  w i t h i n c r e a s i n g g r a i n s|ze, d e c r e a s i n g  from over 2 0 % f o r  6.5 micron m a t e r i a l t o l e s s than 6% f o r 60 micron m a t e r i a l . The  r e s u l t s show t h a t the small g r a i n e d  99.9%  material  behaves s i m i l a r l y t o the 99.7% c o b a l t , but as the g r a i n s i z e increases  the former e x h i b i t s l e s s d u c t i l i t y .  F o r example,  17.5 micron c o b a l t o f 99.7% p u r i t y y i e l d s approximately 20% e l o n g a t i o n whereas 14.5 micron 99.9% c o b a l t f a i l s a f t e r 11%  strain. A f t e r t e s t i n g s e v e r a l specimens o f 99.998%  noting  the low d u c t i l i t y v a l u e s ,  c o b a l t , and  the m a t e r i a l was examined  to determine i f any p h y s i c a l d e f e c t s were p r e s e n t . c a r e f u l p o l i s h i n g and washing  (no etching)  elongated pores became v i s i b l e .  a few small  They were l e s s than 1  micron i n c r o s s - s e c t i o n p e r p e n d i c u l a r  t o the t e n s i l e a x i s  and were a maximum o f s e v e r a l microns i n l e n g t h to the t e n s i l e a x i s . and  the pores. supply  parallel  They d i d not appear t o be contaminated  probably arose d u r i n g  c o l d work i n t r o d u c e d  Upon  a z o n e - r e f i n i n g procedure.  d u r i n g production  The  d i d not c l o s e a l l  Problems attendant t o o b t a i n i n g a f u r t h e r  o f 99.998% m a t e r i a l made i t imperative  m a t e r i a l a v a i l a b l e be used.  t h a t the  Attempts were made t o swage and  draw t h e m a t e r i a l t o c l o s e the pores.  The low d u c t i l i t y ,  paired  w i t h contamination  problems o c c u r i n g a t the  temperatures  r e q u i r e d f o r working, thwarted  every e f f o r t to e l i m i n a t e the  pores.  annealed  Thus, the m a t e r i a l was  p o r o s i t y present.  and t e s t e d w i t h  the  While p a r t i n g the h i g h p u r i t y c o b a l t  rods i n t o l e n g t h s convenient  f o r machining f u r t h e r t e n s i l e  specimens, 50 random s e c t i o n s were taken.  These s e c t i o n s  were examined on planes p e r p e n d i c u l a r and p a r a l l e l to the tensile axis for porosity.  No d e f e c t s were observed  in  80% of the s e c t i o n s . The p o r o s i t y , as a f r a c t i o n of c r o s s - s e c t i o n a l area i s n e g l i g i b l e and  should not a f f e c t the y i e l d  the work hardening  r a t e , but the pores may  d u c t i l i t y by a c t i n g as n u c l e i f o r f r a c t u r e  s t r e n g t h or  i n f l u e n c e the processes.  The d u c t i l i t y of 99 998% m a t e r i a l below 0.33 T  i s low f o r a l l annealing procedures.  T^  (350°C)  As w i t h the other  grades of c o b a l t , there |s a t r e n d to lower d u c t i l i t y the t e s t temperature and g r a i n s i z e i n c r e a s e .  as  Above 0.33  the d u c t i l i t y of 99.998% m a t e r i a l i n c r e a s e s r a p i d l y , r e a c h i n g 10% by 400°C. In t h i s very high p u r i t y m a t e r i a l , 350°C may  be  s u f f i c i e n t f o r the onset of the high d u c t i l i t y r e g i o n observed  by other authors  temperature  81  1 8 2 ,1 0 9  v a l u e s observed deformation  .  surrounding  the t r a n s f o r m a t i o n  They a t t r i b u t e the h i g h e l o n g a t i o n  to the t f a n s f o r m a t i o n proceeding  and r e l i e v i n g s t r e s s c o n c e n t r a t i o n s .  during  T  m  The of  behaviour  grain  size  by M i i l l e r  8 3  .  function  i s i n agreement w i t h d a t a p r e s e n t e d  recently  The  available  and  99.9%  as a  35 m i c r o n s .  was  o f 99.7%  smallest grain Although  size  the m a j o r i t y of the  this,  results  to a certain  sizes  i n t h e a r e a s where c o m p a r i s o n s  made t h e r e i s g e n e r a l a g r e e m e n t .  remains  e x a m i n e d i n h i s work  from the p r e s e n t study are f o r g r a i n  s m a l l e r than  up  cobalt  grain  size,  beyond t h i s ,  c o n s t a n t a t 4 t o 6%.  t h e amount o f r e t a i n e d  Ductility  can  be  decreases  the  This behaviour  ductility also  f e e phase a s . a f u n c t i o n o f  parallels grain  size. Although in  a detailed  c o b a l t was  regarding  not attempted,  the  fracture  shows a f r a c t u r e size 20% by  of 6.5 strain.  of  The  37  s u r f a c e s was  The  failure  cusps  compiled.  failed  is definitely  (47  grain  approximately  ductile,  throughout  i s a large grained  at  36  Figure  cobalt having a  specimen  visible  processes  metallographic evidence  s u r f a c e f o r 99.9%  microns.  the d u c t i l e Figure  study of the f r a c t u r e  as  evidenced  the f r a c t u r e  micron)  surface.  specimen  99.998% c o b a l t t h a t f a i l e d a t l e s s t h a n 5% s t r a i n .  Although  t h e s u r f a c e does n o t e x h i b i t  of d u c t i l e  cusps  as t h e p r e c e e d i n g r e p l i c a  be  considered a ductile  is  some e v i d e n c e o f s h e a r The  fracture. failure  In both  and  R e p l i c a s were t a k e n  obtained after  tests  at  intensity  i t may  still  figures,  i n selected  s p e c i m e n s shown i n F i g u r e s 36  a t room t e m p e r a t u r e . surfaces  t h e same  areas.  37 were from  there  tested  fracture  -196°C, 20°C, and 250°C.  112  Fig.  37  F r a c t u r e s u r f a c e , 99.998% c o b a l t t e s t e d a t 20°C. 47 m i c r o n g r a i n s i z e . 5000X  No  change i n the  general  were u n c o v e r e d . were n o t and  the  Replicas  obtainable oxide  f e a t u r e s of  due  layer that  of the  to the  the  fracture  surface  fracture surfaces  testing  formed on  at  environement  the  400°C  (salt)  specimens f o l l o w i n g  failure.  3.2.1.4  Work H a r d e n i n g  Before  d i s c u s s i n g the  work h a r d e n i n g hardening  behaviour with  was  behaviour  i s presented  Figure  38.  s i z e was  The 22  f o r 6.5  the  of  effect  are  test  99.9%  p o r t i o n of t h e work  test  cobalt.  The  strain  carried  continuing  out  increases  continuing  tensile  strain in  results  anomalous b e h a v i o u r  of m a t e r i a l .  the  hardening  temperatures  small  shown grain  i s more  In F i g u r e  the m a r t e n s i t i c t r a n s f o r m a t i o n  before  anomalous  stress - true  taken from the  micron,  type  of  The  38,  i s quite  increases  c o b a l t e x h i b i t s a r e g i o n where 0 r e m a i n s  actually  the  true  o f a smooth d r o p i n 0 as  constant tests  the  chosen because the for this  yield,  initial  for various  data  pronounced  Instead  i n the  A plot  the  change i n work  i s necessary.  (the s l o p e o f  curve)  the  strain  noted p r e v i o u s l y .  parameter; 0  in Figure  temperature dependence of  r a t e , a d e s c r i p t i o n of  work h a r d e n i n g curves  Behaviour  clear.  beyond  essentially  t o drop i n a normal f a s h i o n .  above - 1 9 6 ° C , t h e work h a r d e n i n g following this  to drop i n value.  A  anomalous r e g i o n  second o b s e r v a t i o n  For  rate before i s that  anomalous b e h a v i o u r p e r s i s t s f o r a l a r g e r p o r t i o n o f  the  Data t a k e n from t h e t r u e - s t r e s s t r u e s t r a i n c u r v e s shewn i n F i g . 22, cobalt  1000 -  •rH  n w  &  O  iH  u. C'. C  o  •i—i  c  J-t E-i  u  m  0) -P ri B  u  o  -P  i  ©  200 100  0  2  5  10  15 True F-train  Fia.  38  (%)  V a r i a t i o n o f work hardening r a t e v.'ith s t r a i n  f o r 99.9% c o b a l t .  115  t e n s i l e curve as the t e s t temperature  At - 1 9 6 ° C ,  increases.  the r e g i o n of constant work hardening r a t e d i s a p p e a r s a t 3% s t r a i n , a t 0°C i t d i s a p p e a r s a t about 5% s t r a i n , a t 400°C a c o n s t a n t v a l u e i s maintained t o over 8% s t r a i n . Both o b s e r v a t i o n s are due t o the m a r t e n s i t i c t r a n s f o r m a t i o n . I t w i l l be shown i n the s e c t i o n d e a l i n g w i t h deformation and the t r a n s f o r m a t i o n t h a t the r e g i o n s where the anomalous work hardening r a t e s e x i s t correspond t o s t r a i n v a l u e s where the r e t a i n e d f e e l a t t i c e i s t r a n s f o r m i n g t o hep a t a high r a t e . For  p o l y c r y s t a l l i n e aggregates t h a t f a i l  in a ductile  manner, the e l o n g a t i o n a t which necking begins i n a t e n s i l e t e s t i s r e l a t e d t o the work hardening r a t e .  Plastic  i n s t a b i l i t y occurs when the slope of the t r u e s t r e s s true s t r a i n curve stress.  (o) becomes equal t o the v a l u e of t r u e  T h i s r e s u l t i s known as C o n s i d e r e s ' C r i t e r i o n .  The d e r i v a t i o n of t h i s r e s u l t i s a v a i l a b l e i n most standard t e x t s * . 31  is  The v e r a c i t y of t h i s r e s u l t f o r c o b a l t  shown i n F i g u r e 38.  Data p o i n t s r e p r e s e n t i n g the  maximum s t r e s s and e l o n g a t i o n a t f a i l u r e f o r the specimens g i v i n g r i s e t o the work hardening curves i n F i g u r e 38 are  i n c l u d e d f o r comparison.  and f a i l  i n accordance w i t h the c r i t e r i o n o u t l i n e d above.  S i m i l a r curves were produced For  C l e a r l y the specimens neck  for a l l grain sizes  investigated.  99.7% and 99.9% c o b a l t , r e s u l t s s i m i l a r t o those i n  F i g u r e 38 were uncovered.  The r e s u l t s c l e a r l y show t h a t the  i n c r e a s e d d u c t i l i t y observed f o r 99.7% and 99.9% p o l y c r y s t a l c o b a l t a t low temperatures  i s a r e f l e c t i o n of Considere's C r i t e r i o n .  The r e s u l t s f o r 99.9,98% c o b a l t do not obey C o n s i d e r e d Criterion.  Specimens of t h i s m a t e r i a l f a i l w h i l e 0 i s s t i l l  an order of magnitude h i g h e r than the t r u e s t r e s s , and or  no necking i s observed.  little  These r e s u l t s s u b s t a n t i a t e the  p r o p o s a l t h a t the p o r o s i t y p r e s e n t i n t h i s h i g h p u r i t y m a t e r i a l promotes f r a c t u r e a t low s t r a i n v a l u e s . The r a t e a t which p o l y c r y s t a l c o b a l t work hardens a f u n c t i o n of temperature of  Curves  s i m i l a r shape and magnitude are a l s o observed f o r a l l  other m a t e r i a l s t e s t e d . clear: of  i s shown i n F i g u r e 39.  as  Two  g e n e r a l o b s e r v a t i o n s are  The work hardening r a t e i s not a s t r o n g f u n c t i o n  g r a i n s i z e or p u r i t y .  On the other hand, the work  hardening behaviour changes s h a r p e l y w i t h dropping v e r y s t e e p l y from -196°C 0.2 5 T  temperature,  (0.04 T ) to approximately  and d e c r e a s i n g a t a lower r a t e above t h i s  temperature.  m  c  T h i s observed change i n behaviour i s recorded f o r work hardening r a t e s taken a t 2% s t r a i n . i s not as c l e a r .  The temperature  At 10% s t r a i n , the e f f e c t  a t which t h i s break i n  behaviour occurs i s approximately the same f r a c t i o n of the m e l t i n g p o i n t a t which the two yield  stage behaviour of the  s t r e s s and flow s t r e s s i s observed.  F i g u r e 40 t r a c e s  the change i n work hardening behaviour versus at  v a r i o u s v a l u e s of s t r a i n to o u t l i n e how  temperature  the two  behaviour of the work hardening r a t e d i s a p p e a r s as increases.  strain  For low s t r a i n v a l u e s , the anomalous e f f e c t shown  i n F i g u r e 38 i s a l s o r e f l e c t e d i n F i g u r e 40. data i n F i g u r e 40 i s f o r 99.9% size.  stage  The work hardening  c o b a l t having a 6.5 micron  grain  The work hardening curves f o r t h i s m a t e r i a l l o s e the  strain strain 2%  1000  strain  - 2% - 10%  strain strain  800 0)  c r-f  600  o  400  200  -200  -100  0  100 Temperature  Fiq.  39  The  work h a r d e n i n g  behaviour  200  300  400  (°C) of c o b a l t  as a f u n c t i o n  of  temperature  Fig.  40  Temperature (°C) V a r i a t i o n i n work h a r d e n i n g b e h a v i o u r w i t h i n c r e a s i n g s t r a i n . 99.9% c o b a l t , 6 . 5 m i c r o n g r a i n s i z e .  two s t a g e c h a r a c t e r i s t i c between Upon r e f e r r i n g  t o T a b l e V I I I i t may  stage c h a r a c t e r i s t i c at  10% s t r a i n .  work h a r d e n i n g  strain. that  above w h i c h t h e two s l o p e s on t h e  The r a n g e  sizes i s  of temperature  i s also  relating  to the flow s t r e s s  3.2.1.5  D i s c u s s i o n and Summary  listed.  i s included  juncture, i t i s clear  i s different  t h e two  i s no l o n g e r e v i d e n t  c u r v e s merge f o r v a r i o u s g r a i n  s l o p e change i s o b s e r v e d  of c o b a l t  be s e e n  of the flow s t r e s s  The s t r a i n  shown i n T a b l e X I I .  At t h i s  10% and 15%  that  a t which the  Similar information f o r comparison.  the t e n s i l e  above and b e l o w 0.25  T  .  behaviour  Pronounced  m differences  i n the temperature  flow s t r e s s ,  dependence of y i e l d  and work h a r d e n i n g r a t e h a v e b e e n  The d i s t i n c t i o n b e t w e e n b e h a v i o u r  stress,  uncovered.  above and b e l o w 0.25  T m  disappears  as s t r a i n  increases,  values  of s t r a i n  as t h e g r a i n  stress  and f l o w s t r e s s  exhibit  d e p e n d e n c e b e l o w 0.25 t h e work h a r d e n i n g  rate  d e p e n d e n c e b e l o w 0.25 As a f u r t h e r behaviour  than  carried  T^ than  approach  out.  temperature  and s t r a i n e d  temperature  was  s t r a i n was  size  increases.  a less above.  intense  The  yield  temperature  temperature  above.  to c l a r i f y T  the d i f f e r e n c e s i n  , a number o f  step-pull  S p e c i m e n s were y i e l d e d a s m a l l amount.  t h e n c h a n g e d and a f u r t h e r  introduced.  lower  On t h e o t h e r hand,  shows a s t e e p e r  above and b e l o w 0.25  t e s t s were  and d i s a p p e a r s a t  T h i s p r o c e d u r e was  The  at a given  test  increment of continued,  TABLE X I I The Two S t a g e B e h a v i o u r o f F l o w S t r e s s and Work H a r d e n i n g R a t e as a F u n c t i o n of. T e m p e r a t u r e Grain Size (Microns)  Purity (%)  S t r a i n (%) and T e m p e r a t u r e (T/T ) S t r a i n (%) a n d T e m p e r a t u r e Above Which t h e Two Stacre Above Which t h e Two S t a a e C h a r a c t e r i s t i c o f t h e Work C h a r a c t e r i s t i c o f t h e Flow Hardening Rate i s n o t Observed S t r e s s i s n o t Observed m  True  Strain  (%)  T/T  True  Strain  (%)  m  99.9  10  0.28  - 0.29  5  0.25  - 0.28  7.0  99.7  10  0.23  - 0.26  10  0.22  - 0.26  9.0  99.998  2  0.24  - 0.25  2  10. 3  99.7  5  0.22  - 0.27  5  0.27  - 0.28  14.5  99.9  5  0.27  - 0.28  5  0.28  - 0.29  17.5  99.7  2  0.26  2  0.27  23.5  99.998  2  0.25  2  0.25  24.0  99.9  2  0.30  2  0.27  47.0  99.998  2  0.28  2  0.32  Curves  P l o t t e d For  2, 5 , 10, 15,  20% S t r a i n  m  T/T ' m  6.5  Note:  (T/T )  0.25  Only.  O  121 alternating occured. pull  temperature  between two v a l u e s , u n t i l  The r e s u l t s were t h e n p l o t t e d  specimens.  Data  from  identical  f o rthe step-  specimens t h a t had  undergone a l l d e f o r m a t i o n a t t h e i n d i v i d u a l being  s t u d i e d were a l s o  step-pull tested  t e s t s as w e l l  i n t h e normal  t o remove t h e a f f e c t  temperatures  plotted.  41 a n d 42 p r e s e n t t h e d a t a  Figures  failure  as s t r e s s - s t r a i n  fashion.  from a s e r i e s o f curves f o r specimens  The d a t a h a s b e e n  corrected  o f change i n s h e a r m o d u l u s w i t h  temperature. Three  types of t e s t s i)  a r e shown:  Tests involving  a temperature  -196°C/20°C where b o t h t e m p e r a t u r e s r  ii)  Tests with both  change between  were b e l o w 0 . 2 5 T . m above 0.25 T ,  temperatures  250°C/385°C. iii) T e s t s where t h e low t e m p e r a t u r e 20°C (0.17 T ) was b e l o w t h e b r e a k i n t h e c u r v e s , t h e h i g h m 3  t e m p e r a t u r e 250°C  (0.30 T ) above. m  From F i g u r e 41 t h e r e s u l t s formed  at  imply t h a t  the structures  -196°C and 20°C a r e v e r y s i m i l a r .  The a g r e e m e n t  between t h e s t e p - p u l l c u r v e s a n d t h e i n d i v i d u a l strain test,  curves i s very close. determined  Appendix  2,  failure  The segments o f t h e s t e p - p u l l  by t h e i n t e r s e c t  e x h i b i t work h a r d e n i n g  with the individual  tests.  stress  technique outlined i n slopes t h a t agree  At strain values  some v a r i a n c e o c c u r s .  closely  approaching  250 -  100  Step-pull - i n i t i a l  y i e l d @ 20°C, second t e s t  Step-pull - i n i t i a l  y i e l d @ -196°C, second t e s t  O  S t e p - p u l l - i n i t i a l y i e l d @ 250°C, second t e s t Mean of 2 spec. @ -196°C  •  Mean o f 2 s p e c . @ 20°C  A  Mean o f 2 spec. @ 250°C  O  Mean o f 2 s p e c . @ 385°C  @ -196°C, e t c . @ 20°C, e t c . @ 3S5%C, e t c .  150  A.  100  50  O  O  O  A  A  A  O  O _L 2  4  JL 6  _L 8 True S t r a i n  Fig.  41  Temperature change t e s t s ,  JL 10  _L 12  14  (%)  99.9% c o b a l t - 6.5 m i c r o n g r a i n  size.  16  18  • A  S t e p - p u l l - i n i t i a l y i e l d @ 250°C, second t e s t Mean o f 2 spec. @ 20°C Mean o f 2 spec. @ 250°C  @ 20°C, e t c .  200 •H  X  w in o  150  w 0) 3 rH  100  50  o4  JL  0 Fig.  2 42  _L  _L  4  6  JL 8 True S t r a i n  10 (%)  12  Temperature change t e s t , .99.9% c o b a l t - 6.5 m i c r o n g r a i n  _L  14  16  18  size. ro  CO  The  r e s u l t s a t 250°C/385°C do n o t s u p e r i m p o s e a s  a c c u r a t e l y a s t h o s e f o r -196°C/20°C. of  the step p u l l curve f a l l  for  s l i g h t l y b e l o w t h e mean d a t a  s p e c i m e n s p u l l e d a t 250°C, w h e r e a s t h e 385°C s e g m e n t s  fall it  The 250°C s e g m e n t s  above c o r r e s p o n d i n g d a t a f o r i n d i v i d u a l t e s t s .  i s proposed  hardening  Thus,  that the structures giving r i s e to strain  a t 385°C m u s t be l e s s r e s t r i c t i v e t o f u r t h e r  d i s l o c a t i o n motion  t h a n t h e s t r u c t u r e s f o r m e d a t 250°C.  A s m a l l amount o f r e c o v e r y i s a l s o o c c u r i n g a t t h e s e high temperatures,  b u t t h e change i n s t r e s s l e v e l  t h i s avenue i s v e r y s m a l l The  from  (See S e c t i o n 3.1.2).  t e s t s f o r temperatures  straddling  0.25 T ,  (20°C/250°C) show a l a r g e v a r i a n c e w i t h t h e i n d i v i d u a l s t r a i n curves of  ( F i g u r e 42).  the curve f a l l  individual fall  The room t e m p e r a t u r e  specimens.  The 250°C s e g m e n t s o f t h e c u r v e  It  i n d i v i d u a l data.  s l o p e s shown by t h e i n d i v i d u a l  are a l s o i n disagreement tensile  segments  much b e l o w t h e c o r r e s p o n d i n g d a t a f o r  above t h e c o r r e s p o n d i n g  hardening  stress  The w o r k  step p u l l  segments  w i t h t h e data from t h e normal  tests. appears,  that the barriers to dislocation  motion  f o r m i n g a t 250°C a r e f a r l e s s r e s t r i c t i v e t h a n t h o s e o c c u r i n g at and  20°C.  A f t e r undergoing  then f u r t h e r  hardening  straining  a s m a l l amount o f s t r a i n a t 250°C t h e s p e c i m e n a t 20°C, t h e w o r k  s l o p e f o r t h e room t e m p e r a t u r e  than would occur  f o r a specimen h a v i n g had a l l s t r a i n  i n t r o d u c e d a t 20°C, b u t t h e s t r e s s l e v e l hardening  segment i s h i g h e r  i s lower.  The w o r k  s l o p e i s e q u i v a l e n t t o t h e s l o p e f o r a 20°C  tensile  t e s t a t a lower v a l u e of s t r a i n .  The s t r e s s  levels  r e q u i r e d t o c o n t i n u e d e f o r m a t i o n a t 250°C f o l l o w i n g some s t r a i n h a r d e n i n g a t 20°C r e f l e c t the more structure  i n t r o d u c e d a t 20°C.  restrictive  The s t r e s s l e v e l s a r e  higher  than those observed f o r a s t a n d a r d t e s t a t 250°C and become l e s s r e p r e s e n t a t i v e of a 250°C t e s t as more s t r a i n  is  i n t r o d u c e d a t 20°C. T h i s s e t of t e s t s appear t o s u b s t a n t i a t e o b s e r v a t i o n s made e a r l i e r .  several  The f a c t t h a t the r e s u l t s of  the  s t e p - p u l l t e s t s w i t h b o t h temperatures below 0 . 2 5 T , superimpose v e r y a c c u r a t e l y w i t h i n d i v i d u a l s t r e s s c u r v e s throughout  the major p o r t i o n of the  strain  stress-strain  curves i m p l i e s t h a t s i m i l a r b a r r i e r s to d i s l o c a t i o n motion a r e c o n t r o l l i n g the f l o w s t r e s s a t b o t h t e m p e r a t u r e s .  The  temperature dependence of the f l o w s t r e s s r e f l e c t s the degree t o which the b a r r i e r s become t r a n s p a r e n t t o d i s l o c a t i o n s as temperature i n c r e a s e s .  The o b s e r v a t i o n t h a t the work  h a r d e n i n g r a t e s a r e a l s o c o i n c i d e n t w i t h those f o r a normal s t r e s s s t r a i n c u r v e ,  suggest t h a t the r a t e a t w h i c h  b a r r i e r s to d i s l o c a t i o n motion are forming, are s i m i l a r at both temperatures. different  If  either result is incorrect a  s e t of r e s u l t s would o c c u r , u n l e s s some v e r y  complex t h e r m a l b e h a v i o u r i s p o s t u l a t e d f o r the mechanism.  controlling  For example:  I f the r a t e a t which o b s t a c l e s are formed  w i t h i n c r e a s i n g s t r a i n i s higher a t -196°C than a t 20°C, the segment of s t r a i n a t 20°C f o l l o w i n g a segment a t -196°C would n e c e s s a r i l y e x h i b i t a s t r e s s h i g h e r than t h a t o b t a i n e d a t s i m i l a r s t r a i n f o r a normal t e n s i l e t e s t . it  On t h i s  basis,  i s reasonable to assume t h a t the mechanisms c o n t r o l l i n g  the major p o r t i o n of the s t r e s s s t r a i n curves are the same a t both temperatures. Any p o s t u l a t e d combination of processes must be capable of producing s t r o n g b a r r i e r s to deformation t o j u s t i f y the h i g h s t r e s s l e v e l s measured and one  component  process must be temperature dependent, as i t i s c l e a r the b a r r i e r s t o d i s l o c a t i o n motion are more e a s i l y  that  overcome  as temperature i n c r e a s e s . For the t e s t s where one temperature i s above 0.25  T  m  and the other below, the r e s u l t s r e f l e c t a d i f f e r e n c e i n behaviour a t the two temperatures.  To y i e l d the  behaviour observed r e g a r d i n g flow s t r e s s l e v e l s , possibilities exist.  E i t h e r more or d i f f e r e n t  two  barriers  are forming a t 20°C than a t 250°C, or the o b s t a c l e s formed a t 20°C are more e a s i l y overcome a t 250°C. The higher work hardening r a t e observed f o r a  segment  of the step p u l l t e s t s a t 20°C, f o l l o w i n g a segment a t 250°C, i m p l i e s t h a t the s t r a i n i n t r o d u c e d a t 250°C forms ' a s t r u c t u r e t h a t c o u l d be formed by f a r l e s s s t r a i n a t 20°C. For example:  I f the 20°C segments of the step p u l l  tests  i n F i g u r e 42 are transposed back onto the curve f o r the normal t e n s i l e t e s t , m a i n t a i n i n g the measured  stress  levels,  127  coincidence of the curves i s observed. This  latter  behaviour,  o b s e r v a t i o n r e g a r d i n g t h e work  reinforces  to d i s l o c a t i o n motion but  that  the t o t a l  the proposal that  similar  are forming a t both  number  hardening barriers  temperatures,  o f b a r r i e r s produced  i s lower  a t 250°C. To  o b t a i n e q u a l amounts o f s t r a i n a t b o t h  temperatures  t h e r e must be e q u i v a l e n t amounts o f d i s l o c a t i o n If  e q u i v a l e n t amounts o f d i s l o c a t i o n m o t i o n  but  fewer  barriers  form,  this  implies  has  come i n t o p r o m i n e n c e a t t h i s  amounts o f d i s l o c a t i o n m o t i o n  without  f o r m i n g t h e same d e n s i t y results  above 0 . 2 5 above. that The  T  m  similar  As t h e temperature  form  deformation  i n stress  behaviour  increases,  levels required  i s due t o a s e c o n d  stress r e l i e f  above 0.25  allowing  temperatures,  of barriers.  f o r a given input of s t r a i n ,  decrease  some mechanism  a s a t low  f o r t h e t e s t s where b o t h  reflect  take p l a c e ,  higher temperature,  similar  The  that  motion.  temperatures  are  to that discussed  t h e number  of obstacles  continues to decrease. to continue  mechanism t h a t p r o v i d e s  T . m  As w o u l d be e x p e c t e d  from  the data presented f o r  other aspects of the t e n s i l e deformation of cobalt, the m e a s u r e d work h a r d e n i n g at  rates are high, approaching  2% s t r a i n f o r t e s t s a t -196°C.  may  give r i s e  low  stacking  Two  important  G/10  factors  that  t o t h e h i g h work h a r d e n i n g v a l u e s a r e t h e fault  energy  and t h e m a r t e n s i t i c t r a n s f o r m a t i o n .  128 Both f a c t o r s are important The of  i n d i v i d u a l l y and are a d d i t i v e .  low s t a c k i n g f a u l t energy ensures t h a t the m a j o r i t y d i s l o c a t i o n s p r e s e n t i n c o b a l t are d i s s o c i a t e d .  Continued  movement of extended d i s l o c a t i o n s a f t e r  i s d i f f i c u l t and g i v e s r i s e to work hardening.  intersection  As  the  m a r t e n s i t i c t r a n s f o r m a t i q n proceeds on d i f f e r e n t  (111)  planes, the m a r t e n s i t i c l a m a l l a e formed i n t e r s e c t  and  growth of the l a m a l l a e i s i n h i b i t e d .  Dislocations within  the l a m a l l a e must c r o s s a boundary, s i m i l a r to a twin boundary, to move out of the m a r t e n s i t e , or i n t i a t e across the boundary.  S i m i l a r i l y , other d i s l o c a t i o n s o u t s i d e  the l a m a l l a e must move through these boundaries s l i p a c r o s s them t o allow c o n t i n u i n g Another way  of viewing  the v a r i o u s m a r t e n s i t e  boundaries  or  initiate  deformation.  the m u l t i v a r i a n t t r a n s f o r m a t i o n ,  as r e l a t e d to work hardening, of  slip  i s to accept the  formation  l a m a l l a e as formation of  i n the c r y s t a l l a t t i c e .  T h i s boundary  i s s i m i l a r to a c o n t i n u i n g g r a i n refinement.  new formation  I f the  t r a n s f o r m a t i o n i s viewed i n t h i s manner, i t would be expected  t h a t the strong e f f e c t s t h a t g r a i n boundaries  impose  upon p o l y c r y s t a l l i n e m a t e r i a l d u r i n g the i n i t i a l p o r t i o n of  the s t r e s s s t r a i n curve, may  as long as the t r a n s f o r m a t i o n The d i f f i c u l t y  continue to be  evident  proceeds.  a r i s i n g when d i s l o c a t i o n s encounter  o b s t a c l e s of the type a r i s i n g from m a r t e n s i t i c t r a n s f o r m a t i o n s has been observed Marcincowski  1 0 5 1 1 0 6  by s e v e r a l authors ' 3 6  observed  3 7 11 0 5  •  1 0 6  .  t h a t widely d i s s o c i a t e d  129 dislocations  on d i f f e r e n t  v e r y e f f e c t i v e l y when t h e y  (111)  planes  intersect.  l o c k each He  t h a t m a r t e n s i t i c l a m a l l a e form f o r m i d a b l e d i s l o c a t i o n motion. must o c c u r  7 8  observed  barriers  proposes that d i s l o c a t i o n  to allow continued  Barrett similar  He  also  other  to generation  deformation.  w o r k i n g w i t h copper-germanium p o s t u l a t e d  strong obstacles to deformation  i n t h i s system where  a similar martensitic transformation occurs.  The  studies carried  similar  o u t by V o t a v a  3 6  '  3 7  infer that  transmission  hardening  mechanisms a r e r e s p o n s i b l e f o r the h i g h work  hardening  rates i n cobalt.  130 3.2.2  Deformation and the A l l o t r o p i c  Transformation  Many o f the r e s u l t s o u t l i n e d to t h i s p o i n t , may  be  e x p l a i n e d i n terms of the m a r t e n s i t i c t r a n s f o r m a t i o n .  As  d e s c r i b e d e a r l i e r a l l t e s t specimens u t i l i z e d f o r t h i s study c o n t a i n e d a l a r g e f r a c t i o n of fee phase.  The  r e t a i n e d f e e phase f o r a l l a n n e a l i n g treatments  was  summarized  initial  i n Table V.  A number of s t e p - p u l l t e s t s were c a r r i e d out t o t r a c e the progress of the t r a n s f o r m a t i o n towards completion.  The  data are presented as a f u n c t i o n of s t r a i n t o a l l o w d i r e c t comparison to the s t r e s s - s t r a i n c u r v e s .  (Figures 43, 44).  The progress of the t r a n s f o r m a t i o n as a f u n c t i o n of s t r e s s d i d not y i e l d data t h a t c o u l d be r e a d i l y a n a l y s e d . fee  The r e t a i n e d  phase i s a l s o p l o t t e d a g a i n s t a l o g a r i t h m i c s c a l e f o r  s t r a i n t o a l l o w o b s e r v a t i o n of the progress of the t r a n s f o r m a t i o n at  low s t r a i n v a l u e s .  ( F i g u r e s 45-47).  The data  l i n e a r r e l a t i o n s h i p s when p l o t t e d i n t h i s manner.  yields This  r e s u l t i m p l i e s t h a t the r e l a t i o n s h i p between s t r a i n and the fee  phase may log  be r e p r e s e n t e d by an equation of the form:  e = l o g A + m(% fee)  ....6)  or e q u i v a l e n t l y ,. m(%fcc) e = A (10) ....7) Where: e = t r u e s t r a i n A = s t r a i n v a l u e where % fee = 0 % fee = (% f e e ) . ... . - (% f e e ) . , initial transformed / 1 r t X  C e r t a i n l i m i t s must be p l a c e d on the equations.  The  volume % fee phase may o n l y vary between the amount present b e f o r e t e s t i n g and the amount present a t f a i l u r e .  For  Puritv (%)  0 I  1 0  1  5  1 10  1 . 1 5  True S t r a i n Fig.  44  Grain r i z e fu)  I 20  fee  initial (%)  I 25  (%)  Room t e m p e r a t u r e t e n s i l e s t r a i n i n d u c e d t r a n s f o r m a t i o n various grain sizes.  for cobalt  of •<-> u> to  drain size (M)  0.05  0.1  0.2  0.5  1.0  2.0  True S t r a i n Pig.  45  Room t e m p e r a t u r e of v a r i o u s g r a i n  5.0  10  fee  20  initial (%)  50  (%)  t e n s i l e s t r a i n induced t r a n s f o r m a t i o n sizes (semilog).  for  cobalt M LO LO  Grain  size  fee  initial  rurity  (%)  (y)  (%)  99.7 99.9 99.998  1  I  I I I I  0.05  0.1  1  1  0.2  1  1  0.5  I  I I I I  1.0  I  2.0  True S t r a i n Fig.46  Tensile strain (semilog).  I  I  I  I  I I I I  5.0  10  I  I  l  20  l  I  I I I I  50  (%)  induced t r a n s f o r m a t i o n of c o b a l t  a t room  temperature U)  •  True S t r a i n Fig.  47  Tensile strain (semilog).  induced  385°C  (%)  transformation  for cobalt at various  temperatures, CO  polycrystal retained  c o b a l t t h e maximum f e e p h a s e t h a t may be  f o l l o w i n g a normal a n n e a l i n g 60%.  approximately  0.25 T . equation  well,  The  takes is  results  of this  47.  A  linear  a b o v e 0 . 2 5 T as m  series of tests little  place before macroscopic y i e l d  temperature.  above  justified.  As s t r e s s i n c r e a s e s , v e r y  shown i n F i g u r e  45 -  shown i n F i g u r e s  t h i s may n p t be  general  follows.  10% a t t e m p e r a t u r e s  h a s b e e n assumed f o r t e s t s  although  varies  c  f o r the curves  relationship  t o over  at failure  X I I I p r e s e n t s t h e s o l u t i o n s t o t h e above  Table  m  The amount p r e s e n t  2% a t -196°C  from around  treatment i s  a r e as  transformation  occurs.  This  result  45 f o r 99.9% c o b a l t t e s t e d a t room  The c u r v e s  have b e e n e x t r a p o l a t e d t o t h e  s t r a i n v a l u e where t h e amount o f r e t a i n e d f e e p r e s e n t to t e s t i n g  occurs.  the  a t which t r a n s f o r m a t i o n begins  strain  0,07%  For the v a r i e t y  and 0.11% s t r a i n .  of grain sizes  Similar results  prior  shown,  i s between are obtained f o r  m a t e r i a l t e s t e d b e l o w 0 . 2 5 T . A t 0.2% s t r a i n , a m ' n o t i c a b l e amount o f t r a n s f o r m a t i o n h a s t a k e n p l a c e a t a l l t e m p e r a t u r e s b e l o w 0 . 2 5 T . Above 0 . 2 5 T the situation m m  all  is  not as c l e a r  induced The  although  transformation  the data  begins  implies that the strain  a t higher  r a t e a t which the t r a n s f o r m a t i o n  strain  increases.  strain  values.  p r o c e e d s slows as  At fracture the transformation  remains  incomplete.  3.2.2.1 To levels,  Purity isolate  any d i f f e r e n c e s i n b e h a v i o u r  small grained  specimens o f  99.7%,  due t o i m p u r i t y  99.9% and 99.998%  TABLE XIII Behaviour of the S t r a i n Induced Transformation i n C o b a l t :Expressed as an Equation of the F orm: m(%fcc) E = t r u e s t r a i n , % f c c = Volume % f c c £ = A(10) Grain S i z e (u)  Purity (%)  7.0  9 9.7  44.8  -0.055  20  0.07  8.0  12.3  6.5  99.9  33.5  -0.060  20  0.09  6.0  8.1  9.0  99.998  16.7  -0.066  20  0.07  7.5  9.4  <6.5  99.9  85.0  -0.059  20  0.08  7.5  10.0  <6.5  99.9  64.5  -0.063  20  0.09  5.5  8.5  6.5  99.9  33.5  -0.060  20  0.09  6.0  8.1  10,0  99.9  51.3  -0.098  20  0.11  2.5  6.6  24 .0  99.9  38.8  -0.104  20  0.07  4.5  6.5  60.0  99.9  21.8  -0.120  20  0.09  3.0  No S o l ' n  6. 5  99.9  33.5  -0.06  -196  0.09  6.0  8.1  6.5  99.9  33.5  -0.06  20  0.09  6.0  8.1  6.5  99.9  33.5  -0.06  100  0.09  6.0  8.1  6.5  99.9  87.0  -0.06  250  0.29  0  No S o l ' n  6.5  99.9  195.0  -0.06  385  0.71  0  No S o l ' n  A  m  Test Temp. °C  e a t v/hich Transformation begins (%)  Volume % transformed a t 0.2% e  £  tx  c o b a l t were s t e p - p u l l e d t o f a i l u r e . 43  in Figures  43,  prior  results  i t i s clear  may  be  be  shown  drawn f r o m t h e d a t a .  t h a t a l l grades of m a t e r i a l  to complete t r a n s f o r m a t i o n .  7 v o l u m e % a t room t e m p e r a t u r e . s e e n t h a t a somewhat j o w e r  limit  when  between 5  a value  47,  In F i g u r e  From fail  Fracture occurs  t h e volume o f r e t a i n e d f e e p h a s e r e a c h e s and  are  46.  and  Three o b s e r v a t i o n s Figure  The  i t  may  -196°C and  applies at  a h i g h e r l i m i t a t 250°C and 385°c. Secondly, 0.09%  and  Table  strain  transformation  and  induced  final  i s independent of p u r i t y  observation  i s t h a t the progress  transformation with  found  to a f f e c t  p h a s e i n a s t r o n g manner. transformation  following  an  annealing  strain  of the  earlier  the As  observation  purity  treatment.  As  there  completion i s less  5 v o l u m e % f e e , where f r a c t u r e o c c u r s .  The  describing  result  5 volume % r e t a i n e d f e e  continuing of  deformation  this value  i n the  fee  purity material, a smaller  transformation  show t h i s  i s accepted  as  equations  retained input  approximately  equations  the  a t room t e m p e r a t u r e ,  appropriate  This  of c o b a l t i n c r e a s e s ,  i s required to b r i n g t h i s m a t e r i a l to  the  not  that  amount o f r e t a i n e d the  strain  grades.  proceeds f u r t h e r towards  fee a v a i l a b l e i n higher  If  46,  (Figure  r e s p e c t t o s t r a i n does  i s i n c o n t r a s t to the  p u r i t y was  of  0.06%  betv/een  a p p r e c i a b l y between t h e v a r i o u s p u r i t y  result  the  begins  XIIJ. A  vary  the  very limit  clearly. for  substitution i n Table  XIII.  should y i e l d the measured e l o n g a t i o n . r e s u l t s are 7.7% m a t e r i a l , and  Upon s u b s t i t u t i o n ,  f o r 99.998% m a t e r i a l , 17.9%  23.9%  f o r 99.7%  cobalt.  for  the  99.9%  I f these v a l u e s  are  compared to the d u c t i l i t y v a l u e s f o r the f i n e g r a i n e d m a t e r i a l presented  i n Table XI reasonable  correspondence i s  observed. The  f a c t t h a t the progress of the s t r a i n  induced  t r a n s f o r m a t i o n does not vary s i g n i f i c a n t l y with p u r i t y i s not unexpected.  The d r i v i n g f o r c e tending to complete  the t r a n s f o r m a t i o n upon c o o l i n g i s very s m a l l . been estimated  I t has  t h a t i t wpuld be e q u i v a l e n t to an a p p l i e d  s t r e s s of s e v e r a l hundred p s i i t would be expected  2 8 - 3 1  .  At these s t r e s s l e v e l s  t h a t s m a l l d i f f e r e n c e s i n the  would y i e l d measurable d i f f e r e n c e s i n behaviour. i t has been observed  The  In f a c t ,  t h a t i n c r e a s i n g p u r i t y does a l l o w  t r a n s f o r m a t i o n to proceed cooling.  lattice  f u r t h e r towards completion  s t r e s s l e v e l s present d u r i n g s t r a i n  the  upon;  induced  t r a n s f o r m a t i o n are an order of magnitude l a r g e r and t h e r e f o r e any d i f f e r e n c e s i n the manner i n which the induced  t r a n s f o r m a t i o n proceeds due  should not be 3.2.2.2  to i m p u r i t y  strain  content  significant.  Grain Size  The progress of t h e  t r a n s f o r m a t i o n with s t r a i n f o r  v a r i o u s g r a i n s i z e s i s shown i n F i g u r e s 44 and i n i t i a l fee phase a v a i l a b l e p r i o r to t e s t i n g markedly as the g r a i n s i z e i n c r e a s e s .  The  45.  The  decreases  only  important  d i f f e r e n c e between t h i s s e t of curves and  those d e a l i n g  with p u r i t y i s t h a t the r a t e a t which the s t r a i n transformation  proceeds decreases as g r a i n s i z e  induced increases.  Although the s t r a i n v a l u e s a t which t r a n s f o r m a t i o n and  the fee phase remaining at f a i l u r e are s i m i l a r ,  slopes of the curves change as g r a i n s i z e The  begins the  increases.  d i f f e r e n c e i n slopes means t h a t a l a r g e r imput of  s t r a i n i s r e q u i r e d to y i e l d an e q u i v a l e n t amount of induced  transformation  size.  f o r m a t e r i a l having  a larger grain  T h i s r e d u c t i o n i n the r a t e of s t r a i n  transformation in available  strain  induced  i s not s u f f i c i e n t to overcome the decrease fee phase.  Thus, the measured  elongation  to f a i l u r e decreases as the g r a i n s i z e i n c r e a s e s because the amount of r e t a i n e d fee reaches the v a l u e where f a i l u r e occurs with  less strain  A dotted to r e p r e s e n t Miiller .  imput.  l i n e has been i n c l u d e d i n F i g u r e 44 and the data  for large grained material tested  He quotes an i n i t i a l  8 3  fee content  t o 30,000 microns.  to r e p r e s e n t grain  T h i s l i n e may  by  of approximately  10% and measured 4 to 6% e l o n g a t i o n f o r m a t e r i a l from 300  45  ranging  be assumed  a l i m i t on the v a r i a t i o n of behaviour  with  size.  3.2.2.3  Temperature  S t e p - p u l l t e s t s were performed above and  below 0.25  to a s c e r t a i n the d i f f e r e n c e s i n t r a n s f o r m a t i o n t h a t occur.  T y p i c a l x-ray r e s u l t s f o r 99.7%  T ,  behaviour  and  99.9%  cobalt  are shown i n F i g u r e s 48 and in Figure  49.  The r e s u l t s are p l o t t e d  47.  C o b a l t t e s t e d a t -196°C, 20°C, and s i m i l a r behaviour. presented  100°C e x h i b i t  In Table XIII a s i n g l e equation i s  f o r behaviour  a t a l l t h r e e temperatures.  r e s u l t s d i f f e r f o r m a t e r i a l t e s t e d at 250°C and  385°C  (0.37  T ).  Although  are s i m i l a r a t a l l temperatures,  The  (0.3 T ) m  the slopes of the  curves  a much l a r g e r amount  of fee phase i s r e t a i n e d a t any v a l u e of s t r a i n f o r t e s t s above 0.25  T . m At f a i l u r e ,  the t r a n s f o r m a t i o n i s 98% complete f o r  m a t e r i a l t e s t e d a t -196°C, approximately m a t e r i a l t e s t e d a t room temperature and  95% complete f o r 100°C, and  80 to 90% complete f o r m a t e r i a l t e s t e d above 0.25 1  only T . m  T h i s v a r i a t i o n r e f l e c t s the s t r e s s l e v e l s a t t a i n e d a t the d i f f e r e n t temperatures.  At lower temperatures the s t r e s s  l e v e l s are much higher which i s e q u i v a l e n t to a p p l y i n g a larger d r i v i n g force for transformation. This f i n a l t h a t may  set of r e s u l t s , p r o v i d e s f u r t h e r i n f o r m a t i o n  be compared to the v a r i a t i o n i n t e n s i l e p r o p e r t i e s  with temperature. martensite  The volume f r a c t i o n of s t r a i n  t h a t forms i n the i n i t i a l  0.2%  induced  s t r a i n during a  t e n s i l e t e s t i s l i s t e d i n Table X I H . C l e a r l y , the behaviour below 0.25 T d i f f e r s from t h a t above. To o b t a i n macroscopic m ^ v i e l d below 0.25 T , s t r a i n induced t r a n s f o r m a t i o n must m occur. Above 0.25 T , t h i s bulk t r a n s f o r m a t i o n does not m appear necessary.  20°C T e s t  Fig.  48  (% S t r a i n / % fee) 250°C T e s t  X - r a y d a t a f o r 99.9% c o b a l t , 6.5 m i c r o n g r a i n s i z e .  s t e p - p u l l e d a t 20°C and 250  143  20°C T e s t  F i g . 49  (% S t r a i n / % fee) 250°C T e s t  X - r a y d a t a f o r 99.7% c o b a l t , 7 micron g r a i n s i z e .  s t e p - p u l l e d a t 20°C and 250°C  The amount of s t r a i n induced m a r t e n s i t e formed as a f u n c t i o n of s t r a i n i s p l o t t e d i n F i g u r e 50 f o r v a r i o u s t e s t temperatures.  The amount of t r a n s f o r m a t i o n t h a t has taken  p l a c e a t any value of s t r a i n , i s s i m i l a r f o r t e s t s a t -196°C,  20°C, and 100°C.  I t can be seen t h a t the volume  f r a c t i o n of s t r a i n induced m a r t e n s i t e i s much s m a l l e r , a t any v a l u e of s t r a i n , f o r m a t e r i a l t e s t e d above 0 . 2 5 T . m J  The  s a l i e n t f e a t u r e s of the s t r a i n induced  i n p o l y c r y s t a l c o b a l t have been presented.  transformation  The r e l a t i o n s h i p  between s t r a i n and r e t a i n e d fee phase may be r e p r e s e n t e d by an equation of the form e = A ( 1 0 )  m  ( % f c c )  l i t t l e w i t h p u r i t y or t e s t temperature, with g r a i n s i z e .  .  » « varies m  but changes r a p i d l y  "A" r e p r e s e n t s the s t r a i n t h a t c o u l d be  introduced into p o l y c r y s t a l cobalt i f f r a c t u r e coincided with completion  of the m a r t e n s i t i c t r a n s f o r m a t i o n .  The  i n i t i a l fee phase present f o l l o w i n g an annealing  procedure  and the s t r a i n v a l u e a t which the s t r a i n induced  transformation  begins determine "A". and  "A" v a r i e s v/ith p u r i t y , g r a i n s i z e ,  t e s t temperatures above 0 . 2 5 T . ^ m Eefore comparing these o b s e r v a t i o n s t o the r e s u l t s  obtained from the t e n s i l e t e s t s , a thorough d i s c u s s i o n of the m a r t e n s i t i c t r a n s f o r m a t i o n as r e l a t e d t o deformation of p o l y c r y s t a l c o b a l t i s r e q u i r e d .  3.2.2.4  Von  Mises  According  Criterion  t o Von  Mises C r i t e r i o n ,  5 independent  requires  s t r a i n without  shear  a  polycrystal  s y s t e m s t o u n d e r g o homogeneous  change i n v o l u m e .  Polycrystal  a s l i g h t v o l u m e change d u r i n g d e f o r m a t i o n and strict  s e n s e , Von  The i s very For of  thus  s h o u l d n o t be  undergoes  in a  applied.  change i n volume a c c o m p a n y i n g d e f o r m a t i o n o f small,  material  1/3  about  used  cobalt  o f 1% f o r c o m p l e t e t r a n s f o r m a t i o n .  in this  study o n l y about  25%  to  40%  the bulk transforms d u r i n g d e f o r m a t i o n e q u i v a l e n t to a  reduction  cannot  shape c h a n g e o f Basal This  0.10% t o 0 . 1 3 % .  i n volume of  transformation  and  Mises C r i t e r i o n  cobalt  slip  system  yield  individual  a major c o n t r i b u t i o n  p r o v i d e s o n l y two  s l i p mode o b s e r v e d  extension p a r a l l e l  independent  shear  volume change upon t r a n s f o r m a t i o n f r o m a small contraction expansion  parallel  to the c a x i s .  of b a s a l s l i p  s a t i s f y Von  Mises  and  i n hep  to the c  systems.  The  f e e t o hep  (Figure  13).  axis  provides a small  The  t h e volume c h a n g e d o e s  not  Criterion.  i t i s accepted that  difficult  the  in cobalt.  p e r p e n d i c u l a r t o t h e c a x i s and  combination  If  to  the  grains.  i s the major  p r o v i d e s no  This f a c e t of  cobalt *,  C r i t e r i o n must a r i s e  1  from  non  basal s l i p  the s a t i s f a c t i o n other  sources.  i s extremely o f Von  Mises  147 As o u t l i n e d i n the review by P a r t r i d g e , other deformation 7  modes have been observed t o s a t i s f y Von Mises  Criterion  where i n s u f f i c i e n t s l i p systems are a v a i l a b l e .  Kink  Boundary f o r m a t i o n , g r a i n boundary s l i d i n g , as w e l l as c o n t r i b u t i o n s from twinning shear are a l l r e c o g n i z e d as processes t h a t may to  supply the needed degrees of  allow deformation of a p o l y c r y s t a l l i n e  According to Kocks , 7  number of independent  c r o s s s l i p may slip  Over the temperature  freedom  aggregate.  a l s o reduce the r e q u i r e d  systems. range i n v e s t i g a t e d , g r a i n boundary  s l i d i n g should not be an important source of s t r a i n i n c o b a l t . Due  to the very low s t a c k i n g f a u l t energy of c o b a l t , c r o s s  s l i p may  a l s o be discounted.as a major route t o an  independent  shear system.  I f these processes are d i s c a r d e d  as unfavourable, f u r t h e r independent  shear systems  may  a r i s e from twinning and k i n k boundary f o r m a t i o n . Twinning elements y i e l d i n g both c o n t r a c t i o n  and  expansion p e r p e n d i c u l a r t o the b a s a l plane have been observed i n cobalt;  {1012}, {1011}, and .{112n}twins have a l l been  observed d u r i n g s i n g l e c r y s t a l deformation.  A l a r g e shear  value f o r {112n} type twins has been p o s t u l a t e d . 7  l a r g e twinned volume may  Thus, a  enhance the d u c t i l i t y of c o b a l t  considerably. Reed-Hill may  9  p o s t u l a t e s t h a t the amount of s t r a i n  that  be accommodated by twinning i s a r r i v e d a t as f o l l o w s : e •= 1//2  (V)  (S)  8)  148 VThere: e = s t r a i n 1//2 = average Schraid f a c t o r f o r p o l y c r y s t a l s V = volume f r a c t i o n twinned S = shear v a l u e f o r twinning mode c o n s i d e r e d as an example. twinning  R e e d - H i l l s u b s t i t u t e s v a l u e s f o r {1012}  i n zirconium.  S = V =  .17 .50 Therefore:  e =  (0.707) (0.50) (0.17) =  Thus {1012} twinning may zirconium.  account f o r up to 6%  A s i m i l a r equation  e = 1//2  (V)  { 1 0 i 2 }  0.06  (S)  strain in  f o r c o b a l t would  { 1 0 i 2 }  + 1//2  (V)  be:  { 1 Q l l }  (S)  { l o I l }  + 9)  In c o b a l t , the volume f r a c t i o n twinned would have to be very l a r g e as occurs percent  i n zirconium to y i e l d more than a  strain.  The r e t a i n e d fee phase t h a t i s d i s t r i b u t e d the g r a i n s of the low temperature phase may change.  throughout  a i d g r a i n shape  The r e t a i n e d areas of fee have no l a c k of  systems a v a i l a b l e , but s l i p and related.  t r a n s f o r m a t i o n are  Deformation of the fee phase i s probably  with c o n t i n u a t i o n of the t r a n s f o r m a t i o n . i s p o s s i b l e i n the fee phase without b a s a l s l i p t r a c e s should be observed. a given  few  {111}  different  If  slip closely synonomous  deformation  transformation, I f s l i p occurs  non on  plane i n the fee phase, t r a n s f o r m a t i o n on a  {111}  v a r i a n t becomes d i f f i c u l t * . 1  etc.  The manner i n which the disappearance  of the f e e phase  may p r o v i d e s t r a i n i n c o b a l t may be s t a t e d i n two ways. ongoing  The  t r a n s f o r m a t i o n may be viewed as s l i p on v a r i o u s {111}  planes, thus y i e l d i n g a number o f independent  shear  systems.  The c o n t i n u i n g t r a n s f o r m a t i o n may a l s o be viewed as a type of  twin f o r m a t i o n .  The s i m i l a r i t y between twinning and  m a r t e n s i t i c formation i s very c l o s e i n c o b a l t because the fee  t o hep t r a n s f o r m a t i o n i s a low energy t r a n s f o r m a t i o n  r e q u i r i n g o n l y a simple shear, w i t h no a d d i t i o n a l complex s h u f f l e s i n the plane o f shear. used by R e e d - H i l l t o account  (Table V I ) .  The formula  f o r the manner i n which twinning,  i n a d d i t i o n t o s l i p p r o c e s s e s , may s a t i s f y Von Mises  Criterion,  should be a p p l i c a b l e t o the m a r t e n s i t i c t r a n s f o r m a t i o n i n cobalt.  T r a n s f o r m a t i o n occurs on many {111} planes i n a  g i v e n f e e g r a i n ; thus we have a deformation process t h a t y i e l d s c o n t r a c t i o n and expansion  i n various directions i n  the parent g r a i n . The  t h e o r e t i c a l shear v a l u e f o r the t r a n s f o r m a t i o n i s  S = 0.353 and shear up t o 35% was observed for  a p p l i e d s t r e s s e s of s e v e r a l thousand  crystals.  by A l t s t e t t e r  2 8 - 3 1  p s i i n single  F o r the high s t r e s s e s i n v o l v e d d u r i n g p o l y c r y s t a l  deformation,  the t r a n s f o r m a t i o n may c l e a r l y p r o v i d e l a r g e  amounts o f shear. I f the formation of m a r t e n s i t i c p l a t e s i n c o b a l t i s c o n s i d e r e d e q u i v a l e n t t o the f o r m a t i o n o f twins the maximum s t r a i n a v a i l a b l e from t h i s source may be determined as for  twinning:  e = 1//2  (S) (V)  £ = 1//2  (S)  Where:  T x  8)  (V)  T x  1  0  )  S = 0.353 v£* = 0.25 t o 0.40  Therefore:' e = 0.25 (V) Tx  11)  For every 4% o f the fee phase transformed a s t r a i n o f 1.0% c o u l d t h e o r e t i c a l l y be obtained from t r a n s f o r m a t i o n alone.  In a l l m a t e r i a l produced  f o r the p r e s e n t study, between  30% and 60% f e e phase was r e t a i n e d i n the annealed m a t e r i a l . As the t r a n s f o r m a t i o n i s f o r c e d near completion d u r i n g deformation, anywhere from 7 1/2% t o 15% s t r a i n c o u l d t h e o r e t i c a l l y be accomplished  through o p e r a t i o n o f the  martensitic transformatiqn.  i n f a c t , i f the t r a n s f o r m a t i o n  does y i e l d the t h e o r e t i c a l maximum shear, '£ = 0.25 it  (v* ) , t  i s p o s s i b l e f o r the t r a n s f o r m a t i o n t o p r o v i d e a l l the  shear necessary i n the i n i t i a l p o r t i o n o f the t e n s i l e curve f o r example: e = A(10) and  e = 0.25  7)  m ( % f c q )  (V  fc  )  .....11)  but  (%fcc). ... . - (%fcc) = V. initial tx therefore (%fcc) = ( % f c c ) . . . . , - V. initial tx and (%fcc) = (%fcc). .,. , - 4^ initial S u b s t i t u t e i n equation #7 e  = A(10)  m ( % f c a  i  n  i  t  i  a  l  - > 4E  12)  Solve f o r log  E + 4m£ - [ l o g A + m(% f e e ) . ... ,] = 0  ....13)  151 If the equation i s s o l v a b l e f o r e, the v a l u e found correspond  will  t o the s t r a i n v a l u e a t which the t r a n s f o r m a t i o n  can no longer p r o v i d e a l l the shear r e q u i r e d .  The progress  of  decreases  the t r a n s f o r m a t i o n f o r a g i v e n s t r a i n imput  with increasing s t r a i n . of  Thus, the v a l u e found  t r u e s t r a i n t h a t may be i n t r o d u c e d without  some other shear mechanism t o operate.  i s the amount requiring  I f the equation i s  not s o l v a b l e f o r any p o s i t i v e value of e, then some shear mechanism other than the m a r t e n s i t i c t r a n s f o r m a t i o n must be r e q u i r e d a t y i e l d and -throughout final  the t e n s i l e curve.  column i n Table X I I I l i s t s the v a l u e o b t a i n e d  equation 13 above.  The column i s t i t l e d  e. . tx  The  from  From Table X I I I  i t may be seen t h a t there a r e two s i t u a t i o n s where no solution exists.  The f i r s t case i s f o r l a r g e g r a i n  T h i s i s not unreasonable i s very low.  Secondly, .  sizes.  as the i n i t i a l r e t a i n e d f e e phase the ecruations f o r t e s t s above 0.25 T m  cannot be s o l v e d f o r p o s i t i v e s t r a i n v a l u e s . o b s e r v a t i o n i n d i c a t e s a change i n behaviour  This  latter  a t higher  temperatures. In  the a n a l y s i s above i t has been assumed t h a t a l l  t r a n s f o r m a t i o n t a k i n g p l a c e would c o n t r i b u t e t o the t e n s i l e strain.  T h i s i s c l e a r l y not the case and the b a s a l  mode i s undoubtedly  o p e r a t i v e throughout  slip  the s t r e s s - s t r a i n  curve. Kink boundary formation has been observed by T h e i r i n g e r  2 4 1 2 5  i n c o b a l t a t a l l temperatures  w i t h the amount of kink formation  i n c r e a s i n g w i t h temperature.  The o b s e r v a t i o n t h a t t h i s  .  d e f o r m a t i o n mode i s more common a t h i g h t e m p e r a t u r e s usually  e x p l a i n e d as  follows:  to  n u c l e a t e and  propogate  it  i s g e n e r a l l y accepted  requires higher stress temperatures, sufficient  twins that  i t i s proposed  has  i s not w e l l  that  stress  twins  understood,  At  levels  higher  are  not  ( i . e . {112n}) and  the  t a k e s p l a c e as an a d j u n c t  i t s h o u l d be  been o b s e r v e d  recalled  in single  crystal  a v e n u e s whereby c o h e r e n c y  be m a i n t a i n e d  that  in polycrystal  in addition  {1122} <1123>  c o b a l t by  to a slip  Seeger  1  h  .  at g r a i n boundaries  cobalt are manifold.  i) ii)  Basal  Twinning  Modes,  {1012}, L e n t i c u l a r  iv)  Twins  Twins  Volume T r a n s f o r m a t i o n o f r e t a i n e d  fee regions  on v a r i o u s {111}  planes.  Corrugated  {1122}, <1123>, S e c o n d  Slip,  Pyramidal v)  Twins  {112n}, T h i n  {1121}, Z i g - Z a g  shear  below:  slip  {1011},  iii)  may  A  summary o f t h e p r o b a b l e d e f o r m a t i o n modes i s p r e s e n t e d  yielding  to  deformation.  o f t w i n n i n g modes, c o r r u g a t e d  The  stress required  than does growth.  to nucleate c e r t a i n  Finally, variety  the  the n u c l e a t i o n process  levels  formation of kink boundaries continuing  Although  is  Duplex S l i p  in retained  fee  phase.  Order  153 3.2.2.5  Metallographic  Observations  P r e s e n t a t i o n o f the m e t a l l o g r a p h i c o b s e r v a t i o n s made d u r i n g the d e f o r m a t i o n of p o l y c r y s t a l c o b a l t has been d e f e r r e d t o t h i s p o i n t as the p r i o r  information presented  i s r e q u i r e d t o e x p l a i n the s u r f a c e f e a t u r e s t h a t a r i s e . F i g u r e 51 i s p r e s e n t e d t o show the m a c r o s c o p i c shear may o c c u r d u r i n g t r a n s f o r m a t i o n .  that  Gross amounts of  t r a n s f o r m a t i o n tend t o obscure a l l o t h e r s u r f a c e  relief  as s t r a i n i n c r e a s e s .  Fig.  51  M a r t e n s i t e shear markings i n t r o d u c e d by a s u r f a c e s c r a t c h i n 99.9% c o b a l t . 1900X  154 The  s t r u c t u r e s produced  d u r i n g deformation of  c o b a l t a r e n o t documented i n t h e l i t e r a t u r e .  o f t r a n s f o r m a t i o n on v e r y by B e b r i n g and hardness and  Sebileau  7 1  cases  *  and  l a r g e g r a i n s has .  The  Annealed  8 2  81  s t r u c t u r e s a r e shown i n a few  the  been  structures  8 9  .  Jagged,  been photographed delineated  shear type, f r a c t u r e  by L o z i n s k y .  r a d i o g r a p h y t e c h n i q u e and  by  by  with  Lozinsky  zones  f e e g r a i n s by  compared  with the s t r u c t u r e determined  outlined  A u c o u t u i r e r and  the high temperature  affects  associated  i n d e n t a t i o n s have b e e n p h o t o g r a p h e d  Wilcox  polycrystal  have  also  Lacombe  an  the observed  ,  11  2  autofee  electropolishing  boundaries  t h e same  areas. T h r e e m a j o r o b j e c t i v e s were p u r s u e d work: A t t e m p t s throughout size,  and  test  temperature.  A variety  of  to o b t a i n the r e q u i r e d data.  strained  s e v e r a l p e r c e n t i t was  After  a  removed f r o m  specimen the  examined m i c r o s c o p i c a l l y  replicated.  was  The  until  specimen failure  a t - 1 9 6 ° C , room t e m p e r a t u r e , The  1 0 0 ° C , 250°C, and  to obtain r e p l i c a s  segment o f a s t e p - p u l l  and  test.  and  the  realized  from  as  then  procedures  T e s t s were c a r r i e d  o b j e c t i v e s were n o t f u l l y  impracticable each  then r e t e s t e d  occured.  grain  s p e c i m e n s were  I n s t r o n machine, x-rayed,  continued  experimental  were made t o f o l l o w a d e f i n e d s u r f a c e a r e a  the t e n s i l e curve while v a r y i n g p u r i t y ,  step-pulled was  d u r i n g the  out  380°C. i t was  t h e same•area  found  after  The replicas  two methods of examination a r e complimentary.  The  show much more d e t a i l than the o p t i c a l o b s e r v a t i o n s ,  but  as s t r a i n i n c r e a s e s the r e p l i c a s become so complex  the  o v e r a l l s i t u a t i o n becomes obscure.  that  The o p t i c a l work  c l a r i f i e s t h i s gross p i c t u r e w h i l e d e l e t i n g  the f i n e  structure. 3.2.2.5.1 The  Purity  and G r a i n  Size  e f f e c t s of d i f f e r i n g p u r i t y on the s u r f a c e f e a t u r e s  of p o l y c r y s t a l difference  cobalt  i s that  a r e minimal.  The o n l y  noticable  the i n t e n s i t y o f s u r f a c e rumpling a t  f a i l u r e i s lower f o r an i n c r e a s e i n p u r i t y .  This  r e f l e c t s the lower amount o f t r a n s f o r m a t i o n t h a t  difference occurs  prior to f a i l u r e . S i m i l a r l y , any d i f f e r e n c e s  observed between specimens o f  d i f f e r e n t grain  s i z e were d i f f e r e n c e s  only.  s i z e i n c r e a s e s , the amount of s t r a i n induced  As g r a i n  i n scale  and i n t e n s i t y  t r a n s f o r m a t i o n , and the e l o n g a t i o n p r i o r t o f a i l u r e decrease yielding  l e s s severe m i c r o s t r u c t u r e s .  Based on these i n i t i a l  findings,  c o n c e n t r a t e d on the v a r i a t i o n s t r a i n and temperature.  further  work was  i n observed deformation w i t h  156 3.2.2.5.2  Optical  Figure specimen At obvious  52  y i e l d  y i e l d ,  the  v i s i b l e .  A  This  is  band  reoriented  A  small  twin  so  of  in  zig-zag  and  upper that  grained  only  set  the  (2)  one  are  large  annealing  twin  set  of  barely  grain which  completely. has  been  transformation.  two  the  the  has  changed  systems  (1  r a d i c a l l y .  and  2)  i n  the  central  system  has  also  left  to  some  small  twin  reflected  shape  value.  Twins  single  crystal  R o s i  1  5  .  {1121} Figure  specimen been  second  forming  in  configuration.  l e n t i c u l a r  a  also  severely  f a i l u r e ,  propogated  had  the  exhibits  situation on  large  250°C.  traverses fee  a  twin.  The  grains  is  shown  grain.  caused  the  surface are  The  is  t i l t e d  focus.  At  by  shear is  a  cooling  the  of  at  grain  (1),  an  constraint  grain  rumpled out  s t r a i n  A  f a i l u r e  band  the  of  progress  largest  probably  during  increasing  to  rumpled  amount  c l e a r l y .  the  markings  2.3%  large  traces  from  shear  At  Metallography  at  of  cobalt  Both  by  presents  in  Davis  s t r a i n  boundaries has  not  1  have  and  to  taken  i n f e r r i n g  type 1  2.3%  a  been  has take  on  high  a  observed  that  a  a  different  Three single  area  important grain.  in  shear  the  a  shear  polycrystal  determined  up  in  titanium  the  twins  plane.  f a i l u r e .  operative  growth,  zig-zag  at  two  twin  investigators  habit 53  from The  during this  forming  same  planes  have  157  Fig.  52  D e f o r m a t i o n of 99.998% c o b a l t , 850X  158  Fig.  53  Deformation markings i n 99.998% c o b a l t  at f a i l u r e .  1000X  cn  X rt  • H  CD  C CD EH  Fig.  54(a)  Fig.  54  1.9% strain  Fig.  54(b)  G r a i n shape change i n 99.9% c o b a l t .  6.6% s t r a i n 1000X  159 52 a n d 53 show c o b a l t h a v i n g  Figures size, of  lowest d u c t i l i t y ,  any m a t e r i a l u s e d  show v e r y shear  similar  and l e a s t  strain  i n the present  behaviour,  the largest grain induced  study.  although  t h e r u m p l i n g and  54 shows t h e amount o f g r a i n shape c h a n g e t h a t  may o c c u r w i t h  little  increase i n tensile  strain.  t e n s i l e a x i s i s shown and t h e d i s l o c a t i o n be l i m i t e d  this  to a single  slip  The  activity  appears  system i n a m a j o r i t y o f  grain. Although  limited basal  slip,  observed exhibit test  o p t i c a l metallography  use i n determining  that occurs  and  A l l other materials  increase i n intensity. Figure  to  transformation  was f o u n d  the presence  t o be o f  o r absence o f non-  F i g u r e s 55 and 56 show t h e c o m p l e x i t y i n polycrystal cobalt.  a t low s t r a i n v a l u e s . a v e r y complex t w i n n e d temperature  Very  few t w i n s a r e  At fracture structure.  have l i t t l e  affect  a t e s t a t -196°C,  F i g u r e 56 a f t e r  a l l specimens Purity,  on t h i s  F i g u r e 55 shows t h e t w i n n i n g  deformation.  of twinning  a test  grain  facet  size,  of the  present following  a t 350°C.  a t 350°C.  F i g u r e 56  shows a s i n g l e g r a i n a f t e r  testing  have t a k e n on a l e n t i c u l a r  c o n f i g u r a t i o n and a r e p r o b a b l y  {1012} t w i n s , w h e r e a s t h e t h i n s t r a i g h t t w i n s r e p r e s e n t ' { 1 0 1 1 } and {112n} habit planes  are represented  appearance o f the t h i n high i n t e r n a l n u c l e a t e them.  twins  habit planes. i n this  Several  twins  probably  Over a d o z e n  single grain.  a t high temperature  The  infers  s t r e s s c o n c e n t r a t i o n s were a v a i l a b l e t o  that  160  F i g . 56  Twinning i n c o b a l t a t 350°C.  850X  161 The i n f o r m a t i o n g a i n e d by o p t i c a l m e t a l l o g r a p h y may be summarized as i)  follows:  P u r i t y and g r a i n s i z e do not a f f e c t  observed d e f o r m a t i o n modes. size simply increases  the  A r e d u c t i o n i n p u r i t y or g r a i n  the i n t e n s i t y o f the  structures  observed. ii)  L i t t l e d i f f e r e n c e was noted between  above and below 0.25 T . m  deformation  S u r f a c e r u m p l i n g was somewhat  severe a t h i g h temperature which s i m p l y r e f l e c t s volumes o f s t r a i n induced m a r t e n s i t e  the  less  smaller  formed at. t h e s e  temperatures. iii)  Deformation i s very heterogeneous.  Some  g r a i n s e x h i b i t g r o s s amounts o f d e f o r m a t i o n a t low s t r a i n v a l u e s w h i l e n e i g h b o u r i n g g r a i n s appear iv)  undeformed.  .Macroscopic shear o c c u r s on two o r more  p l a n e s w i t h i n the r e g i o n s d e l i n e a t e d by an fee g r a i n boundary. The amount of shear t h a t may o c c u r on any p l a n e i s v e r y v)  Twins are observed i n c o b a l t specimens  a t a l l t e m p e r a t u r e s between - 1 9 6 ° C and 380°C.  tested  The amount o f  t w i n n i n g observed a t y i e l d i s n e g l i g i b l e but i n c r e a s e s strain. plane,  Z i g - z a g t w i n n i n g , assumed t o occur i n the i s a common o b s e r v a t i o n a t a l l t e m p e r a t u r e s .  {1012} t w i n s as w e l l as s t r a i g h t planes  with  {1121}- h a b i t Lenticular  t h i n twins of probable  {1011} and {112n} are a l s o o b s e r v e d .  large.  habit  Although a  m u l t i p l i c i t y o f t w i n n i n g modes are o b s e r v e d , the t w i n n i n g volume remains  small.  162 3.2.2.5.3 The  Replica  Observations  f e a t u r e s observed  by r e p l i c a techniques are  d i f f i c u l t t o c o r r e l a t e w i t h those d e l i n e a t e d o p t i c a l l y . As shown i n F i g u r e 57 f o r a s m a l l g r a i n e d specimen of 99.7%  c o b a l t , the e f f e c t of s t r a i n on the s u r f a c e topography  i s v e r y pronounced.  To a l l o w comparison t o the o p t i c a l  o b s e r v a t i o n s r e p l i c a s taken a f t e r l i m i t e d amounts o f s t r a i n are presented.  In F i g u r e 58, the m a j o r i t y o f shear has  taken p l a c e on two s l i p systems approximately angles.  Where one r e g i o n  at right  a b u t t s the o t h e r , twins have been  i n i t i a t e d to r e l i e v e s t r e s s .  Although  evidence  of twinning  does e x i s t i n t h i s case, a t o t a l l a c k o f v i s i b l e s t r e s s relief  i s more common.  In F i g u r e 59, shear has taken  on t h r e e d i s t i n c t systems.  The amount o f shear  i n a l l cases, y e t no evidence i n t e r s e c t i o n i s observed.  The  i s large  o f twinning a t p o i n t s o f  The observed  F i g u r e 59 i n v o l v i n g two or t h r e e shear most commonly observed  place  p a t t e r n shown i n systems i s the  surface feature i n p o l y c r y s t a l cobalt.  topography i s c l e a r l y a r e s u l t of the m u l t i v a r i a n t  martensitic transformation.  The l a r g e step h e i g h t s  the passage o f many d i s l o c a t i o n s over a s i n g l e s l i p T h e r e f o r e , i t may be i n f e r r e d t h a t the s t r e s s system t r a n s f o r m a t i o n on some planes  i s cabable  d i s l o c a t i o n p r o d u c t i o n on these  planes.  reflect plane. producing  of continuing  163  F i g . 57  Deformation of 99.7% c o b a l t a t 250°C.  3000X  1 6 4  Fig.  59  T y p i c a l s u r f a c e shear markings i n c o b a l t . 6500X  -196°C T e s t .  In c o n t r a s t t o the markings shown i n F i g u r e 59 are the twins shown i n F i g u r e s 60 and 61.  S l i p on m a r t e n s i t i c  t r a n s f o r m a t i o n planes i s c h a r a c t e r i z e d by n o t i c a b l e shear on many p a r a l l e l planes w i t h gross amounts on o c c a s i o n a l p l a n e s . The twins, on the other hand, are c h a r a c t e r i z e d by a s i n g l e volume of sheared l a t t i c e .  In F i g u r e 61, a l a r g e  lenticular  twin appears i n the c e n t e r of the g r a i n and another twin has formed  i n z i g - z a g f a s h i o n between the l e n t i c u l a r twin and  a g r a i n boundary. The r e p l i c a work was  undertaken  t o a s c e r t a i n i f non-  b a s a l s l i p occured i n p o l y c r y s t a l c o b a l t . was  A second g o a l  to determine whether the f i n e d e t a i l s of deformation  d i f f e r above and below 0.25 Non-basal  s l i p was  T . m observed d u r i n g t e s t s a t 250°C  (0.30 T ) but no s i m i l a r o b s e r v a t i o n s were made a t -196°C, m ' 20°C, or 100°C. system.  F i g u r e 62 shows s l i p markings on a second  The non-basal t r a c e s are assumed to occur on the  {1122} <1123> system, as t h i s i s the o n l y non-basal  slip  system t h a t has been observed i n s i n g l e c r y s t a l c o b a l t "*. 1  The appearance grains. of  of non-basal s l i p i s not observed i n a l l  T h i s i s to be expected i n view of the h e t e r o g e n e i t y  deformation. One  f u r t h e r o b s e r v a t i o n may  f e a t u r e s above and below 0.25  T . m  be drawn from the s u r f a c e Below 0.25  a l l shear m  markings were very s t r a i g h t , and remained occured.  T  so u n t i l  failure  Although l a r g e amounts of shear occured on some  planes they e x h i b i t e d very l i t t l e bending or F i g u r e 63 demonstrates  this situation.  At  waviness.  temperatures  166  Fig.  60  Twinning i n c o b a l t a t -196°C.  Fig.  61  Twinning i n c o b a l t a t  250°C.  3700X  3700X  Fig.  62  N o n - b a s a l s l i p i n p o l y c r y s t a l c o b a l t t e s t e d a t 250°C. 7500X  above 0.25 T  the s i t u a t i o n d i f f e r e d .  m  l a r g e amounts o f s h e a r were v i s i b l e one  s e t of shear markings o f t e n  orientation. This  Figure  In areas,  on s e v e r a l  t o o k up a c u r v e d  of the s l i p  b a n d s may  i n t h e amount o f g l i d e p o l y g o n i z a t i o n  allowing  visible  changes i n s l i p  metallographic  observations Purity  surface  induced  1  s i z e had l i t t l e  allotropic  fee  grains  fee  g r a i n boundary.  number  zig-zag  transformation.  provides  hep  structure  metals.  1h  on t h e o b s e r v e d increased  due t o l e s s  strain  several basal Shear  s y s t e m and i s e x t r e m e l y  observed  The l e n t i c u l a r  an  on more  heterogeneous.  modes o c c u r e d with  within  orientations within  i s commonly  of twins increased  i s related to  The m u l t i v a r i a n c e  a t a l l temperatures  strain;  t w i n s a s w e l l a s many s t r a i g h t t h i n  observed.  and  occuring.  A v a r i e t y of twinning The  less intense  of the surface  the  affect  A s g r a i n s i z e and p u r i t y  t o p o g r a p h y became  t h a n one b a s a l  earlier  made i n s i n g l e c r y s t a l m a t e r i a l * ' .  and g r a i n  majority  forpolycrystal  the behaviour postulated  transformation  The  band o r i e n t a t i o n .  evidence presented  d e f o r m a t i o n mechanisms. the  occuring,  Summary  c o b a l t agrees with with  o r wavy  a r i s e f r o m an  increase  The  systems,  64.  non l i n e a r i t y  3.2.2.5.4  where  l e n t i c u l a r and t w i n s were  t w i n s were {10l2} t w i n s common t o  The t w i n s t a k i n g  up a z i g - z a g  c o n f i g u r a t i o n were  169  assumed t o belong to the {1121} twinning p l a n e , as twins of s i m i l a r c o n f i g u r a t i o n were i d e n t i f i e d c o b a l t by D a v i s .  The t h i n s t r a i g h t twins probably  1 1  to the { l u l l } ,  i n single crystal  {1122}, or {1124} twin systems as a l l t h r e e  have been observed  i n cobalt single c r y s t a l s  Non-basal s l i p was observed  4 , 1 k  above 0.25 T  . but not  m  c  below.  belong  I t i s postulated  t h a t the s l i p occurs on the  {1122} <1123> system, as c o r r u g a t e d s l i p has been observed in single c r y s t a l cobalt *. A further observation o u t l i n i n g a difference i n 11  behaviour above and below 0.25 T i s the non l i n e a r i t y of m basal  s l i p traces.  The observed  bending  and waviness  may r e f l e c t c o n c e n t r a t i o n s of d i s l o c a t i o n s of s i m i l a r s i g n on p a r a l l e l s l i p p l a n e s .  171  3.2.2.6  D i s c u s s i o n and  Summary  I t remains to compare the data r e g a r d i n g the induced  strain  t r a n s f o r m a t i o n t o t h a t determined from t e n s i l e  procedures.  The  e a s i e s t way  to a v o i d c o n f u s i o n  while  d i s c u s s i n g the numerous o b s e r v a t i o n s made i n t h i s study i s to d e a l w i t h v a r i o u s measured parameters i n a t a b u l a r form. A summary of the experimental i n Table XIV.  r e s u l t s i s presented  A s e r i e s of f o o t n o t e s are i n c l u d e d f o r  those o b s e r v a t i o n s t h a t do not lend themselves to the tabular  format.  3.2.2.6.1  The  Yield  Stress  A g r e a t d e a l of i n f o r m a t i o n has been gathered the y i e l d s t r e s s .  The most important  d i f f e r e n c e i n behaviour  regarding  o b s e r v a t i o n i s the  above and below 0.25  T  .  (Table  XIV.)  m T h i s r e s u l t i s m i r r o r e d i n the r e s u l t s f o r the induced  transformation.  the s t r a i n induced s t r a i n and  Below 0.25  strain  T , the i n i t i a t i o n of m  t r a n s f o r m a t i o n occurs a t 0.05%  i s w e l l underway at the 0.2%  to  offset yield  0.10% stress.  On the other hand, as the temperature i s i n c r e a s e d above 0.25  T , the i n i t i a t i o n of the t r a n s f o r m a t i o n i s delayed  to higher v a l u e s of s t r a i n .  A t 0.2%  strain, l i t t l e  i f any  t r a n s f o r m a t i o n has occured i n specimens t e s t e d a t 0.30 or 0.37 T . T h i s combination of r e s u l t s leads to the m c o n c l u s i o n t h a t the e s s e n t i a l l y athermal observed  below 0.25  of fee c o b a l t .  T , i s due m  yield  behaviour  to the onset of bulk  transformation  172 TABLE Xfff  Summary o f Experimental R e s u l t s As P u r i t y Increases  As G r a i n S i z e At T e s t Temperatures Increases <0.25 T >0.25 T m m  % retained fee  decreases rapidly  decreases rapidly  N/A  0.2% y i e l d stress  decreases slightly  decreases rapidly  ^ c o n s t a n t decreases rapidly  Elongation to Failure  decreases  decreases rapidly  decreases decreases  Work Hardening Rates  little effect  little effect  decreases decreases rapidly slowly  T o t a l Work Hardening  decreases  decreases  decreases decreases  Ultimate Strength  decreases  decreases  decreases decreases  S t r a i n a t which l i t t l e effect Tx. Begins  little effect  constant  increases  Volume Tx. a t 0.2% s t r a i n  little effect  little effect  large  nil  Volume Tx. a t Failure  decreases  decreases  decreases decreases  Rate of S t r a i n Induced Tx.  little effect  decreases  little effect  little effect  Volume Tx. a t any S t r a i n  little effect  little effect  constant  decreases rapidly  N/A  - F r a c t u r e s u r f a c e s e x h i b i t d u c t i l e f a i l u r e a t a l l temperatures. - High v a l u e s a r e observed f o r c. and K i n a H a l l - P e t c h r e l a t i o n s h i p and both parameters decrease r a p i d l y above 0.25 T . - The r e l a t i o n s h i p between percentage f e e and strai^^may be represented by an equation o f the form e = A(10) °  173  The behaviour i s athermal because the d r i v i n g  force  f o r t r a n s f o r m a t i o n a r i s i n g from thermodynamic c o n s i d e r a t i o n s i s very small when compared t o the s t r e s s l e v e l s i n v o l v e d . Above 0.25 T , i t i s p o s t u l a t e d t h a t i n i t i a t i o n o f m d i s l o c a t i o n a c t i v i t y on the corrugated  s l i p plane can occur  a t s t r e s s l e v e l s below those r e q u i r e d f o r m a r t e n s i t i c transformation.  Thus, s l i p on the second order  system c o n t r o l s y i e l d above 0.25 T . in yield  pyramidal  The sharp decrease  s t r e n g t h measured i s due t o the temperature dependence  of the P e i e r l s s t r e s s on the corrugated  slip  plane.  T h i s r e s u l t was o u t l i n e d d u r i n g d i s c u s s i o n o f the e f f e c t of g r a i n s i z e on the y i e l d s t r e n g t h .  The r e s u l t s  i n S e c t i o n 3.2.2.4 on the s t r a i n induced  presented  transformation are  c o n s i s t e n t with the p o s t u l a t e d behaviour,  but do not d i s m i s s  the p o s s i b i l i t y t h a t some other s t r o n g l y temperature dependent mechanism may be r e s p o n s i b l e f o r the behaviour. The  large increase i n y i e l d  decreases was d i s c u s s e d e a r l i e r .  s t r e s s as g r a i n s i z e A change i n g r a i n s i z e has  l i t t l e a f f e c t on the manner i n which the s t r a i n  induced  t r a n s f o r m a t i o n proceeds i n the r e g i o n o f y i e l d . I t was noted d u r i n g d i s c u s s i o n o f the t e n s i l e t h a t the temperature a t which the y i e l d  results  s t r e s s changes  behaviour i n c r e a s e d as the g r a i n s i z e i n c r e a s e d .  This  r e s u l t i s a l s o c o n s i s t e n t w i t h the behaviour p o s t u l a t e d above.  The change i n temperature dependence occurs a t  higher temperatures as the g r a i n s i z e i n c r e a s e s because the s t r e s s l e v e l s accomplished d u r i n g deformation  differ  radically.  In F i g u r e 65, two  l i n e s have been drawn to r e p r e s e n t  corrugated s l i p and bulk t r a n s f o r m a t i o n a t y i e l d .  As  a p p l i e d s t r e s s i n c r e a s e s y i e l d w i l l occur when Von  Mises  C r i t e r i o n can be s a t i s f i e d .  At low temperatures,  occurs when t r a n s f o r m a t i o n begins. ^  Above 0.25  the  yield  T , i t is m  p o s t u l a t e d t h a t y i e l d occurs when the s t r e s s l e v e l i s s u f f i c i e n t to i n i t i a t e d i s l o c a t i o n motion on the corrugated s l i p plane.  I f m a t e r i a l of a d i f f e r e n t g r a i n s i z e i s  t e s t e d , a d i f f e r e n t s e t pf curves  apply.  As o u t l i n e d e a r l i e r , the y i e l d s t r e s s may as a r i s i n g from a combination  of f a c t o r s ; o^,  be c o n s i d e r e d the  lattice  f r i c t i o n and K, a f a c t o r r e p r e s e n t i n g the d i f f i c u l t y which s l i p may determined 0.25  T .  be i n i t i a t e d a c r o s s a boundary.  t h a t both  was  and K decrease more r a p i d l y above  As g r a i n s i z e i n c r e a s e s , the y i e l d  r a p i d l y r e f l e c t i n g the l a r g e v a l u e of K. g r e a t e r than 0.25  It  with  At  s t r e s s drops temperatures  T , the v a l u e of K i s d e c r e a s i n g  and t h e r e f o r e a l e s s severe drop i n y i e l d  rapidly  stress i s  observed. A second s e t of l i n e s r e p r e s e n t i n g the behaviour 24 micron,  99.9%  c o b a l t are shown i n F i g u r e 65.  These  l i n e s are c o n s i s t e n t w i t h the t e n s i l e o b s e r v a t i o n s the p o s t u l a t e d P u r i t y has formation.  and  behaviour. little  e f f e c t on the s t r a i n induced t r a n s - .  T h i s p a r a l l e l s the r e s u l t s f o r the 0.20%  s t r e s s , where p u r i t y was parameter.  of  not found  to be an  important  yield  stress required to i n i t i a t e i n 6.5 )j c o b a l t .  corrugated  stress required to i n i t i a t e i n 24 JJ c o b a l t .  corrugated  stress required to i n i t i a t e bulk tranformation i n 6  stress required to i n i t i a t e bulk transformation i n 24 JJ c o b a l t .  _L  Temperature Fig.  65  0.25  T  Mechanisms c o n t r o l l i n g y i e l d  m in polycrystal  slip  cobalt  slip  176 3.2.2.6.2 The  Flow S t r e s s observed  t e n s i l e c h a r a c t e r i s t i c s of the flow s t r e s s  are d i r e c t l y r e l a t e d to the s t r a i n induced The  two  transformation.  stage temperature dependence of flow s t r e s s d i s a p p e a r s  as s t r a i n i n c r e a s e s , p a r a l l e l i n g the o b s e r v a t i o n t h a t as  strain  i n c r e a s e s the amount of t r a n s f o r m a t i o n t a k i n g p l a c e d u r i n g I  any  s t r a i n increment i s a l s o dropping.  As  i s o c c u r i n g a t higher s t r a i n s , the athermal a t t r i b u t e d to the s t r a i n induced  less  transformation  behaviour  transformation also disappears,  and the flow s t r e s s becomes more r e p r e s e n t a t i v e of the other c o n t r o l l i n g deformation behaviour  noted  mechanisms.  The  i n the i n i t i a l p o r t i o n of the  anomalous stress-strain  curve i s simply a f u r t h e r m a n i f e s t a t i o n of the high r a t e of t r a n s f o r m a t i o n a t low s t r a i n v a l u e s . 3.2.3.6.3  E l o n g a t i o n to F a i l u r e  Based on the o b s e r v a t i o n s made i n t h i s study,  i t is  not s u r p r i s i n g t h a t the d p c t i l i t y of p o l y c r y s t a l c o b a l t quoted elsewhere forms no r e c o g n i z a b l e p a t t e r n . E l o n g a t i o n v a r i e s v i a a complex between p u r i t y , g r a i n s i z e , and reasons  behind  the behaviour  inter-relationship  t e s t temperature.  The  o n l y become c l e a r when the  amount of fee phase present, and  the manner i n which  t h i s fee phase d i s a p p e a r s w i t h s t r a i n , i s  understood.  P o l y c r y s t a l c o b a l t f r a c t u r e s when e i t h e r o f two c r i t e r i a are s a t i s f i e d .  F i r s t , polycrystal cobalt w i l l  fail  when the volume p e r c e n t o f f e e phase i s reduced t o a c r i t i c a l value.  I f the s t r e s s l e v e l i s v e r y high,  -196°C, the t r a n s f o r m a t i o n before f a i l u r e occurs.  may come w i t h i n  as i n t e s t s a t 2% o f completion  A t room temperature, the c r i t i c a l  v a l u e i s about 5% r e t a i n e d f e e phase. The  second l i m i t i s due t o C o n s i d e r e ' s C r i t e r i o n ;  When the work hardening r a t e becomes equal t o the a p p l i e d t r u e s t r e s s , i n s t a b i l i t y occurs and f a i l u r e becomes imminent. The  measurable parameter t h a t determines the d u c t i l i t y  of p o l y c r y s t a l c o b a l t i s the amount of f e e phase p r e s e n t i n the m a t e r i a l  f o l l o w i n g an annealing  procedure.  The r a t e  a t which the f e e phase d i s a p p e a r s w i t h s t r a i n does not vary w i t h p u r i t y , or t e s t temperature. measured f o r high p u r i t y m a t e r i a l  Thus, the lower d u c t i l i t y simply r e f l e c t s the  reduced amount o f f e e phase a v a i l a b l e f o l l o w i n g treatment.  The observed decrease i n d u c t i l i t y as t e s t  temperature i n c r e a s e s The  a r i s e s from Considere's C r i t e r i o n .  r a t e a t which the s t r a i n induced  transformation  proceeds decreases as the g r a i n s i z e i n c r e a s e s , retained  heat  a l s o the  f e e phase p r e s e n t p r i o r t o t e s t i n g decreases as  the g r a i n s i z e i n c r e a s e s .  The l a t t e r f a c t o r i s l a r g e r and .  thus, the measured e l o n g a t i o n as g r a i n s i z e  increases.  f o r p o l y c r y s t a l c o b a l t decreases  178  3.2.2.6.4  Work Hardening  Behaviour  E a r l i e r , work hardening behaviour  i n polycrystal cobalt  was compared t o t h a t f o r metals undergoing martensitic transformation.  a similar  The e s s e n t i a l l y c o n s t a n t work  hardening r a t e observed a t low s t r a i n was a t t r i b u t e d t o the t r a n s f o r m a t i o n t a k i n g p l a c e a t a h i g h r a t e . monitored  The  progress o f the t r a n s f o r m a t i o n v e r i f i e s t h a t the  m a j o r i t y o f s t r a i n induced m a r t e n s i t e forms d u r i n g the i n i t i a l p o r t i o n o f the t e n s i l e The  two stage temperature  curve. dependence o f the work  hardening r a t e was a l s o a t t r i b u t e d t o the m a r t e n s i t i c t r a n s f o r m a t i o n , as was the disappearance behaviour as s t r a i n i n c r e a s e d . observed behaviour  of the two stage  These r e s u l t s p a r a l l e l the  of t h e flow s t r e s s and may be e x p l a i n e d  i n l i k e manner. A t low temperatures  and low s t r a i n v a l u e s , the r a t e  a t which m a r t e n s i t e p l a t q s a r e forming i s very h i g h , thus, the s t r u c t u r e through which d i s l o c a t i o n s must move i s increasing i n i n t e n s i t y very q u i c k l y .  As s t r a i n i n c r e a s e s ,  the r a t e a t which the t r a n s f o r m a t i o n proceeds rapidly.  Simultaneously,  drops o f f  the s t r e s s l e v e l i s i n c r e a s i n g ,  i n i t i a t i n g other deformation mechanisms t o r e l i e v e concentrations.  stress  E v e n t u a l l y , t h e r e i s i n s u f f i c i e n t f e e phase  a v a i l a b l e t o a l l o w f u r t h e r deformation or the work hardening r a t e becomes equal t o the s t r e s s l e v e l and f a i l u r e o c c u r s .  Above 0.25 T , the onset of bulk t r a n s f o r m a t i o n occurs m at  some p o i n t f o l l o w i n g y i e l d w h i l e c o r r u g a t e d s l i p i s  p o s t u l a t e d as o c c u r i n g throughout  the s t r e s s s t r a i n c u r v e .  Transformation does take p l a c e , but a t any v a l u e of s t r a i n far  l e s s s t r a i n induced m a r t e n s i t e has formed than a t  temperatures  below 0.25 T .  The s t r u c t u r e formed d u r i n g  deformation above 0.25 T^ i s probably l e s s i n t e n s i v e t h a t formed below. boundaries  than  Less l a t t i c e d e b r i s and fewer m a r t e n s i t e  due t o t r a n s f o r m a t i o n are produced.  In a d d i t i o n ,  s l i p may be more e a s i l y i n i t i a t e d a c r o s s boundaries due to  the r e d u c t i o n i n P e i e r l s s t r e s s on the c o r r u g a t e d  plane.  slip  T h e r e f o r e the measured work hardening r a t e s are  lower above 0.25 T . m Although  l i t t l e v a r i a t i o n i n work hardening r a t e s  were recorded f o r changes i n p u r i t y or g r a i n s i z e , the t o t a l work hardening between y i e l d and f r a c t u r e  (and the  u l t i m a t e strength) decreases w i t h an i n c r e a s e i n e i t h e r parameter.  T h i s r e s u l t r e f l e c t s the r e d u c t i o n i n  d u c t i l i t y t h a t accompanies i n c r e a s i n g p u r i t y or g r a i n  size.  4  Conclusions i)  A l t h o u g h t h e hep a l l o t r o p e i s t h e s t a b l e b e l o w 417°C, p o l y c r y s t a l hep c o b a l t  form f o r c o b a l t  a theoretical possibility  Cobalt  T  e x i s t s a s a two p h a s e  m i x t u r e o f f e e a n d hep c r y s t a l l a t t i c e s heat t r e a t i n g  The amount o f f e e p h a s e r e t a i n e d  an a n n e a l i n g  p u r i t y and i n c r e a s i n g metastable 60%,  following  normal  procedures.  ii) following  i s only  treatment decreases with grain  size.  i n cobalt increasing  The maximum amount o f  f e e p h a s e t h a t may be r e t a i n e d  i s approximately  t h e minimum 10%. iii)  cobalt  The r e t a i n e d  i n polycrystal  t r a n s f o r m s m a r t e n s i t i c a l l y t o t h e hep m o d i f i c a t i o n  as d e f o r m a t i o n may be compared  i s introduced  retained  yielding tensile properties  to other metals that  transformation.  form  f e e phase p r e s e n t  e = A (10)  °~  .  transformation  The y i e l d  i n t e r f e r e s with  and o t h e r common hep m e t a l s . stress of cobalt,  temperature,  temperature dependence.  and t h e  by a n e q u a t i o n o f t h e  The t r a n s f o r m a t i o n  c o m p a r i s o n s between c o b a l t iv)  undergo a s i m i l a r  The r e l a t i o n s h i p between s t r a i n  f e e p h a s e may be d e s c r i b e d  below t h e  h a s two d i s t i n c t  regions of  Below 0 . 2 5 T^ t h e y i e l d  stress  is  e s s e n t i a l l y temperature  by  the stress necessary  to i n i t i a t e  of  retained  Above 0 . 2 5 T i ti s postulated m  f e e phase. L  the  strong  decreasing  {1122} s l i p  i n d e p e n d e n t and i s d e t e r m i n e d bulk  temperature dependence observed  transformation that  i s due t o t h e  v a l u e o f t h e P e i e r l s s t r e s s on t h e c o r r u g a t e d planes.  that  v) in  cobalt  w h i c h do  The  strengthening effect  i s large,  as  not e x h i b i t  i s the case  of g r a i n  boundaries  f o r o t h e r hep  a multiplicity  of s l i p  metals  s y s t e m s a t room  temperature. vi)  The  amount o f r e t a i n e d aggregate.  A  larger  temperature vii)  polycrystal behaviour during  The  observed  may  of other m a t e r i a l s that  deformation.  A  two  rate The  shear  t o hep  be compared  a t low  from  the  dependence of  strain values.  fee  grain-boundary.  t r a n s f o r m a t i o n from dislocation  A number o f t w i n n i n g modes a r e o b s e r v e d from  0.04  form  Non-basal  the twinned slip  secondary  slip  T . m m volume i s s m a l l  in  t o 0.38  T  (several  but  not  m  system  order pyramidal  T  o c c u r s above 0.25  1  <1123> s e c o n d  to  these planes.  A l t h o u g h many t w i n s  The  Criterion.  t h a t o c c u r s on b a s a l p l a n e s  c  below.  as  r a t e s measured f o r  t h e s e p l a n e s combined w i t h c o n t i n u e d  at a l l temperatures  x)  in ductility  to Considere's  o r i e n t a t i o n w i t h i n an  fee  ix)  yields  commonest s u r f a c e f e a t u r e i n d e f o r m e d  intense s u r f a c e shears a r i s e  p r o d u c t i o n on  the  transform m a r t e n s i t i c a l l y  i s observed  i s the heterogeneous  on  to  polycrystal  stage temperature  The  cobalt  decrease  work h a r d e n i n g  c o b a l t a r e h i g h and  more t h a n one  i s related  p r o p o r t i o n of fee phase  i s i n c r e a s e d i s due  viii)  of  initial  The  t h e work h a r d e n i n g  cobalt  of c o b a l t  f e e phase p r e s e n t i n the  higher d u c t i l i t y . test  ductility  i s p o s t u l a t e d as the  system.  {1122}  182 5  Suggestions The  further  f o r F u t u r e Work  results  study.  required  o f t h e p r e s e n t s t u d y open many a v e n u e s f o r  The  most o b v i o u s  are o u t l i n e d i)  below.  I f more i s t o be  modes, s t u d i e s must be specimens. directions  of the o p e r a t i v e s l i p  via  A  second  been found  to produce  a series  alloying to higher of  grained and  strain  procedures  modes i n c o b a l t .  information could  induced t r a n s f o r m a t i o n . i t should  be  high strength cobalt o r by v a r i a t i o n s  f e e p h a s e and  should  thus y i e l d  From t h e h i g h K v a l u e s and  polycrystal  extend  Winning  C e r t a i n combinations  cobalt,  i t i s reasonable  a d d i t i o n s of a l l o y i n g  a l l o y s with extremely To  deformation  in  yield high  coupled with high strength. iii)  judicious  large  i n the present study  of ausforming  l a r g e volumes o f r e t a i n e d  i n pure  and  some v e r y d u c t i l e ,  annealing procedures.  ductility  the  allowing accurate  §rea where u s e f u l  i s i n a study of the  From what has possible  out with very  involved i n deformation  ii) gained  carried  l e a r n e d about  T h i s w o u l d a l l o w d i r e c t measurement o f p l a n e s  verification  be  a r e a s where f u r t h e r work i s  the u s e f u l  elements  high t e n s i l e  a. v a l u e s t o assume  should y i e l d  p r o p e r t i e s a t room  s t r e n g t h to higher temperatures  a d d i t i o n s t h a t w o u l d move t h e a l l o t r o p i c temperatures  the m e t a s t a b l e ,  found that cobalt temperature. requires  transformation  while maintaining useful proportions  high temperature,  phase.  I  183  Appendix 1 X-Ray A n a l y s i s The f o l l o w i n g o u t l i n e f o r the q u a n t a t a t i v e  a n a l y s i s of  volume f r a c t i o n s of fee and hep c o b a l t i s based on work done by Sage and G u i l l a u d  9 0  and a thorough t r e a t m e n t o f  the  procedure p r o v i d e d by L a n n e r s . 8 4  The method u t i l i z e s d i f f r a c t i n g p l a n e s t h a t are s i m i l a r l y by any p r e f e r r e d o r i e n t a t i o n p r e s e n t .  affected  Anomalies  t h a t occur i n c e r t a i n d i f f r a c t e d i n t e n s i t i e s are d i s c u s s e d and a l l o w e d f o r .  The anomalies uncovered by Sage and G u i l l a u d  and Lanners a r e the same and i t i s assumed t h a t t h e i r a  results  P P l v t o the m a t e r i a l u t i l i z e d f o r the p r e s e n t work. When the fee c u b i c s t r u c t u r e (111) p l a n e s become  transforms  (0002) p l a n e s .  t o hep,  certain  The number o f d i f f r a c t i n g  atoms i s c o n s t a n t but the t r a n s f o r m a t i o n p r e s e r v e s o n l y two p l a n e s out o f e i g h t e x i s t i n g i n the c u b i c s t r u c t u r e . p o s i t i o n o f the d i f f r a c t e d l i n e does net change but i n t e n s i t y i s reduced by a. f a c t o r o f 1  (0002)  The it's  four.  1  =  "(111)  ^  . . . . 1)  In a m i x t u r e of mx grans of c u b i c c o b a l t and m ( l - x )  grams  of hep c o b a l t , the common l i n e c o n s i s t s of a f r a c t i o n due the c u b i c phase and a f r a c t i o n due to the hep 1  J  (111) (0002)  =  4mx R  (  1  _  = x  )  4x ~ ( 1  X )  to  phase.  ••••  2)  184 The r e l a t i v e i n t e n s i t i e s o f p a i r s o f l i n e s i n the or fee phase have been c a l c u l a t e d .  hep  F o r example:  D-=2.22 (200)  I ( 1 1 I  '  (  0  0  0  2  3)  = 0.28  )  •(1011)  4)  The c a l c u l a t i o n g i v i n g r i s e t o the r a t i o s above assumes t h a t the i n t e n s i t y o f the l i n e s a r e independent o f diffraction  angle.  The v a l u e s f o r t h e s e r a t i o s  the  determined  by Sage and G u i l l a u d a r e 1.85 and 0.27 r e s p e c t i v e l y Combine e q u a t i o n 1  x  _ -""(111) (0002)  X  4 I  1  _ 1 (lll) (200)  . -""(lOll) (0002)  I  4  I  1  I  (200) )  I  _ 1.93  .  1  (200) (1011) I  (200) (1011)  T  I  ( 1 0 1 1  5)  = 2 (200) I  X  I  (10ll)  6)  Edwards and L i p s o n the l i n e s and  5 5  c a r r i e d out t h e i r a n a l y s i s u s i n g  (200) and (1010) .  The p r e s e n t work compares  (1011) as d i d Lanners and Sage and G u i l l a u d .  (1011) l i n e i s f o u r t i m e s as i n t e n s e as  of hep phase i s  (200)  The  (1010) and a l l o w s  f o r more a c c u r a t e c a l c u l a t i o n s e s p e c i a l l y when the  amount  small.  Both Lanners * and T r o i a n o * found anomalous 81  f o r the  .  2,3,4.  \ . 2.22 . 4 0.28 x  9 0  (111) and  51  (0002)  lines.  intensities  By e l e c t r o n m i c r o s c o p y they  d e t e r m i n e d t h a t the r e s u l t s were due t o a p r e f e r r e d  presentation  185  of  t h e s e p l a n e s t o t h e x - r a y beam, t h a t  existed. implies  T h i s abnormally  affected for  the  T  (111) and 0  (111)  ,  2  H I  22  =  (1011).  L e t t h e enhanced  i s not  intensities  (1011)  .7)  CT U  X  2.22  (0002)  from  8  '  (111)  (111)  (0002) p l a n e s be:  2  =  1  (200) and  (0002) and  for quantitative analysis  by t h e s e a n o m a l i e s .  (0002)  1 1  high p r e s e n t a t i o n of  normal p r e s e n t a t i o n o f  The e q u a t i o n u s e d  i s , a texture  (200)  .8)  C I (200) HI (1011)  .9)  n  0.28  equation 6  ' (111) I'(0002)  4 . •  X  from  !l-x)  equations  x (1-x)  1 4  9 and  H C.  I' I'  10)  10  (Ill) (0002)  1 4  2.22 0.28  H r  C (200) II I (1011)  (200)  6)  (1011) The d i f f e r e n c e between Guillaud  and L a n n e r s 2  (1-x) x (1-x)  i s as  the e q u a t i o n s used  1.5  (lOll) I  Lanners  8  used  bv  used  by Sage and  k  6)  (200) (1011)  and  follows:  (200) 1  by Sage  9 0  Guillaud 11)  186  Although calculation  a l a r g e d i s c r e p a n c y appears  shows t h a t t h e d i f f e r e n c e s  a c c e p t a b l e when c o n s i d e r e d scatter  in results.  F o r example  Measured R a t i o  (200) 1  Equation  8 2  ,  observed  are  o f the normal  experimental  see t h e c h a r t below.  % f e e , eqn. 6  % f e e , e q n . 11  (1011)  1  66  60  1/2  50  43  1/10  16  13  6 was c h o s e n f o r t h e p r e s e n t work b e c a u s e a l l  the r e c e n t i n v e s t i g a t i o n s Beckers  in light  obvious,  Miiller  8 3  .  have u s e d  this  formula, i . e .  187 Appendix 2 Measurement of T e n s i l e Parameters The  f o l l o w i n g paragraphs  by an I n t e r s e c t Method  o u t l i n e a method f o r determining  the y i e l d s t r e s s versus temperature taken from a s t e p - p u l l  r e l a t i o n s h i p from data  test.  F i g u r e 66 shows the step p u l l r e s u l t s f o r a specimen initially  s t r a i n e d a t -196°C and r e t e s t e d a f t e r  the temperature  i n 40°C s t e p s .  increasing  The data i s p l o t t e d as t r u e  s t r e s s versus t r u e s t r a i n , and i s a l s o normalized f o r the change i n G w i t h i n c r e a s i n g temperature.  Due  t o the  p a r a b o l i c shape of the curve upon r e t e s t i n g , the proper s l o p e t o apply to the i n d i v i d u a l segments of curve was  found  from g r a p h i c a l data s i m i l a r to t h a t shown i n F i g u r e 38.  From  curves of t h i s type, the work hardening r a t e a t any value of s t r a i n and temperature  can be determined.  T h i s s l o p e was  a p p l i e d t o each i n d i v i d u a l segment of the curve to  then  determine  the s t r e s s l e v e l corresponding t o the s t a r t of the s t r a i n segment under s c r u t i n y . is  shown i n F i g u r e 67.  The manner i n which the data i s determined The data f o r the complete  F i g u r e 66 i s presented i n Table  XV.  Because a l l data i s to be normalized t o 0.2% v a l u e i s t a b u l a t e d f o r the i n i t i a l At f i r s t of  t e s t shown i n  strain,  s t e p p u l l i n Table  XV.  g l a n c e , i t would appear t h a t i f the summation  A a v a l u e s due  t o work hardening was  s u b t r a c t e d from the  i n t e r s e c t y i e l d s t r e n g t h a p l o t of y i e l d s t r e n g t h versus temperature  this  would r e s u l t .  Some curve i s found i f t h i s i s  done, but i t i s i n c o r r e c t , because no work hardening  rate  175  L  True S t r a i n Fig.  66  Step-pull tensile test.  99.9%  (%)  c o b a l t , 6.5  micron g r a i n  size.  0 f o r 100°C  test  a t vL0%  strain  0 f o r 140°C t e s t ^11% s t r a i n  Intersect Yield Strength  10.4  9.3  True 67  Strain  10.4  11.5  (%) ->  D e t e r m i n a t i o n of the step-pull data.  intersect  yield  strength  from  at  T A B L E XV  Typical  D a t a From  Specimen  ADJ - 99.9% c o b a l t  Pull  Temp. (°C)  True  1  -196  0 - 1 .5  2  -196  1.5  3  -140  4  a Step-Pull  annealed  Test  1 h r . a t 600°C Maximum stress (ksi)  Aa  104.0  123.9  19.9  104.0  102.0  - 2 .2  126.9  131.4  4.5  107.0  101.2  2.2  - 3 .4  125. 0  131.1  6.1  100.6  98.4  -100  3.4  - 4 .6  127. 0  131.4  4.4  96.5  97.8  5  -  60  4.6  - 5 .7  126.9  129.8  2.9  92.0  96.7  6  -  20  5.7  - 6 .9  125.1  127.3  2.2  87.3  96.2  7  20  6.9  - 8 .2  122.7  123.9  1.2  82.7  95.1  (#)  Strain  Tensile  (%)  Intersect yield stress (ksi)  Uncorrected yield stress (ksi)  Corrected for 9 a n d t o C).2% (ksi)  3  60  8.2  - 9 .3  118.8  119.4  0.6  77.6  93.4  q  100  9.3  - 10.4  114.4  115.0  0.6  72.6  92.5  10  140  10.4  11.5  108.4  109.7  1.3  66.0  90.2  11  180  11.5  12.7  102.1  103.7  1.6  58.4  86 .5  12  220  12. 7  13.8  93.8  95.2  1.4  48.5  81.0  13 •  260  13. 8  14.6  84.8  85 .2  1.4  38.1  74.6  14  300  14.6  16.1  75.6  77.7  2.1  27 .5  65.9  15  340  16.1  17.7  66.2  67.4  1.2  10.0  59.5  16  380  17.7  19.0  52.9  55.5  2.6  1.5  46.9  17  400  19.0  fail  50.7  50.. 8  —  -3.3  45.6  -  -  -  'O O  corrections  have been a p p l i e d  calculation  best explains  the  0.2% y i e l d  stress  t o t h e Ac v a l u e s .  t h e problem.  a t -100°C.  ?.. s a m p l e  F o r example,  From T a b l e  XV  determine  the stress  3 l e v e l s measured  a r e 1 2 7 X 10  p s i a t 3.4% s t r a i n  t o 131.1  3 X 10  p s i a t 4.6% s t r a i n .  -100°C, strain a  To d e t e r m i n e  theyield  s u b t r a c t o u t t h e work h a r d e n i n g i n t r o d u c e d a t -140°C  yield  (-100°C)  =  a  A  "  - 1 9 6 ° C  G  •  I T  /  yield  "  J  ( A a  -196°C  yield  occurs  o  f o r the  Thus,  +  -140°C  A a  )  strength ^  ^ hardening introduced a t (Similarly f o r a l l AaTOr)  w  From Table A  a n d -196°C.  iys  Where a. = intersect lys  that  stress at  r  - 1 9 6 ° C .  XV  inno-s  =  127.0  -  (19.9  4.5  +  +  6.1)  =  96.5  X  (~100°C) psx  This tests  v a l u e does n o t appear  thus The  a  (400°C)  an error  =  5  0  ,  "  7  has been  m i s s i n g element  strong function  all  but f o r the  a t 400°C. °yield  and  out of line;  work h a r d e n i n g  E A  <0  °T°C  made.  i s that  t h e work h a r d e n i n g  o f temperature  and s t r a i n .  introduced prior  Therefore,  to the test  under  s c r u t i n y must be n o r m a l i z e d t o t h e t e m p e r a t u r e yield  stress For  stress  (-100-C)  =  a  +  iys  A  a  a t - 1 0 0 ° C , v;e t h e n -  [ A a  - i 4 o ° c  -196°C  (  0  0  V- here 7  a t which t h e  i s desired.  the yield  Vleld  rate i s  -  i  o  o  ^  (  °  c  -14O°C  I  0  g  have:  ° -196°C 0  C  2  %  £  @  ) 1  @ 2%e  1  > @ l%e  %  £  10"  i « r , o ^ o -, -100°C @ l%e i s the work h a r d e n i n g^ r a t e m a t e r i a l t e s t e d a t -100°C a t 1% s t r a i n . ( S i m i l a r l y for other 0 , )  0  a  T <  a  yield  If  (-100-C)  =  1 2 7  '° -  2  0  '  2  C  &  =  for  %£  9  7  '  8  c a l c u l a t i o n s of t h i s t y p e a r e c a r r i e d out f o r  segment, r e a s o n a b l e agreement i s found between the  each  results  of the s t e p - p u l l t e s t s and the r e s u l t s from many i n d i v i d u a l tests.  The advantage of the s t e p - p u l l t e s t i s t h a t  a l l o w s a more a c c u r a t e d e t e r m i n a t i o n of the  it  temperature  a t which the 0.2% y i e l d s t r e s s changes temperature dependence. Clearly, approximate.  t h i s n o r m a l i z i n g pr ocedur e i s Nevertheless,  only  the m a n i p u l a t i o n of d a t a i n  t h i s manner a l l o w s a second approach t o the p r o b l e m .  The  agreement between the c u r v e s shown i n F i g u r e 29 i m p l i e s t h a t the major c o r r e c t i o n s have been i n c l u d e d .  193 References 1.  B. 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