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Strain induced martensitic transformation in Cu-Al-Ni Oishi, Kazumasa 1970

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STRAIN INDUCED MARTENSITIC TRANSFORMATION  IN C u - A l - N i  BY  KAZUMASA OISHI Waseda U n i v e r s i t y , Tokyo, B.Eng., 1965 Waseda U n i v e r s i t y , Tokyo, M.Eng., 1967  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE  i n the Department  of  METALLURGY  We accept t h i s thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA J u l y , 1970  In  presenting  this  thesis  an a d v a n c e d d e g r e e the L i b r a r y I  further  for  agree  scholarly  by h i s of  shall  this  written  at  the U n i v e r s i t y  make i t  tha  p u r p o s e s may be g r a n t e d  for  of  of  of  Columbia,  British  available  It  financial  gain  Metallurgy  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, Canada  by  for  Columbia  S e p t e m b e r 2 1 , 1970  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  permission.  Department  Date  freely  fulfilment  perm.i s s i o n . f o r e x t e n s i v e  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  (i)  ACKNOWLEDGEMENT  The a u t h o r e x p r e s s e s h i s s i n c e r e thanks f o r the a d v i c e and encouragement g i v e n by h i s r e s e a r c h d i r e c t o r , Dr. L.C. Brown. Thanks a r e a l s o extended t o P r o f e s s o r and Dr. A. A k h t a r f o r t h e i r k i n d a d v i c e  R. B u t t e r s , Dr. B, Hawbolt  throughout the e x p e r i m e n t s .  He a l s o wishes t o acknowledge a s s i s t a n c e g i v e n by many f e l l o w graduate s t u d e n t s and t e c h n i c i a n s . S p e c i a l thanks a r e extended t o Mr. K. I s h i m a r u , c h i e f - s u p e r v i s o r of t h e Oyama Aluminum E x t r u s i o n P l a n t o f Furukawa Aluminum Co., L t d . , Tokyo, Japan, f o r g i v i n g a l e a v e o f absence University of B r i t i s h  t o study a t the  Columbia.  F i n a n c i a l a s s i s t a n c e was r e c e i v e d i n the form o f an a s s i s t a n t s h i p from t h e N a t i o n a l Research C o u n c i l o f Canada (grant number A2459) and is gratefully  acknowledged.  (ii) ABSTRACT  A study has been made o f s u p e r - e l a s t i c i t y and t h e strain-memory effect  i n C u - A l - N i a l l o y s i n t h e c o m p o s i t i o n range 14 wt. % A l and 2 t o  6 wt. % N i .  These a l l o y s have a b c c s t r u c t u r e on quenching and show  a low temperature t r a n s f o r m a t i o n sitic the  structure.  t o a b o d y - c e n t e r e d orthorhombic marten-  I t i s t h i s transformation  that  i s responsible f o r  s u p e r - e l a s t i c and strain-memory e f f e c t s . Tests  on b o t h s i n g l e and p o l y c r y s t a l l i n e specimens showed  the maximum s u p e r - e l a s t i c i t y o c c u r r e d  c l o s e to Ag .  that  At h i g h e r  temperatures t h e e f f e c t g r a d u a l l y  d e c r e a s e d , w h i l s t a t lower tempera-  tures  The magnitude o f t h e e f f e c t was  i t decreased very q u i c k l y .  i n s i n g l e c r y s t a l specimens (> 6 % ) , b u t s m a l l  large  i n p o l y c r y s t a l specimens  (< 1.5%).  The s u p e r - e l a s t i c e f f e c t was caused by  martensite  (SIM).  Two types o f SIM were o b s e r v e d :  stress-induced thin plates of  thermoelastic  m a r t e n s i t e which was always r e v e r s i b l e , and wide p l a t e s  of burst-type  martensite.  This burst-type  m a r t e n s i t e was  responsible  f o r t h e major p o r t i o n o f SIM, and whether i t was r e v e r s i b l e o r not on removal o f t h e s t r e s s c o n t r o l l e d the amount o f s u p e r - e l a s t i c i t y observed. The  strain-memory e f f e c t o c c u r r e d  martensitic  state  (temperature <M^)  on d e f o r m a t i o n e i t h e r i n t h e  o r i n the temperature range where  the m a r t e n s i t e once formed was s t a b l e  (temperatures c l o s e t o M ) . g  D e f o r m a t i o n caused r e o r i e n t a t i o n o f the m a r t e n s i t e p l a t e s and when the specimen was h e a t e d , the m a r t e n s i t e d i s a p p e a r e d and the specimen reverted basis  back t o i t s o r i g i n a l shape.  This  e f f e c t was e x p l a i n e d  of development of m a r t e n s i t e p l a t e s o f f a v o r a b l e  stressing.  on the  o r i e n t a t i o n on  (iii)  TABLE OF CONTENTS page 1.  INTRODUCTION  2.  EXPERIMENTAL  1  2.1 Specimen P r e p a r a t i o n  -..  12  2.2 T e n s i l e T e s t P r o c e d u r e  13  2.3 M e t a l l o g r a p h y  14  2.3.1. Measurement o f M ,M^,A ,A^ Temperatures 2.3.2. D i r e c t O b s e r v a t i o n o f S t r e s s Induced M a r t e n s i t e . . . . . . . . . . . . S  S  14 16  2.4 X-Ray D i f f r a c t i o n and H a b i t P l a n e Determination 2.5 O b s e r v a t i o n of the S t r a i n Memory E f f e c t . . 3.  18 20  RESULTS 3.1 P o l y c r y s t a l l i n e M a t e r i a l  23  3.1.1. M , M ^ , A , A £ Temperatures  23  3.1.2. S t r e s s v s . S t r a i n Curves  26  3.1.3. V i s u a l O b s e r v a t i o n o f S t r e s s Induced M a r t e n s i t e . . . . . . . . . . . . .  32  g  s  3.2 S i n g l e C r y s t a l M a t e r i a l . . . . . . . . . . . . . . . . . .  39  3.2.1. S t r e s s v s . S t r a i n C u r v e s . . . . . . .  39  3.2.2. V i s u a l O b s e r v a t i o n o f S t r e s s Induced M a r t e n s i t e . . , .  45  3.3 X-Ray A n a l y s i s  of Martensite  50  3.4 C r y s t a l l o g r a p h i c H a b i t P l a n e o f M a r t e n s i t i c Structure  53  3.5 E f f e c t o f C r y s t a l O r i e n t a t i o n on SuperElasticity...  56  3.6 S t r a i n Memory E f f e c t  58  (iv) TABLE OF CONTENTS(continued)  4.  DISCUSSION 4.1 Nature o f S t r e s s v s . S t r a i n Curves  64  4.2 Nature o f  74  Stress-Induced  Martensite  a) S t r e s s - i n d u c e d t h e r m o e l a s t i c m a r t e n s i t e and b u r s t - t y p e m a r t e n s i t e b) Mechanism o f s u p e r - e l a s t i c i t y  74 76  c) Morphology o f s t r e s s - i n d u c e d m a r t e n s i t e i n poly-and  single  crystals.  4.3 S t r a i n Memory E f f e c t  85 87  APPENDICES I.  I.  C a l c u l a t i o n o f Expansion and C o n t r a c t i o n Due To the Formation o f y ' A l o n g t h e P r i n c i p a l P o l e s of a b . c . c . C r y s t a l . . . C a l c u l a t i o n o f t h e Maximum Amount o f D e f o r m a t i o n P r o v i d e d by Twinning Alone  REFERENCES  89 (  91 98  (v)  LIST OF FIGURES  page  1.  P o r t i o n of t e n s i l e c u r v e f o r a t e r n a r y Cu-Zn-Sn a l l o y deformed a t room temperature showing c h a r a c t e r i s t i c h y s t e r e s i s loop a f t e r u n l o a d i n g  4  2.  Cu-Al-Ni e q u i l i b r i u m  phase diagram  3.  Stress v s . s t r a i n curve f o r a s i n g l e c r y s t a l of a C u - A l - N i a l l o y a t 38°C  4.  The e r r o r i n apparent e l o n g a t i o n ,  5.  Apparatus  6.  Dilatometer  7.  Specimen h o l d e r used to determine h a b i t  8.  Schematic p r o j e c t i o n  9.  Schematic i l l u s t r a t i o n of the s t r a i n memory e f f e c t by bending  to measure M ,  10. ' M a r t e n s i t i c s t r u c t u r e c o o l i n g to 10°C 11.  12.  13.  14.  15.  s  £, p l o t t e d  „  7  .  11  ....  16  as a  Mf, A , and A f temperatures s  17 plane  19  of a p l a n e i n two s u r f a c e s  o b t a i n e d i n Cu-14Al-2Ni  20 22  on ,  24  The volume change due t o t r a n s f o r m a t i o n from m a r t e n s i t e to b . c . c . on h e a t i n g  25  E f f e c t of temperature on the t e n s i l e b e h a v i o r o f Cu-14.5Al-3Ni p o l y c r y s t a l specimens  27  E f f e c t o f temperature on the t e n s i l e b e h a v i o r o f Cu-14Al-2Ni p o l y c r y s t a l specimens  28  E f f e c t o f r e p e a t e d s t r a i n s on the magnitude o f the permanent s e t  29  The t o t a l s t r a i n s a t a s t r e s s o f 4 x 10 p . s . i . and the r e s i d u a l s t r a i n s a t z e r o s t r e s s f o r Cu-14.5A13Ni p o l y c r y s t a l specimens  30  4 16.  The t o t a l s t r a i n s a t a s t r e s s o f 4 x 10 p . s . i . and the r e s i d u a l s t r a i n s a t zero s t r e s s f o r Cu-14Al-2Ni p o l y c r y s t a l specimens  >.  31  (vi)  LIST OF FIGURES  (continued)  page  17.  18.  19.  20.  21.  22.  23.  24.  25.  26.  27.  28.  29.  S t r e s s v s . s t r a i n curve f o r a Cu-14Al-2Ni specimen, showing the s t r e s s a t which the photographs i n F i g . 18 were taken D i r e c t o b s e r v a t i o n of s t r e s s - i n d u c e d martensite f o r Cu-14Al-2Ni p o l y c r y s t a l specimens  33  35,36  D e t a i l e d photographs o f n e e d l e - l i k e m a r t e n s i t e (Cu-14Al-3Ni)  37  Change i n l e n g t h o f n e e d l e - l i k e m a r t e n s i t e on l o a d i n g and u n l o a d i n g  38  S t r e s s v s . s t r a i n curve o f a s i n g l e c r y s t a l taken to 16% s t r a i n a t 22°C (Cu-14.5Al-3Ni)  40  E f f e c t o f temperature on the. t e n s i l e b e h a v i o r o f Cu-14.5Al-3Ni s i n g l e c r y s t a l specimen  41,42  The t o t a l s t r a i n s o f 5.5% and the r e s i d u a l s t r a i n s a t z e r o s t r e s s f o r Cu-14.5Al-3Ni s i n g l e c r y s t a l specimens  44  S t r e s s v s . S t r a i n curve f o r a Cu-14.5Al-3Ni s i n g l e c r y s t a l specimen, showing the s t r e s s a t which the photographs i n F i g . 2 5 were taken  46  D i r e c t observation of stress-induced martensite f o r a Cu-14.5Al-3Ni s i n g l e c r y s t a l taken w h i l e s t r e s s i n g i n I n s t r o n machine  47  D i r e c t observation of stress-induced martensite f o r Cu-14.5Al-3Ni s i n g l e c r y s t a l  48  D i f f r a c t i o n t r a c e s showing (a) b . c . c . m a t r i x , (b) low temperature m a r t e n s i t e , (c) s t r e s s - i n d u c e d m a r t e n s i t e , and (d) powder f i l e d a t 22°C  51  Stereographic r e p r e s e n t a t i o n of martensite h a b i t p l a n e , showing the o r i e n t a t i o n o f t h e r m a l m a r t e n s i t e , and o f s t r e s s - i n d u c e d b u r s t - t y p e m a r t e n s i t e and the s t r e s s a x i s f o r each o f the specimens examined  55  S t r e s s v s . s t r a i n c u r v e showing t h e e f f e c t o f c r y s t a l o r i e n t a t i o n on s u p e r - e l a s t i c i t y  57  (vii)  LIST OF FIGURES  (continued)  page  30.(a) . S t r a i n memory e f f e c t on bending o f C u - A l - N i p o l y c r y s t a l specimen 3 0 . ( b ) . S t r a i n memory e f f e c t on s t r e s s i n g o f C u - A l - N i single  c r y s t a l specimen  60  3 0 . ( c ) . S t r a i n memory e f f e c t on s t r e s s i n g single  61  3 0 . ( d ) . S t r a i n memory e f f e c t on s t r e s s i n g  31.  32.  of Cu-Al-Ni  c r y s t a l specimen  polycrystal  of Cu-Al-Ni  specimen  62  M a c r o s t r u c t u r e o f thermal m a r t e n s i t e and deformed martensite i n a single c r y s t a l  63  T y p i c a l s t r e s s v s . s t r a i n curves s i n g l e ( b ) c r y s t a l specimens  66  f o r p o l y ( a ) and  33.  S t r e s s v s . s t r a i n curve i n the t h e r m a l l y . i n d u c e d m a r t e n s i t e a t 2.5°C o b t a i n e d by Busch^^  34.  V a r i a t i o n o f s t r e s s r e q u i r e d t o produce s t r e s s - i n d u c e d m a r t e n s i t e as a f u n c t i o n o f temperature both i n p o l y and s i n g l e c r y s t a l specimens  35.  E f f e c t o f temperature on s u p e r - e l a s t i c i t y f o r p o l y c r y s t a l specimens..  36.  E f f e c t o f temperature c r y s t a l specimens  37.  38.  39.  on s u p e r - e l a s t i c i t y  67  70  for single 73  Schematic i l l u s t r a t i o n o f the e f f e c t o f l o a d i n g and u n l o a d i n g s t r e s s on t h e b e h a v i o r o f s t r e s s - i n d u c e d martensite  77  The orthorhombic c e l l o f the o r d e r e d y'-martensite t o g e t h e r w i t h o n e - h a l f o f an o r d e r e d b . c . c . c e l l i n a p p r o x i m a t e l y the c o r r e c t o r i e n t a t i o n r e l a t i o n s h i p . . . .  78  Schematic b.c.c.  40.  59  i l l u s t r a t i o n of transformation  to orthorhombic  martensite  I l l u s t r a t i o n showing t h a t always c l o s e  from the  a {133} h a b i t  80 plane i s  to 4 5 ° with respect to the t e n s i l e a x i s . .  81  (viii)  LIST OF FIGURES ( c o n t i n u e d )  page  41.  42.  S t e r e o g r a p h i c r e p r e s e n t a t i o n of the c a l c u l a t e d d i r e c t i o n a l expensions (a) and the observed expansions (b)  83  Suggested mechanism f o r t h i c k e n i n g o f the subM i c r o s c o p i c twins i n an accomodation region b e h i n d the i n t e r f a c e  84  4 3 . . Morphology  of b u r s t - t y p e s t r e s s - i n d u c e d m a r t e n s i t e i n  (a) s i n g l e and  (b) p o l y c r y s t a l specimens  4 4 . ( a ) . The r e l a t i o n s h i p s between K]_, and  K^,^,  the p l a n e o f s h e a r  4 4 . ( b ) . The r e l a t i o n o f sphere and twinned e l l i p s o i d . . . . . 45.  86  91 91  O r i e n t a t i o n r e l a t i o n s h i p between l a t t i c e s o f twinned y ' and m a t r i x b . c . c  95  (ix)  LIST OF TABLES  page 1.  S t r u c t u r e o f thermal m a r t e n s i t e and d e f o r m a t i o n m a r t e n s i t e i n 3:2 e l e c t r o n compounds  2  2  M a r t e n s i t i c phases  8  3  Compositions and M^,M ,A ,Af temperatures o f p o l y c r y s t a l l i n e material studied  23  V a l u e s o f d s p a c i n g and i n t e n s i t y work and i n R a c h i n g e r ' s work  52  4  5  6  7  i n Cu-Al a l l o y s g  S  i n the present  Angle of s t r e s s - i n d u c e d martensite w i t h respect to the s t r e s s a x i s  53  M , M f , A , A £ temperatures orientation studies  56  s  g  o f t h e specimens used f o r  D i r e c t i o n a l l i n e a r change by t h e b . c . c . t o orthorhombic t r a n s f o r m a t i o n  89  - 1 -  1.  The  3:2  INTRODUCTION  ( e l e c t r o n : a t o m r a t i o ) e l e c t r o n compounds are found  in a  l a r g e number of a l l o y systems,"'" such as Cu-Zn, Ag-Cd, Ag-Zn, Au-Cd, C u - A l , Cu-In, Cu-Sn. e i t h e r ordered  They n o r m a l l y have a bcc s t r u c t u r e which can  (6') o r d i s o r d e r e d ( 3 ) .  s t a b l e a t h i g h temperatures  and  The  B phase i s g e n e r a l l y  i n many systems decomposes on slow  c o o l i n g by a e u t e c t o i d r e a c t i o n , o r by a polymorphic to  a different  crystal structure.  h i g h temperature on quenching.  and on c o o l i n g may  s t r e s s and may  j u s t above M temperature  s  Even i n these systems, however, the  g s t r u c t u r e produced  i n t h i s way  temperature  i s very  unstable  undergo t r a n s f o r m a t i o n to an orthorhombic  tetragonal martensitic structure. to  transformation  8 phase can o f t e n be r e t a i n e d t o room  The  The  6 phase i s a l s o v e r y  or sensitive  t r a n s f o r m to the m a r t e n s i t i c s t r u c t u r e on  , o r to a c o m p l e t e l y new  deformation.  The  be  close-packed  deformation  s t r u c t u r e on h i g h  a l l o y systems i n which such m a r t e n s i t i c  13 t r a n s f o r m a t i o n s have been s t u d i e d are l i s t e d i n T a b l e 1. has  suggested  a reason  f o r the v e r y easy d e c o m p o s i t i o n  the b . c . c . s t r u c t u r e s are v e r y s u s c e p t i b l e to a b s o l u t e v a l u e of shear c o n s t a n t s v a r y from one d i r e c t i o n t o another, instability  Zener  of the B phase:  {110}<110>  shear.  The  crystallographic  so a c o n v e n i e n t measure o f the tendency  towards  i s the " a n i s o t r o p y f a c t o r " which i s the r a t i o of the  shear  - 2T a b l e 1.  S t r u c t u r e o f Thermal M a r t e n s i t e and D e f o r m a t i o n M a r t e n s i t e Produced  Alloy  i n 3:2  E l e c t r o n Compounds  Thermal M a r t e n s i t e  Ag-Cd  orthorhombic  Ag-Zn  complex  Au-Cd  Deformation M a r t e n s i t e  2  h.c.p.(52 wt.  hexagonal  %  Cd)  2  3 f.c.c.  orthorhombic"'  (up to 45 a t  at %  b.c.c. ordered^  Cu-Zn  h.c.p.  Cu-Al  (52 wt.  % Zn)  47.5  orthorhombic  (47.5 a t % C d )  9  '  10  f.c.  tetragonal  f.c.  orthorhombic  un.c.p.  8  (39 wt.  %  Zn)  4  h.c.p.(above  tetragonal(49.0 at % Au-Zn  4 Zn)  %  Cd)  8  Zn) f.c.  1 1  t e t r a g o n a l ( 4 0 wt. 1 1  %  See T a b l e 2, p. 8  In-Tl  f. c.t. , f inely  twinned  m o d u l i i n the (100}<010> and  12  h i g h e r the tendency  towards  f.c.t.  1 2  {110}<110> d i r e c t i o n s .  u n i t y f o r an i s o t r o p i c e l a s t i c  (20.7 a t  %)  This i s equal to  body, and the h i g h e r the f a c t o r ,  instability.  the  The f . c . c . m e t a l s have v e r y  low a n i s o t r o p y f a c t o r s , e.g., A l : 1 . 2 9 , Cu:3.2, Au:2.90, as t h e r e i s no tendency  towards  instability.  h i g h a n i s o t r o p y f a c t o r , e.g., tendency  towards  instability  However, 8 b . c . c . s t r u c t u r e s have a v e r y Cu-Zn a l l o y : 8 . 4 , so t h e r e i s a h i g h on c o o l i n g or on d e f o r m a t i o n .  The m a r t e n s i t i c r e a c t i o n on c o o l i n g has been s t u d i e d i n i n the F e - N i and Cu-Zn. systems. start  temperature,  show l i t t l e  On  6  c o o l i n g below M , g  detail  the m a r t e n s i t e  t h i n p l a t e s o f t h e r m o e l a s t i c m a r t e n s i t e form.  o r no temperature h y s t e r e s i s and grow as the  These  temperature  Zn)  - 3i s lowered and parent  s h r i n k on h e a t i n g .  s t r u c t u r e transforms  p l a t e s suddenly  burst  temperature  until  the t r a n s f o r m a t i o n i s complete at M^.  to the  8 phase does not  start  take p l a c e t i l l  On  to the  the  B phase b e g i n s  at A  g  transforms  20 to 50°C above M^  200°C above M^  i n Fe-Ni,  and  the  reheating, reversion  c o n s i d e r a b l e temperature h y s t e r e s i s w i t h b u r s t - t y p e Reversion  to form a t  and more and more of the s t r u c t u r e  Cu-Zn system and more than  of  by t h e r m o e l a s t i c m a r t e n s i t e . On f u r t h e r  c o o l i n g , much t h i c k e r m a r t e n s i t e (M^),  However, o n l y a few p e r c e n t  i n the  i . e . , there i s a martensite.  i s complete a t A^.  No  d e t a i l e d i n v e s t i g a t i o n s have been made to determine i f t h e r m o e l a s t i c and b u r s t - t y p e m a r t e n s i t e have d i f f e r e n t i t has been suggested  14  crystal structure.  • that thermoelastic martensite  s t r u c t u r e , i n t e r m e d i a t e between the m a t r i x  is a  phase and  However,  transitional  burst-type  martensite. M a r t e n s i t e has been produced by d e f o r m a t i o n Cu-Zn-Sn systems"'""' at temperatures up  i n the Cu-Zn-Si  to 85°C above M  .  The  martensite  produced by s t r e s s d i s a p p e a r s when the s t r e s s i s r e l e a s e d and rise  this  to a v e r y l a r g e p s e u d o - e l a s t i c e f f e c t w i t h e l a s t i c s t r a i n s of  to 15% b e i n g p o s s i b l e i n c o a r s e g r a i n e d p o l y c r y s t a l specimens. s t r e s s v s . s t r a i n curve  i s shown i n F i g . 1.  Metallographic  and  A  gives up typical  observations  have shown t h a t the u n u s u a l l y l a r g e e l a s t i c s t r a i n s o c c u r by means o f the f o r m a t i o n of r e v e r s i b l e t h e r m o e l a s t i c m a r t e n s i t e .  The d i f f e r e n c e  between the t e r n a r y Cu-Zn a l l o y s and b i n a r y Cu-Zn a l l o y i s due l a r g e amount of t h e r m o e l a s t i c m a r t e n s i t e  i n the former c a s e .  d e t a i l e d mechanism i s g i v e n f o r the s u p e r - e l a s t i c e f f e c t  to  the  No  and'specifically  - 4 -  Percentage Strain Figure  1.  P o r t i o n of t e n s i l e curve f o r a ternary  Cu-Zn-Sn a l l o y  deformed a t room temperature showing c h a r a c t e r i s t i c hysteresis  loop  a f t e r unloading."''"' •  Pops does n o t i n d i c a t e whether i t o r i g i n a t e s from the a c t u a l  formation  of m a r t e n s i t e o r from the d e f o r m a t i o n o f m a r t e n s i t e which has a l r e a d y been produced. 12 Super-elastic and  e f f e c t s have a l s o been observed i n In-20.7 T l  Au-47.5 Cd^ on deforming the m a r t e n s i t i c  I n - T l i s complex.  phase.  The s t r u c t u r e i s f . c . c . above M  The e f f e c t i n and f . c .  tetragonal  below the M  temperature.  s  The  low  temperature m a r t e n s i t i c  s t r u c t u r e e x h i b i t s f i n e markings which are twin r e l a t e d . specimen i s s t r e s s e d , d e f o r m a t i o n does not movement but  r a t h e r by growth of one  o f the o t h e r . -10°C -5°C  (M  The  tetragonal When the  o c c u r by d i s l o c a t i o n  twin o r i e n t a t i o n at the  expense  a l l o y s show r u b b e r - l i k e b e h a v i o r between -5°C  = 65°C).  I n t h i s temperature  range  and  f  to -10°C jthe o r i g i n a l c o n f i g u r a t i o n of twin i n t e r f a c e s becomes  " s t a b i l i z e d " and when t h i s c o n f i g u r a t i o n i s a l t e r e d by means of a p p l i e d s t r e s s , i t w i l l r e t u r n to the s t a b i l i z e d one when the i s removed.  The  stress  i n v e s t i g a t i o n i n the Au-Cd system i s l e s s d e t a i l e d .  t h i s case the orthorhombic m a r t e n s i t i c phase shows r u b b e r - l i k e The  m a t e r i a l i s e a s i l y deformed below M  b e h a v i o r i s r e v e a l e d by when the saying  an  g  (60°C).  The  a d d i t i o n a l s t r e s s i n g and  l o a d i s removed.  behavior.  rubber-like  the m a t e r i a l snaps back  This rubber-like behavior i s explained  t h a t the f a v o r a b l y o r i e n t e d r e g i o n s  increase  i n s i z e at  expense o f l e s s f a v o r a b l y o r i e n t e d r e g i o n s . L e s s f a v o r a b l y  temperature c u b i c s t r u c t u r e .  by  the  oriented  means o r i e n t e d such t h a t the a p p l i e d s t r e s s tends to shear the back toward the h i g h  region  A f t e r r e l e a s e of  a p p l i e d s t r e s s , r e l a x a t i o n o f the b o u n d a r i e s between the f a v o r a b l y unfavorably  oriented regions  occurs.  No  of the n a t u r e of the f a v o r a b l y o r i e n t e d  detailed explanation  1 6 , 1 7  '  1 8  the and  i s given  regions.  Another phenomenon r e l a t e d to s u p e t e l a s t i c i t y i s the memory e f f e c t found i n T i - N i ,  In  Ti-Nb,  1 9  Cu-Zn,  20  strain  Au-Cd,  21  and  22 Ag-Cd.  The  major p o r t i o n o f the r e s e a r c h  B - T i - N i which has M One  g  has  been c a r r i e d out  j u s t below room temperature and  A  g  on  around 90°C.  suggested mechanism i s t h a t when t h i s a l l o y i s p l a s t i c a l l y  deformed  - 6-  at room temperature, s t r e s s - i n d u c e d m a r t e n s i t e i s formed w i t h the o r i e n t a t i o n o f the m a r t e n s i t e such as t o a l l o w the specimen  to expand  18 or c o n t r a c t as r e q u i r e d .  Lange e t . a l .  have observed a r e l a t i o n  between t h e a p p l i e d s t r e s s and t e x t u r e o f the low temperature  phase,  and f i n d a marked d i f f e r e n c e i n the t e x t u r e o f m a r t e n s i t e formed by t e n s i o n and compression.  On h e a t i n g above A^, the m a r t e n s i t e t r a n s f o r m s  back t o t h e 8-phase and t h e specimen  r e t u r n s to i t s i n i t i a l  A s i m i l a r e x p l a n a t i o n o f the shape memory e f f e c t  shape.  i n Ti-Nb has been  20 p r e s e n t e d by Baker.  I n t h i s case t h e e f f e c t i s observed under two  d i f f e r e n t sets of conditions.  F i r s t l y , when the f u l l y  martensitic  a l l o y i s deformed below M^, i t r e c o v e r s i t s pre-deformed shape when heated above A . Secondly, i f t h e a l l o y i s deformed between M and s s A , a s t r a i n i n d u c e d m a r t e n s i t e i s formed and on h e a t i n g above A the s s deformation i s recovered.  He s t u d i e d t h e r e l a t i o n between s t r e s s and  t e x t u r e , and a g a i n e x p l a i n e d the phenomena i n terms o f an o r i e n t a t i o n of the m a r t e n s i t i c s t r u c t u r e d u r i n g d e f o r m a t i o n such as t o g i v e the r e q u i r e d e l o n g a t i o n and c o n t r a c t i o n . T h i s t h e s i s i s concerned w i t h s t r e s s - i n d u c e d m a r t e n s i t e and the shape memory e f f e c t i n C u - A l - N i a l l o y s .  These a r e b a s i c a l l y  Cu-Al  a l l o y s w i t h s m a l l amounts (2 ^ 6 wt. %) o f N i added to c o n t r o l t h e M temperature.  g  F i g . 2(a) shows the e q u i l i b r i u m phase diagram o f the 23  Cu-Al system.  I t w i l l be seen t h a t the 3-phase f i e l d  hood o f 25 atomic p e r c e n t A l .  i s i n the n e i g h b o r -  T h i s has a d i s o r d e r e d b . c . c . s t r u c t u r e  at h i g h e r temperatures which decomposes eute c t i o d a l l y i n t o the c o p p e r - r i c h s o l i d s o l u t i o n and the h a r d 6 - i n t e r m e d i a t e phase a t 565°C.  By quenching  - 7-  Ca) S e c t i o n a t 0 percent n i c k e l F i g u r e 2.  (b) S e c t i o n a t 3 percent n i c k e l  Cu-Al-Ni  e q u i l i b r i u m phase diagram  (c) S e c t i o n a t 6 percent n i c k e l ( a f t e r Alexander  23  ).  from the homogeneous b . c . c . r e g i o n , an o r d e r e d b . c . c . s t r u c t u r e may produced, and  t h i s may  d i s o r d e r temperature  t r a n s f o r m i n t u r n to m a r t e n s i t e .  of the b . c . c .  3-phase i s f a i r l y h i g h  The  be  order-  (^ 500°C a t  24 12 wt.  % Al  ) , and  the M  temperature  c o n t e n t , from ^ 500°C a t 10 wt. ^ 100°C at 14 wt.  2A % Al.  decreases w i t h i n c r e a s i n g A l  % A l to ^ 300°C a t 12 wt.  % A l , to  I t i s p o s s i b l e to r e t a i n the o r d e r e d  g-phase  - 8at  l e a s t p a r t i a l l y a t room temperature by g o i n g to v e r y h i g h A l  contents.  H° ver, w e  t h i s i s not r e a l l y p r a c t i c a l s i n c e the 6 •> <5  e q u i l i b r i u m t r a n s f o r m a t i o n b e g i n s at h i g h e r temperatures w i t h Al  c o n t e n t and i t may  not be p o s s i b l e to suppress t h i s r e a c t i o n on  Adding N i to Cu-Al a l l o y s d e p r e s s e s M 25 temperature, (see the  increasing  g  to w e l l below room  26 '  a l t h o u g h N i has l i t t l e  F i g . 2(b) and  ( c ) ) . Even  though  e f f e c t on the g-phase r e g i o n  the N i i s c o m p l e t e l y s o l u b l e i n  Cu, i t i s the i n t e r a c t i o n parameters between the Cu-Ni, and  which might w e l l a f f e c t  the shape o f the phase diagram.  r e p l a c i n g a s m a l l p e r c e n t a g e o f Cu by N i has l i t t l e phase r e g i o n .  quenching.  In 82.5  Ni-Al  However,  e f f e c t on the g-  Cu-14.5 A l - 3 N i (wt. %) a l l o y , M  g  i s approximately  27 -10°C.  T h i s a l l o y i s 100%  g on quenching  to room  temperature.  The m a r t e n s i t i c s t r u c t u r e s o b t a i n e d i n Cu-Al a l l o y s are d e s c r i b e d in  T a b l e 2.  T a b l e 2.  M a r t e n s i t i c Phases  Phase, Composit i o n range (wt.%)  i n Cu-Al A l l o y s .  S t r u c t u r e Type  Remarks  g*,  'vLl  f.c.c.  heavily  Bj,  llvL3  tetragonal, ordered  11.86 wt. % A l , heavily faulted,  y', 13vL4.5  close-packed  hexagonal  orthorhomic, ordered  faulted,^ 2  g  13.6VL4.7 wt % A l , twinned 11.36 wt % A l , f i n e l y twinned ' 2 g  2  '  9  - 9As shown i n T a b l e  2, the y' m a r t e n s i t e  s t r u c t u r e with a twinning and  deformation  faulted  twins  plane  of t h i s by  has  an i n t e r n a l l y  twinned  jjp.0 1 l ^ h . c . p . , or ^(201^,  orth.  c o l d - r o l l i n g g i v e s r i s e to new  i n a d d i t i o n to heavy f a u l t i n g of the  heavily  y'-transformation  twins. S i m i l a r m a r t e n s i t i c s t r u c t u r e s a r e o b t a i n e d when N i i s added to 25 the Cu-Al a l l o y s .  Garwood and H u l l  t r a n s f o r m a t i o n i n Cu-12.8 A l - 7 . 7 N i the  g-phase would t r a n s f o r m  relatively  f i n e and  martensite  types of  the m a r t e n s i t e  f r e e from s u b s t r u c t u r e  (g').  that  martensite When the  formed  was  However, on  tempering  subsequent c o o l i n g to room temperature, a  coarse-grained martensite The  different  found  h i s t o r y o f the specimen.  c o o l e d to 0°C,  the quenched a l l o y and  formed.  (wt. %) a l l o y s , and  i n t o two  depending upon the p r i o r thermal quenched specimen was  i n v e s t i g a t e d the m a r t e n s i t i c  (y') w i t h a d i s t i n c t  g' h a b i t p l a n e was  found  secondary s t r u c t u r e  to be ^{155}g, w h i l s t the  e x h i b i t e d a h a b i t p l a n e between {133}g and  {144}g.  was  y' Rachinger  26 and Duggin  r e p o r t e d o n l y the Y ' m a r t e n s i t i c s t r u c t u r e i n a  a l l o y c o n t a i n i n g 14.5 internally  3 Ni  twinned s t r u c t u r e but  f a u l t s i n the b . c . c . martensite.  A l and  The  ordered  %).  introduced  They d i d not mention  the p o s s i b i l i t y of s t a c k i n g  found  to l i e between {122}g and  alloys.  They o b t a i n e d  a s s o c i a t e d w i t h 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 .  {133}g.  1%  strain  Removal of the s t r e s s  r e s t o r e the o r i g i n a l dimensions of the specimen.  be a c h i e v e d  y'  s t u d i e d the abnormal e l o n g a t i o n i n c o a r s e -  g r a i n e d p o l y c r y s t a l l i n e Cu-Al-Ni  d i d not  the  phase p r o v i d i n g n u c l e a t i o n s i t e s f o r the  h a b i t p l a n e was 32  Arbuzova and Khandros  (wt.  Cu-Al-Ni  o n l y by h e a t i n g above the temperature of the  This could reverse  - 10 -  transformation. orientation the  This  e f f e c t was c o n s i d e r e d  to be due t o p r e f e r r e d  i n r e g i o n s which had undergone m a c r o s c o p i c d i s p l a c e m e n t i n  c o u r s e o f t r a n s f o r m a t i o n under 26 Rachinger  load.  33 and Busch  studied  super-elasticity i n single crystal  specimens o f 82.5 Cu-14.5 A l - 3 N i (wt. % ) . (highest  They o b t a i n e d b e s t  results  e l a s t i c s t r a i n s ) when the s i n g l e c r y s t a l s were o i l quenched  from temperatures j u s t above t h e 8/(8 + 6) boundary and 6 p r e c i p i t a t i o n o c c u r r e d d u r i n g the quench. studied  the s u p e r - e l a s t i c  I n t h e s e o i l - q u e n c h e d specimens, Rachinger  e f f e c t u s i n g a 3-point bending method and  o b t a i n e d e l a s t i c s t r a i n s o f up t o 4%.  U s i n g p r e c i s e l y the same  specimens Busch o b t a i n e d much h i g h e r e l a s t i c s t r a i n s up t o 24%.  He  o b t a i n e d these s t r a i n s both i n t h e b . c . c . b e t a phase and i n t h e m a r t e n s i t e phase a t low temperatures. vs.  F i g . 3 shows t h e e l a s t i c s t r e s s  s t r a i n curve o b t a i n e d i n a s i n g l e c r y s t a l a t 38°C.  Busch  attempted t o study s u p e r - e l a s t i c i t y i n f a i r l y l a r g e g r a i n  also  size poly-  c r y s t a l l i n e specimens, but was u n s u c c e s s f u l because he was u n a b l e t o cast  suitable alloys. The  p r e s e n t i n v e s t i g a t i o n was undertaken t o c a r r y  study o f s t r e s s - i n d u c e d and  out an i n t e n s i v e  martensite i n 8 Cu-Al-Ni a l l o y s .  p o l y c r y s t a l l i n e specimens were examined.  Both  single  Specimens were water  quenched from t h e h i g h temperature t o r e t a i n 100% 8 and t o e l i m i n a t e difficulties phase.  of interpretation associated  Tensile  w i t h t h e p r e s e n c e o f the <5-  t e s t s have been c a r r i e d o u t over a wide temperature range  and  d i r e c t o b s e r v a t i o n o f the s t r e s s - i n d u c e d  The  s t r a i n memory e f f e c t was a l s o  loading.  m a r t e n s i t e was made.  examined by bending and by u n i a x i a l  - 11 -  F i g u r e 3.  S t r e s s v s . s t r a i n curve f o r a s i n g l e c r y s t a l o f C u - A l - N i 33 a l l o y a t 38°C  (Busch  b e h a v i o r up to 24%  ).  strain.  Note the p e r f e c t  elastic  - 12 -  2.  2.1  EXPERIMENTAL  Specimen P r e p a r a t i o n Master a l l o y s weighing approximately  h i g h p u r i t y Cu  (99.98%), A l ( 9 9 . 9 9 % ) ,  100  and N i  g were p r e p a r e d  (99.99%).  c a r r i e d out i n a i r u s i n g an i n d u c t i o n heated g r a p h i t e P o l y c r y s t a l l i n e specimens were made by a copper mould to g i v e a b i l l e t T h i s b i l l e t was  ( < 0 . 3 % ) due  product  c h i l l c a s t i n g the melt i n i n . x 0.2  in.  i n d i c a t e d o n l y a s m a l l l o s s of  to v o l a t i l i z a t i o n , and  a s u i t a b l e a l l o y 0.5  by  the Bridgman method.  i n . diameter x 3.5  c a s t i n g i n a g r a p h i t e mould. tube i n vacuum, p l a c e d  e x a m i n a t i o n o f the m i c r o -  homogeneous.  S i n g l e - c r y s t a l s were p r e p a r e d  The  was  crucible.  of dimensions 3 i n . x 1.5  s t r u c t u r e showed the a l l o y to be  3 in./hour  Melting  h o t - r o l l e d at 850°C down to a t h i c k n e s s of 0.030 i n .  Chemical a n a l y s i s of the f i n a l weight  using  i n . l o n g were p r e p a r e d  T h i s r o d was  i n the f u r n a c e ,  sealed i n a quartz  and  allowed  to s o l i d i f y  from a p o i n t to g i v e a s i n g l e c r y s t a l w i t h  m o l t e n zone was  Rods of by glass at  random o r i e n t a t i o n .  kept at 1200°C.  27 Rachinger  found machining of the  rubber-like behavior. make t h i n s t r i p s and machining" was  g-phase d i f f i c u l t  to  the  To a v o i d t h i s , specimens were f i r s t machined.to then h e a t - t r e a t e d  to o b t a i n the  used to a v o i d the p o s s i b i l i t y  h e a t - t r e a t m e n t and  due  t e n s i l e specimens w i t h  3-phase.'  Spark-  of r e c r y s t a l l i z a t i o n  .85  i n . gauge l e n g t h  during  and  -  .30  i n x .085  13 -  i n . c r o s s s e c t i o n were produced.  The  t h i n specimens  were s o l u t i o n h e a t - t r e a t e d at 850°C f o r 2 minutes i n a s a l t p o t , quenched d r a s t i c a l l y  i n t o 10%  c a u s t i c soda s o l u t i o n .  specimens were checked by m e t a l l o g r a p h y and  by  The  and  single-crystal  t a k i n g Laue back-  r e f l e c t i o n photographs at d i f f e r e n t p o s i t i o n s a l o n g  the l e n g t h of  the  specimen.  2.2  T e n s i l e Test The  Procedure  t e n s i l e t e s t s were g e n e r a l l y c a r r i e d out on an  Instron  -2 machine at a c r o s s - h e a d not n o r m a l l y s t r a i n and  t a k e n to f r a c t u r e but  the l o a d was  at room temperature and (-80°C to 2 1 ° C ) , 100°C).  speed of 0.02  x 10  in./min.  r a t h e r to some f i x e d s t r e s s or  then r e l e a s e d .  T e s t s were c a r r i e d out  i n the f o l l o w i n g media:  heated water  (21°C  Temperature c o n t r o l was  ±  to 1 0 0 ° C ) ,  in air  cooled e t h y l a l c o h o l and  s i l i c o n e o i l (above  1°C.  A major d i f f i c u l t y i n t e n s i l e t e s t i n g  i s caused by  the machine, such t h a t the measured e l o n g a t i o n i s due deformation  Specimens were  o f the components i n the machine.  "softness" i n  partly  to  This uncertainty  avoided  i n the p r e s e n t work i n 2 ways:-  (< .5%)  at room temperature, s t r a i n s on the specimen were measured  d i r e c t l y u s i n g an extensometer. produced by  the machine was  e l o n g a t i o n i n a standard  1) For  2) For o t h e r  t e s t s a t low  was  t e s t s , the e r r o r  measured by d e t e r m i n i n g  specimen at d i f f e r e n t  the r e a l and  stresses.  e l o n g a t i o n was  measured from the r e c o r d e r  e l o n g a t i o n was  found by measuring the d i s t a n c e between two  scratched  strains  The  c h a r t , w h i l s t the  apparent  apparent  true parallel  l i n e s at various s t r e s s e s with a t r a v e l l i n g microscope.  The  -  14 -  e r r o r i n the apparent  e l o n g a t i o n i s p l o t t e d i n F i g . 4 as a f u n c t i o n o f  stress.  s t r a i n s a r e always g r e a t e r than the a c t u a l  The apparent  4 s t r a i n s w i t h t h e e r r o r , b e i n g l e s s than 1% f o r s t r e s s e s under 5.5 x 10 p s i , the maximum s t r e s s used  i n the p r e s e n t work.  A l l stress vs. strain  curves p r e s e n t e d i n t h i s t h e s i s have been c o r r e c t e d t o g i v e t r u e s t r a i n v a l u e s u s i n g F i g . 4.  2.3  Metallography M e t a l l o g r a p h i c examination was c a r r i e d out t o determine  M^, A , and A^ temperatures, g  and t o observe  the M , g  the phase t r a n s f o r m a t i o n  d i r e c t l y w h i l e s t r e s s i n g the specimen. Mechanical  p o l i s h i n g w i t h alumina was found  to be the most  c o n v e n i e n t and r e l i a b l e method o f p r e p a r i n g a s u r f a c e f o r m e t a l l o graphic observation.  The e t c h a n t used was F e C ^ . . ^ g, HC1...10 c c ,  H2O...IOO c c , which gave a r e d d i s h - p i n k c o l o u r t o the b . c . c .  g-phase  and a y e l l o w c o l o r to t h e m a r t e n s i t e .  2.3.1.  Measurement o f M , M_, A , A,. Temperature s f s f  In the p r e s e n t work, M , M^, A , and A^ were measured by d i r e c t g  o b s e r v a t i o n u s i n g a microscope  g  as i l l u s t r a t e d  i n F i g . 5.  The c o l o r s  of t h e g m a t r i x and m a r t e n s i t e a r e q u i t e d i f f e r e n t so f o r m a t i o n and disappearance  o f m a r t e n s i t e c o u l d be e a s i l y  D i l a t o m e t r y was used  t o check these o p t i c a l measurements.  p o l y c r y s t a l l i n e specimen w i t h dimensions 1.555  detected. A  0.235 i n . x 0.20 i n . x  i n . was h o t - r o l l e d , spark-machined, and h e a t - t r e a t e d to o b t a i n  -  15 -  6 •  o X4  V o  CO CL  00  o  co" UJ  °  L^—— - —^-^-  cc  (o) 0 8 " gauge lenglh x soft copper A stainless steel 401 ( b ) 0-4"gauge length o soft copper • stainless steel 401  02  F i g u r e 4.  0.4  0.6  0.8  1.0  The e r r o r i n apparent e l o n g a t i o n ,  6,  ( o b t a i n e d by s u b t r a c t -  i n g t r u e e l o n g a t i o n from apparent e l o n g a t i o n ) f u n c t i o n of s t r e s s .  p l o t t e d as a  -  16 -  microscope  alumel-chromel  copper plate specimen holder  gloss beaker  F i g u r e 5.  xooling or heoting liquid media  Apparatus t o measure M , g  the g-phase.  thermocouple  M^, A , and A^ temperatures. g  F i g . 6 shows the d i l a t o m e t e r used.  The change i n  l e n g t h d u r i n g c o o l i n g and h e a t i n g was measured by an extensometer, and a graph of l e n g t h v s . temperature was produced on an x-y r e c o r d e r .  2.3.2.  D i r e c t Observation of Stress-Induced Martensite  I t was found e a s i e s t  to study the m a r t e n s i t e r e a c t i o n i n d u c e d  by s t r e s s i n g when the t r a n s f o r m a t i o n o c c u r r e d near room temperature. I t was then s u f f i c i e n t merely to s t r e s s an e t c h e d specimen  i n the  I n s t r o n machine and r e c o r d the change o f m i c r o s t r u c t u r e u s i n g a low power m i c r o s c o p e equipped w i t h a l o n g working d i s t a n c e l e n s and a 35 mm camera.  objective  I n some experiments t h e m a r t e n s i t e i n t e r f a c e  moved suddenly w h i l e s t r e s s i n g a t c o n s t a n t c r o s s - h e a d speed and i t was  - 18 -  i m p o s s i b l e to o b t a i n s a t i s f a c t o r y photographs. graphs were taken on the H o u n s f i e l d be stopped o r changed  I n t h i s case photo-  tensometer i n which  straining  a t any time.  Most o f the t e s t s were c a r r i e d o u t on p o l y c r y s t a l l i n e of Cu-14 A l - 2 N i (wt.%) and s i n g l e c r y s t a l s o f Cu-14Al-3Ni These had M  g  could  j u s t below room temperature.  A few t e s t s were  specimens (wt.%). carried  out i n which the specimen was s t r e s s e d above o r below room temperature. In t h i s case i t was n e c e s s a r y t o photograph  the specimen  through a  l i q u i d medium, andthe q u a l i t y o f the m i c r o g r a p h s was poor.  2.4.  X-Ray D i f f r a c t i o n and H a b i t P l a n e D e t e r m i n a t i o n X-ray d i f f r a c t i o n s t u d i e s were made on p o l y c r y s t a l l i n e  a f t e r quenching, a f t e r c o o l i n g below the M^ temperature, d e f o r m a t i o n o f a t e n s i l e specimen, and a f t e r f i l i n g specimen.  specimens  after  a quenched  X-ray d i f f r a c t i o n t r a c e s were made u s i n g a N o r e l c o  d i f f r a c t o m e t e r , o p e r a t i n g a t a s c a n n i n g speed o f 1 deg. 20/min. w i t h CuK  a  radiation. S i n g l e c r y s t a l l i n e specimens were used t o f i n d  m a r t e n s i t e produced by c o o l i n g and by s t r e s s .  the h a b i t p l a n e s o f  Specimens were p r e p a r e d  by p o l i s h i n g two s u r f a c e s a t 90° to one another,- the 90° a n g l e b e i n g e x a c t l y m a i n t a i n e d by h a v i n g t h e specimen  sandwiched  t h i c k n e s s e s o f p l a s t i c d u r i n g diamond p o l i s h i n g .  between 1 i n .  The o r i e n t a t i o n s o f  the s i n g l e c r y s t a l s were found by b a c k - r e f l e c t i o n Laue photographs. In specimens to have M  s  i n which thermal m a r t e n s i t e was examined, i t was n e c e s s a r y  j u s t below room temperature and A  s  just  above room  tempera-  t u r e so t h a t a s m a l l amount o f m a r t e n s i t e c o u l d be produced on c o o l i n g  -  19 -  the specimen and r e t a i n e d on r e t u r n i n g t o room temperature.  Stress-  induced m a r t e n s i t e was produced by p l a c i n g the specimen i n the h o l d e r shown i n F i g . 7 and a p p l y i n g s t r e s s u n t i l  transformation occurred,  stress - induced martensite  F i g u r e 7.  Specimen h o l d e r used t o determine h a b i t p l a n e . i s a p p l i e d by screwing  Stress  the b o l t .  making sure t h a t t h e l o a d was s t r i c t l y a x i a l .  Thehabit  p l a n e o f the  m a r t e n s i t e was found by measuring a n g l e s a and 8 as seen i n F i g . 8 u s i n g a 10X m i c r o s c o p e equipped  with a p r o t r a c t e r eye-piece.  35 Following the standard  two-surface  method,  the p o l e s o f the h a b i t  p l a n e s were p l o t t e d on a s t e r e o g r a p h i c p r o j e c t i o n and f i n a l l y the p o s i t i o n o f the p o l e s o f the h a b i t p l a n e s were determined the s t a n d a r d  (001) s t e r e o g r a p h i c p r o j e c t i o n .  on  - 20 -  yellow colored stress-induced ^ marten site stress axis  stress axis  reddish pink colored b.c.c.  F i g u r e 8.  Schematic  p r o j e c t i o n o f a p l a n e i n two  a x i s i s at r i g h t angles to  2.5,  surfaces.  Stress  NS.  O b s e r v a t i o n o f the S t r a i n Memory E f f e c t P r e l i m i n a r y o b s e r v a t i o n s showed t h a t i n s u i t a b l e  ranges  temperature  the C u - A l - N i a l l o y s had a shape memory e f f e c t , i . e . a l l o y s  deformed a t a low temperature and then heated r e t u r n e d t o t h e i r shape.  A more d e t a i l e d e x a m i n a t i o n o f t h i s e f f e c t was  two  different  1)  Quenched p o l y c r y s t a l l i n e specimens .030  35 mm  made u s i n g  procedures.  r a d i u s of c u r v a t u r e a t low temperature l i q u i d bath.  i n . t h i c k were bent t o a  and then s l o w l y heated i n a  D u r i n g h e a t i n g , the specimen was  photographed  camera and the p e r c e n t r e c o v e r y a t a g i v e n temperature  by measuring  initial  the a n g l e  (0) between the two  ends of the s t r i p  with a calculated as shown  - 21 -  i n F i g . 9, t h i s a n g l e b e i n g p r o p o r t i o n a l t o the magnitude o f the surface 2)  strain.  The s t r a i n memory e f f e c t was a l s o measured  specimens o f p o l y - and s i n g l e c r y s t a l  on u n i a x i a l l y  specimens.  stressed  The specimens were  s t r a i n e d a t two t e m p e r a t u r e s ; - a t low temperature (< M^) and a t room temperature between  (M < T < A ) . s s  A f t e r u n l o a d i n g , changes o f d i s t a n c e  p a r a l l e l s c r a t c h e d l i n e s were measured w i t h a t r a v e l l i n g  m i c r o s c o p e as the specimens warmed up i n water c o n t a i n e d i n a f l a s k .  - 22 -  Temp.= To  Tern p. = Ti  Temp.=Tf Surface s t r a i n at T , o  e  I  o  •l ' il i! i  T  Mi  tdso0^)  A£,,  t(180°-9 )  e  1  f  T  e  T  f  f  2l £  Recovery a t  t(180°-9 ) * o 2%  o" l £  1  11 e  o  - e  f  2 x 10 i n  x 10  (%)  where SL: l e n g t h o f specimen, Ail: e l o n g a t i o n a t s u r f a c e , t  : t h i c k n e s s o f specimen.  F i g u r e 9.  Schematic i l l u s t r a t i o n bending.  o f the s t r a i n memory e f f e c t by  - 23 -  3. RESULTS 3.1. P o l y c r y s t a l l i n e M a t e r i a l 3.1.1.  M . M, s f r  A , A- Temperatures s f r  Direct observations  o f 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 were made  on the a l l o y s shown i n T a b l e c a r r i e d out by Can. T e s t L t d .  Table  3.  3. - Chemical a n a l y s i s o f the a l l o y s was On c o o l i n g , i t was found  Compositions and M , M , A , and A  f  that a large  Temperatures o f p o l y -  c r y s t a l l i n e material studied. Nominal Composition  Chemical A n a l y s i s Cu Al Ni (wt.%)  Cu-14Al-2Ni  83.85  13.78  2.37  Cu-14.5Al-3Ni  82.85  14.15  3.00  Cu-14.5Al-3Ni  —  —  m a r t e n s i t e was seen.  martensite.  A  s  s  A  Used i n  f  (°C) 5  21  35  43  -82 -70  -60  -48  Tensile  59  -54  -29  Dilatometry  at M ,  formed.  g  Tensile tests and v i s u a l observation  and on d e c r e a s i n g  tests  the tempera-  No t r a c e o f t h e r m o e l a s t i c  A t y p i c a l martensite  10 and can be seen to c o n s i s t o f v e r y burst-type  M  —  amount o f t r a n s f o r m a t i o n o c c u r r e d t u r e more and more m a r t e n s i t e  Mf  s t r u c t u r e i s shown i n F i g .  thick plates c h a r a c t e r i s t i c of  -2k-  F i g u r e 10.  M a r t e n s i t i c s t r u c t u r e o b t a i n e d i n Cu-14Al-2Ni on c o o l i n g to 10°C.  -25-  F i g . 11 shows the r e s u l t s  o f the d i l a t o m e t r y experiment and g i v e s  the volume change due to t r a n s f o r m a t i o n from m a r t e n s i t e I t w i l l be n o t i c e d t h a t measurement  of A s  of  to b . c . c .  and A„ by d i r e c t f  observation  the m i c r o s t r u c t u r e i s i n good agreement w i t h o b s e r v a t i o n by d i l a t o -  metry.  The volume change due to t r a n s f o r m a t i o n i s l e s s  I  1  '  '  i  •  -80  -6 0  I  1  |  1—  '  i  1  1——i  -40  1  -20  than  1  0.1%.  r  1  I  0  20  TEMPERATURE,°C  F i g u r e 11.  The volume change due to t r a n s f o r m a t i o n from m a r t e n s i t e to b . c . c . on h e a t i n g .  Note the c o i n c i d e n c e of A  g  and A^ tempera-  t u r e s w i t h d i r e c t o b s e r v a t i o n , and the s m a l l volume change of  the t r a n s f o r m a t i o n  (<  0.1%).  40  3.1. 2.  Stress vs. S t r a i n  The  Curves  s t r e s s v s . s t r a i n c u r v e s a t two  shown i n F i g . 12 and 13.  The  d i f f e r e n t compositions  reason f o r c h o o s i n g two  c o m p o s i t i o n s i s to see the e f f e c t of v a r y i n g M , g  t u r e s on the s t r e s s - i n d u c e d m a r t e n s i t e .  M^,  different A  and A^  a)  tempera-  Both the 3Ni and 2Ni  alloys  have f o u r t y p i c a l types o f s t r e s s v s . s t r a i n curves depending the temperature  are  upon  range.  Temperature > A^,  F i g . 12-(a), 13-(a).  4 S t r e s s i n c r e a s e s l i n e a r l y w i t h s t r a i n r i g h t up to 4 x 10 upon u n l o a d i n g  the s t r e s s d e c r e a s e s f o l l o w i n g a s i m i l a r  A l t h o u g h a s m a l l permanent s e t remains  curve.  i n the sample a f t e r  complete  u n l o a d i n g , the curves show t y p i c a l e l a s t i c b e h a v i o r i n t h i s 4 range.  The  t o t a l s t r a i n s a r e l e s s than 1% a t 4 x 10  s t r a i n s o f 1.2% 3Ni a l l o y .  A  temperature  p s i , and  elastic  have been measured a t f r a c t u r e as shown a t 22°C f o r the  These specimens thus show some enhanced e l a s t i c i t y  a normal m e t a l has an e l a s t i c i t y o f l e s s than b)  p s i and  < Temperature < A  since  0.2%.  , F i g . 12-(b) , 1 3 - ( b ) .  S  I  The  s t r e s s v s . s t r a i n curve i s made up o f two  linear  portions,  a low s t r e s s p o r t i o n c o r r e s p o n d i n g to a modulus s i m i l a r to t h a t  found  i n case a) , and a second p o r t i o n w i t h an extremely s m a l l modulus.  At  4 a s t r e s s of 4 x 10  p s i the t o t a l s t r a i n i s much l a r g e r than a ) .  Upon u n l o a d i n g , the s t r e s s drops l i n e a r l y down to 1 x 10 d e c r e a s e s more s l o w l y . s e t r e m a i n i n g i n the c)  M  then  a s m a l l permanent  specimen.  < Temperature < A , g  Upon i n i t i a l On  However, t h e r e i s always  p s i and  F i g . 12-(c), 13-(c).  l o a d i n g , the s t r e s s i n c r e a s e s l i n e a r l y w i t h  f u r t h e r l o a d i n g , the s t r e s s v s . s t r a i n r e l a t i o n i s v e r y  strain.  different  - 27 -24<C  75-5'  22°C  A  C  203°C  to)  t  t  .78 °C  %  STRAW  ensile behavior o  ?igure 12.  m a t u rru ree on the t e a  {  ^  _  ( f l  Effect  s t a l specimens, 3Ni  poiy  c r  different  ^ temperature  ranges  referred  to i n  the  are  the  text.  STRAIN , F i g u r e 13.  %  E f f e c t of temperature on the t e n s i l e b e h a y i o r o f Cu-14Al-2Ni p o l y c r y s t a l (a),  ( b ) , ( c ) , (d) are the d i f f e r e n t temperature ranges r e f e r r e d  specimens.  to i n the  text.  F i g u r e 14.  The e f f e c t o f r e p e a t e d s t r a i n s permanent s e t . serrations  on the magnitude o f the  The permanent s e t d e c r e a s e s as t h e  decrease.  o strains at stress of 4xl0 psi 4  x strains at zero stress  Mf M As Af s  -200  -160  -120  -80  -40  0  40  80  120  160  200  T E M P E R A T U R E , °C 4 F i g u r e 15.  The t o t a l s t r a i n s a t a s t r e s s o f 4 x 10 f o r Cu-14.5Al-3Ni p o l y c r y s t a l specimens.  p s i and t h e r e s i d u a l s t r a i n s a t zero  stress  - 31 -  1  T  r  TEMPERATURE, °C  F i g u r e 16.  The t o t a l s t r a i n s a t a s t r e s s o f 4 x 10 residual crystal  s t r a i n s a t zero s t r e s s specimens.  p s i and,the  f o r Cu-14Al-2Ni  poly-  - 32 -  from cases a) and b) . a given load  Suddenly  appear  At  which i s about  4 times t h a t  found f o r  Upon u n l o a d i n g , the s t r e s s drops almost l i n e a r l y and a  l a r g e permanent s e t remains  i n the t e s t sample a f t e r u n l o a d i n g .  F i g . 14 shows c l e a r l y t h a t p r e s e n c e of s e r r a t i o n s .  The  the permanent s e t i s due  specimen was  c o n t i n u a l l y repeated u n t i l f r a c t u r e .  s e t was  v e r y l a r g e but as the s t r a i n i n c r e a s e d ,  s e r r a t i o n s d e c r e a s e d and Temperature < M The  and  Initially  unloaded.  the permanent  the a m p l i t u d e o f the  the permanent s e t got s m a l l e r and  smaller,  , F i g . 12-(d), 13-(d).  stress increases  of a d u c t i l e m a t e r i a l . l i n e a r l y and  t o the  s t r a i n e d 0.85%  T h i s was  d)  on the c u r v e s .  the s t r a i n i s much l a r g e r than f o r cases a) and b ) , and  the s t r a i n a t f r a c t u r e i s 4.8% case a ) .  serrations  w i t h s t r a i n w i t h a smooth c u r v e c h a r a c t e r i s t i c  Upon u n l o a d i n g , the s t r e s s drops  almost  t h e r e i s a l a r g e permanent s e t .  4 F i g . 15 and 16 show the s t r a i n s a t a s t r e s s of 4 x 10 the r e s i d u a l s t r a i n s a t zero s t r e s s on u n l o a d i n g . seen t h a t  p s i and  I t can be c l e a r l y  the m a r t e n s i t e i s more d u c t i l e than the g-phase and maximum  d u c t i l i t y o c c u r s i n the neighborhood  of the M  temperature.  t h e r e i s no l a r g e amount o f s u p e r - e l a s t i c i t y i n the m a t e r i a l temperature  3.1.3. The  However, i n any  range.  V i s u a l Observation of Stress-Induced M a r t e n s i t e s t r e s s v s . s t r a i n curve f o r a Cu-14Al-2Ni  at room temperature  (M s  specimen  < T < A ) i s shown i n F i g . 17. s  deformed  The numbers on  the curve show the s t r e s s at which the photographs  shown i n F i g . 18-(a)  and  different  (b) were taken.  on the same specimen.  (a) and  (b) were taken at two  These photographs  reveal  that  two  positions  types o f  I  I  circled numbers refer to fig.l8-(o) other numbers to fig.|8-(b)  I  2  STRAIN, F i g u r e 17.  3  4  %  S t r e s s v s . s t r a i n curve f o r a Cu-14Al-2Ni specimen, showing the s t r e s s a t which the photographs i n F i g . 18 were taken C i r c l e d numbers r e f e r to F i g . 1 8 - ( a ) , and o t h e r nunihpT-s f n V-i cr 1 S_ Ch\  - 34 -  s t r e s s - i n d u c e d m a r t e n s i t e a r e formed under the a p p l i c a t i o n of n e e d l e - l i k e m a r t e n s i t e b e f o r e s e r r a t i o n s b e g i n , and , a f t e r serrations begin,the  martensite  w i t h the s e r r a t i o n s .  The  lenticular  , being l e n t i c u l a r martensiteAassociated  s t r e s s vs. s t r a i n curves before  serrations  appear^have a v e r y s m a l l permanent s e t a f t e r u n l o a d i n g , but s e r r a t i o n s appear^  stress;  have a l a r g e permanent s e t .  The  those  after  lenticular  m a r t e n s i t e p l a t e s appear to be b u r s t - t y p e m a r t e n s i t e and have the following  characteristics:  4 1)  They appear suddenly  at a s p e c i f i c  which depends upon the t e s t o f the i n d i v i d u a l 2)  stress  temperature,  10  psi i n this  h a b i t p l a n e , and  case)  orientation  grain.  They o f t e n have the z i g - z a g c h a r a c t e r i s t i c of b u r s t - t y p e m a r t e n s i t e  produced  on  cooling.  3)  They are not r e v e r s i b l e a t room temperature  4)  Not  (M  = 20°C, A  g  = 35°C).  a l l g r a i n s are covered w i t h b u r s t - t y p e m a r t e n s i t e a t the  f r a c t u r e s t r e s s , a l t h o u g h the b u r s t phenomenon seems a l r e a d y to have f i n i s h e d b e f o r e the f r a c t u r e p o i n t as seen i n F i g . 17. N e e d l e - l i k e m a r t e n s i t e s have q u i t e d i f f e r e n t  features.  More  d e t a i l e d photographs of n e e d l e - l i k e m a r t e n s i t e s taken on the H o u n s f i e l d tensometer f o r Cu-14Al-3Ni a l l o y , are shown i n F i g . 19. m a r t e n s i t e s were observed 1)  The  to have the f o l l o w i n g c h a r a c t e r i s t i c s :  f i r s t n e e d l e - l i k e m a r t e n s i t e s are observed  boundaries  (see 1  ).  Subsequently,  e x i s t i n g m a r t e n s i t e p l a t e s (see 2 2)  Needle-like  to grow from  some m a r t e n s i t e n u c l e a t e s  grain from  ).  Growth o f the p l a t e s i s t e r m i n a t e d by i n t e r f e r e n c e w i t h  m a r t e n s i t e p l a t e s and w i t h the g r a i n b o u n d a r i e s  (see 3 - 1  other ).  Figure l8-a D i r e c t observation Cu-lAAl-2Ni  of stress-induced martensite  p o l i c r y s t a l taken while  for a  x60  s t r e s s i n g i n I n s t r o n machine.  F i g u r e l8-b  D i r e c t o b s e r v a t i o n of s t r e s s - i n d u c e d m a r t e n s i t e C u - l ^ A l - 2 N i p o l y c r y s t a l taken w h i l e  for a  x60  s t r e s s i n g i n I n s t r o n mashine.  0 psi  0.62x10  0 psi  psi  0.89x10 p s i  F i g u r e 19.  D e t a i l e d photographs of n e e d l e - l i k e m a r t e n s i t e s  (Cu-14Al-3Ni).  i s r e v e r s i b l e and has a l a r g e h y s t e r e s i s to the s t r e s s .  Note the m a r t e n s i t e  x5  - 38 -  Figure  20.  Change i n l e n g t h of n e e d l e - l i k e m a r t e n s i t e unloading.  on l o a d i n g  and  - 39 3)  They a r e c o m p l e t e l y r e v e r s i b l e .  Upon removal  o f the e x t e r n a l  s t r e s s , the r e v e r s e t r a n s f o r m a t i o n s t a r t s w i t h some h y s t e r e s i s as shown i n F i g . 20.  S h o r t e n i n g always  starts f i r s t ,  f o l l o w e d by a  decrease i n width.  3.2. S i n g l e C r y s t a l M a t e r i a l 3.2.1. The  Stress vs. Strain  Curves  Cu-14.5Al-3Ni specimens used have c h e m i c a l c o m p o s i t i o n s o f  82.94 Cu-14.09 Al-2.97  N i (wt.%) and the f o l l o w i n g f i x e d  M„ = -10°C, M = 11°C, A = 30°C, and A^ = 45°C. f s s f  temperatures:-  F i g . 21 shows t h e  s t r e s s v s . s t r a i n curve o b t a i n e d up t o 15% s t r a i n a t room  temperature  (22°C).  A s t r a i n o f 5.5% c o r r e s p o n d s t o t h e end o f s e r r a t i o n s and  complete  t r a n s f o r m a t i o n to m a r t e n s i t e .  Most t e s t s were c a r r i e d out t o  5.5% s t r a i n as shown i n F i g . 22. The  s t r e s s v s . s t r a i n curves f o r s i n g l e c r y s t a l specimens a r e  q u i t e d i f f e r e n t from those 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 . curves can s t i l l be d i v i d e d i n t o f o u r temperature the M , M , s t r  a)  g  upon  F i g . 22-(a).  s t r e s s v s . s t r a i n curve i s i n i t i a l l y  0.5% s t r a i n .  With  l i n e a r up t o a p p r o x i m a t e l y  f u r t h e r l o a d i n g the curves become n e a r l y  and s m a l l s e r r a t i o n s a r e v i s i b l e .  horizontal  The m a t e r i a l i s h i g h l y e l a s t i c and  on u n l o a d i n g from 5.5% s t r a i n r e t u r n s p r a c t i c a l l y length.  ranges depending  A , and A,, temperatures, s I  Temperature < A , The  However, t h e  to i t s i n i t i a l  On u n l o a d i n g t h e s t r e s s drops l i n e a r l y a t f i r s t  and then  becomes n e a r l y h o r i z o n t a l w i t h s e r r a t i o n s l e s s v i s i b l e than on the loading curve.  Finally  the s t r e s s drops l i n e a r l y w i t h a s l o p e e q u a l  F i g u r e 22.  Effect  o f temperature on the t e n s i l e b e h a y i o r of Cu-14.5Al-3Ni  specimen.  Ca),  O),  Cc),  Cd)  show d i f f e r e n t  single  temperature ranges.  crystal  - zv -  - 43 -  to  the f i r s t  p a r t of the l o a d i n g c u r v e .  permanent s e t g r a d u a l l y i n c r e a s e s and b)  M  < Temperature < A s  At e l e v a t e d temperature  the  the h o r i z o n t a l r e g i o n d e c r e a s e s .  , F i g . 22-(b). s  The  s t r e s s v s . s t r a i n curve i s l i n e a r a t the b e g i n n i n g , becomes  c o m p l e t e l y h o r i z o n t a l , and  then shows v e r y l a r g e s e r r a t i o n s .  The  s t r a i n i s c o m p l e t e l y r e v e r s i b l e upon u n l o a d i n g i n the h o r i z o n t a l r e g i o n b e f o r e s e r r a t i o n s appear.  However, once the s e r r a t i o n s  p r e s e n t , the specimen a c q u i r e s a permanent s e t .  are  As the specimen  c o n t i n u e s to be s t r a i n e d , the amplitude  o f the s e r r a t i o n s becomes  s m a l l e r and  s e r r a t i o n becomes h i g h e r .  the l o a d to produce another  At h i g h s t r a i n s , the s t r e s s a g a i n i n c r e a s e s l i n e a r l y w i t h s t r a i n , upon u n l o a d i n g , the s t r e s s drops remaining c)  r a p i d l y w i t h a l a r g e permanent s e t  unloading.  < Temperature < The  and  after  M. g  s t r e s s v s . s t r a i n c u r v e s show a s m a l l l i n e a r p o r t i o n a t  t h e r e i s no h o r i z o n t a l r e g i o n .  The  amplitude  much s m a l l e r than t h a t f o r the curve a t 20°G. temperature, seen.  and  the s m a l l e r the amplitude  and  Upon u n l o a d i n g , the s t r e s s drops  first,  of the s e r r a t i o n s i s  The  lower  the  test  the l e s s the s e r r a t i o n s  are  r a p i d l y w i t h a l a r g e permanent  set. d)  Temperature < W e l l below M^,  s t r e s s i n c r e a s e s w i t h s t r a i n w i t h a smooth c u r v e ,  and upon u n l o a d i n g the s t r e s s drops permanent s e t . are found  almost  However, a t temperatures  (see curve f o r - 2 5 ° C ) , a l t h o u g h  l i n e a r l y with a l a r g e  j u s t below  , some s e r r a t i o n s  the s e r r a t i o n s are much s m a l l e r  o: total strain y: residual strain at zero stress M  F i g u r e 23.  The  f  M  8  A  8  A  f  t o t a l s t r a i n s of 5.5% and the r e s i d u a l s t r a i n s a t zero s t r e s s f o r Cu-14.5A1-  3Ni s i n g l e c r y s t a l specimens.  - 45 -  than those o b t a i n e d f o r M Fig.  £  f  < T < M . s  23 shows the r e s i d u a l s t r a i n s a t zero s t r e s s on u n l o a d i n g .  I t can be c l e a r l y seen t h a t maximum d u c t i l i t y of t h e M  s  temperature.  o c c u r s i n the  neighborhood  T h i s phenomenon i s t h e same as f o r p o l y -  crystalline material.  3.2.2. The  V i s u a l Observation of Stress-Induced  Martensite  s t r e s s v s . s t r a i n curve f o r a Cu-14.5Al-3Ni specimen deformed  at room temperature  (M < T < A ) i s shown i n F i g . 24. s s  The numbers  on the curve show the s t r e s s a t which photographs shown i n F i g . 25 were taken.  Another and much c l e a r e r s e t o f p i c t u r e s i s shown i n F i g . 26.  No s t r e s s v s . s t r a i n curve was o b t a i n e d i n t h i s c a s e , because t h e specimen was s t r e s s e d and photographed i n t h e same h o l d e r as used i n the d e t e r m i n a t i o n o f the h a b i t p l a n e shown i n F i g . 7.  Fig.  25 and 26  r e v e a l t h a t two types o f s t r e s s - i n d u c e d m a r t e n s i t e a r e formed under the a p p l i c a t i o n o f s t r e s s , v i z . n e e d l e - l i k e m a r t e n s i t e and b u r s t - t y p e m a r t e n s i t e , as was the case f o r p o l y c r y s t a l l i n e samples.  The appearance  and b e h a v i o r , however, a r e q u i t e d i f f e r e n t . J u s t b e f o r e the h o r i z o n t a l r e g i o n appears  on the s t r e s s v s . s t r a i n  c u r v e , n e e d l e - l i k e m a r t e n s i t e n u c l e a t e s a t the edge o f the specimen. The  f e a t u r e s o f n e e d l e - l i k e m a r t e n s i t e s i n the s i n g l e c r y s t a l m a t e r i a l s  are: 1)  The f i r s t n e e d l e - l i k e m a r t e n s i t e t h a t i s observed  grows from the  specimen edge. 2)  Needles  grow from one edge to the o t h e r o f the specimen.  i n c r e a s e s , more and more n e e d l e - l i k e m a r t e n s i t e s  appear.  As the l o a d  - 46 -  Figure  24.  Stress  v s . s t r a i n curve f o r a Cu-14.5Al-3Ni s i n g l e  crystal  specimen, showing the s t r e s s a t which the photographs i n Fig.  25 were taken.  Note the f o r m a t i o n o f b u r s t - t y p e m a r t e n s i t e .  Note: Release  of s t r e s s due t o the f o r m a t i o n of  burst martensite and  causes the needleshaped m a r t e n s i t e  i n i t i a l stage o f b u r s t m a r t e n s i t e t o d i s a p p e a r .  Note the c o n s t a n t p r o g r e s s of boundary between b u r s t m a r t e n s i t e and m a t r i x w i t h an angle 65  0  t o the s t r e s s  F i g u r e 25.  of  direction.  D i r e c t o b s e r v a t i o n of s t r e s s - i n d u c e d m a r t e n s i t e taken w h i l e s t r e s s i n g i n I n s t r o n machine f o r a Cu-14.5Al-3Ni s i n g l e c r y s t a l ,  x  30  •oe x  - 49 -  3)  The v e r y f i n e m a r t e n s i t e p l a t e s a r e c o m p l e t e l y  o t h e r and have a d e f i n i t e angle  parallel  to each  t o the s t r e s s a x i s (56° f o r t h i s  specimen). 4)  I n t h e h o r i z o n t a l r e g i o n o f the s t r e s s v s . s t r a i n c u r v e , the f i n e  n e e d l e - l i k e m a r t e n s i t e s become v e r y dense and j u s t b e f o r e the d r a s t i c drop o f s t r e s s t h e r e a r e many dense groups o f f i n e p a r a l l e l 5)  They a r e c o m p l e t e l y  martensite  plates.  r e v e r s i b l e upon u n l o a d i n g , and on r e l o a d i n g ,  forms a g a i n a t the same l o c a t i o n s .  On continuous  l o a d i n g , t h e s t r e s s suddenly  of b u r s t - t y p e m a r t e n s i t e m a r t e n s i t e i s densest i n F i g . 26-(9)).  drops and a r e g i o n  forms i n the areas where the n e e d l e - l i k e  (see F i g . 26-(8) and a b a n d - l i k e b u r s t  A d d i t i o n a l f e a t u r e s o f the b u r s t - t y p e  martensite  martensite  formation are: 1)  At the same time  t h a t b u r s t - t y p e m a r t e n s i t e appears, n e e d l e - l i k e  martensite disappears 2)  Initially  shape. and  Only  (see F i g . 2 5 - ( 5 ) , ( 6 ) ) .  s e v e r a l b u r s t - t y p e p l a t e s may form h a v i n g one o f these d e v e l o p s  the others disappear,  martensite  a wedge-like  to give a band-like martensite,  i . e . , the i n i t i a l  stage of b u r s t - t y p e  f o r m a t i o n i s r e v e r s i b l e a t room temperature  (see F i g . 26-(8)  and ( 9 ) ) . 3)  Having once formed, t h e b a n d - l i k e b u r s t - t y p e m a r t e n s i t e  at low s t r e s s i n t o the untransformed r e v e r s i b l e a t room 4) a  propagates  b . c . c . m a t r i x and i s n o t  temperature.  B u r s t - t y p e m a r t e n s i t e n u c l e a t e s the n e e d l e - l i k e m a r t e n s i t e which has different  h a b i t plane  (see F i g . 2 6 - ( 8 ) ) .  from a l r e a d y - e x i s t i n g n e e d l e - l i k e m a r t e n s i t e  - 50 -  5)  Both edges o f the b u r s t - m a r t e n s i t e r e g i o n a r e p a r a l l e l and have a  d e f i n i t e angle t o the s t r e s s a x i s (65° f o r t h i s specimen).  The angle  f o r b u r s t - t y p e m a r t e n s i t e and f o r the n e e d l e - l i k e m a r t e n s i t e i s not the same. 6)  The b u r s t - t y p e m a r t e n s i t e o f t e n c o n t a i n s a pronounced  secondary  s t r u c t u r e o f f i n e l a m e l l a e (see F i g . 2 6 - ( 9 ) ) .  3.3. X-Ray A n a l y s i s o f M a r t e n s i t e The (c).  X-ray  d i f f r a c t i o n r e s u l t s are given i n F i g . 27-(a),  (b) , and  The range o f 20 shown (30° t o 50°) i n c l u d e s the most i n t e n s e peaks  of the v a r i o u s s t r u c t u r e s . quenched  F i g . 27-(a) shows the p a t t e r n from the  3-phase w i t h d i s t i n c t  l a t t i c e r e f l e c t i o n s a r e observed  (200),  (220) r e f l e c t i o n s .  s i n c e Cu, A l , and N i have  a b s o r p t i o n c o e f f i c i e n t s w i t h Cu r a d i a t i o n 49.3).  No  super-  similar  (Cu : 52.7, A l : 48.7, N i :  The thermal m a r t e n s i t e s t r u c t u r e o b t a i n e d on c o o l i n g i s shown 26  i n F i g . 27-(b).  Rachinger  has examined the thermal m a r t e n s i t e i n  t h i s system and has concluded structure.  t h a t i t has an o r d e r e d  orthorhombic  In T a b l e 4 values of d s p a c i n g and i n t e n s i t y o b t a i n e d i n  the p r e s e n t work a r e compared w i t h the v a l u e s found by Rachinger. agreement i s o n l y moderate d e s p i t e the f a c t t h a t the c o m p o s i t i o n i n the p r e s e n t work i s almost  the same as t h a t used by  A t e n s i l e specimen (Cu-14Al-2Ni) w i t h M room temperature taken.  Rachinger.  immediately  F i g . 27-(c) shows a r e l a t i v e l y i n t e n s e (220) b . c . c .  together with martensite r e f l e c t i o n s at e s s e n t i a l l y s i m i l a r (see T a b l e 5 ) .  used  = 21°C was f r a c t u r e d a t  (22°C) and a d i f f r a c t o m e t e r t r a c e was  to the thermal m a r t e n s i t e  The  reflection angles  F i g u r e 27.  Diffraction  t r a c e s showing (a) b . c . c . m a t r i x ,  (c) s t r e s s - i n d u c e d m a r t e n s i t e , and  (b) low  (d) powder f i l e d  temperature m a r t e n s i t e ,  a t 22°C.  T a b l e 4.  Values of d s p a c i n g and i n t e n s i t y i n the p r e s e n t work and i n Rachinger's work ( f o r CuK^ radiation.  Rachinger (Thermal m a r t e n s i t e ) obs  hkl  d  O  P r e s e n t work (Thermal m a r t e n s i t e ) ^obs  (A)  41.1  (200)  2.18  s+  42.9  (002)  2.11  vs  2  6  K  hkl  obs  d  O  (Deformation ^"obs  obs  hkl  d  O  40.1  2.25  s  40.3  2.24  s  41.6  (200)  2.17  s+  43.2  (002)  2.09  vs  43.1  2.02  vs  44.2  (002)' b.c.c.  45.4  (201)  1.94  vs  46.3  (201)  1.96  vs  46.3  ' (201)  2.09  vs  2.05  vs  2.00  vs  1.96  w  1.93  w  >  47.0 48.6  ''"obs  (A)  (A)  44.9  46.7  martensite)  1.87  w+  - 53 -  T a b l e 5.  Angle of s t r e s s - i n d u c e d m a r t e n s i t e w i t h r e s p e c t to the  stress  axis. Specimen  Angle  No.  to s t r e s s a x i s  needle-martensite  burst-- m a r t e n s i t e  1  51°  47°  2  48°  54°  3  49°  50°  4  57°  22°  A diffTactometer t r a c e was  produced  a f t e r f i l i n g the quenched  g-  s t r u c t u r e of the same 2Ni a l l o y a t room temperature  (Fig. 27-(d)).  I n t h i s case the q u a l i t y of the p a t t e r n i s poor due  to c o l d work.  However, i t appears produced  t h a t the s t r u c t u r e o b t a i n e d i s c l o s e to t h a t  on t e n s i l e d e f o r m a t i o n , w i t h peaks a t e s s e n t i a l l y  similar  p o s i t i o n s as thermal m a r t e n s i t e p l u s some r e t a i n e d (3-phase. i s no apparent  tendency  to form a new  There  close-packed s t r u c t u r e ( f . c . c .  or h.c.p.) such as has been found i n the Cu-Zn,  8  Ag-Zn,  4  o r Ag-Cd  2  systems.  3.4.  C r y s t a l l o g r a p h i c H a b i t P l a n e of M a r t e n s i t i c S t r u c t u r e The h a b i t p l a n e of thermal m a r t e n s i t e was  c r y s t a l s and approximately  i s shown i n F i g . 2 8 - ( a ) . 2° and corresponds  The  o b t a i n e d f o r two  single  experimental e r r o r i s  to the diameter  of the c i r c l e  round  27 the e x p e r i m e n t a l p o i n t s .  Rachinger  p l a n e s of thermal m a r t e n s i t e and  has  a l s o determined  the h a b i t  found a spread i n h i s r e s u l t s  due  - 54 -  probably  to m a t r i x s t r e s s e s s e t up d u r i n g p l a t e f o r m a t i o n .  The  r e s u l t s are i n moderate agreement w i t h those of Rachinger,  thus  t e n d i n g to c o n f i r m the e x p e r i m e n t a l  technique.  The h a b i t p l a n e s f o r s t r e s s - i n d u c e d n e e d l e - l i k e and m a r t e n s i t e were a l s o determined F i g . 28-(b) and 45°  (c).  present  i n s i n g l e c r y s t a l s and  burst-type  are shown i n  Both types of m a r t e n s i t e tend to form c l o s e to  to the p l a n e of a p p l i e d s t r e s s , i . e . , i n the p l a n e o f maximum  shear s t r e s s as seen i n T a b l e The onto  5.  p r e s e n t r e s u l t s show an e x t r e m e l y wide s c a t t e r when p l o t t e d  the s t a n d a r d s t e r e o g r a p h i c t r i a n g l e s , and  the p r e s e n t r e s u l t s  to say t h a t t h e r e i s one  e i t h e r needle or burst-type m a r t e n s i t e . would be n e c e s s a r y  i t i s impossible  unique  from  h a b i t plane f o r  A v e r y l a r g e number o f specimens  to e s t a b l i s h t h i s p o i n t .  I t does appear, however,  t h a t the h a b i t p l a n e f o r b o t h n e e d l e and b u r s t m a r t e n s i t e i s i r r a t i o n a l . 32 T h i s o b s e r v a t i o n agrees w i t h Arbuzova. s t a t e the h a b i t p l a n e was  He  said  t h a t i n the  loaded  o r i e n t e d d i f f e r e n t l y w i t h r e s p e c t to the  a x i s of the specimen, but g i v e s no d a t a .  There i s no q u e s t i o n t h a t the  h a b i t p l a n e s f o r n e e d l e - l i k e and b u r s t - t y p e m a r t e n s i t e are  different.  T h i s can a l s o be seen by r e f e r e n c e to the photographs i n F i g . 26 which show the c o m p l e t e l y d i f f e r e n t h a b i t s f o r the two  types of m a r t e n s i t e .  I t s h o u l d , f i n a l l y , be emphasized t h a t the spread i n the p r e s e n t r e s u l t s i s genuine and are r e l a t i v e l y is  i s not caused  by the e x p e r i m e n t a l  s m a l l , but i s p r o b a b l y due  changed by the presence  to the f a c t t h a t the h a b i t p l a n e  o f s t r e s s which i s caused  t i o n and by the a p p l i e d u n i a x i a l  stress.  e r r o r s which  both by  transforma-  (o)  Rachinger 001  t h e r m a l burst martensite  (b)  stress-induced burst martensite stress-induced needle-like martensite stress  axis  (c) F i g u r e 28.  Stereographic r e p r e s e n t a t i o n of martensite h a b i t plane. thermal m a r t e n s i t e , i l l u s t r a t i n g  (a) Shows the o r i e n t a t i o n o f  the p r e s e n t r e s u l t s t o g e t h e r w i t h those o f R a c h i n g e r .  (b) Shows the o r i e n t a t i o n o f s t r a i n - i n d u c e d b u r s t - t y p e m a r t e n s i t e f o r the f o u r s p e c i mens examined,  (c) Shows the o r i e n t a t i o n o f s t r a i n - i n d u c e d m a r t e n s i t e and t h e s t r e s s  ' a x i s f o r each o f the specimens examined.  - 56 -  3.5.  E f f e c t o f C r y s t a l O r i e n t a t i o n on  Super-Elasticity  Three s i n g l e c r y s t a l s w i t h d i f f e r e n t  s t r e s s axes were t e s t e d  to examine the e f f e c t o f c r y s t a l o r i e n t a t i o n on  super-elasticity.  In p r e v i o u s experiments, i t has been found t h a t the f i x e d (M , M,. s' f  A , A,.) have a c r i t i c a l s' f  curves.  T h e r e f o r e , two  M  s  temperatures  i n f l u e n c e on the s t r e s s v s .  strain  temperature ranges were t e s t e d , one 9°C above  and the o t h e r 15°C above the A,, temperature. f  f i x e d temperatures o f the t h r e e specimens  used.  T a b l e 6 shows the F i g . 29 shows the  s t r e s s axes and s t r e s s v s . s t r a i n c u r v e s o b t a i n e d .  T a b l e 6.  M , M,.,  , and k  A  f  orientation  M  No.  s  temperatures o f the specimens  used f o r  studies  M  f  A  s  A  f  1  9  -9  28  51  2  13  2  21  40.5  3  13  2  25.5  44  (°C)  I t i s perhaps u n f o r t u n a t e t h a t a wider range of c r y s t a l  orientations  c o u l d not be examined, but w i t h the p r e s e n t t e c h n i q u e of n u c l e a t i n g c r y s t a l s a t a p o i n t , the c r y s t a l a x i s always had a tendency to be c l o s e to  <001> .  Specimen No.  2 appear to have the h i g h e s t e l a s t i c i t y ; however, w i t h  the p r e s e n t r e s u l t s no e f f e c t o f c r y s t a l o r i e n t a t i o n on the f o r m a t i o n and b e h a v i o r o f s t r e s s - i n d u c e d m a r t e n s i t e can be  seen.  3h  65°C ( I5°C above A ) f  j  i  I  L  6  55°C (I5°C above A ) f  0  f  6  (2)  (I)  I 8 ° C ( 9°C above M ) s  I  55°C ( I 5 ° C obove A )  22°C ( 9 ° C above Mg)  22°C ( 9 ° C above M ) s  Uw-l.  4  6 0  2 STRAIN ,  F i g u r e 29.  6 0  4  2  4  6  %  S t r e s s v s . s t r a i n curve showing the e f f e c t o f c r y s t a l o r i e n t a t i o n  on  super-elasticity.  - 58 -  3.6.  S t r a i n Memory E f f e c t A s t r a i n memory e f f e c t  produced e i t h e r by c o o l i n g  i s found i n m a r t e n s i t e which has or by  Preliminary investigation  stressing.  o f the memory e f f e c t i n specimens  had been l>ent below M^  showed t h a t  r e c o v e r y began at the A  g  shown i n F i g . 3 0 - ( a ) .  Uniaxially  b e h a v i o r upon h e a t i n g . single  as the specimen was  temperature and was stressed  F i g . 30-(b) and  c r y s t a l s w i t h two d i f f e r e n t  specimen was  specimens showed  about 15°C below M^  first  strained  before s t r e s s i n g ) ,  5.6%  and unloaded at the same temperature.  However, when the specimen was  g  The  color  and  strained  the specimen had a m a r t e n s i t i c  at a  had c o m p l e t e l y  (yellow color martensite reddish pink  (i.e.,  on  specimen.  Recovery o f the o r i g i n a l shape began around A  complete around A^.  as different  (c) show the r e s u l t s  then heated u n t i l > t h e m a r t e n s i t i c s t r u c t u r e  disappeared v i s u a l l y b.c.c).  g  slowly heated,  complete a t A^,  c r y s t a l s , the specimen was  temperature 10°C above M  which  f i x e d t e m p e r a t u r e s , and F i g . 30-(d)  shows the r e s u l t on a p o l y c r y s t a l l i n e In the s i n g l e  been  was 5.6%  structure  r e c o v e r y began around 10°C above A^ upon h e a t i n g  and was  not complete u n t i l 16°C above A,.. f  Measurement o f the M , s  M^,  and A^ temperatures soon a f t e r  o f the memory e f f e c t  A , g  showed no change. the  The h y s t e r e s i s  testing  o f r e c o v e r y seemed to depend  upon  e x t e n t o f s t r a i n , i . e . , the more s t r a i n the specimen had, the  h i g h e r the temperature f o r the specimen to s t a r t r e c o v e r i n g . same s i t u a t i o n o c c u r r e d i n the p o l y c r y s t a l shows the m a r t e n s i t i c s t r u c t u r e crystals  specimen.  F i g . 31-(a)  formed a t h e r m a l l y on c o o l i n g  and F i g . 31-(b) shows the d i f f e r e n t  The  single  t e x t u r e s on the t e n s i o n  and compression s i d e s on bending the specimen ( a ) .  - 59 -  F i g u r e 30-(a)  S t r a i n memory e f f e c t on b e n d i n g o f C u - A l - N i p o l y c r y s t a l specimens.  1  - 60  F i g u r e 30-(b).  -  S t r a i n memory e f f e c t on s t r e s s i n g  of C u - A l - N i  single crystal  - 61  Figure 30-(c).  -  S t r a i n memory e f f e c t on s t r e s s i n g of C u - A l - N i s i n g l e c r y s t a l specimen.  ON  F i g u r e 31.  M a c r o s t r u c t u r e of thermal m a r t e n s i t e and deformed m a r t e n s i t e i n a s i n g l e c r y s t a l . (a) Below M^  low temperature m a r t e n s i t e w i t h random o r i e n t a t i o n i s shown.  deformation at 22°C, p l a s t i c deformation i s r e a l i z e d by the f o r m a t i o n of oriented martensite.  (b)  After  preferred x  30  - 64 -  4.  4.1.  Nature of S t r e s s v s . S t r a i n Curves O p t i c a l microscopy  of  DISCUSSION  Cu-Al-Ni  shown c l e a r l y  t h a t the t e n s i l e  behavior  a l l o y s i s a s s o c i a t e d w i t h the f o r m a t i o n of s t r e s s - i j i d u c e d  martensite.  For s i m p l i c i t y  following discussion. depend on the t e s t The  has  The  the term SIM  w i l l be adopted i n the  a c t u a l s t r e s s v s . s t r a i n curves  temperature  r e l a t i v e to M , g  M^,  A , g  and  obtained A^.  e f f e c t of a m a r t e n s i t i c r e a c t i o n on s t r e s s v s . s t r a i n  curves  35 has been d e s c r i b e d by S c h e i l Fe-Ni a l l o y s . 1)  He p o s t u l a t e d : -  At and below M , g  the a u s t e n i t e l a t t i c e becomes m e c h a n i c a l l y  w e l l as thermodynamically martensite without 2)  w i t h r e f e r e n c e to transformations i n  u n s t a b l e and  shears over s p o n t a n e o u s l y  as into  the a p p l i c a t i o n of e x t e r n a l s t r e s s .  At temperatures  not too f a r above M  , a critical  r e s o l v e d shear  s t r e s s w i t h i n the e l a s t i c range i s r e q u i r e d to promote the  transforma-  t i o n to m a r t e n s i t e . 3)  At temperatures  s u f f i c i e n t l y f a r above M , g  the c r i t i c a l r e s o l v e d  shear s t r e s s f o r m a r t e n s i t e f o r m a t i o n i n c r e a s e s to a l e v e l above t h a t required  for slip  and p l a s t i c  flow r a t h e r than the t r a n s f o r m a t i o n o c c u r s  when e x t e r n a l s t r e s s i s a p p l i e d . In  Fe-base a l l o y s the m a r t e n s i t e produced  by d e f o r m a t i o n  i s stable  - 65 -  once formed. M  The  i s large (T  r e a s o n f o r t h i s i s t h a t the d i f f e r e n c e between T  i s the temperature  q  thermodynamically  above which m a r t e n s i t e i s  u n s t a b l e under zero s t r e s s ) .  In Fe-20Ni-0.5C  (wt. %) a l l o y s , f o r i n s t a n c e , the m a r t e n s i t e produced d e f o r m a t i o n of the a u s t e n i t e forms o n l y up  However, i n t h i s s t e e l , T  q  and  o  by  elastic  to 37°C above M  (-37°C 37  i s c o n s i d e r a b l y h i g h e r , ^ 200°C,  36  ).  so  t h e r e i s no p o s s i b i l i t y of the m a r t e n s i t e d i s a p p e a r i n g on removal of the s t r e s s and  thus g i v i n g s u p e r - e l a s t i c b e h a v i o r .  the l a r g e d i f f e r e n c e between M  and T  Q  The  reason f o r  i n Fe-base a l l o y s i s due  to the  s l a r g e volume change a s s o c i a t e d w i t h the t r a n s f o r m a t i o n to m a r t e n s i t e 38 (^ 4.2%  volume change i n the Fe-Ni a l l o y  In Cu-Al-Ni  ).  a l l o y s , the volume change i n the t r a n s f o r m a t i o n has  been measured as l e s s than 0.1%,  and  so the d i f f e r e n c e between M s  and T  q  i s v e r y s m a l l (^ 4°C i n the p r e s e n t work).  then, m a r t e n s i t e may  be produced  by d e f o r m a t i o n  removal of the s t r e s s , thus g i v i n g s u p e r - e l a s t i c U s i n g the concepts  In t h i s  and d i s a p p e a r  1)  Below  d i s c u s s e d above, the s t r e s s v s . s t r a i n curves obtained t y p i c a l s t r e s s v s . strain  s i n g l e c r y s t a l are r e p e a t e d i n F i g . 3 2 - ( a ) and  (b).  M: f  T h i s temperature martensitic structure. little  on  behavior.  i n the p r e s e n t work, can be e x p l a i n e d . For convenience curves f o r p o l y - and  system  elasticity.  range corresponds  to d e f o r m a t i o n of the  fully  The m a r t e n s i t e i s h i g h l y d u c t i l e but shows v e r y  However, as d i s c u s s e d l a t e r  deformed m a r t e n s i t e r e t u r n s to i t s i n i t i a l back to the b . c . c . s t r u c t u r e .  ( i n s e c t i o n 4.3),  the  s i z e on h e a t i n g to r e v e r t  T h e r e f o r e , the d e f o r m a t i o n  i s not  to a s l i p mechanism but i s r a t h e r caused by r e o r i e n t a t i o n of  the  due  T  1  1  F i g u r e 32.  1  1  Typical  1  1  1  stress-strain  1  1  1  curves f o r p o l y  1  1  (a) and  1  single  1  1  1  (b) c r y s t a l  i  1  specimens.  1  1  1  r  - 67  -  3  1 2  4  STRAIN. F i g u r e 33-  Stress vs. induced  strain  martensite  33 by  Busch  .  curve  5 % i n the  a t 2.5°C  thermally  obtained  - 68 -  martensite. to  Only a s m a l l s t r e s s i s r e q u i r e d f o r l a t t i c e  occur i n t h i s  temperature range.  rearrangement  A f t e r d i s p l a c e m e n t of the atoms  from t h e i r e q u i l i b r i u m p o s i t i o n by the e x t e r n a l s t r e s s , they can no l o n g e r r e t u r n to t h e i r o r i g i n a l 2)  Between M  and s  positions.  M,.: r  A v e r y s m a l l a p p l i e d s t r e s s i s n e c e s s a r y to form SIM,  and  the  lower the temperature, the s m a l l e r the a p p l i e d s t r e s s r e q u i r e d .  In  t h i s temperature range, once the m a r t e n s i t e forms, i t i s thermodynamically s t a b l e , and a l a r g e permanent s e t remains w i t h no behavior appearing.  super-elastic  As might be e x p e c t e d , the permanent s e t o b t a i n e d  here i s g r e a t e r than t h a t o b t a i n e d below the M^  temperature,  since  o n l y f a v o r a b l e p l a t e s w i l l be formed on d e f o r m a t i o n , whereas a l l o r i e n t a t i o n s w i l l be formed below M^ the  deformation.  3)  Between M s  and some o f t h e s e w i l l h i n d e r  and A : s  The c r i t i c a l r e s o l v e d shear s t r e s s to induce SIM i n c r e a s e s as the temperature i n c r e a s e s .  At M  g  progressively  the SIM i s s t a b l e , but  as the temperature i n c r e a s e s , the m a r t e n s i t e becomes u n s t a b l e on removal of the s t r e s s , and so the a l l o y s t a r t s  to show enhanced  elas-  ticity. 4)  Above A : s High s t r e s s e s a r e n e c e s s a r y to n u c l e a t e SIM and the m a r t e n s i t e once formed i s u n s t a b l e on removal o f s t r e s s . C l o s e to A , the a l l o y i s s  highly e l a s t i c .  However, as the temperature i n c r e a s e s ,  plastic  d e f o r m a t i o n i n v o l v i n g d i s l o c a t i o n movement o c c u r s e i t h e r i n the m a t r i x or  i n the a l r e a d y formed m a r t e n s i t e , and so the permanent s e t on  removal of s t r e s s becomes p r o g r e s s i v e l y  larger.  -  I t s h o u l d be mentioned  69  -  here t h a t Busch  b e h a v i o r on d e f o r m a t i o n below M  found  super-elastic  as shown i n F i g . 33.  The  material  used by him was  d i s p e r s i o n hardened  i n the m a t r i x .  T h i s made h i s specimens much h a r d e r than those used i n  the p r e s e n t work.  It i s d i f f i c u l t  by h a v i n g f i n e <5 p r e c i p i t a t i o n  to say d e f i n i t e l y why  the 6  p r e c i p i t a t i o n would have such an e f f e c t on the s u p e r - e l a s t i c i t y .  .One  possibility  SIM,  i s that  the p r e c i p i t a t i o n h i n d e r s the development o f  but g i v e s much more f o r m a t i o n o f e l a s t i c m a r t e n s i t e n u c l e a t i n g a t the i n t e r f a c e s between p r e c i p i t a t i o n and m a t r i x . reasons can be g i v e n f o r d i f f e r e n c e s i n the two Formation of SIM  However, no sets of  results.  i s a s s o c i a t e d w i t h the onset o f s e r r a t i o n s o r a  sudden change i n s l o p e o f the s t r e s s v s . s t r a i n c u r v e . and  definite  F i g . 34-(a)  (b) show the s t r e s s at which d e v i a t i o n s from l i n e a r i t y on the  s t r e s s v s . s t r a i n curves o c c u r on l o a d i n g p l o t t e d as a f u n c t i o n o f temperature i n b o t h p o l y - and s i n g l e c r y s t a l specimens. temperature,  the l e s s s t r e s s i s r e q u i r e d  the t e n s i l e c u r v e .  The  lower  the  to i n d u c e s e r r a t i o n s i n  increment of s t r e s s w i t h temperature i n c r e a s e  i s much l a r g e r i n p o l y c r y s t a l m a t e r i a l and 0.04  The  ('v. 0.1  psi/°C f o r s i n g l e c r y s t a l specimens).  psi/°C f o r p o l y c r y s t a l T h i s i s because  grain  b o u n d a r i e s i n p o l y c r y s t a l m a t e r i a l i n t r o d u c e r e s t r a i n t s to m a r t e n s i t e formation.  I t i s f o r the same r e a s o n t h a t SIM  m a t e r i a l o n l y up to 20°C above  but i n s i n g l e c r y s t a l m a t e r i a l i t  forms at l e a s t up to 100°C above the t h a t i n Cu-Zn-Sn and Cu-Zn-Si becomes zero a t M  s  forms i n p o l y c r y s t a l  tempeature.  Pops^^ found  a l l o y s the s t r e s s to n u c l e a t e m a r t e n s i t e  , r a t h e r than a t M,. as found i n the p r e s e n t work. . f ^  p r e s e n t o b s e r v a t i o n s seem more r e a s o n a b l e .  Between M  and M^  some  The  •  Mf  F i g u r e 34.  •  M  s  I  A  8  I  I  I  1  Af  V a r i a t i o n of s t r e s s required  I  Mf  I  I  M  I s  taken from F i g . 22.  A  I 8  I  I  I  I  1  1  Af  to produce SIM as a f u n c t i o n  f o r p o l y c r y s t a l specimen are taken from F i g . 12. are  I  o f temperature.  Results  Results f o r single c r y s t a l  specimen  I  - 71 -  martensite exists p r i o r  to d e f o r m a t i o n .  m a r t e n s i t e an e x t r a d r i v i n g  However, to produce  force i s required.  more  T h i s can come e i t h e r  from the a p p l i e d s t r e s s or by c o o l i n g down the specimen. t i o n of the c u r v e to zero s t r e s s does not come to M  Thus e x t r a p o l a -  , but r a t h e r to s  Fig.  35 and  of tempeature  36 show the amount o f s u p e r - e l a s t i c i t y  f o r p o l y - and s i n g l e c r y s t a l specimens.  as a f u n c t i o n These curves were  o b t a i n e d by s u b t r a c t i n g the r e s i d u a l s t r a i n s a t zero s t r e s s from t o t a l s t r a i n s as shown p r e v i o u s l y i n F i g . 15 and The  M... f  the  23.  e f f e c t o f SIM i s s i m i l a r i n b o t h p o l y - and s i n g l e  crystal  material. A  g  The maximum s u p e r - e l a s t i c e f f e c t o c c u r s around A . Below s the e l a s t i c i t y r a p i d l y d e c r e a s e s u n t i l the temperature r e a c h e s M^,  and below M.. t h e r e i s no change i n the c u r v e . f &  SIM  i s r e v e r s i b l e upon u n l o a d i n g , and  As the temperature since  t h e r e f o r e , the s t r a i n i s e l a s t i c .  r i s e s w e l l above A , g  p l a s t i c deformation occurs.  temperature  range, A  down t o M^,  less stress i s required  g  < T < A^.  At and above A , . s  l e s s SIM w i l l be  induced,  Thus-the maximum e f f e c t  As the temperature to i n d u c e SIM.  d e c r e a s e s below The  w e l l below A ,  r a p i d l y as the temperature  g  approaches  d e c r e a s e s r a p i d l y as the temperature  and T .  A  g  stress vs.  s t r a i n curve f o r s i n g l e c r y s t a l m a t e r i a l c l e a r l y shows the of SIM a t temperatures  i s i n the  the r e v e r s i b i l i t y  reversibility decreases  Therefore, s u p e r - e l a s t i c i t y  decreases.  The low  temperature  thermal m a r t e n s i t e i s h i g h l y d u c t i l e and does not show s i g n i f i c a n t elasticity. The p o l y c r y s t a l m a t e r i a l shows a much s m a l l e r s u p e r - e l a s t i c effect  than the s i n g l e c r y s t a l m a t e r i a l .  i n the morphology of the SIM  T h i s i s due  to a d i f f e r e n c e  and w i l l be d i s c u s s e d i n the next  section.  Mf M A A f s  s  3 Ni-ALLOY  —i -200  1  i  _  -160  -120  -80 TEMPERATURE, °C ^  F i g u r e 35.  '  8  "°  '  K  '°  E f f e c t of temperature on s u p e r - e l a s t i c i t y f o r p o l y c r y s t a l  ,  &  0  specimens.  2  0  6  -  4.2.  74 -  Nature o f SIM In t h i s s e c t i o n p o s s i b l e mechanisms f o r e x p l a i n i n g the s u p e r -  elastic a)  Stress-induced The  be  e f f e c t i n C u - A l - N i a l l o y s w i l l be thermoelastic  discussed.  martensite  and b u r s t - t y p e  martensite  m a r t e n s i t i c s t r u c t u r e formed by thermal t r a n s f o r m a t i o n  will  randomly d i s t r i b u t e d over a l l p o s s i b l e h a b i t p l a n e s i n t h e o r i g i n a l  cubic structure.  I f the t r a n s f o r m a t i o n  i s caused by m e c h a n i c a l  d e f o r m a t i o n , however, the p o s s i b l e o r i e n t a t i o n f o r the m a r t e n s i t i c s t r u c t u r e w i l l be v e r y much r e s t r i c t e d , s i n c e o n l y o r i e n t a t i o n s contribute be  that  t o a r e l e a s e o f a p p l i e d s t r e s s by atomic d i s p l a c e m e n t  will  formed. Direct observations  r e v e a l t h a t two types o f SIM a r e formed on  t e n s i l e loading:- thermoelastic type m a r t e n s i t e .  As d i s c u s s e d  n e e d l e - l i k e martensite i n the I n t r o d u c t i o n ,  have been observed i n some t h e r m a l m a r t e n s i t e s , time t h a t they have been d e f i n i t e l y r e p o r t e d  and b u r s t -  similar structures  b u t t h i s i s the f i r s t  i n deformation-induced  martensite. Before burst-type  martensite  of f i n e n e e d l e - l i k e m a r t e n s i t e s martensite  appears a u t o c a t a l y t i c a l l y , numbers  have appeared.  The n e e d l e - l i k e  appears a t low s t r e s s e s and i s always f u l l y  a limited stress hysteresis  (see F i g . 20). T h i s  r e v e r s i b l e , with  i n d i c a t e d t h a t the  s t r a i n energy a s s o c i a t e d w i t h t h e a p p l i e d s t r e s s always b a l a n c e s the volume f r e e energy of the t r a n s f o r m a t i o n , reduces the volume o f the m a r t e n s i t e . e l a s t i c martensite and  this implies  and r e d u c i n g  the s t r e s s  The h a b i t p l a n e f o r thermo-  i s d i f f e r e n t from t h a t f o r b u r s t - t y p e  t h a t the two m a r t e n s i t e s  martensite,  p r o b a b l y have d i f f e r e n t c r y s t a l  - 75 -  structures.  I t seems l i k e l y  t h a t the t h e r m o e l a s t i c n e e d l e - l i k e  m a r t e n s i t e i s i n t e r m e d i a t e i n c r y s t a l s t r u c t u r e between the b . c . c . m a t r i x and  the orthorhombic  burst-type martensite, s i m i l a r  t r a n s i t i o n structure with a monoclinic unit c e l l m a r t e n s i t e i n Cu-Zn a l l o y s .  i s v e r y u n s t a b l e and  Because o f t h i s unique  i t can be expected  s e n s i t i v e to r e v e r s e s t r e s s .  the p o l y c r y s t a l specimen above the A expected  over a wide temperature  g  n a t u r e of  the  that n e e d l e - l i k e martensite  t h e r m o e l a s t i c m a r t e n s i t e g i v e s some e l a s t i c i t y  r a t h e r than ^ 0.2%  lattice  a l o n g the p a t h to t h e i r u l t i m a t e p o s i t i o n  i n the n e e d l e - l i k e m a r t e n s i t e . displacement,  i n thermal  Hence the atoms i n the b . c . c .  1 0  a r e d i s p l a c e d p a r t o f the way  atomic  suggested  3^  to the  The  to the a l l o y s  temperature  i n a normal m e t a l ) .  f o r m a t i o n of (e.g.,  shows 1.4% The  effect  elasticity occurs  range but i s not v e r y l a r g e s i n c e the amount  of t h e r m o e l a s t i c m a r t e n s i t e f o r m a t i o n i s never more t h a n ~ l % . type m a r t e n s i t e forms at s t r e s s e s above t h a t n e c e s s a r y t h e r o e l a s t i c m a r t e n s i t e and presumably corresponds g r e a t e r atom displacement  Burst-  to n u c l e a t e  to c o n s i d e r a b l y  from the b . c . c . s t r u c t u r e .  type m a r t e n s i t e n u c l e a t e s i t c o n t i n u e s to grow a t low  Once the b u r s t stresses.  The  s t r e s s r e l i e v e d by f o r m a t i o n of the b u r s t - t y p e m a r t e n s i t e i s w e l l below the s t r e s s n e c e s s a r y  to induce n e e d l e - l i k e m a r t e n s i t e and  so a l l of  the  r e v e r s i b l e n e e d l e - l i k e m a r t e n s i t e d i s a p p e a r s at the same time t h a t the b u r s t - t y p e m a r t e n s i t e i s formed. The  l o a d drops o c c u r r i n g i n s e r r a t i o n s i n the s t r e s s v s .  curves are a p p a r e n t l y accompanied by  strain  the f o r m a t i o n and p r o p a g a t i o n o f  14 b u r s t - t y p e martensite..  Cohen  has p r e d i c t e d t h a t an a p p l i e d s t r e s s  w i t h i n the e l a s t i c range might c o n t r i b u t e to the d i s p l a c e m e n t  i n the  -  embryos and thus enable at temperatures  -  them to a c h i e v e  above M  s  n u c l e a t i o n of burst-type a)  76  .  Two  the c r i t i c a l  size for nucleation  mechanisms can be suggested  martensite:-  Embryos h a v i n g p r e f e r r e d o r i e n t a t i o n s w i t h r e s p e c t to the  a x i s c o u l d t r a n s f o r m to b u r s t - t y p e m a r t e n s i t e , i f one the h a b i t p l a n e  f o r n e e d l e - l i k e m a r t e n s i t e and  m a r t e n s i t e are not b)  f o r the  that f o r burst-type  c o u l d a c t as a n u c l e u s  s i n c e as suggested  p o s s i b l y an i n t e r m e d i a t e one the b u r s t - s t r u c t u r e .  indicating  for burst-type  above, the n e e d l e - l i k e s t r u c t u r e i s and would be expected  to a i d f o r m a t i o n of  E x p e r i m e n t a l l y , the b u r s t m a r t e n s i t e  appeared a u t o c a t a l y t i c a l l y martensite,  assumes t h a t  the same.  Needle-like martensite  martensite  stress-  from the densest  always  r e g i o n of n e e d l e - l i k e  t h a t t h i s i s the more p r o b a b l e mechanism f o r  nucleation. The  above o b s e r v a t i o n s o f the e f f e c t of the t h e r m o e l a s t i c  m a r t e n s i t e and b u r s t - t y p e m a r t e n s i t e on s t r e s s v s . s t r a i n curves different b)  temperatures  Mechanism of  are summarized i n the diagram shown i n F i g . 37,  super-elasticity  I t has been shown by X-ray d i f f r a c t o m e t r y , o p t i c a l and h a b i t p l a n e a n a l y s i s t h a t b u r s t - t y p e SIM has s t r u c t u r e as thermal m a r t e n s i t e .  by  I f we 26  Rachinger,  probably  microscopy, the same  assume the s t r u c t u r e of  martensite  determined  martensite  t r a n s f o r m a t i o n can g i v e an abnormal expansion  directions.  at  i t can be shown t h a t the  b.c.c,  in particular  Upon l o a d i n g a: D e f o r m a t i o n o f b . c . c . s t r u c t u r e w i t h no martensite formation, b: E l a s t i c d e f o r m a t i o n o f b . c . c . s t r u c t u r e before n e e d l e - l i k e martensite formation, c: N e e d l e - l i k e m a r t e n s i t e . d: B u r s t - t y p e m a r t e n s i t e . e; D e f o r m a t i o n o f t h e r m a l m a r t e n s i t e .  i  TEMPERATURE in ui or H  i  c  • o o  i  f  g i  A rt— i  ~~—  1  1  M  f  M  A  s  h  Upon u n l o a d i n g f : Untransformed b . c . c . s t r u c t u r e , g: B u r s t - t y p e m a r t e n s i t e . h: Reversed b . c . c . i : Mixed phase w i t h b u r s t - t y p e m a r t e n s i t e and r e v e r s e d b . c . c . j : Untransformed t h e r m a l m a r t e n s i t e .  Af  TEMPERATURE Figure 37.  Schematic  i l l u s t r a t i o n o f the e f f e c t of l o a d i n g and u n l o a d i n g s t r e s s on the  b e h a v i o r o f SIM.  -  Fig. martensite cell  38  '  78 -  shows the orthorhombic  cell  y'~  of the o r d e r e d  ( t h i c k l i n e s ) t o g e t h e r w i t h o n e - h a l f o f an o r d e r e d b . c . c .  ( t h i n l i n e s ) i n approximately  the c o r r e c t  w i t h a ( 1 0 1 ) b a s e b e f o r e t r a n s f o r m a t i o n and transformation.  The  orientation  relationship  a ( O O I ) Y ' base a f t e r  l a t t i c e parameters o f o r d e r e d  orthorhombic  X  Cu Al  F i g u r e 38.  The  orthorhombic  c e l l o f the o r d e r e d y -martensite  (thick  1  l i n e s ) t o g e t h e r w i t h o n e - h a l f of an o r d e r e d b . c . c . ( t h i n l i n e s ) i n approximately relationship.  The  the c o r r e c t  orientation  o t h e r h a l f o f the b . c . c . c e l l  by r e f l e c t i o n i n the p l a n e  (OOl)y'  cell  i s found  -  m a r t e n s i t e a r e a = 5.31  -  79  A, b = 4.22  A, and  c = 4.41  A,  and  t h a t of the  o  o r d e r e d b . c . c . phase i s 5.80  A.  to  understand  the t r a n s f o r m a t i o n  26 mechanism, c o n s i d e r the c e l l shown i n F i g . 39 Fig.  - ( a ) , ( b ) , and ( c ) .  39-(a) shows a u n i t of s t r u c t u r e w i t h a (101)3^ base b e f o r e  transformation.  F i g . 39-(b) and  (c) show the p r i s m a t i c c e l l s  ABCDEFGH i n F i g . 39-(a) i n the 3^ s t r u c t u r e b e f o r e t r a n s f o r m a t i o n and  the y' s t r u c t u r e a f t e r t r a n s f o r m a t i o n , r e s p e c t i v e l y .  and  [111]3^  shears a c t on i t i n (121)3^ and  the a n g l e FGH  from  70°32' to 62°6'.  The  [Ill]8^  (121)3^ p l a n e s  to  decrease  c e n t r a l atom ( A l ) i n the  o  p r i s m a t i c c e l l i s then d i s p l a c e d 0.74 This displacement  A i n the  [100]y  ?  direction.  w i l l o c c u r i n the same d i r e c t i o n on a l t e r n a t e  (001)y'  o  planes.  F i n a l l y , a 0.06  (OOl)y' p l a n e s o c c u r s .  A i n c r e a s e i n the i n t e r p l a n a r s p a c i n g o f I t i s expected  that t h i s process w i l l  occur  on a l l {110}S^planes t o produce s i x p o s s i b l e v a r i a n t s . The  t r a n s f o r m a t i o n from the b . c . c . to orthorhombic  o c c u r s p r i m a r i l y by a shear mechanism.  From the d e t a i l e d t h e o r y o f 40  the c r y s t a l l o g r a p h y o f the m a r t e n s i t e t r a n s f o r m a t i o n t h a t the maximum shear s t r e s s g e n e r a l l y has the h a b i t p l a n e . at 45°  i t i s found  a direction parallel  to  In a t e n s i l e specimen the p l a n e of maximum shear i s  to the a p p l i e d s t r e s s .  Thus the most l i k e l y h a b i t p l a n e of  the m a r t e n s i t e to form w i l l be a t 45°  to the s t r e s s a x i s .  always be a p o s s i b l e h a b i t p l a n e c l o s e to 45°. F i g . 40 where  structure  i t i s assumed t h a t the SIM has  p l a n e as the thermal m a r t e n s i t e . t e n s i l e a x i s were c o n s i d e r e d and  There w i l l  This i s i l l u s t r a t e d i n the same {133}  habit  Twelve d i f f e r e n t o r i e n t a t i o n s of i t can be seen t h a t i n every  t h e r e i s a p o s s i b l e h a b i t p l a n e between 43°  the  case  to 50° w i t h r e s p e c t to the  - 80 -  X = CuAtom  (a) Showing  the u n i t o f s t r u c t u r e i n the 8-^ phase which t r a n s f o r m s to  the y' s u p e r l a t t i c e c e l l . (101) 8  The base o f the c e l l  i s indicated i n  r  4 22A  [210jK (b) Showing  a prismatic  shape o f the c e l l  2-E8A  c e l l ABCDEFGH i n the g  i n the y'  c e n t r a l atom a r e indicated  >T structure.  by d o t t e d  l i n e s . The base o f the c e l l  the dimensions o f the same c e l l  formation. Figure  39.  The f i n a l  s t r u c t u r e and the p o s i t i o n o f the  i n d i c a t e d i s (101) i n the g^ phase and (001) i n . t h e y (c) Showing  tfOO],  1  phase.  as F i g . 6-(a) a f t e r trans-  The c e l l base i s ( O O I ) Y ' -  Schematic i l l u s t r a t i o n o f t r a n s f o r m a t i o n to orthorhombic  martensite.  from the b . c . c .  - 81 -  F i g u r e 40.  I l l u s t r a t i o n showing t h a t a c l o s e to 45° w i t h r e s p e c t  {133}  h a b i t p l a n e i s always  to the t e n s i l e  axis.  - 82  tensile axis. plane  Experimentally  f o r SIM w i t h o n l y one  -  i t has been shown (Table 5) t h a t the h a b i t  e x c e p t i o n l i e s between 47°  to 57°  to  the  tensile axis. It  i s worth emphasizing t h a t a l t h o u g h  t r a n s f o r m a t i o n from the b . c . c .  the volume change i n the  to orthorhombic s t r u c t u r e i s s m a l l  (by  26 calculation 0.1%  1.92%  i n c r e a s e , and  i n c r e a s e ) , t h e r e are ,  i n the p r e s e n t work measured as about l a r g e expansions and  contractions i n  some p a r t i c u l a r d i r e c t i o n s a s shown i n Appendix I . From the data i n Appendix 1 the maximum expansions i n v a r i o u s d i r e c t i o n s are o b t a i n e d is  l a r g e s t a l o n g the  and  summarized i n F i g . 4 1 - ( a ) .  [001]3^ d i r e c t i o n and  s t r e s s a x i s moves from  The  expansion  g r a d u a l l y decreases  [001]3^ i n the s t e r e o g r a p h i c t r i a n g l e .  experiments showed s u p e r - e l a s t i c i t y of up  to 5.8%  as  the  The  independent of  c r y s t a l o r i e n t a t i o n as shown i n F i g . 40-(b) or i n F i g . 29 i n s e c t i o n 3.5.  Approximately  3.5%  e l a s t i c i t y w i l l be  the maximum w h i c h can  e x p l a i n e d i n terms of p r e f e r e n t i a l d i r e c t i o n a l expansion of  the 5.8%  direction).  e l a s t i c i t y cannot be e x p l a i n e d The  the d e f o r m a t i o n  deformation of SIM  (except i n the  with l i t t l e  transformation The two  the  cannot proceed  rest  [001]3^  of SIM must thus be c o n s i d e r e d .  However,  by normal s l i p i n v o l v i n g  d i s l o c a t i o n s , s i n c e a specimen w i t h s t r a i n of 5.8% completely  and  be  recovers  almost  or no permanent s e t when the r e v e r s e y'  3^  occurs.  deformation  mechanism of the SIM  mechanisms can be suggested.  The  s t r u c t u r e must be  first  one  i s due  complex, but  to the a c t u a l  n a t u r e o f the t r a n s f o r m a t i o n . The m a r t e n s i t i c s t r u c t u r e formed 33 i n i t i a l l y c o u l d be a m e t a s t a b l e phase as proposed by Arbuzova e t . a l . , and might t r a n s f o r m  to the s t a b l e f i n a l m a r t e n s i t i c s t r u c t u r e by  the  - 83 -  (65)  (4 3)(4-0)(4-5)  F i g u r e 41.  (29)  , (5-8) %  S t e r e o g r a p h i c r e p r e s e n t a t i o n o f the c a l c u l a t e d expansions  applied stress.  (a) and t h e observed expansions  s i t e s o f t h e new phase. to  a final  load w i l l  (b).  A f t e r d i s p l a c e m e n t o f atoms from t h e i r  p o s i t i o n by t h e e x t e r n a l s t r e s s , they occupy  directional  metastable  new ones on t h e l a t t i c e  I f the l o a d i s removed b e f o r e  transition  s t a b l e s t r u c t u r e , the p a r t o f the c r y s t a l s formed disappear.  The e x i s t e n c e o f t h e m e t a s t a b l e  phase, however, has n o t been c o n f i r m e d . propagation of deformation twins. f i n e twin r e l a t e d s u b s t r u c t u r e .  under  martensitic  The second p o s s i b i l i t y  i s the  The y' m a r t e n s i t i c s t r u c t u r e has a The mechanism i s t h a t twins o f most  - 84 -  f a v o r a b l e o r i e n t a t i o n w i l l grow at the expense of the u n f a v o r a b l y o r i e n t e d ones, as o r i g i n a l l y proposed alloys.  by Read and  As shown i n Appendix I I , an attempt  s t r a i n i n t w i n n i n g i n the y' m a r t e n s i t e , and l a r g e s t r a i n s c o u l d be o b t a i n e d to e x p l a i n the o b s e r v e d  was  Burkart  12  i n In-Tl  made to c a l c u l a t e  i t was  found  the  that very  (up to ^ 4 0 % ) , more than t h a t r e q u i r e d  elasticity.  Thus o n l y a r e l a t i v e l y s m a l l amount  of p r e f e r e n t i a l twin growth would be r e q u i r e d to g i v e the r e s t of the strain. is  The  important  completely  preceded  t h i n g to be emphasized i s t h a t the 5.8%  reversible.  by s l i p  Thus, twin p r o p a g a t i o n s h o u l d not  i n v o l v i n g d i s l o c a t i o n s , and  does not o c c u r r i g h t up  strain be  a l s o the growth o f  to the i n t e r f a c e between the m a r t e n s i t e  m a t r i x but o n l y w i t h i n the m a r t e n s i t e .  Fig.42  5.8%  twins and  shows a p o s s i b l e  orthorhombic  b.c.c. F i g u r e 42.  Suggested mechanism f o r t h i c k e n i n g of s u b - m i c r o s c o p i c i n an accomodation r e g i o n b e h i n d the i n t e r f a c e ( a f t e r Basinski^ ). 1  twins  - 85 -  mechanism f o r i n c r e a s e  i n t w i n w i d t h behind the i n t e r f a c e .  Very  wide twins c o u l d n o t e x i s t r i g h t up t o the i n t e r f a c e , s i n c e  the atomic  mismatch would then become v e r y l a r g e and r e v e r s i b i l i t y would be hindered. c)  Morphology o f SIM i n p o l y -  and s i n g l e c r y s t a l m a t e r i a l .  One o f the major d i f f e r e n c e s is  that  SIM g i v e s  crystal material. and  i n poly-  and s i n g l e c r y s t a l m a t e r i a l  a much l a r g e r s u p e r - e l a s t i c i t y i n the s i n g l e This  i s m a i n l y due t o t h e d i f f e r e n c e i n morphology  behavior of burst-type  SIM.  The v e l o c i t y o f the i n t e r f a c e between  m a r t e n s i t e and the b . c . c . phase a t any i n s t a n t o f time i s dependent upon the magnitude o f the shear s t r e s s . there  In s i n g l e c r y s t a l m a t e r i a l  a r e no r e s t r a i n t s t o the p r o p a g a t i o n o f n e e d l e - and  m a r t e n s i t e , and once formed, o n l y  one b u r s t - t y p e  p r o p a g a t e s under u n i a x i a l s t r e s s . moves smoothly w i t h a p p l i e d  Therefore,  are several  martensite  plate  the i n t e r f a c e boundary  s t r e s s u n t i l t h e whole m a t e r i a l i s  t r a n s f o r m e d as shown i n F i g . 4 3 - ( a ) . there  burst-type  In p o l y c r y s t a l material,  r e s t r a i n t s due t o the g r a i n b o u n d a r i e s .  however,  The v e l o c i t y  of the i n t e r f a c e a t any i n s t a n t o f time i s dependent upon the magnitude of the shear s t r e s s . grains  T h i s w i l l be d i f f e r e n t f o r d i f f e r e n t l y  i n the p o l y c r y s t a l m a t e r i a l .  i n some g r a i n s w i l l be slow. slip  In'unfavorably oriented  occurs before m a r t e n s i t i c  growth o f t h e p l a t e .  Consequently, p l a t e  transformation  Furthermore g r a i n s  the boundary o f t h e c r y s t a l w i l l f o r m a t i o n once s t a r t e d w i l l  motion developed at the boundaries.  propagation  grains,  local  and t h i s p r e v e n t s  further  of d i f f e r e n t o r i e n t a t i o n at  a c t as c o n s t r a i n t s .  be q u i c k l y  oriented  A chain  of t r a n s -  stopped by the r e s i s t a n c e t o  The s t r e s s must be i n c r e a s e d  to  -  86  -  b.c.c. grain-boundary  burst-type mortensite stress direction  parallel interphase boundaries  valley-like bursMype martensite  (a) single-crystal  Figure  43.  (b) poly-crystal  Morphology o f b u r s t - t y p e SIM i n (a) s i n g l e and  (b)  p o l y c r y s t a l specimens.  overcome t h i s r e s i s t a n c e . stress.  The s t r e s s r e q u i r e d  C o n s e q u e n t l y , not a l l g r a i n s  may  be above the f r a c t u r e  completely transform  into  martensite. Another p o i n t which must be d i s c u s s e d type m a r t e n s i t e i n p o l y c r y s t a l l i n e m a t e r i a l Individual burst-type plates  i s the morphology o f  burst-  as shown i n F i g . 4 3 - ( b ) .  form i n such a way  as to r e l i e v e the  m a t r i x s t r e s s which has been s e t up by the a d j a c e n t m a r t e n s i t i c  plate.  - 87 -  r  For example z i g - z a g p a t t e r n s c o n s i s t i n g o f two v a r i a n t s whose s t r e s s f i e l d s c a n c e l one another  o u t , a r e extremely  common.  Thus,  lenticular  b u r s t - t y p e m a r t e n s i t e p l a t e s a r e much more s t a b l e to u n l o a d i n g s t r e s s e s than the s i n g l e m a r t e n s i t e p l a t e s formed i n s i n g l e c r y s t a l m a t e r i a l . Consequently,  s i n g l e c r y s t a l m a t e r i a l shows much g r e a t e r  super-  elasticity.  4.3. Strain-Memory The  s t r a i n memory e f f e c t o c c u r s when a specimen i s deformed i n a  temperature and to  Effect  range where t h e m a r t e n s i t e i s thermodynamically  then heated  stable  t o cause i t to r e v e r t back to the b . c . c . phase and so  i t s o r i g i n a l shape.  The d e f o r m a t i o n  can o c c u r i n two  temperature  ranges, below M^ and c l o s e t o M . g  Below M^,  the s t r u c t u r e i s f u l l y m a r t e n s i t e b e f o r e  I t has been shown i n T i - N i  1 8  and i n T i - N b  1 9  deformation.  that martensite p l a t e s  f a v o r a b l y o r i e n t e d to t h e a p p l i e d s t r e s s grow a t t h e expense o f unfavorably oriented p l a t e s .  The p r e s e n t work showed a s i g n i f i c a n t  change i n m i c r o s t r u c t u r e i n a bent  s i n g l e - c r y s t a l specimen as shown  i n F i g . 31 i n s e c t i o n 3.6., which w i l l  support  When the specimen i s deformed above M s  this idea.  b u t below T , nono  r e v e r s i b l e SIM i s formed w i t h a h a b i t p l a n e c o r r e s p o n d i n g t o the p l a n e s o f maximum shear s t r e s s . to  6% show complete r e c o v e r y .  super-elasticity. orientation w i l l  S i n g l e c r y s t a l specimens s t r a i n e d up Thus the same problem o c c u r s as w i t h  M a r t e n s i t e p l a t e s forming w i t h f a v o r a b l e g i v e .a s t r a i n of 3 to 4% which i s c o m p l e t e l y  reversible.  However, a l a r g e r s t r a i n than t h i s i s r e q u i r e d and as suggested  before  -  88 -  t h i s must come from d e f o r m a t i o n The  temperature  o f the i n d i v i d u a l m a r t e n s i t e p l a t e s £  a t which r e c o v e r y takes p l a c e on h e a t i n g  depends on whether the specimen b e i n g deformed o r i g i n a l l y had t h e m a r t e n s i t i c o r the b . c . c . s t r u c t u r e . occurs at ^ A  i f the d e f o r m a t i o n  g  With u n i a x i a l l o a d i n g , r e c o v e r y  i s c a r r i e d o u t i n an o r i g i n a l l y  b . c . c . s t r u c t u r e , b u t a t a s i g n i f i c a n t l y h i g h e r temperature A ) i f the specimen was o r i g i n a l l y m a r t e n s i t e . g  t h a t i n the former  The r e a s o n f o r t h i s i s  case, o n l y f a v o r a b l y o r i e n t e d m a r t e n s i t e  and g i v e s the n e c e s s a r y d e f o r m a t i o n .  system.  forms  I n the l a t t e r case, however,  u n f a v o r a b l y o r i e n t e d p l a t e s a r e a l s o p r e s e n t and w i l l reorientation.  (10°C >  hinder  There i s thus a much h i g h e r i n t e r n a l s t r e s s i n the  T h e r e f o r e , a l a r g e s t r a i n energy  back t o b . c . c ,  opposing  the t r a n s f o r m a t i o n  i . e . , t h e s i t u a t i o n i s s i m i l a r t o h e a t i n g up a  specimen under s t r e s s i n a t e n s i l e machine, where i t would be found t h a t r e c o v e r y t o the b . c . c . m a t r i x would o c c u r a t temperatures  above t h e  normal A . s Specimens which were bent below M^ began to r e c o v e r a t A  g  r e c o v e r y was complete near A^, thus showing d i f f e r e n t b e h a v i o r t e n s i l e specimens. s t r a i n s i n bending approximately  The reason f o r t h i s i s p r o b a b l y  from  t h a t the average  a r e s m a l l , v a r y i n g from 0 a t t h e c e n t e r to  1% a t t h e s u r f a c e (specimen  c u r v a t u r e =40 mm)  and  t h i c k n e s s = 0.8 mm, r a d i u s o f  and so u n f a v o r a b l y o r i e n t e d m a r t e n s i t e p l a t e s do n o t  hinder reorientation s i g n i f i c a n t l y ,  thus c a u s i n g r e c o v e r y a t lower  temperature. *Note added i n p l o o f : Some q u a l i t a t i v e o b s e r v a t i o n s o f the strain-memory e f f e c t i n C u - A l - N i a l l o y s have j u s t been r e p o r t e d by Otsuka and Shimizu ( S c r i p t a Met., June 19705*4 They suggest t h a t the e f f e c t i s caused by the development of one twinned o r i e n t a t i o n i n the m a r t e n s i t e p l a t e a t the expense of the o t h e r i n agreement w i t h the p r e s e n t i d e a s .  - 89 -  APPENDIX I .  Calculation  o f Expansion and C o n t r a c t i o n Due t o t h e Formation o f  y' Along the P r i n c i p a l P o l e s o f a b . c . c . C r y s t a l . The c a l c u l a t e d  expansions and c o n t r a c t i o n s a r e g i v e n w i t h r e s p e c t  to t h e o r i e n t a t i o n r e l a t i o n s h i p between Plus  and y' i n T a b l e 7.  s i g n s show a d i r e c t i o n a l expansion and minus s i g n s show a c o n t r a c -  t i o n caused by t h e b . c . c . to orthorhombic t r a n s f o r m a t i o n .  T a b l e 7.  D i r e c t i o n a l l i n e a r change by t h e b . c . c . t o orthorhombic transformation  b.c.c.  orthorhombic  [001],[002] [010] [200]  [101] [010] [101]  [015],[0,2,10] [0,10,2] [10,2,0] [501] [2,10,0] [105]  [525] [1,10,1] [525] [203] [1,10,1] [203]  [013],[026] [062] [103] [260] [620] [301]  b.c.c.  orthorhombic  % change/b  2.66 6.15  +6.55 -8.28 +5.69  29.57 29.57 29.57 14.79 29.57 14.79  30.70 27.29 30.70 15.43 27.29 15.43  +3.80 -7.80 +3.80 +4.30 -7.80 +4.30  [323] [161] [102] [161] [323] [102]  18.34 18.34 9.17 18.34 18.34 9.17  19.07 17.09 9.52 17.09 19.07 9.52  +3.98 -6.82 +3.82 -6.82 +3.98 +3.82  [115],[115] [151] [511]  [213] [051] [203]  15.07 15.07 15.07  15.66 13.95 15.43  +3.92  [012],[210] [240] [042] [012] [402] [204]  [111] [141] [141] [111] [103] [103]  6.47 12.97 12.97 6.47 12.97 12.97  6.76 12.25 12.25 6.66 13.41 13.41  5.80°. 2.90 5.80  6.158  -7.43 +2.39 +4.48  -5.55 -5.55 +2.94  +3.39 +3.39  - 90 -  T a b l e 7.  (Continued)  b.c.c.  orthorhombic  b.c.c.  orthorhombic  % change/b.c.c.  [124],[248] [284] [214] [482] [842] [412]  [345] [183] [113] [183] [345] [113]  26. 5 £ 26. 5 13. 3 26. 5 26. 5 13. 3  27. oX 25. 2 13. 6 25. 2 27. 0 13. 6  -1. 89 +4. 90 -2. 25 +4. 90 -1. 89 -2. 25  [135],[135] [153] [315] [351] [531] [513]  [233] [152] [114] [152] [233] [114]  17. 17 17. 17 17. 17 17. 17 17. 17 17. 17  17. 37 16. 36 17. 65 16. 36 17. 65 17. 37  +1. 16 -4. 72 +2. 80 -4. 72 +2. 80 +1. 16  [113],[113] [131] [311]  [112] [030] [112]  9. 62 9. 62 9. 62  9. 89 9. 03 9. 89  +2. 81 -6. 13 +2. 81  [123],[246] [264] [426] [462] [321] [624]  [144] [163] [125] [163] [122] [125]  21. 70 21. 70 21. 70 21. 70 10. 85 21. 70  20. 44 20. 84 22. 20 20. 84 10. 90 22. 20  -5. 81 -3. 96 +2. 30 -3. 96 +0. 46 +2. 30  [112],[224] [121] [422]  [123] [021] [123]  14. 24 6. 48 14. 24  13. 41 6. 78 13. 41  -5. 83 +4. 63 -5. 83  [Oil],[022] [202] [220]  [121] [002] [121]  8. 19 4. 10 8. 19  8. 09 4. 22 8. 09  -1. 22 +2. 93 -1. 22  [122],[244] [442] [212]  [143] [143] [012]  17. 40 17. 40 8. 70  17. 10 17. 10 8. 84  -1. 72 -1. 72 +1. 60  [111],[111]  [011]  5. 02  5. 07  +0. 99  - 91 -  APPENDIX I I . Calculation Twinning Fig.  o f the Maximum Amount o f Deformation  P r o v i d e d by  Alone  44-(a) i l l u s t r a t e s two important  r e f e r e n c e p l a n e s t o be  42 considered i n a twin of the f i r s t  kind.  The shear a s s o c i a t e d w i t h  t w i n n i n g l e a v e s two l a t t i c e p l a n e s u n d i s t o r t e d , i . e . , a l l d i s t a n c e s and a n g l e s i n these p l a n e s a r e l e f t  unchanged by the t w i n n i n g shear.  One  p l a n e i s K^, the t w i n n i n g p l a n e which c o n t a i n s the shear d i r e c t i o n q ^ . The  other i s  before twinning,  a f t e r twinning.  The a n g l e between  the two u n d i s t o r t e d p l a n e s , 26, i s unchanged by t h e t w i n n i n g and i s related  to the magnitude o f the shear s t r a i n .  F i g u r e 44-(a). The r e l a t i o n s h i p s between K , K , n n , and the 9  p l a n e o f shear.  F i g u r e 44-(b) The r e l a t i o n o f sphere and twinned e l l i p s o i d .  -  A convenient  92  -  p i c t u r e o f the t w i n n i n g  process,  e n a b l i n g the c r y s t a l l o -  graphy t o be seen, i s t o imagine a s i n g l e c r y s t a l sphere o f u n i t  radius  43 as shown i n F i g . 44-(b). plane.  A diametral section represents  The upper h a l f o f the sphere i s then d i s t o r t e d  p o s i t i o n w i t h r e s p e c t to the lower h a l f .  the t w i n n i n g  i n t o the twinned  I f s i s the amount o f shear,  the o r i g i n a l c o o r d i n a t e s o f a p o i n t p ( x y z ) on the s u r f a c e o f the upper hemisphere become p'Cx'y'z') by t h e t r a n s f o r m a t i o n :  x' = x y' = y + s z z' = z  where  BB' unit radius  = 2tan  AA' OC  2  AC OC  (90°-26) = 2cot20  (1)  Thus, the r a t i o o f the l e n g t h o f the p o s i t i o n v e c t o r s o f p' and p i s  J_  £  x' +y' +z' 2 2 2 x +y +z 2  =  r  I  (  2  .1/2 '  x +y +2syz-Ks +l)z l / 2 2  =  2  2  2  2  }  x +y +z 2  2  2  - 93 Introducing angular  c o o r d i n a t e s , A and x  y = I cosX  z = a sinx  equation  (2) becomes  =  (1 + 2 s i n x  c o s  ^  + s^sin^x)^^  The maximum e x t e n s i o n and compression i . e . , when \ = x,  as ±s obvious  (3)  a r e o b t a i n e d i n the p l a n e x = 0,  from F i g . 4 4 - ( b ) .  Under  these  conditions  -jj-—  =  (1 + 2s s i n x cosx + s ^ s i n ^ x ) " ^ ^  /#)  The maximum and minimum v a l u e s o f the e x t e n s i o n a r e thus o b t a i n e d by d i f f e r e n t i a t i o n of e q u a t i o n (4)  2 2 s  or  cos  2 x  +  s  s i n 2 x  2 „ 1-tan _x , 2 2s ^ + s 2 1+tan x  =  0  2tanx  = 0 n  2 1+tan x  or *  S  2 1+tan x , hence  tanx  =  (tan  s ±  2 X  - s tan  /s  2  ^  + 4  X  - 1) = 0  (5)  .  (o)  - 94 -  But,  from e q u a t i o n ($f)  jL  =  (  1  +  itanx_  a  +  s  2  t a n ^ l / 2  1+tan x =  n  +  s  from e q u a t i o n  t a n [ 2 + s t a n ] .1/2 1 + tan^x X  v  and  1+tan x  X  W  ;  (JPf ^  2 s tanx = tan x ~ 1  hence  f - = (1 + £ v  -  8  tan [2+s tan ] 2+s-tanx X  X  (1 + s t a n )  1/2  (8)  1 / 2  X  hence the maximum v a l u e o f the e x t e n s i o n i s g i v e n by  Using the f i n a l equation be  (9), t h e maximum s t r a i n by t w i n n i n g a l o n e can  calculated.  30 Greninger  r e p o r t e d t h a t t h e y' phase has a hexagonal c l o s e -  packed l a t t i c e which i s t w i n - r e l a t e d .  The Y ' B - ^ l a t t i c e -  i s summarized s t e r e o g r a p h i c a l l y i n F i g . 45-(a). the important  p o l e s o f the b . c . c .  B  p r o j e c t i o n as t h e s u r f a c e o f p o l i s h .  relationship  On t h i s a r e p l o t t e d  l a t t i c e with the plane o f A l s o p l o t t e d a r e the b a s a l p l a n e  p o l e s o f the f o u r y' o r i e n t a t i o n s c o n t a i n e d i n p l a t e s p a r a l l e l to t h e  (a) P r o j e c t i o n shows b a s a l p l a n e s and a c t i v e t w i n n i n g p l a n e of the f o u r y' o r i e n t a t i o n s c o n t a i n e d i n one s e t of p a r a l l e l m a r t e n s i t e plates. Plane of p r o j e c t i o n i s p l a n e o f set of martensite p l a t e s . Great c i r c l e r e p r e s e n t s c l o s e - p a c k e d d i r e c t i o n s common to both l a t t i c e s  F i g u r e 45.  (b) Procedure to f i n d A ( a f t e r t w i n n i n g ) from Ai ( b e f o r e t w i n n i n g ) . O r i e n t a t i o n A2 i s produced from A i by a r o t a t i o n o f 180° about the normal t o (1011). An a n g l e between A-^ and A i s the A_0A' i n F i g . 43-(b) 2  2  O r i e n t a t i o n r e l a t i o n s h i p between l a t t i c e s o f twinned  y'  a n  ^  matrix b.c.c.  -96-  p o l i s h e d s u r f a c e , as w e l l as the p o l e s of the two a c t i v e twinning planes.  and  i.e., orientation  i s produced  the normal to ( 1 0 1 1 ) .  ( 1 0 1 1)  are twins, with twinning plane  (10 1 1);  from A^ by a r o t a t i o n o f 180° about  T h i s procedure  can be seen i n F i g . 4 5 - ( b ) .  From these c o n s i d e r a t i o n s , we get 180° - 46 = 60° Hence 20 = 60°  I n s e r t i n g 20 i n e q u a t i o n  (1) g i v e s  s = 2cot 60° = 2 x 0.577 = 1.154  Hence from e q u a t i o n (9)  (  Z\ Z 'max  K  n U  1.15[1.15 + ( 1 . 1 5 2  2  +  4)  1 / 2  Kl/2 '  = 1.732 Thus i f y' c o n v e r t s c o m p l e t e l y  from one o r i e n t a t i o n to a  o r i e n t a t i o n , t h e r e i s a 73% i n c r e a s e i n l e n g t h .  T h i s seems a v e r y  h i g h f i g u r e when one c o n s i d e r s t h a t t w i n n i n g i n Zn elongation.  twinned  g i v e s o n l y a 7.4%  However the t w i n n i n g p l a n e s i n Zn and y' m a r t e n s i t e a r e 44  d i f f e r e n t , the amount of shear f o r the y' i s much l a r g e r . same (.10 1 1) and  t w i n n i n g p l a n e and (0001)  Mg  has the  p l a n e as y' m a r t e n s i t e  s = 1.07, and the maximum e l o n g a t i o n i s 28%, so the f i g u r e s f o r y  m a r t e n s i t e a r e not unreasonable.  In f a c t the amount o f e l o n g a t i o n  1  - 97 -  o b t a i n a b l e on deforming the y' m a r t e n s i t e There a r e two twinned m a r t e n s i t e  w i l l be much l e s s than 73%.  o r i e n t a t i o n s present  i n the o r i g i n a l  28 structure to  ) , so t h a t i f t h i s  o n l y one twinned o r i e n t a t i o n , the e l o n g a t i o n w i l l  ^ 47%. to  ( i n the volume r a t i o ^ 2 : 1  transforms  o n l y be i> 24 o r  Furthermore i t i s u n l i k e l y t h a t the t e n s i l e a x i s w i l l  correspond  t h e d i r e c t i o n o f maximum e l o n g a t i o n and so the a c t u a l e l o n g a t i o n  i n the t e n s i l e d i r e c t i o n w i l l  be much lower.  - 98 -  REFERENCES  1.  Hume-Rothery, W. , Raynor, G.V., The S t r u c t u r e of M e t a l s and A l l o y s , I n s t , of M e t a l s , London, (1962) 198.  2.  Masson, D.B.,  3.  K i n g , H.W.,  4.  Brown, L.C., Stewart, M.J., T r a n s . AIME 242  5.  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R e e d - H i l l , R., P h y s i c a l M e t a l l u r g y P r i n c i p l e , D. Van Nostrand Co., New J e r s e y , 1966, p. 404.  43.  H a l l , E.O., T w i n n i n g , B u t t e r w o r t h s S c i e n t i f i c P u b l i c a t i o n s London, 1954, p. 43.  44.  Otsuka, K., Shimizu, K., S c r i p t s Met., 4^ June (1970) 469.  }  

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