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Deformation of the two phase composite Al -CuAl2 Baker, Victor Thomas 1966

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DEFORMATION OF THE TWO PHASE COMPOSITE A l - C u A l  BY V. T. BAKER B.A.Sc.,, The-University of B r i t i s h Columbia, ,1958  •A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR-THE DEGREE OF • MASTER OF APPLIED!.SCIENCE '  in,the Department of •' METALLURGY  "  We accept t h i s thesis as conforming.to the standard required from candidates for the degree of MASTER.OF APPLIED:.SCIENCE.  THE UNIVERSITY OF BRITISH COLUMBIA April,  1966  2  In p r e s e n t i n g t h i s t h e s i s requirements Columbia, for  in p a r t i a l  f u l f i l m e n t of  f o r an advanced degree at the U n i v e r s i t y of  I agree t h a t the L i b r a r y  r e f e r e n c e and s t u d y .  I further  s h a l l make i t  freely  the  British available  agree that p e r m i s s i o n f o r  ex-  t e n s i v e c o p y i n g of t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by the Head of my Department o r by h i s  representatives.  understood t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r cial  g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n  Department of  Metallurgy  The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada Date  M a y 11,  1966  It  is  finan-  permission.  ABSTRACT  The deformation behavior, of the two phase composite"Al-CuAl has been studied at ordinary and elevated temperatures using r o l l i n g and extrusion.  ' P l a s t i c i t y was exhibited.by the C u A l  2  sheath 2  only at  elevated,temperatures and c o n t r i b u t e d . l i t t l e to o v e r a l l deformation.of the composite. - Results have been interpreted i n terms of simple models f o r combined stresses  involving a.hydrostatic  component.  .Examination  of microstructures.of extruded composites indicated that i t may be possible to study, the flow of metals i n deformation processes by the use of suitable  composites.  CONTENTS Page INTRODUCTION  1  .........................  1 6 7  Deformation o f Two Phase A l l o y s Flow i n E x t r u s i o n - Scope o f P r e s e n t Work  EQUIPMENT AND PROCEDURE  9  ..........................................  9 9 10 11 12 13  • P r e p a r a t i o n o f the A l l o y • Sheath R o l l i n g . : . Extrusion: Equipment . E x t r u s i o n Procedure Tensile Testing .......................................... Metallography  ... M - - " " '  RESULTS  Undeformed M i c r o s t r u c t u r e Sheath R o l l i n g . E x t r u s i on T e n s i l e Tests  Ik 1^ 15 18  22  DISCUSSION  C o a r s e n i n g o f the A l - C u A l Composite S t r e s s and Flow i n Composites O b s e r v a t i o n s on Sheath Rolled.Composite .................. E x t r u s i o n Deformation Behavior Comparisons w i t h Other-Work • E f f e c t s of P a r t i c l e Size . . . Tensile Tests ............................................ E x t r u s i o n Flow P a t t e r n ................................... R e p r e c i p i t a t i o n Phenomenon 2  22 22 27 28 31 32 32 36 37  CONCLUSIONS  39  APPENDIX A. . E x t r u s i o n C y l i n d e r Design  k-0  APPENDIX B  h2  REFERENCES  .  kj>  ii  LIST OF FIGURES Number 1.  Page . Steady State E f f e c t i v e Natural Strain Trajectories  2.  Al-CuAl  3-  Extrusion Cylinder Assembly  • h.  2  .... .  . ke  Phase Diagram .  • ^7  As. Cast A l - C u A l ( 6 0 volume # CuAl ) 2  5-  - As' Cast. Showing Lamellae  6.  - A f t e r 2k hr..Soak at 500°C  2  ................... .  h8  .  k8  .....  . h  F i n a l Coarsened Microstructure  8.  F i n a l Coarsened. Microstructure Showing Amoeboid Shaped Phase P a r t i c l e s ................................ . Rolled:  ........................  . kQ  7-  9-  ^5  70$ Reduction at 20°C  9  ^9  . . . . . . . . . . . . . . . . . . . . . . . . . • - 50 .............  .......  50  10.  .Rolled: . 80$ Reduction at 20°C  ll.  • Rolled:  95$ Reduction at. 20°C  ...................  50  12.  • Rolled:  95$ Reduction at 200°C  .........................  51 51  13. 14.  . Extrusion Ram Pressure vs. Temperature  .................  52  15.  • One-half of Butt Cross-Section: 500°C Extruded B i l l e t (Coarsened Material) .............................  53  One-half of Butt Cross-Section: 350°C Extruded B i l l e t (As Cast Material) ................................  5^  .Cross Section of Butt of B i l l e t Extruded, at 350°C from As Cast Material with Superimposed. Natural Strain Trajectory. Diagram, of F r i s c h and Thomsen ...........  55  18.  Location A: 300°C Extrusion from.Coarsened Material . . . .  56  19-  Location B: 300°C Extrusion  56  20.  Location C: 300°C Extrusion  21.  Location D: 300°C Extrusion  .............................  57  22.  Die Entrant Corner: 350°C Extrusion . . . . . . . . . . . . . . . . . . . .  57  16.  17-  ...........................  56  •List-, of Figures  (cont'd)  Number  Page  23.  Die Entrant. Corner: 400°C Extrusion . . . . . . . . . . . . . . . . . . . .  57  2k.  Die Entrant Corner: - k^O°C Extrusion . . . . . . . . . . . . . . . . . . . .  58  25.  • Die Entrant Corner: 500°C Extrusion  26.  Location A: 500°C Extrusion  27.  • Location B: 500°C.' Extrusion  28.  Location C : 500°C Extrusion  - 29.  • Location -D:. 500°C Extrusion  .30. 31. 32.  • Location.D Showing. Details of Microstructure  58 59 59 "... . .59 60 ..........  Tensile Test Results  61  . Microstructure :. Tensile Specimen from..325°C Extruded ;  Rod. (Type A) .33. 34.  35 . 36.  60  Tensile Specimen from k^>0°C Extruded.Rod (Type D)  62 .....  62  Tensile Specimen from..350°C Extruded Rod of As Cast • Material (Type H)  62  . Mean Free Path E f f e c t  63  Matrix. Shear Rupture Zone i n Composite Rolled at 20°C .. 6k  iv  • ACKNOWLEDGEMENT  ' I would l i k e t o thank my research d i r e c t o r , Professor Lund, f o r the help and advice generously provided.in the preparation.of t h i s thesis.  Professor Brown and Dr. Tromans gave h e l p f u l comments and  suggestions on some aspects of the work. .Valuable assistance was received, on equipment f a b r i c a t i o n and procedures  from''P. - Bruin,. P. Musil. and A. L a c i s . • F i n a n c i a l assistance provided by the Defence Research'. 3card •'  Grant No. 7501-02 and National'Research Council Grant.No. A-2449, was g r e a t l y appreciated.  INTRODUCTION Deformation.of Two-Phase Alloys Considerable research e f f o r t has been devoted to studying the mechanical properties of two phase a l l o y s .  -Workers i n this f i e l d are  faced.with d i f f i c u l t i e s due to the number of variables which must be considered.  These are as .a)  follows:  Mechanical properties of the two phases  b)  Volume f r a c t i o n of the p a r t i c u l a t e phase  c)  Size of the p a r t i c l e s  d)  Shape of the p a r t i c l e s  e)  Bond e x i s t i n g between the two phases  As a r e s u l t , i o n l y those systems that permit reasonable control: of the variables, and measurement of them, have been, selected for investigation. Unfortunately t h i s has led .to experiments covering very limited.ranges of important v a r i a b l e s , p a r t i c u l a r l y volume f r a c t i o n .  There has been a  r e s u l t i n g lack of agreement among investigators where generalized conclusions are drawn from.experimental work. An example of t h i s i s the v a r i e t y of expressions which have resulted from attempts by researchers in the f i e l d to generalize the' effects of p a r t i c l e size and spacing on bulk mechanical properties.  The volume  fraction f, and the p a r t i c l e size d, of the discontinuous phase are independent variables. free path,  They may be combined.into such functions as mean  \ - 2d A. = (1-f)  2,3  Gensamer  Ipd = \y^~  2  or i n t e r p a r t i c l e spacing. D  states that y i e l d stress  y 6  s  i •  i s a function of mean free path  2 i n the expression  <5^ = -A log X- + B,. where A and; B are constants.  contributors have developed, expressions using Starr and-Dorn , in 4  in  X  ^  _ 1  6*. = -A  / ^ and. Unckel 2  5  <T^ = Ae~  and'Skoda , by contrast state that  e.g. Shaw,. Shepard, ;  , ( 0 . 0 8 < . n «C 0.15),. Lenel and A n s e l l  X"  in  X ;  Other  B A  + C  Keeler ' 6  7  2 2  and • Meikle John  CY i s a function of volume f r a c t i o n f  8  1  only^particle size d being without e f f e c t . Studies of the deformability of alloys containing a very small volume percent of p a r t i c u l a t e phase were carried'out by Malkiewicz and Rudnik .  Using d i f f e r e n t complex-silica and.alumina inclusions in s t e e l s ,  the r e l a t i v e deformation of the p a r t i c l e s and matrix was investigated with the mechanical properties of the two phases varied by changing the r o l l i n g temperature.  Conclusions were that .these minimal volume non.-metallic p a r t i c l e s  deform, to a degree varying from zero to equality with the matrix.  They found  that•at a fixed, temperature the deformability of the inclusions decreases as t h e i r melting, temperature increases. In contrast,. P i c k e r i n g  10  found that iron: oxide inclusions in steels  became i n c r e a s i n g l y - p l a s t i c as rolling-temperature decreased from.1150  0  to 850°C,  presumably due to the flow stress of the matrix increasing more rapidly than that of the inclusions with cooling. In an excellent p a p e r  11  Warric and. Van Vlack presented-the results  of deformation studies on NaCl-type s a l t • inclusions present up to ^>0 vol. in FCC metal matrices.  'fo  Extrusion was used to provide a superimposed  hydrostatic stress on the inclusions, which was varied by alterations in extrusion r a t i o , die angle, temperature and matrix metal-salt combinations. The  3 authors were able, to produce variation in p a r t i c l e behavior from r i g i d through b r i t t l e fracture, to p l a s t i c flow.  The conclusions of t h e i r  work include: : a) Manner of deformation of NaCl-type salt  inclusions  depends on the hydrostatic stress l e v e l superimposed upon the flow stress. b) . Inclusion deformation and extrusion pressure are e s s e n t i a l l y independent of inclusion s i z e . c) As the die included-angle approached.l80°,  inclusion  elongation near the sample surface became many times ' greater than.the average elongation, due to the very high stresses at the die entrant edge. Work done on systems containing more than ,50$ of an i n t e r metallic p a r t i c u l a t e phase i s quite l i m i t e d . this area was carried-out by Clarebrough  12  Some e a r l i e r research i n  and Clarebrough and P e r g e r . 13  Using natural two-phase alloys including duplex brass and silver-magnesium, they worked with- i n t e r m e t a l l i c contents up to 70$ by volume using wire drawing (to 80$> maximum reduction i n area) as the deformation method. Measurement of r e c r y s t a l l i z a t i o n temperature for the two deformed.phases was used to measure the severity of phase deformation.. The conclusions of t h e i r work were:  a) Below 35 volume "jo i n t e r m e t a l l i c phase, i n i t i a l deformation is carried e n t i r e l y by the metallic matrix.  As deformation  increases towards 75f° reduction i n area, the intermetallic phase deformation approaches the t o t a l matrix deformation.  b) Above 35 volume fo i n t e r m e t a l l i c phase, b o t h phases  deform  e q u a l l y f o r a l l degrees o f t o t a l d e f o r m a t i o n .  K r o c k and S h e p a r d  1 4  l a t e phase c o n t e n t .  a l s o s t u d i e d d e f o r m a t i o n o f composites o f h i g h p a r t i c u U s i n g l i q u i d - p h a s e s i n t e r e d composites o f 58 to- 80  volume °lo t u n g s t e n p a r t i c l e s i n a. W-Ni-Fe FCC a l l o y m a t r i x t h e y s t u d i e d , t h e s t r e n g t h and d e f o r m a t i o n p r o p e r t i e s by t e n s i l e t e s t i n g . . The c o n c l u s i o n s drawn from t h e i r work were:  a) S i g n i f i c a n t d u c t i l i t y i n two phase composites i n which the s t r o n g e r phase c o n s t i t u t e s a major p o r t i o n o f the volume appears t o be dependent on two  factors.  1. The s t r o n g e r phase must be d e f o r m a h l e . 2. The weaker phase must be a b l e t o s u s t a i n w i t h o u t f a i l u r e the h y d r o s t a t i c  stress  component n e c e s s a r y t o promote i t s s t r e n g t h t o t h a t o f the s t r o n g e r o v e r t h e e n t i r e course of deformation.  b) A l t h o u g h the m a t r i x i s l e s s s t r o n g t h a n a t u n g s t e n p a r t i c l e , the s t r e s s - s t r a i n curve of the composite i s independent o f m a t r i x volume p e r c e n t o r mean f r e e p a t h .  The  composite  s t r e n g t h i s determined s o l e l y , by the s t r e n g t h o f t h e s t r o n g e r . phase. c) Tungsten p a r t i c l e s d i s p l a y i n c r e a s e s i n low temperature d u c t i l i t y when deformed w i t h i n t h e composite.  5 In a study of the mechanical properties, of i n t e r m e t a l l i c s , Petty  1 5  c a r r i e d out t e n s i l e tests on a variety of.two phase a l l o y s ,  p a r t i c u l a r l y aluminum-copper, with intermetallic p a r t i c l e contents'up to over 70 volume $.  His e a r l i e r work  to "soften" at about 300°C ., 1  16  suggested.that C u A l  2  begins  Tensile t e s t i n g was carried-out from room  temperature up to ^00°C to observe the e f f e c t of t h i s i n t e r m e t a l l i c " phase softening on the composite behavior.  Results from the aluminum-copper system f o r high volume "jo CuAl  2  content included: a) At temperatures below 300°C b r i t t l e f a i l u r e of the composite occurred,, but at a fracture strength much higher than for the matrix a l l o y . .. C u A l  2  p a r t i c l e s in  the t e n s i l e specimens displayed b r i t t l e cracks in'.the f a i l u r e area. . b) From.: 300° to 400°C b r i t t l e f a i l u r e ceased and d u c t i l i t y increased. c) Above 400°C superplasticity appeared, i n that elongation ^  of the composite-before fracture became very large (up to ^OO/o), much higher than for the matrix a l l o y by itself  . (100$),  :  CuAl  2  p a r t i c l e s displayed some elong-  ation in the composite specimens, but not equal to the composite deformation. . Superplasticity should be discussed b r i e f l y . reviewed the subject quite thoroughly.  Underwood  17  has  The phenomenon' is the appearance  6  of very large elongations (several hundred $) at low flow stress•on-tensile testing.some eutectic and near-eutectic two phase alloys such as Al-Cu and Al-Zn.  Presnyakov (see 17 above) has found that•quenching from the single  phase region was necessary for s u p e r p l a s t i c i t y to occur.  He has postulated  that the complete breakdown of the metastable state requires high temperatures and applied stress of a d e f i n i t e magnitude,, s u p e r p l a s t i c i t y occurring due to the weakening of the atomic bonds taking place during.the reconstruction of the c r y s t a l l i n e l a t t i c e . Petty  1 5  However,, i n the above work,  has demonstrated that s u p e r p l a s t i c i t y appears i n these two phase  alloys when they have been, slow cooled, and attributes the e f f e c t primarily to s l i g h t deformability of the i n t e r m e t a l l i c phase at the elevated temperature. It  should.be pointed.out that the appearance of the super-  e l a s t i c i t y phenomenon is inherently associated with t e n s i l e t e s t i n g only. It  i s not the high p l a s t i c i t y but rather the absence of necking which•allows  elongation to proceed so far without f a i l u r e .  However a mechanism causing  s u p e r p l a s t i c i t y i n t e n s i l e t e s t s , i f operating during other forms of deformation, could result in low flow stress of the composite. Flow in.Extrusion An understanding of the d e t a i l s of flow and stress during the extrusion process was important to the experimental work carried."out by the writer.  A. most f r u i t f u l source of such information i s the excellent study  of stress and deformation i n extrusion carried out by F r i s c h and T h o m s e n ' . 18  They used the established technique of a s p l i t b i l l e t marked with a fine grid which would allow measurement of a l l l o c a l deformations a f t e r  19  incremental motions of the d i e . . Inverted extrusion of  with a c i r c u l a r die  8.2 extrusion r a t i o and l80° entrant angle was used.  Complete studies  were carried out at room temperature on pure lead and pure aluminum respectively.  For each metal the e f f e c t of extrusion rate on flow, pattern  and wall, pressure was observed and compared with the • theoretical, predictions of the Levi-Mises equations.  From t h e i r calculations they plotted the  pattern of steady state e f f e c t i v e natural s t r a i n t r a j e c t o r i e s . . ( s e e  Fig.l)  Conclusions of t h e i r work which are of interest here are: a) Flow patterns for pure lead and pure aluminum were- i d e n t i c a l , hence the occurrence of s t r a i n hardening did not e f f e c t the flow pattern.  Apparently.the flow pattern i s independent  of the flow properties of the metal being extruded.and i s only influenced, by geometry and the degree of external friction. b) E f f e c t i v e s t r a i n rates and t o t a l strains were highest at the corner of the die. c) Chamber wall pressure was equal to the applied ram pressure less the flow stress of the metal. - Scope of Present Work It w i l l be noted from the above resume of work previously done in. the f i e l d o f composite deformation, that,,.with the exception of Warric and Van V l a c k , no one has attempted to study the effects of superimposed 1 1  hydrostatic pressure on the deformation of composites containing a b r i t t l e second phase. - Work has also been concentrated.on systems with well below  8 50 volume $ b r i t t l e phase.  The experiments of Clarebrough and P e r g e r ' 1 2  1 3  on. high volume $ b r i t t l e phase was very limited due to fracturing of the composite in tension during wire drawing. • It was therefore decided that deformation studies of a twophase system of t h i s type, with a high volume f r a c t i o n of particulate' phase, would prove i n t e r e s t i n g .  The Al-CuA.12 system (see Fig.2) was chosen-for the  following reasons: .a) The eutectic i s about. 60 volume $ i n t e r m e t a l l i c . b) The eutectic structure would result i n uniform d i s t r i b u t i o n of the two phases macroscopically. c) Petty's t e n s i l e  1 5  and hardness  16  studies are available for  t h i s system. d) Melting-and casting procedures are convenient. e) . The melting temperature (548°C) is low enough to permit study of the phase-softening .effects of heating over.a  1  temperature range convenient.for equipment and handling. • It was decided to use a composition s l i g h t l y hypereutectic in  copper to produce a few. coarse- primary throughout the composite.  phase p a r t i c l e s randomly scattered  This would permit the d i r e c t comparison of the  deformation behavior of large p a r t i c l e s with small.  9 EQUIPMENT AND PROCEDURE Preparation of the A l l o y : The a l l o y , 33.6 weight $ Cu, was made from 99.97$ pure aluminum (Alcan "Super Purity") and 99-99$ pure copper (Metals  Disintegrating  Corp.) melted i n a graphite crucible under a KCl-NaCl flux and poured at 58O C into a cold 2 x. 2 x k graphite mold. . Two castings,, one each from 0  two heats were produced. A f t e r removing, a portion f o r l a t e r studies the castings were soaked, f i r s t at  5OO  0  C for 2k hours, then at 530° C for a period of six  days to produce the f i n a l coarsened microstructure.  C y l i n d r i c a l b i l l e t s were  machined from,the soaked castings. Sheath R o l l i n g : B i l l e t s of the coarsened.composite 1.5 inches long were pressed into the center of 3-5 inch lengths of stainless I.D.  and 0.050 inch wall thickness, for r o l l i n g .  s t e e l tubing, 0.5 inch The dual purpose of the  s t e e l sheath was: a) To constrain a x i a l and l a t e r a l flow of the composite, thereby providing a hydrostatic pressure superimposed upon the r o l l i n g  stresses.  . b) To provide insulation against heat loss during elevated temperature r o l l i n g .  R o l l i n g at room temperature was done at a low speedj35 fpni, to reduce f r i c t i o n a l heating of the workpiece.  As an added.precaution, specimens  10 were cooled- in water a f t e r each pass through the r o l l s . . By contrast,  rolling  at elevated.temperature was carried-out at a higher speed (14-5 fpm) to m i n i mize temperature losses by reducing the period of contact between the hot workpiece and the cold r o l l s .  Samples were prepared for microscopic examination by sectioning at the center of the rolled.specimens,, p a r a l l e l to the r o l l i n g d i r e c t i o n . . Extrusion:. Equipment The extrusion equipment was designed, with: a) inverted extrusion to reduce maximum ram pressure b) die entrant angle of l80° (because of interest in the resulting flow pattern) c) bore size of 0.75 inches In order to study the extrusion behavior of the composite over the widest temperature range, requirements placed on the s t r u c t u r a l materials chosen included: .a) A b i l i t y of the extrusion cylinder assembly including cylinder, die, ram and plug to withstand prolonged heating up to 530° C without loss of mechanical properties or oxidation damage. This would permit heating the entire assembly with the b i l l e t i n s t a l l e d and hence avoid temperature gradients. - (This rendered unsuitable most steels used for commercial extrusion since usual i n d u s t r i a l practice i s to place a, heated-.billet into cooler extrusion equipment to reduce heat damage to the • assembly).  11 b) High ram pressures.available at a l l operating temperatures without resorting to complex design. .c) Resistance to g a l l i n g and seizure of sliding-contact  surfaces  The s t e e l f i n a l l y selected for cylinder l i n e r , dies, ram and "'plug, was a'T-15 "type high-speed t o o l s t e e l (Atlas "Sabre") containing ;  C-1.25,. W-10-.0 ,Cr-4„75,- Mo-2.5, .V-4.3, Co-5.5. r  This a l l o y has a hardness of  Rockwell. C'63 retained up to 540 C when tempered twice .for one,hour at 600°C . 9  Because of high, carbide content.it is very.resistant to g a l l i n g and wear.  The ram, with a y i e l d strength of about 340,000 p s i in compression, . permitted a- design"maximum ram pressure of 300,000 p s i (with careful a l i g n ment to assure a x i a l loading). • The complete assembly is. shown in F i g . 3 with d e t a i l s of the design of the composite! cylinder discussed, in-Appendix A. .The design proved .successful, being operated, at up to 290,000 p s i ram pressure and up.to 500°C without f a i l u r e or deterioration.  Wear resistance of dies and cylinder bore  was excellent. - Extrusion Procedure A preliminary heating and cooling rate study of the cylinder assembly was carried out.  It was placed in a box furnace set at 500°C with  a thermocouple attached, to the c o n t a i n e d . b i l l e t .  Seventy-five minutes were  required for the b i l l e t temperature to come within 5°C of the c o n t r o l l e r temperature.  On.removal of the assembly from the furnace f i v e minutes elapsed  before any drop in b i l l e t temperature could.be detected.  For each extrusion  12  run thereafter l / ^ hours furnace soak at the set temperature was carried 1  out without further b i l l e t temperature measurements.  Upon removal from the furnace the assembly was covered with an asbestos jacket and i n s t a l l e d on an insulated.plate i n a 100-ton hydraulic jack.  Alignment was checked and extrusion c a r r i e d out, the entire i n t e r v a l  being about three minutes.  Ram. t r a v e l was l i m i t e d by a c o l l a r such as to  leave a butt i n the cylinder s u f f i c i e n t l y long to permit observation-of flow .patterns'.--during extrusion.  • Several l u b r i c a n t s , including graphite flake and carbowaxes were tried.  None proved s a t i s f a c t o r y and most of the extrusion work was carried  out without l u b r i c a t i o n .  A l l r e s u l t s reported were from an extrusion die of 1 2 : 1 reduction r a t i o .  Tensile Testing Extruded rod, from each temperature run was machined into c y l i n d r i c a l t e n s i l e specimens of the.shape and dimensions i n inches shown below. 9  .82r  .  o  F  JL  CM OJ o  T 0..9.  Because of the low t e n s i l e d u c t i l i t y of the a l l o y at room temperature, the large shoulder radius was used to reduce stress concentration effects to a minimum.  The grips  were designed to bear on.the specimen only near the  13 edge o f t h e shoulders, t o a v o i d d i s t u r b i n g . t h e h i g h e r s t r e s s e d reduced portion.  The specimens were not f u r t h e r p o l i s h e d as t h e e x c e l l e n t  a b i l i t y o f t h e a l l o y produced a v e r y smooth as-machined  A l l t e s t i n g was c r o s s head speed- o f 0.1  machin-  surface.  c a r r i e d out on an I n s t r o n T e s t i n g machine a t a  in/min.  Metallography A l l specimens were p r e p a r e d f o r e x a m i n a t i o n i n t h e same manner. S e c t i o n i n g was  c a r r i e d out on an a b r a s i v e c u t - o f f wheel f o l l o w e d by rough  g r i n d i n g . o n a wet b e l t .  . C y l i n d r i c a l specimens were p r e p a r e d so as t o expose  a s e c t i o n a t t h e c e n t r e - l i n e . Kerosene l u b r i c a t i o n was used, on a b r a s i v e papers and t h e f i n a l p o l i s h was w i t h 0.05 on a l a p p i n g w h e e l .  micron. ( L i n d e B) a l u m i n a and w a t e r  E t c h i n g was w i t h K e l l e r s E t c h w i t h 15 second immersion.  P h o t o m i c r o g r a p h y was . P o l a r o i d film:asa200.  c a r r i e d out on a -Reichert m i c r o s c o p e u s i n g  Ik RESULTS Undeformed:Microstructure The structure as-cast and a f t e r p a r t i a l and complete coarsening i s shown in Fig.k to 8 .  It may be noted on heating,, although the eutectic  lamellae have broken up and substantial coarsening has occurred, the .©.-phase has not-tended to spherodize.  The amoeboid, p a r t i c l e shape results from the  strongly l o c a l l y oriented l a t t i c e structure of the eutectic p l a t e l e t s (see  Discussion).  The p r i m a r y © , -phase dendrites in the as-cast material are seen to have survived i n the coarsened.structure as larger p a r t i c l e s scattered amongst uniformly f i n e r ones. Sheath R o l l i n g Reduction i s defined.here i n terms of the o r i g i n a l composite b i l l e t diameter less the measured thickness of the r o l l e d .composite. 95$ reduction means that the r o l l e d composite thickness is 5$  0  Hence  ? "the o r i g i n a l  b i l l e t diameter. The constraining e f f e c t of the stainless extensive reduction even at 20°C (room temperature).  s t e e l sheath permitted Figures 9 to H  show  the e f f e c t of progressively increasing reduction at 20°C on the microstructure. It up.  is seen that at 70$ reduction the large -©-phase p a r t i c l e s have been broken By 80$,.average p a r t i c l e size is smaller and shear rupture of the matrix  has appeared,(see  App.B).  Between 80 and 95$; p a r t i c l e size has remained,almost but shear rupture of the matrix has continued. the C u A l  2  composite.  unchanged  No p l a s t i c deformation of  has apparently occurred even at t h i s severe deformation of the  15 Proceeding t o Figures 12 and 15 the microstructure r e s u l t i n g from 95$ reduction a t 200°C and.300°C r e s p e c t i v e l y , i s shown. •Shear rupture of the matrix has ceased, at 200°C but even a t 300°C there i s no appearance of elongation i n the -0- -phase p a r t i c l e s . R e l a t i v e average p a r t i c l e s i z e was estimated by averaging the number, of phase boundary i n t e r c e p t s per u n i t length of a large number of traverses of each micrograph.  Although the-©-phase  p a r t i c l e s are not  s p h e r i c a l , the values should be representative of r e l a t i v e p a r t i c l e s i z e s . These are l i s t e d . b e l o w . i n Table I along with r e l a t i v e average number of p a r t i c l e s per u n i t volume ( o<C d ) 3  TABLE I R e l a t i v e P a r t i c l e Size  R e l a t i v e No. P a r t i c l e s Per Unit Volume  20°C - 70$ reduction  100  100  20°C - 80$ reduction  •85  160  20$C - 95$ reduction  84.5'  166  200°C - 95$ reduction  120  58  300°C - 95$ reduction  165  22  Extrusion With the exception of the 500°C extrusion run, pressure  behavior  during ram t r a v e l was the same. The pressure reached a peak a t which flow began t o occur, then i t q u i c k l y dropped o f f , reaching a steady value s l i g h t l y  16 below,the i n i t i a l peak, pressure.  Both pressures are reported in Figure 14  for each extrusion.temperature. .The r i s e i n pressure at 500°C is probably attributable to flow around the outside of the die which began to occur at this temperature. Most metallography•was  carried-out on the butt of the b i l l e t  remaining in the extrusion cylinder.  In each case t h i s butt was sectioned  through the center for examination. To display the general deformation pattern of the extruding a l l o y , two composite photomicrographs were prepared of half of the entire butt cross sections, one for the coarsened material extruded at ^>00°C and the other for as-cast material extruded at 350°C.  These are shown in Figures 15 and 16.  The flow pattern i s p a r t i c u l a r l y clear i n the as-cast material. This is due to areas of orderly lamellae progressively breaking up.into small fragments of C u A l  2  in the oC  matrix, and hence losing the "sheen" which i s  c h a r a c t e r i s t i c of p e a r l i t i c structures.  Also contributing i s the realignment  of the primary -9- -phase p a r t i c l e s in a d i r e c t i o n of flow with some p l a s t i c deformation apparently occurring. The section from which Figure 16 was obtained.was re-photographed in i t s entirety using oblique l i g h t . - This i s shown i n Figure 17 with the pattern of natural s t r a i n t r a j e c t o r i e s of F r i s c h and Thomsen ( F i g . l ) ( f o r a s l i g h t l y d i f f e r e n t extrusion,ratio)  superimposed.  -To observe the changes in the microstructure with progressive increases in composite deformation the sectioned butt of the coarsened  17 m a t e r i a l extruded a t 300°C was traversed from rear face t o the corner a t the die entrance i n four equi-spaced micrographs as shown i n the sketch below.  (C)  1  These are i l l u s t r a t e d - i n Figures. 18 t o 21. - Proceeding.from the undeformed area a t A through t o C, there appears the i n i t i a t i o n of f r a c t u r i n g of the C u A l  2  p a r t i c l e s , but l a r g e r ones  have not been broken down. At D, the corner of the die entrance, where deformation has reached the maximum, extensive f r a c t u r i n g has occurred and no large p a r t i c l e s remain.  N e g l i g i b l e p l a s t i c deformation of the-©-phase  has apparently taken place even a t t h i s very high bulk deformation. The e f f e c t of increasing temperature on the structure i n t h i s die corner entrant area i s shown i n Figures 21 t o 25- There i s a smooth progressive change i n appearance of the-©-phase. At 350°C-400°C p l a s t i c deformation of -©-first becomes n o t i c a b l e . -At 450°C and 500°C t h i s deformation becomes i n c r e a s i n g l y marked and at the l a t t e r temperature has reached approximately 4.5 t o 1 elongation. •A traverse was a l s o c a r r i e d out on the 50O°C e x t r u s i o n butt according t o Figures 26 t o 29- The undeformed structure at A has changed at B, where the C u A l  2  p a r t i c l e s have been n o t i c a b l y elongated i n the flow  18 direction.  This e f f e c t increases at C, the arms of the C u A l  becoming folded and fused together.  2  particles  This folding and elongation produces  the lens-shaped p a r t i c l e s observed at D, the die entrance. At D has appeared a form of r e p r e c i p i t a t i o n not seen i n areas with less bulk deformation. . Small p a r t i c l e s of C u A l the oc  2  are scattered through  phase matrix and attached to the l a r g e r . p h a s e p a r t i c l e s .  This area  i s shown at higher magnification i n Figure JO. . Returning to Figures 22 to 25 the evidence of r e p r e c i p i t a t i o n can be seen to increase progressively with increasing temperature at the same composite deformation.  Tensile Tests Below are l i s t e d the types of t e n s i l e specimens which were prepared, grouped according to thermal history. Group I:  from rod extruded from coarsened material, machined,  then soaked at 250°C for 1 hour. Type A - 325°C extrusion temperature Type B - J50°C extrusion temperature Type C - 400°C extrusion temperature Type D - h^>0°C extrusion temperature Type E - ^>00°C extrusion temperature Group II:  from rod extruded from as-cast material, coarsened  s l i g h t l y by soaking for  1  /  2  hour at 550°C followed by cooling i n a i r to  •20°C to supersaturate the O C  _ phase; then machined.  . Type F - 350°C extrusion temperature Type G - 400°C extrusion temperature  19 Group III:  as group- II  but, machining was followed, by a  250°  soak  for 1 hour. Type H - 350°C extrusion temperature Type J -400°C extrusion temperature Aluminum-copper alloys are known to age at room temperature, and to rapidly overage at temperatures above 200°C.  Groups I and III  were heated  to 250°C since t h i s i s s u f f i c i e n t l y high to uniformly overage the matrix a l l o y but not so high as to increase the s o l u b i l i t y of copper i n the o C significantly  (see Figure 2).  phase  Hence no appreciable change i n - © - phase p a r t i c l e  size or shape would be expected to occur.  The results of t e n s i l e testing are shown i n Table III Typical microstructures are shown i n Figures 32 to 35-  and Fig.31.  The rod produced by  the 300°C extrusion could not be subjected to t e n s i l e t e s t i n g as i t was severely " a l l i g a t o r e d " and cracked. A l l t e n s i l e specimens f a i l e d i n the region of constant section and the results were notably reproducible considering the inherently low d u c t i l i t y of the a l l o y s . The o C -phase mean free path transverse to the specimen axis (  z\ T  ) was determined from the micrographs by averaging transverse phase  boundary intercept counts. . If N i s the number of phase boundaries/inch at 500 magnification, and the X T  o C -phase comprises O.k volume f r a c t i o n , then  can be estimated from;  The resulting values are in Table- l i t  . TABLE Type A  8.6 /*  Type D Type H  1.8/  21  TABLE I I I Specimen Type and Number  Al A2  Length between • Shoulders (ins.)  Diameter (ins.)  Area (ins. ) 2  Fracture Load (lbs)  Fracture Stress (psi)  A3  0.957 0.953 0.963  .0992 .0998 .1008  • 772 .780 .798  .228 185 238  29,500 23,700 29,800  Bl B2 B3  0.885 0.953 0.953  .1002 .0968 .0964  .789 • 734 • 732  .223 214 210  28,300 29,200 28,700  Cl C2 C3  0.952 O.965 • 0.955  '..0982 ,0977 ,0970  • 757 .748 • 738  209 215 210  .27,700 28,800 28,500  DI D2  0.904 0.888 0.918  .0997 .0994 .1002  .780 • 776 .788  270 255 -272  34,600 32,900 34,500  E3  0.927 0,910 0.943  .1001 .1063 .1062  .786 .890 .889  255 282 284  32,500 31,700 32,000  Fl F2  0.939 0.936  .1000 .1011  .785 .885  416 388  53,100 49,900  Gl G2  0.951 0.916  .1090 .1080  • 932 .917  495 504  53,100 54,900  HI H2  0.960 0.950  .0915 .0991  .660 • 773  300 327  45,500 42,400  Jl J2  0.925 0.912  .1073 .1075  .905 .911  415 • 405  45,800 44,500  D3 El E2  A  Estimated.from non-linear portion of load-elongation curve on chart  Elon- ^ .. t gation  CD cn cn  13 O H  P3  o ct  DISCUSSION Coarsening of the A l - C u A l  2  2  2  Composite  Graham, and K r a f t  2 1  and i t s coarsening behavior.  have studied,the microstructure of t h i s a l l o y Using d i f f r a c t i o n techniques they found that  the strongly orientated l a t t i c e structure of the cast lamellae persisted during coarsening. . If,  however, freezing rate is f a i r l y rapid (such as used  .to produce the experimental material'here) the residual orientation.effects are conf ined. to portions of the highly coarsened•particles, r e s u l t i n g i n a reluctance to spheroidize. In addition, they established.that l i t t l e interconnection of the phase p a r t i c l e s remains when the eutectic has been highly coarsened. Hence i t is reasonable to assume that, macroscopically, the coarsened composite p r i o r to deformation is e s s e n t i a l l y isotropic with disconnected  phase p a r t i c l e s in an  o o -phase matrix.  Stress and Flow in Composites When stresses acting at a point in a.metal are other than pure tension or compression, p l a s t i c flow can no longer be predicted from the maximum t e n s i l e or compressive stress.  Mathematical analysis based on the  concept of the metal being an isotropic continuum has produced, a number of equations attempting.to predict p l a s t i c flow under combined stress.  .The most  successful has been the VonMises or Strain Energy C r i t e r i o n , which expressed . i n i t s most general form i s :  do  -  '  -  ^  xy  L  y 2  . (1)  23  where ^  ^ j_ . and 7~* ,• *  is the flow stress and  0  are the t e n s i l e and shear  stresses acting at the point i n question when any arbitrary- orientation of cartesian coordinate system i s referred t o . It  can be shown.that an orientation of the coordinate system exists  where no shear stresses act,.and the t e n s i l e stresses become the p r i n c i p a l stresses,  (1)  then becoming:  1  £ o  =  (  ^  1  "  6  I  j  )  +  i  6  l  l  ~  6  1  1  1  )  2  +  (  ^ -III-cil  J  ) 2  . . . . . . (2)  Inspection of (2) indicates that the addition of a common quantity (hydrostatic stress) to each ^ ^ ^  o-  on the right w i l l not change the value of  Hence i t i s only-the v a r i a t i o n among the p r i n c i p a l stresses which  causes flow, any superimposed hydrostatic pressure having, no e f f e c t . This prediction has been experimentally demonstrated, to be correct for r e a l metals. When stresses are applied to a two phase a l l o y  causing,plastic  flow of the body, differences in mechanical properties between the two phases profoundly a f f e c t - t h e i n t e r n a l stresses. .The e f f e c t of a superimposed hydrostatic stress i s therefore no longer n e g l i g i b l e .  Consider a composite comprising a d u c t i l e metal matrix and. a large volume f r a c t i o n of a p a r t i c u l a t e phase.  Six mechanical properties of the  system.are important i n c o n t r o l l i n g the deformation behavior of the composite.  2k  . 1. 2. • 3.  flow strength of the p a r t i c l e phase shear rupture•strength of the p a r t i c l e phase flow strength of the matrix  k. . shear rupture strength of the matrix 5.  t e n s i l e fracture strength of the p a r t i c l e phase  6.  interphase bond.strength  If a hydrostatic stress i s superimposed upon the stresses  causing  flow to make' a l l p r i n c i p a l stresses compressive then t e n s i l e fracture of the p a r t i c l e s becomes energetically impossible.  If  the hydrostatic stress i s  s u f f i c i e n t l y high,.no voids can develop as they w i l l immediately b e . f i l l e d by flow of the matrix.  Interphase bond, strength becomes unimportant.  The  remaining important mechanical properties are:  • 1-  o  (particle)  2.  - S-^  (particle)  3.  ^  "  (matrix)  k.  . Spj  (matrix)  0  The deformation behavior r e s u l t i n g from different r e l a t i v e values of these properties can be predicted from the following s i m p l i f i e d models, where i t i s assumed that the p a r t i c l e s are i s o t r o p i c and equiaxed.and the matrix i s  isotropic. ^ 'o higher than  -^High  0  "  High shear stress transmitted to the p a r t i c l e s by the matrix on bulk deformation w i l l rupture the p a r t i c l e s in shear without p l a s t i c a l l y deforming them.  25 Compressive p r i n c i p a l stresses of d i f f e r e n t magnitude result in shear stress acting on the p a r t i c l e .  This sheer stress acting, i n mutuallyperpendicular directions causes the rupture of the p a r t i c l e s as shown in A and B to occur simultaneously.  \B - r e s u l t i n g i n C«equiaxed smaller p a r t i c l e s stretched out i n d i r e c t i o n ' of composite elongation.  Fracturing of the p a r t i c l e s i n . t h i s manner would r e l i e v e the l o c a l shear stress b u i l t up by deformation.  Continuing bulk'deformation would repeat  the process - on each f r a c t u r e d . p a r t i c l e , tending to steadily, reduce average particle size. If  the matrix i s being cold worked i t s flow stress w i l l steadily  increase u n t i l i t s shear rupture strength i s exceeded.  Progressive bulk  deformation w i l l cause progressive shear rupture in the matrix, without  26 further f r a c t u r i n g and size reduction, of the p a r t i c l e s away from .the matrix shear rupture regions.  UJ-  Low  ^ o  In the above model, the e f f e c t of lowering  ^  0  would be to  reduce the shear stress imposed upon the p a r t i c l e s and, hence reduce f r a c t u r i n g rate during bulk deformation.  For the same f i n a l bulk deformation,  average p a r t i c l e size would be larger.  .  £  •  o  /  lower than S  , N  J "  -^High . £  0  Particles, w i l l yield, rather than fracture under-the high transmitted shear stress and w i l l elongate i n the d i r e c t i o n of bulk elongation. Low  £ o  P a r t i c l e s w i l l not fracture and, l i t t l e elongation w i l l occur since shear stress transmitted to the particles, w i l l be-low. Without hydrostatic stress composite deformation results from combinations of p r i n c i p a l stresses which inc/Lude t e n s i l e stress, and. two further modes of p a r t i c l e - m a t r i x behavior are possible.  Consider the previous  composite model subjected,to pure t e n s i l e stress: -Low -tensile rupture strength i n p a r t i c u l a t e phase with good interphase bond:  «^* ^-^ rr  I  27  Interphase bond, prevents separation near, crack and hence i n h i b i t s necking of matrix. - Interphase bond, poor r e l a t i v e to t e n s i l e rupture strength of particles:  Separation.of the two phases could occur permitting l o c a l necking of the matrix. Observations on Sheath Rolled.Composite The deformed microstructures can be examined with the foregoing analysis i n mind.  The restraining e f f e c t of the s t e e l sheath during r o l l i n g  has produced high hydrostatic stress superimposed upon the bulk, flow stress. No p u l l i n g "apart of-0- phase and matrix has occurred, indicating that a l l p r i n c i p a l stresses are compressive. At 20°C the  6>o  (c>^ phase) is high r e l a t i v e to S-^  the r e s u l t i s that extensive shear rupture of the C u A l  2  (-©- phase);  phase occurs, producing  equiaxed fragments, the average size of which diminishes u n t i l about 80fo reduction is reached. matrix so that  g o  This degree of bulk deformation has strain-hardened the  has reached'b^.  This i s evidenced by the i n i t i a t i o n of  rupture zones in the matrix.,(Figures 9 and 10)  2 8  Further "bulk deformation.to 9 5 $ produces further matrix shear rupture but no further composite deformation away from the matrix rupture zones ( F i g u r e " 1 1 ) .  This i s shown by the-©- phase p a r t i c l e size being e s s e n t i a l l y  unchanged between 8 0 and 9 5 $ reduction (Table  I).  -Complete absence of p l a s t i c deformation of C u A l at  20°C  S'  R  is lower than •At  200°C  (o  2  indicates that  Q.  r o l l i n g temperature matrix rupture no longer appears.  This indicates that dynamic recovery and r e c r y s t a l l i z a t i o n of the oC- phase must.be taking place, preventing  U  0  from reaching  . Oo  i  s  becoming  s t r a i n - r a t e sensitive. • In.the temperature range  200°C  through  3 0 0  C effective  o  at  0  this s t r a i n rate is dropping and hence the shear stress imposed.upon the-6phase p a r t i c l e s i s also dropping.  A lower fracturing rate is the r e s u l t , as  evidenced by. the f i n a l - © - p h a s e p a r t i c l e size increasing.with temperature. (Table I and Figures • At  1 2 - 1 3 ) .  must s t i l l be lower.than R p l a s t i c deformation of t h i s phase has appeared. 300°C  S  Q  since no evidence of  Extrusion Deformation Behavior It  i s desirable to f i r s t consider the macroscopic- deformation  behavior of the composite during extrusion.  From:Figure 1 7 i t i s clear that  the composite and pure metal flow patterns are at least very similar.  Hence  i t i s j u s t i f i e d - t o estimate l o c a l deformation i n t e n s i t y in the composite from the F r i s c h and Thompsen figure.  29 For 8.2.:1 extrusion r a t i o , deformation i n the die corner entrance area reached  4, equivalent to about 50:1 elongation.  -With  the present work, the 12.1 extrusion r a t i o used.would r e s u l t i n a maximum bulk elongation  > 50:1.  The composite microstrifctures observed i n the die  corner area (Figures 21 to 25) at d i f f e r e n t temperatures can be examined with t h i s bulk-elongation i n mind. From,!350°C where the f i r s t p l a s t i c behavior of the-©-phase becomes evident, there i s a steady increase in •©-phase elongation, reaching about,4.5:1 at 500°C.  This i s a.small f r a c t i o n of the bulk elongation,  indicating that no relationship exists between the deformation of the two phases over, t h i s temperature range. lowering of  g  Q  There would be expected a progressive  with increasing temperature.  However, t h i s i s  obviously  being offset,here by a drop i n applied.shear stress from the matrix due to a similar drop i n e f f e c t i v e . It  o Q  of the  o C phase a t . t h i s s t r a i n rate.  i s reasonable to assume that the mechanism of shear rupture  i n the two p h a s e s ' w i l l be relatively.temperature insensitive when compared to t h e i r flow stresses. • If, t h i s is accepted, t h e n . i t is possible to present in the form of a.schematic diagram- a.unified.picture of phase deformation-in t h i s composite,., over the temperature range 20° to 500°C .  To be considered  are the t o t a l bulk- s t r a i n and. s t r a i n rate which w i l l be s i g n i f i c a n t  factors  at low and high temperatures respectively i n determining deformation behavior. .Here only, the microstructure observed.at severe bulk deformation w i l l be used, namely,. 95$ reduction in r o l l i n g , and at the die entrance corner in extrusion. (Figures 11 to 15, 21 to 25 and 36).  Hence the diagram can be v a l i d only  30  The dominant deformation process w i l l be that . requiring the least stress.  In the diagram t h i s changes from b r i t t l e fracture of the-©-  -phase to p l a s t i c flow of the matrix as temperature increases. as an explanation for the fact that no l a r g e p h a s e any temperature.  temperatures i n spite of  oC  elongation occurs at  This process i s never-the preferred one.  The fact that some p l a s t i c flow of C u A l  of the system.  This serves  Q Q >  &  Q  2  occurs at- high  must be attributed, to the ani'sotropy  Bulk.flow w i l l result i n l o c a l i z e d , h i g h s t r a i n rates i n the  phase and, hence increased shear stress where i t passes around  .projecting arms of the C u A l producing a stringing-out  2  particles.  The C u A l w i l l y i e l d l o c a l l y , 2  of the p a r t i c l e s i n the d i r e c t i o n of flow.  31 Comparisons With Other. Work It  i s desirable to compare these results with pertinent work by  others i n the f i e l d . . a) It  is demonstrated from the results of r o l l i n g and extruding  at and below 300°C that no amount of bulk deformation can induce p l a s t i c flow of C u A l  2  i n this temperature range.  This should be compared.with the  extrusion studies of Warric and Van V l a c k matrices.  1 1  on NaCl-type solids i n FCC  They showed that.these solids could be made to display large  p l a s t i c deformations at ordinary temperatures i f the superimposed hydrostatic stress was s u f f i c i e n t l y high to prevent t e n s i l e cracking. • It must be concluded t h a t - C u A l  2  does not have the p l a s t i c .flow mechanisms posessed by  alkali halides.since.it  ruptures  in shear without--any • p l a s t i c  flow occurring even under high hydrostatic  stress.  b) Primarily on the basis of t e n s i l e tests P e t t y contended that above 300°C "softening" of C u A l is possible.  2  1 5  '  1 6  has  occurs and p l a s t i c deformation  This is supported by the extrusion studies here, in that  p l a s t i c i t y "is first•observed.to appear between 300 and 350°C at high bulk deformation (Figures 21 and 22).  The significance of t h i s temperature is  emphasized by the complete absence of  -phase p l a s t i c i t y at "lower temperatures  even when high bulk deformation is imposed. c) The results of the higher temperature deformation studies indicate that the mechanical properties of the two phases are of major importance i n c o n t r o l l i n g deformation behavior. • It has been shown here for a composite of 60 volume $ particulate phase at 500°C,. where, both .phases are p l a s t i c , that the-©- phase deformation i s more than one order of magnitude smaller than bulk deformation.  32 This i s i n c o n f l i c t with the views of Clarebrough et a l  1 2  and  14  Krock et a l  .  Both predict i d e n t i c a l phase deformation would occur in the  experimental conditions existing here.  It  can only be concluded, that a  p a r t i c u l a r combination of mechanical properties i n the composites.examined by these researchers was responsible for both phases deforming i d e n t i c a l l y . E f f e c t s of P a r t i c l e Size It w i l l be. seen from.Figures 9 to 13 and. 18 to 21 that,, at. temperatures where b r i t t l e rupture- of the  phase i s dominant, none of the  large scattered--©- phase p a r t i c l e s remain a f t e r severe-bulk; deformation. • Apparently.these large p a r t i c l e s tend.to fracture more quickly than small ones and. hence approach.the average p a r t i c l e size as deformation progresses. It  i s reasonable to expect t h i s behavior. . During bulk deforma-  tion,, stress concentration at,the surfaces of the larger p a r t i c l e s would increase more rapidly due to the larger l o c a l disturbance in matrix flow. .Their fracture stress would therefore be more quickly reached. Tensile Tests The e f f e c t on bulk t e n s i l e strength of two system variables, . a) • -Q~ phase•particle size and shape and  b) matrix strength, have been  isolated and examined. . a)  In Groups I and III  the volume f r a c t i o n and mechanical  properties of the two phases are the same.  Only p a r t i c l e size and shape  vary, and i t i s these which determine the transverse matrix mean free path X T  of the composite.  3 3  b)  Groups II  and III  d i f f e r e d . o n l y i n that the matrices of  the former group were not softened "by the 250°C over-aging.process the l a t t e r .  as were  The result was average ultimate t e n s i l e strengths of 52,800  and. 44,600 p s i for Groups II  and III  respectively.  As noted i n the introduction,. a number of researchers have studied the strengthening effects of a dispersed p a r t i c u l a t e phase. . In most cases yield, strength of the composite was the mechanical property measured.  It would be desirable i f the results of some of t h i s work could  be compared with the ultimate (or fracture) strength  <^u_ attained i n the  present t e n s i l e t e s t s .  • In t h i s work, bulk elongation i n the specimens was found to be unmeasurable and no necking could be detected. • Also fracture occurred on a r i s i n g load-elongation slope. ,<d> u_  Hence i t seems reasonable to assume that  values obtained here are closely representative of y i e l d strength. Following t h i s assumption the work of Lenel and A n s e l l  2 2  was  examined, as i t involved consideration of a composite containing a b r i t t l e p a r t i c u l a t e phase.  They developed a model for y i e l d i n g in the ductile matrix  involving d i s l o c a t i o n pileup at the dispersed p a r t i c l e s .  They argue that  although dislocations could.pass around.such p a r t i c l e s leaving loops, no bulk y i e l d i n g could occur u n t i l fracture of these b r i t t l e p a r t i c l e s took place.  Their reasoning required however, that a minimum b r i t t l e p a r t i c l e  spacing was necessary before t h i s fracturing became a c r i t e r i o n of bulk yielding.  The development of Lenel and'- Ansell's argument incorporated the influence of the shear moduli and matrix.Burgers Vector and predicted the r e l a t i o n s h i p between y i e l d stress  d -j  and  X  :  1/2  In the present work,, Figure 35 shows average t e n s i l e types plotted, against  /X T  1/  ^  2  .  u. for.the three  The r e s u l t s suggest the  relationship:  dV = A + B A"" The  1 / 2  implication i s that the Lenel and A n s e l l model i s v a l i d for t h i s  composite and. that fracture of the  phase p a r t i c l e s accompanies the  i n i t i a t i o n of bulk y i e l d i n g .  It may  be possible, however, to develop a.satisfactory- model  based entirely-on continuum.arguments which w i l l explain the observed t e n s i l e behavior of the composite.  Two aspects of the t e n s i l e behavior suggest that p l a s t i c flow of the matrix i s c r i t i c a l i n the i n i t i a t i o n of f a i l u r e :  a)  The good reproduceability of t e n s i l e strength for d i f f e r e n t specimens of each type.  b)  The strong influence of matrix flow strength on composite ultimate  strength.  In apparent opposition i s the demonstrated:brittle: f a i l u r e of the specimens without s i g n i f i c a n t elongation or necking.  35 The model must explain these t e n s i l e c h a r a c t e r i s t i c s . •Consider a small area in.the composite where two adjacent b r i t t l e - * - -phase p a r t i c l e s have cracked, under the applied bulk stress. This cracking could occur, i n the e l a s t i c range of matrix behavior or a f t e r slight local yielding.  If  the intervening matrix,  A T  wide, necks and  elongates under the applied stress the cracking w i l l be permitted to extend to the next nearest -©- phase p a r t i c l e s before rupture of t h i s necked matrix has occurred.  Hence the resistance of the matrix to t h i s l o c a l necking and  elongation w i l l be c r u c i a l in c o n t r o l l i n g the spread.of the crack. •The interphase bond strength i s also of c r i t i c a l importance. Consider the two adjacent cracked.particles where strong and weak bonds e x i s t :  Strong Bond  . Weak Bond  The strong bond w i l l prevent l o c a l i z e d necking and elongation (and crack propogation) u n t i l bond rupture occurs or u n t i l the matrix ruptures without elongation. Consider the e f f e c t of reducing  X T  . .In the diagram above,  the width of the matrix a l l o y between the restraining phase interfaces w i l l be reduced, but the l o c a l i z e d force necessary to i n i t i a t e rupture of the bond.will be unchanged.  Or, considering the model i n three dimensions the  36 cross-sectional area of the matrix.phase transverse to the t e n s i l e d i r e c t i o n w i l l reduce in proportion to  / N - f , whereas the length of  interface bond subjected to load at the periphery of this area w i l l reduce in proportion to  X T  •  The result is that the bulk stress w i l l have  to be increased to b u i l d up s u f f i c i e n t force between the p a r t i c l e s to induce the phase separation and thus permit l o c a l necking.  Composite  ultimate strength w i l l be increased.  The result of r a i s i n g matrix flow stress by suitable heat t r e a t ment would be to enhance the resistance to l o c a l necking and crack propagation.  Again composite strength would be increased. - The above model therefore involves p l a s t i c flow over very  'limited.regions which would be e n t i r e l y undetectable in terms df bulk behavior, and i n t h i s fashion explains the simultaneous appearance of ductile and b r i t t l e t e n s i l e c h a r a c t e r i s t i c s .  The influence of  considered i n t h i s model but.only q u a l i t a t i v e l y .  X T has been  Interphase bond.strength  has been introduced.as an important factor in influencing composite ultimate t e n s i l e strength. Extrusion Flow Pattern The s i m i l a r i t y of the composite flow pattern and the F r i s c h and Thomsen pure metal flow pattern suggests that they might prove on further study to be e s s e n t i a l l y i d e n t i c a l . .If  so, a very useful t o o l would be  provided.for the study of metal working processes i f the l o c a l bulk deformation could be determined accurately from microscopic examination of the deformed particulate phase.  Clarebrough et a l and Krock et a l have  37  demonstrated.that in some composite alloys the particulate phase deformation is identical to bulk deformation.  Hence selection of a suitable composite  would permit this local deformation measurement. The s p l i t - b i l l e t grid-distortion technique of Frisch and Thomsen, besides being laborious, limits investigation to planes across which no shear stresses act. • By, using a composite the body could be sectioned on any plane of interest after complete deformation had taken place. Reprecipitation' Phenomenon An examination of Figures 21 to 25 and 26 to 30 shows that when the composite i s severely deformed at temperatures where Cu solubility in O C  .phase is much larger than room temperature (see Figure 2) a distinctive  form.of reprecipitation occurs, not seen in the undeformed regions. .It consists of.'small--©-phase particles scattered, through the matrix and attached :to the original larger particles. It is desirable to show, that the o<z phase composition must be homogeneous when deformation.occurs:  using 3 x 10 ~  4  cm as a reasonable  value for half the. average inter-particle distance in Figure 26 and diffusion data for this system, .log. D  0  = O.36 -enT^eee  •E.= 3,49 x:10 . cal/mole (Ref.23) 4  Then: log D = log D  Q  E 2.3 RT  Substituting gives D = 3.2 x 1 0 ° _1  Then using: X  2  = 2Dt  gives: t -r2=r 150 sec = 2 / min. x  2  38 With the slow rate of heating i t can be assumed that homogeneity of the  OC  phase e x i s t s a t the time of deformation. Therefore on slow c o o l i n g , the same quantity of C u A l r e p r e c i p i 2  tates i n the undeformed. and deformed.areas.  Apparently i n the former i t i s  uniformly on the o r i g i n a l p a r t i c l e s but in,the l a t t e r at preferred l o c a t i o n s . The deformation temperature precludes r e s i d u a l stresses.  Hence i t i s d i f f i c u l t  to suggest a p o s s i b l e o r i g i n f o r these p r e f e r r e d points of r e p r e c i p i t a t i o n which would survive a f t e r deformation has ceased.  39  CONCLUSIONS Contrary t o the conclusions of the other workers, w i t h two phase a l l o y s of high volume f r a c t i o n second, phase i t was found out. that •a). I n general the two-phases w i l l not deform, to the same extent b) The relative.phase deformation w i l l vary g r e a t l y depending on the mechanical p r o p e r t i e s of the two phases. CuAl  2  i n t e r m e t a l l i c , a t temperatures a t and-below. 300°C does not d i s p l a y  p l a s t i c behavior under combinations of a p p l i e d . s t r e s s which are known to produce flow i n some NaCl type s o l i d s . Matrix flow strength i s an important•variable i n determining  composite  t e n s i l e strength even where there i s a large volume f r a c t i o n b r i t t l e p a r t i c u l a t e phase. Transverse mean free path  i s an important f a c t o r i n determining  .composite-tensile strength, and the A n s e l l and Lenel r e l a t i o n s h i p ( ^  c<  holds true f o r t h i s  composite.  ..Suitably, selected, composites might. provide a. valuable t o o l f o r the study of d e t a i l s of deformation processes.  • ho  APPENDIX-A. • Extrusion Cylinder Design The T 15 tool steel used for parts of the extrusion assembly i s b r i t t l e and notch sensitive,, and hence not reliable under high tensile stress. This precludes the use of a one-piece design for the extrusion cylinder as can be shown by a brief analysis of the stresses in thick walled elastic cylinders under internal pressure. • Starting with the assumptions:  static equilibrium, continuity of  material and Hookian behavior, the resulting equations for.radial and tangential stress at radius r are developed:  j <5  = P  —11  r  (5^.=  a  i£_ _ a 2  1  P  20  /  b  /b + a / r 2  1  - "/b  2  (I-  / B  a  .......(i)  2  .-(2)  2  where a and b are the inner and- outer radii respectively and P the internal pressure.  The maximum tensile stress is in the tangential direction ( ^ Q )  and is. the c r i t i c a l stress for b r i t t l e failure.  Considering the inner surface  where r = a and rearranging  Hence unlimited increases in outer diameter of a fixed bore cylinder w i l l not increase the internal pressure capacity, when the material i s subject to b r i t t l e failure in tension.  kl  The extrusion c y l i n d e r was therefore designed as a composite, using the hard,. wear r e s i s t a n t T 15 s t e e l as. a 0.75 inch bore, 1.5 inch O.D. l i n e r h e a v i l y shrunk i n t o a k inch O.D. c y l i n d e r made of a tough hot work H 12 t o o l s t e e l (Atlas " C r o d i " ) .  The composition of the l a t t e r s t e e l i s  C-Q.45,; Cr-5.0,.W-3.75,, Mo-1,0,. V-0,5, ,.09/0.5,...and ' i t . has a y i e l d strength :  of 230,,000 p s i at540°C when tempered.at 600°C f o r one hour. 1  This design r e s u l t s i n a compressive pre-load.on the l i n e r which would prevent t e n s i l e f r a c t u r e under i n t e r n a l pressure. . I f the t o t a l pressure a c t i n g on the i n s i d e of the outer, c y l i n d e r became s u f f i c i e n t l y , high, s l i g h t y i e l d i n g at the inner surface would occur.but no f r a c t u r e due t o the toughness of the s t e e l employed.  k2  APPENDIX B •- It is well known that metals can frequently be cold roiled to very, large reductions beyond the point at which rupture would be expected to occur.  It has been suggested,that shear rupture followed by  healing may be occurring under such conditions.  In a single phase material  no surviving, evidence of such rupture i s found.  In the absence of contamin-  ation the shear surfaces can heal by recrystallization from, the localized (  heating. - In this Al-CuAl composite, residual bulk shear rupture zones 2  were observed in cold,rolled material a t 80$ reduction and beyond. • This is apparently due to pulverized CuAl particles covering the rupture surface 2:  and preventing healing of the © C phase matrix (see Figure 36). Hence we have direct evidence of the- occurrence of bulk rupture at severe cold.work preserved by the composite deformation behavior.  43  REFERENCES 1.  B.E.Edelson and W.M. Baldwin, jr.,.The Effect of Second-Phases on the Mechanical Properties- of Alloys, Trans.- A.S,M., v.55,.No.l,, p.230.  2.  M. Gensamer,, E.B.- Pearsall,. W-.S. P e l l i n i and J.R. Low,. jr.,, The Tensile Properties- of Pearlite,, Bainite and Spherodite.,. Trans.- A.S-.M., v , 3 0 , 1 9 4 2 , p.983.  3.  M. Gensamer,. Strength and Ductility,. Trans.. A. S-.M.,. v. 3 6 , . 19^6,. p. 3 0 .  4.  R.B.-Shaw, L,A. Shepard,, C.D..Starr and J.E. Dorn,.The Effect of Dispersions on the Tensile Properties of AlCu Alloys, Trans.•A.S.M. v..45, 2953, P-249.  5«  H. Unckel,.Zur Akhangegkeit der Mechanischen Eigenshaften von der Structur bei Zwei-Phasen-Legierugen, Metall, v o l . 5 , 1951, P.146.  6.  J.H. Keeler,.Tensile Properties of Zr-Cr-Alloys: - Effects, Trans.-A.S.M. vol.48, 1956, p.825.  7.  J.H. Keeler,.Tensile Characteristics of Particle Strengthened-Alloys of Zr with Fe..J.of M . v o l . 8 , 1956, p.486.  8.  W.H. Meiklejohn and R.E. Skoda,. Dispersion Hardening, Acta Met. v o l . 7 , 1959, P-675.  9.  T.•Malkiewicz and S. Rudniek, Deformation of Non-Metallic Inclusions During Rolling of Steel,,J..of ISI,. Jan. 1963, p.33.  Particle Strengthening 1  10.  F.B. Pickering,.Some Effects of Mechanical'Working on the Deformation of Non-Metallic Inclusion,. J.-of I.S.I. 1 8 9 , 1958,, p..l48.  11.  R.J. Warric and L-.H. Van Vlack, • Plastic Deformation of Non-Metallic Inclusions within-Ductile'Metals. Trans.-A.S.M..Sept. 1964, p.672.  12.  L.M.-Clarebrough, Deformation and Recrystallization df Alloys Containing Two Phases. Australian J. of Sci. Research. 1950,,Ser. A.,, v o l . 3 , P.73-  13.  L.M. - Clarebrough and G.R. Perger,,Influence of the Volume Fraction of the Phases upon the Deformation of cC-/S Brass. - Australian J. of Sci. Research. 1952..Ser.A. v o l . 5 , p . l l 4 .  14.  Richard H. Krock and L.A. Shepard,. Mechanical Behavior of the Two-phase Composite,, Tungsten-nickel-iron,, Trans . • A.I..M,E. vol.227,. Oct. I963, p.1127.  15.  E.R. Petty,. The Deformation Behavior of Some Aluminum.Alloys Containing • Intermetallic Compounds, J . o f Inst. Met. 1962-63, v o l . 9 1 , , p.. 274.  16. .E.R. Petty,-J.. of Inst.-Met. I96O-6I, v o l . 8 9 , , p.343.  References  (cont'd)  kk  17.  E.E. Underwood, A Review of S u p e r p l a s t i c i t y , J . of M.y Dec.1962, p.914.  18.  J . F r i s c h and E.G..Thomsen,, An.Experimental Study.of Metal Extrusions a t V a r i o u s S t r a i n Rates, Trans. A. S.M..E., vol.76, No. k, . p .599 (May 1954).  19. -E.G.•Thomsen and J.-Frisch,. Stresses and S t r a i n s i n Cold-extruding . 2 S-0 Aluminum, Trans. A.S.M.E. vol.77, p.Ijkj, Nov.I955. 20.  0. Hoffman and G..Sachs,. Introduction to the Theory of P l a s t i c i t y f o r • Engineers,. New York,, McGraw-Hill, 1953, p.80.  21.  L.D..Graham and R.W. Kraft,, Coarsening of E u t e c t i c Microstructures at Elevated Temperatures,.Trans. AIME,.vol.236, Jan. 1966, p.9^.  22.  F.V.  23. -W.J.  Lenel and- G.S. Ansell,, Powder Metallurgy, p.267, New York-London Interscience Publishers I n c . 1 9 6 1 . Jost,. D i f f u s i o n i n Solids,. L i q u i d s and Gasses, Academic Press, I960, p.216.  Figure 1.  Steady--State" Effective Natural Strain .Trajectories Frisch and Thomsen (Refs. 18-19)  Figure.. 2 .  C u A l - A l Phase Diagram 2  P. Hansen (I958 ed.) p.  Figure J .  Extrusion Cylinder Assembly  Figure k.  x 120  As Cast A l - C u A l (60 vol.if, CuAl )  2  2  Figure 5.  x 500  As Cast showing Lamellae  Figure 6.  x 120  A f t e r 2k hr. Soak at 500°C  kQ  Figure 9.  x k-00  Rolled: 70$ reduction at 20°C (rolling direction  Figure 10.  ^  x kOO  Rolled: 80$ reduction at 20°C  Figure 11.  x kOO  Rolled: 95$ reduction at 20°C  Figure 12.  x kOO  Rolled: 95$ reduction at 200°C (rolling direction  Figure 15.  x kOO  Rolled: 95$ reduction at 300°C  /  52  300 Coarsened  250 As Cast  «0  03  Extrusion Temp. 300°C 325 350 400 450 500  RamiPressure Initial 290,000 251,000 182,000 112,000 95,000 95,000  234,000 173,000 104,000 86,000 121,000  350 4oo  152,000 121,000  143,000 117,000  (psi) Final  P.  q  200 X X  ra  o o  150  flow around die  o  x-  8  100  X X  1  300  325  350  1  1 400  :  1  450  1  500  Extrusion Temp. (°C)  Figure l 4 .  Extrusion Ram.Pressure vs, Temperature (12:1 reduction ratio)  Figure 15.  One h a l f of butt c r o s s - s e c t i o n : 500°C extruded b i l l e t . x 20 (coarsened material)  Figure 16.  One half of butt cross-section: x 20 (as-cast material)  350°C extruded b i l l e t  Figure 17.  Cross Section of butt of b i l l e t extruded at 350°C from as cast material with superimposed Natural Strain Trajectory Diagram of F r i s c h and Thomsen ( F i g . l )  vn  Figure 18.  x 360  Location A: 300°C e x t r u s i o n from coarsened m a t e r i a l .  Figure 19.  x 36O  Location B: 300°C e x t r u s i o n  Figure 20. Location C: 300°C e x t r u s i o n  x 36O  56  Figure 21,  x 360  Location D: 300°C extrusion Die entrant corner seen i n lower l e f t .  Figure 22.  x 360  Die entrant corner: 350°C extrusion  Figure 25.  x 36O  Die entrant corner: 400°C extrusion  58  Figure 2 k  x 36O  Die entrant corner: 450°C extrusion  Figure 25-  x  Die entrant corner: 500°C extrusion  36O  Figure 26.  x 36O  Location A: 500°C extrusion  Figure 27. • H |  x 36O  Location B: 500°C extrusion  Figure 28. Location C: 500°C extrusion  x 36O  Figure JO.  x  1600  Location D: Showing details of microstructure  Specimen Type ...  20  30 -4X X  A  i  50  —4—  60 i  x XX X XX  Group  X X X  D  X XX  E  X  Group . II  Group III  X  x x H  X  20  - r 30  X  1  ko  5P  60  Tensile Fracture Strength ( 41u ) (1000 psi) Figure 31»  Tensile Test Results  62 Figure J2.  x 500  Microstructure? Tensile Specimen from 525°C extruded f rod. (Type A) 1 (Specimen a x i s )  Figure J$.  x 500  Tensile Specimen from extruded rod (Type D)  k^>0°C  t  Figure $4.  x 500  I Tensile Specimen from gg 350°C extruded rod of pi as cast m a t e r i a l (subsequently coarsened)  50  -T  40 •H W ft  o o o  I?  50  J  20  J  T  0.2  0.4  0.3  Figure  55-  "  —  —i—  ,  0.6  0-5  0.7  Mean Free Path E f f e c t (Tensile Data P l o t t e d According to L e n e l - A n s e l l Prediction ) 2 2  Figure J6.  Matrix Shear Rupture Zone in Composite Rolled  at 20°C  x 1200  

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