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Grain refinement in β-CuAlNi strain memory alloys Sure, Ganesh Narayan 1984

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GRAIN REFINEMENT IN 0-CuAlNi by STRAIN MEMORY ALLOYS GANESH NARAYAN SURE B. Tech., Indian I n s t i t u t e Of Technology, Kharagpur, 1981 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES Department Of M e t a l l u r g i c a l Engineering We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA January 1984 © Ganesh Narayan Sure, 1984 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I agree t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d 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 f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f ME-TMLv; KC,IC\hL EUgmEEgMC; The U n i v e r s i t y o f B r i t i s h C o l u m b i a 1956 Main Mall V a ncouver, Canada V6T 1Y3 Date J * - /^^ * b i ^ DE-6 (3/81) i i A b s t r a c t A study has been made of the e f f e c t of g r a i n refinement on the mechanical and the s t r a i n memory p r o p e r t i e s of 0-CuAlNi a l l o y s . A d d i t i o n of 0.5 % T i to CuAlNi r e s u l t e d i n c o n s i d e r a b l e g r a i n refinement. This was due to the formation of t i t a n i u m -r i c h p a r t i c l e s which e f f e c t i v e l y r etarded g r a i n growth d u r i n g a n n e a l i n g . By c o n t r o l l e d a n n e a l i n g , a g r a i n s i z e as small as 15nn\ c o u l d be obtained, though some second phase y2 w a s present due to incomplete p r e c i p i t a t e d i s s o l u t i o n . S t r e s s - s t r a i n curves f o r most specimens i n both the s t r a i n memory and ps e u d o e l a s t i c s t a t e s showed a three-stage c h a r a c t e r i s t i c with a region of lower slope between two regions of higher modulus. I t was found the o, (the t r a n s i t i o n s t r e s s between stages 1 and 2) and do/de (the slope of stage 2) increa s e d w i t h g r a i n s i z e according to a (g.s . ) ~ l 2 r e l a t i o n s h i p . The u l t i m a t e t e n s i l e s t r e n g t h and s t r a i n to f r a c t u r e a l s o f o l l o w e d a s i m i l a r H a l l - P e t c h r e l a t i o n s h i p . The a l l o y s showed higher s t r e n g t h i n the m a r t e n s i t i c s t a t e than i n the p s e u d o e l a s t i c one. The presence of second phase p a r t i c l e s had no e f f e c t on the mechanical p r o p e r t i e s and mar t e n s i t e deformation behaviour. F r a c t u r e s t r a i n s as high as 7% were obtained at the f i n e s t g r a i n s i z e s . I t was found that the s t r a i n memory and p s e u d o e l a s t i c recovery p r o p e r t i e s were not a f f e c t e d s i g n i f i c a n t l y by decreasing g r a i n s i z e and the presence of second phase p a r t i c l e s . Maximum recovery s t r a i n s of 6.5% were obtained i n i i i f i n e g r a i n samples. The f a t i g u e l i f e of most specimens was r e l a t i v e l y poor and d i d not vary s i g n i f i c a n t l y w i t h g r a i n s i z e and the presence of t i t a n i u m - r i c h p a r t i c l e s . However, the presence of y2 p a r t i c l e s i n the f i n e s t g r a i n s i z e s of l5-40j/m caused the f a t i g u e l i f e to i n c r e a s e by a f a c t o r of ten, so that specimens c y c l e d to 280 MPa and 0.6% s t r a i n had a l i f e of 40000-50000 c y c l e s . This improvement i n f a t i g u e l i f e appeared to be due t o a change i n the f a t i g u e crack propagation mode. Table of Contents Abst r a c t i i L i s t of Tables v L i s t of Figures v i Acknowledgement v i i i Chapter I INTRODUCTION 1 1.1 General Review 1 1.2 Previous Work 3 1.3 Aim Of The Present Work 6 Chapter II EXPERIMENTAL PROCEDURE 8 2.1 A l l o y P r e p a r a t i o n 8 2.2 Hot R o l l i n g 10 2.3 Pre p a r a t i o n Of T e n s i l e Specimens 11 2.4 Measurement Of Transformation Temperature 17 2.5 T e n s i l e And Recovery Tests And Fractography ....18 2.6 Fatigue Tests 19 Chapter I I I RESULTS AND DISCUSSION 20 3.1 Grain Growth 20 3.2 Ms Determination 26 3.3 T e n s i l e Tests 29 3.3.1 E f f e c t Of Grain Size On T e n s i l e P r o p e r t i e s ...29 3.3.1.1. E x t r a p o l a t e d T r a n s i t i o n S t r e s s , 33 3.3.1.2. Slope Of The "plateau" Region 38 3.3.1.3 o f And & 40 3.3.2. E f f e c t Or Temperature On T e n s i l e P r o p e r t i e s .47 3.3.3. Fractography 58 3.3.3.1 E f f e c t Of Grain Size 58 3.3.3.2 E f f e c t Of Titanium A d d i t i o n 60 3.3.3.3 E f f e c t Of Temperature 62 3.3.3.4 E f f e c t Of Second Phase P r e c i p i t a t e s 63 3.4 Recovery P r o p e r t i e s 65 3.4.1. E f f e c t Of Temperature 65 3.4.2 E f f e c t Of Grain Size 69 3.4.3. E f f e c t Of Increasing S t r a i n 71 3.5 Fatigue P r o p e r t i e s 75 3.5.1. E f f e c t Of Grain Size On Fatigue L i f e 75 3.5.2 Fractography 79 Chapter IV CONCLUSIONS 85 REFERENCES 87 APPENDIX A - RESULTS OF FATIGUE TESTS 89 L i s t of Tables Chemical Composition and Ms of the a l l o y s V a r i a t i o n of Ms with t i t a n i u m a d d i t i o n and g r a i n s i z e v i L i s t of F i g u r e s 1. C r o s s - s e c t i o n of the ingots showing the as cast and homogenised s t r u c t u r e , a)CuAlNi (A1) and b) CuAlNi-Ti (T1 ) , X4.5 12 2. Micrographs showing the m i c r o s t r u c t u r e s of a l l o y T1 a) S t r u c t u r e before r e c r y s t a l l i z a t i o n b) Same s t r u c t u r e i n d e t a i l , R e c r y s t a l l i z e d s t r u c t u r e a f t e r c) 10 s. d) 15 s. and e) 20 s., Note 7 2 slowly disappearing and g r a i n growth t a k i n g p l a c e . M a g n i f i c a t i o n s : a) x145, b)-e) x475 14 3. Micrographs showing m i c r o s t r u c t u r a l d i f f e r e n c e s i n CuAlNi (A2) and CuAlNi-Ti (T2). Note the presence of x-phase p a r t i c l e s i n CuAlNi-Ti a l l o y s , x95 1 6 4. V a r i a t i o n of g r a i n s i z e with i n c r e a s i n g s o l u t i o n treatment time for a l l o y s A1 and T1 22 5. Log-Log p l o t of g r a i n s i z e vs. s o l u t i o n treatment time at 800 C f o r a l l o y s Al and T1 23 6. Schematic i l l u s t r a t i o n showing i n t e r a c t i o n between a g r a i n boundary and a second-phase p a r t i c l e 25 7. S t r e s s - s t r a i n curves at varying g r a i n s i z e s f o r a l l o y s Al and T1 31 8. S t r e s s - s t r a i n curves at varying g r a i n s i z e s f o r a l l o y s A2 and T2 32 9. A schematic s t r e s s - s t r a i n curve 34 10. V a r i a t i o n of e x t r a p o l a t e d t r a n s i t i o n s t r e s s ox with a ) ( g s / t ) and b) (gs/tH/2 36 11. V a r i a t i o n of the gradient do/de wi t h a ) ( g s / t ) and b) ( g s / t ) - l / 2 40 12. V a r i a t i o n of of with a) (gs/t) and b) ( g s / t ) - l / 2 42 13. V a r i a t i o n of e f with a) (gs/t) and b) (gs/t)-T/2 46 14. S t r e s s - s t r a i n curves for A2-L and A2-F at varying temperatures 48 15. S t r e s s - s t r a i n curves for T2-I specimens at varying temperatures , 49 16. V a r i a t i o n of a, and oy with temperature f o r A2-F v i i specimens 51 17. V a r i a t i o n of oy and Of w i t h temperature f o r A2-L specimens 51 18. V a r i a t i o n of a, and o f w i t h temperature i n T2-I specimens 52 19. V a r i a t i o n of do/de w i t h temperature i n A2-L, A2-F and T2-I specimens 57 2 0 . Fractograph showing t e n s i l e f r a c t u r e surface of an A1-L specimen (gs/t = 0.59), x40 59 2 1 . Fractograph showing t e n s i l e f r a c t u r e surface of an A2-F specimen (gs/t=0.076), x40 59 22. Fractograph showing the t e n s i l e f r a c t u r e surface of a T1-F specimen (gs/t = 0.08) Test temperature =22°C, x120 61 23. Fractograph showing the t e n s i l e f r a c t u r e surface of a T1-F specimen (gs/t=0.09) Test temperature=-35°C, X120 61 24. Fractograph showing t e n s i l e f r a c t u r e surface of an A2-F specimen (gs/t=0.l) i n s i n g l e 0-phase c o n d i t i o n , x120 • , 64 25. Fractograph showing t e n s i l e f r a c t u r e surface of an A2-F specimen (gs/t=0.0l7) c o n t a i n i n g y2 p r e c i p i t a t e s , X1 2 0 64 26. S t r e s s - s t r a i n curves of T2-I specimens at v a r y i n g temperatures showing recovered s t r a i n s on unloading and heating 67 27. V a r i a t i o n of percent recovery w i t h temperature f o r T2-I specimens 68 28. V a r i a t i o n of percent recovery with g r a i n s i z e at constant a p p l i e d s t r a i n of 2% 70 29. S t r e s s - s t r a i n curves at i n c r e a s i n g a p p l i e d s t r a i n s of a) 1.5%, b) 4% and c) 6.5% i n T2 specimens 72 30. V a r i a t i o n of percent recovery with i n c r e a s i n g a p p l i e d s t r a i n i n T2 specimens 74 31. A t y p i c a l v a r i a t i o n of s t r a i n w i t h c y c l i n g i n T2 specimens 76 32. V a r i a t i o n of number of c y c l e s to f a i l u r e (N) wi t h v a r y i n g g r a i n s i z e i n A1, A2, T1, T2 and T3 a l l o y s ...78 v i i i 33. Fractograph showing f a t i g u e f r a c t u r e s u r f a c e of A2 specimen (gs/t=0.09), x40 80 34. Fractograph showing f a t i g u e s t r i a t i o n s i n the i n t e r g r a n u l a r region of f r a c t u r e surface? i n A2, x900 .80 35. Fractograph showing f a t i g u e f r a c t u r e s u r f a c e of T2 specimen (gs/t = 0 . l 5 ) , x40 82 36. Fractographs showing f a t i g u e s u r f a c e of T2-F specimens (gs/t=0.0l5) c o n t a i n i n g y2 p r e c i p i t a t e s , M a g n i f i c a t i o n s : a) x40, b) x200 83 i x Acknowledgement I wish to express my s i n c e r e g r a t i t u d e t o P r o f e s s o r L.C. Brown f o r h i s advice and a s s i s t a n c e d u r i n g the course of t h i s i n v e s t i g a t i o n . I would a l s o l i k e to thank p r o f e s s o r R.G. B u t t e r s f o r h i s i n v a l u a b l e help i n experimental work and Mr. Nick Wong fo r h i s h e l p i n p r e p a r a t i o n of the a l l o y s . Thanks are a l s o extended to the members of f a c u l t y and f e l l o w graduate students f o r h e l p f u l d i s c u s s i o n s . The a s s i s t a n c e of the t e c h n i c a l s t a f f i s g r e a t l y a p p r e c i a t e d . A s p e c i a l note of thanks to Mr. Jayant Sathaye f o r h i s h e l p f u l c r i t i c i s m . F i n a n c i a l a s s i s t a n c e provided by NSERC under grant number A2459 and the graduate a s s i s t a n t s h i p awarded by the Department of M e t a l l u r g i c a l Engineering are g r a t e f u l l y acknowledged. 1 I . INTRODUCTION 1.1 General Review The s t r a i n memory e f f e c t (SME) and a s s o c i a t e d phenomena such as p s e u d o e l a s t i c i t y and the two-way s t r a i n memory e f f e c t have been w e l l i n v e s t i g a t e d , p a r t i c u l a r l y i n the l a s t f i f t e e n years and are now f a i r l y w e l l understood from a mechanistic p o i n t of view. 1 - 2 The e f f e c t s are a l l a s s o c i a t e d w i t h a th e r m o e l a s t i c m a r t e n s i t i c t r a n s f o r m a t i o n . A m a r t e n s i t i c t r a n s f o r m a t i o n i s t h e r m o e l a s t i c i f there i s continuous formation and growth of martensite as the temperature i s lowered, and continuous shrinkage and disappearance of mar t e n s i t e as the temperature i s r a i s e d . P s e u d o e l a s t i c behaviour i s a mechanical analogue of the t h e r m o e l a s t i c t r a n s f o r m a t i o n , where the tran s f o r m a t i o n proceeds c o n t i n u o u s l y with i n c r e a s i n g a p p l i e d s t r e s s and reverses c o n t i n u o u s l y when the s t r e s s i s decreased. The s t r a i n memory e f f e c t i s r e a l i s e d i f a macroscopic deformation that i s accompanied by a m a r t e n s i t i c t r a n s f o r m a t i o n i s not reversed by removal of a p p l i e d s t r e s s . Rather the reverse transformation and r e v e r s a l of macroscopic deformation are induced by hea t i n g . The two-way s t r a i n memory e f f e c t occurs i f there i s a macroscopic shape change accompanying the th e r m a l l y induced m a r t e n s i t i c transformation and i s brought i n t o e f f e c t by lowering and r a i s i n g the temperature. 2 The novel and remarkable p r o p e r t i e s of s t r a i n memory a l l o y s present unique p o s s i b i l i t i e s f o r a p p l i c a t i o n s 3 " ' such as leakproof c o u p l i n g s and f a s t e n e r s i n pneumatic and h y d r a u l i c l i n e s , thermomechanical and t h e r m o s t a t i c c o n t r o l d e v i c e s , o r t h o d o n t i c d e n t a l arch w i r e s , medical implants, heat engines and many o t h e r s . A number of these a p p l i c a t i o n s have been f u l l y developed and are being put i n t o p r a c t i c a l use. Two major groups of s t r a i n memory a l l o y s have emerged as s u i t a b l e f o r p r a c t i c a l a p p l i c a t i o n s i n view of t h e i r e x c e l l e n t s t r a i n memory p r o p e r t i e s and l a r g e range of t r a n s f o r m a t i o n temperatures. These are N i T i , N i T i - X (X=ternary element as Cu,Fe) a l l o y s and the copper-based a l l o y s such as CuAlNi, CuZnAl, CuZnSn. The N i T i a l l o y s c u r r e n t l y i n widespread use e x h i b i t e x c e l l e n t s t r a i n memory p r o p e r t i e s w i t h recoverable s t r a i n s of 8-10% and have good d u c t i l i t y and f a t i g u e s t r e n g t h 5 . The good mechanical p r o p e r t i e s of N i T i a l l o y s are u s u a l l y a t t r i b u t e d to t h e i r small g r a i n s i z e and low e l a s t i c a n i s o t r o p y (A=2C„ ,/[C, , - C 1 2 ] 2) However due to high m a t e r i a l cost and c o m p l e x i t i e s i n v o l v e d i n a l l o y p r e p a r a t i o n and f a b r i c a t i o n , they are expensive and economic c o n s i d e r a t i o n s l i m i t t h e i r use i n many a p p l i c a t i o n s . The copper-based a l l o y s on the other hand, e x h i b i t e x c e l l e n t s t r a i n memory r e c o v e r i e s up to 5-7% s t r a i n , but s u f f e r from poor mechanical p r o p e r t i e s such as l i m i t e d d u c t i l i t y and low f a t i g u e s t r e n g t h . The b r i t t l e n e s s of the a l l o y s has been a t t r i b u t e d to high e l a s t i c a n isotropy(A =* 13) and l a r g e g r a i n s i z e 6 . The low m a t e r i a l cost and r e l a t i v e ease of a l l o y p r e p a r a t i o n and f a b r i c a t i o n make these a l l o y s p o t e n t i a l l y economically 3 a t t r a c t i v e and would extend the range of p r a c t i c a l a p p l i c a t i o n s of SME a l l o y s . Recent r e s e a r c h e f f o r t s have mainly been d i r e c t e d towards d e v e l o p i n g copper-based s t r a i n memory a l l o y s w i t h b e t t e r mechanical p r o p e r t i e s . D e t a i l e d i n v e s t i g a t i o n s are being done on t e r n a r y 0-phase Cu A l N i , CuZnAl and CuZnSn a l l o y s , these b e i n g the most promising a l l o y s f o r p r a c t i c a l use. Recent i n v e s t i g a t i o n s ' on the f r a c t u r e c h a r a c t e r i s t i c s of these a l l o y s have suggested that the l i m i t i n g s t r a i n s f o r s t r a i n memory and p s e u d o e l a s t i c e f f e c t s can be improved by g i v i n g them a p r e f e r e d t e x t u r e or o b t a i n i n g f i n e r g r a i n s i z e . 1.2 Previous Work The f i r s t o b s e r v a t i o n s of the e f f e c t of g r a i n s i z e .on s t r e n g t h of s t r a i n memory a l l o y s were c a r r i e d out by Khan and D a l a e y 7 " " . In t h e i r CuAlFe a l l o y s , an e m p i r i c a l r e l a t i o n between g r a i n s i z e and ma r t e n s i t e p l a t e t h i c k n e s s of the form d «= cY(i was found, where cL i s the mar t e n s i t e p l a t e t h i c k n e s s , dp i s the 0-grain s i z e and C * 1. Furthermore, a H a l l - P e t c h type r e l a t i o n between g r a i n s i z e and both y i e l d s t r e n g t h and u l t i m a t e t e n s i l e s t r e n g t M af ) of the a l l o y was shown t o e x i s t . As e x c e s s i v e g r a i n growth of beta phase had been r e p o r t e d d u r i n g heat treatment', Khan and Dalaey* added 3% Fe f o r r e f i n i n g the g r a i n s i z e . 4 More recent work i n o b t a i n i n g e f f e c t i v e g r a i n refinement i n Cu-based 0 - a l l o y s has i n d i c a t e d the f o l l o w i n g methods t o be e f f e c t i v e , i ) S o l u t i o n treatment 1 0 i i ) Two phase s t r u c t u r e s 1 1 i i i ) A l l o y a d d i t i o n s 1 2 - 1 * i v ) Powder Me t a l l u r g y (PM) techniques 1 5 - 1 6 v) Rapid s o l i d i f i c a t i o n methods 1 7 " 1 8 In the s o l u t i o n treatment method 1 0 specimens c o l d r o l l e d 15% were r e c r y s t a l l i z e d by heating f o r v a r i o u s p e r i o d s i n a s a l t bath at 800°C and were subsequently water quenched to give a range of g r a i n s i z e s . White et a l 1 1 took advantage of the presence of alpha phase i n the form of f i n e Widmanstatten p l a t e s to prevent r a p i d g r a i n growth duri n g the s o l u t i o n treatment i n CuZnAl a l l o y s . They showed that a small volume f r a c t i o n of alpha was r e t a i n e d at g r a i n s i z e s below 0.1 mm but the presence of t h i s r e t a i n e d alpha d i d not e f f e c t the s t r a i n memory propert i e s . The e f f e c t of a d d i t i o n of a l l o y i n g elements such as V, Co, T i , W, Zn, P and Fe to CuAlNi a l l o y s has been i n v e s t i g a t e d by Matsumoto et a l 1 9 and Kamei et a l 2 0 . I t was found that T i , Co and V were most e f f e c t i v e f o r forming equiaxed g r a i n s and suppressing g r a i n coarsening of the 0-phase. Sugimoto et a l 1 2 i n v e s t i g a t e d the e f f e c t of 0.5 to 3.99 wt % T i a d d i t i o n s to a CuAlNi a l l o y . E f f e c t i v e g r a i n refinement and improved hot working a b i l i t y were demonstrated and a t t r i b u t e d to the presence 5 of f i n e l y d i s p e r s e d T i - r i c h p a r t i c l e s formed by p e r i t e c t i c r e a c t i o n on s o l i d i f i c a t i o n . Enami et a l 1 3 employed vanadium a d d i t i o n s from 0.2 to 2 wt% i n CuZnAl a l l o y s and found that vanadium was very e f f e c t i v e i n reducing the 0-grain s i z e and that g r a i n boundary c r a c k i n g was remarkably suppressed. Recoverable s t r a i n s up to 5.5% were obtained showing there was no lowering of the s t r a i n memory p r o p e r t i e s by the vanadium a d d i t i o n s . I k a i et a l 1 ' employed a d d i t i o n s of Co, B, Zn f o r g r a i n refinement of CuAINi a l l o y s and demonstrated that g r a i n refinement provides b e t t e r r e s i s t a n c e to thermal degradation on repeated m a r t e n s i t i c t r a n s f o r m a t i o n ( i e b e t t e r r e s i s t a n c e to ag i n g ) . Jassen et a l 1 5 showed e f f e c t i v e g r a i n refinement i n CuZnAl PM a l l o y s w i t h improvement i n t e x t u r e by e x t r u s i o n and they demonstrated sup e r i o r f a t i g u e p r o p e r t i e s f o r these a l l o y s . Duerig et a l 1 6 a l s o produced f i n e g r a i n CuAINi a l l o y s by PM techniques and demonstrated improved d u c t i l i t y up to 7% and high f a t i g u e s t r e n g t h . Rapid s o l i d i f i c a t i o n techniques employed by Oshima et a l 1 7 and Wood 1 8 f o r producing f i n e g r a i n s i z e s i n Cu-based /3-phase a l l o y s have proved very promising, and ribbons produced e x h i b i t e x c e l l e n t s t r a i n memory p r o p e r t i e s with recoverable s t r a i n s up to 6 - 7 % . The c r y s t a l s t r u c t u r e s of martensites forming i n Cu-base a l l o y s are w e l l known. In CuAINi a l l o y s , the s t r u c t u r e of the martensites has been determined as having a long p e r i o d s t a c k i n g 6 order with common basal plane [100]/J. The u n i t c e l l s of these m a r t e n s i t e s have been found to be m o n o c l i n i c . In CuAlNi, the mat r i x to martensite t r a n s f o r m a t i o n i s of the type 0 — • w i t h 0' being 18R type m a r t e n s i t e . At higher s t r e s s l e v e l s , m a r t ensite to martensite t r a n s f o r m a t i o n s of the type j8' —> 7 ' (2H type martensite) a l s o occur. 1.3 Aim Of The Present Work The present i n v e s t i g a t i o n was undertaken with the main aim of studying the e f f e c t of g r a i n s i z e on mechanical p r o p e r t i e s and s t r a i n memory p r o p e r t i e s of Cu-base /3-phase a l l o y s . Recent i n v e s t i g a t i o n s have i n d i c a t e d s i g n i f i c a n t promise i n improving mechanical p r o p e r t i e s by g r a i n refinement without s i g n i f i c a n t l y d e t e r i o r a t i n g the s t r a i n memory p r o p e r t i e s . However no d e t a i l e d i n v e s t i g a t i o n of the i n f l u e n c e of g r a i n s i z e on mechanical p r o p e r t i e s has been made. The f i n e s t g r a i n s i z e s that have been obtained so f a r i n Cu-base 0 - a l l o y s are of the order of 20um , i n a l l o y s produced by powder m e t a l l u r g y 1 6 and by r a p i d s o l i d i f i c a t i o n 1 8 techniques. G r a i n refinement by a l l o y a d d i t i o n s 1 2 " 1 3 a l s o shows some promise i n o b t a i n i n g g r a i n s i z e s of the order of 20^m , but needs to be f u r t h e r i n v e s t i g a t e d . This method was considered as the most s u i t a b l e f o r g r a i n refinement as only a minor m o d i f i c a t i o n of con v e n t i o n a l a l l o y p r e p a r a t i o n techniques was necessary. The CuAlNi system was chosen by v i r t u e of the p o s s i b i l i t y of c l o s e composition c o n t r o l i n order to ob t a i n a wide range of Ms temperatures, and because of the very good and w e l l i n v e s t i g a t e d 7 s t r a i n memory p r o p e r t i e s . CuAINi i s a l s o the most b r i t t l e of the Cu-based beta a l l o y s ; hence i f mechanical p r o p e r t i e s of t h i s a l l o y can be improved by g r a i n refinement, other l e s s b r i t t l e beta a l l o y s might show even b e t t e r mechanical p r o p e r t i e s on g r a i n refinement. A 0.5% t i t a n i u m a d d i t i o n was used f o r g r a i n refinement of the CuAINi a l l o y s , as the recent r e p o r t of Sugimoto et a l 9 showed t h i s to be an e f f e c t i v e g r a i n r e f i n e r and i n d i c a t e d promising p o s s i b i l i t i e s of improvement i n mechanical p r o p e r t i e s . The method used f o r a l l o y p r e p a r a t i o n was s i m i l a r t o that of Sugimoto et a l 1 2 . The i n v e s t i g a t i o n was undertaken with the o b j e c t i v e of a s s e s s i n g the degree of g r a i n refinement o b t a i n a b l e by t i t a n i u m a d d i t i o n s , and to determine the e f f e c t of g r a i n s i z e on t e n s i l e p r o p e r t i e s , s t r a i n memory recovery p r o p e r t i e s and f a t i g u e s t r e n g t h of the a l l o y s . P a r t i c u l a r a t t e n t i o n was p a i d to i n v e s t i g a t i o n of the e f f e c t of t i t a n i u m a d d i t i o n s on mechanical p r o p e r t i e s of the a l l o y by comparing them wi t h the mechanical p r o p e r t i e s of a l l o y s g r a i n r e f i n e d by the s o l u t i o n treatment method 1 0. 8 I I . EXPERIMENTAL PROCEDURE  2.1 A l l o y P r e p a r a t i o n The a l l o y s were prepared from 99.9% pure copper, 99.7% pure aluminum, 99.95% pure n i c k e l and 99.9% pure t i t a n i u m sponge. The CuAINi a l l o y s were prepared by m e l t i n g weighed q u a n t i t i e s of the elements i n a graphite c r u c i b l e i n a high frequency i n d u c t i o n furnace under an argon atmosphere. A 5% l o s s of aluminum duri n g melting due to o x i d a t i o n and evaporation was taken i n t o account. The melt was heated to 1350-1400°C and c a s t i n t o a pre-heated rectangular g r a p h i t e mold of dimensions 140mm x 60mm x 15mm. The c a s t i n g was then homogenized at 1000°C f o r 24 hours. A s i m i l a r procedure was followed i n p r e p a r a t i o n of CuAlN i - T i a l l o y s , except that the t i t a n i u m sponge added to the melt was wrapped i n an aluminum f o i l to prevent o x i d a t i o n of t i t a n i u m . The melt was allowed to remain at 1400°C f o r a longer time to ensure complete d i s s o l u t i o n of the t i t a n i u m . Approximately 1.3 kg of a l l o y was prepared i n each batch. Table 1 shows the chemical compositions of the a l l o y s prepared, as determined by the spectroscopic technique. I t was found t h a t the a l l o y composition was c l o s e to the charge composition i n d i c a t i n g only a s l i g h t l o s s d u r i n g m e l t i n g . Table 1 - Chemical Composition and Ms of the a l l o y s A l l o y Cu A l Ni T i Ms D e s i g n a t i o n wt% wt% wt% wt% A1 82.2 14.2 3.25 - 68 A2 81.7 14.6 3.55 - -15 T1 82.1 14.1 3.15 0.45 75 T2 81 .6 14.6 3.25 0.38 5 T3 80.7 14.8 3.9 0.42 -80 10 2.2 Hot R o l l i n g P i e c e s of dimension 10mm x 15mm x 60mm were cut from the cast ingot and were hot r o l l e d at 850°C i n the s i n g l e beta phase region to produce ±he f i n a l t h i c k n e s s of 1-1.2 mm. Fig.1(a) and (b) shows t y p i c a l cast s t r u c t u r e s of the CuAlNi and CuAlNi-Ti a l l o y s . Both s t r u c t u r e s show l a r g e columnar grains and two phase s t r u c t u r e s a r i s i n g from slow c o o l i n g . Comparison of f i g s . 1 (a) and (b) i n d i c a t e s that CuAlNi-Ti e x h i b i t s a somewhat smaller g r a i n s i z e than the CuAlNi a l l o y . The smaller g r a i n s i z e i s probably due to the presence of T i - r i c h p a r t i c l e s which i n h i b i t g r a i n boundary m i g r a t i o n thus r e t a r d i n g g r a i n growth during homogenization. The f i n e columnar g r a i n s t r u c t u r e of C u A l N i - T i proved to be advantageous during hot r o l l i n g . In the i n i t i a l stages of r o l l i n g when the c a s t s t r u c t u r e was being broken up, the c r a c k i n g tendency along the g r a i n boundaries was l e s s than i n CuAlNi. The extent of deformation that c o u l d be obtained i n a s i n g l e pass without c r a c k i n g was 10% r e d u c t i o n i n t h i c k n e s s i n CuAlNi-Ti compared to approximately 5% i n CuAlNi. Sugimoto et a l 1 2 a l s o confirm b e t t e r f o r m a b i l i t y and l e s s tendency f o r g r a i n boundary cr a c k i n g i n hot r o l l i n g CuAlNi-Ti a l l o y s . A f t e r the cast s t r u c t u r e had been broken up and r e p l a c e d by a r e c r y s t a l l i z e d equiaxed f i n e g r a i n s t r u c t u r e , both CuAlNi and CuAlNi-Ti e x h i b i t e d very l i t t l e tendency to crack along g r a i n boundaries during hot r o l l i n g , and up to 20% r e d u c t i o n i n t h i c k n e s s could be given at each stage of r o l l i n g . I t was a l s o 11 observed that i n a s i n g l e hot r o l l i n g pass the temperature of specimens dropped below 800°C t o a two phase re g i o n and second phase p r e c i p i t a t e s formed. Consequently, not more than a s i n g l e pass c o u l d be given before h e a t i n g the sample back t o the s i n g l e 0-phase r e g i o n . Immediately a f t e r the l a s t r o l l i n g pass, when the d e s i r e d t h i c k n e s s of 1-1.2 mm had been reached, the sheets were quenched i n water so that the hot r o l l e d s t r u c t u r e was r e t a i n e d [ f i g . 2 ( a ) ] . 2.3 Pr e p a r a t i o n Of T e n s i l e Specimens I t was not p o s s i b l e to machine the t e n s i l e specimens by co n v e n t i o n a l methods due to the b r i t t l e n e s s of the m a t e r i a l i n the a s - r o l l e d c o n d i t i o n . I n s t e a d , t e n s i l e specimens were prepared by spark machining. F l a t t e n s i l e t e s t specimens of 20mm gauge length and 5mm width were spark machined from the as-r o l l e d s t r i p s . Considerable e f f o r t was expended i n developing techniques f o r producing f i n e g r a i n s i z e s . I n i t i a l experiments showed that when 0-phase specimens were c o l d worked 10% and then heated to 800°C , r e c r y s t a l l i z a t i o n and g r a i n growth occurred very q u i c k l y and a f t e r a heat treatment time of 15s., the g r a i n s i z e was 150Atm . Thus t h i s procedure was not s a t i s f a c t o r y f o r producing g r a i n s i z e s f i n e r than 150Mm . This was t r u e even f o r the titanium-added CuAINi a l l o y s . ( a ) (b) F i g u r e 1 - C r o s s - s e c t i o n of t h e i n g o t s s h owing t h e as c a s t and h o m o g e n i s e d s t r u c t u r e , a ) C u A l N i (A1) and b) C u A l N i - T i ( T 1 ) , x4.5 13 In a second attempt the a l l o y was annealed at 750°C ( i n the two phase region) f o r 5 min. i n order to o b t a i n ** 10% of second-phase p r e c i p i t a t e s , f o l l o w i n g a suggestion of White et a l 1 1 t h a t the presence of a second phase a f f e c t s the deformation s u b s t r u c t u r e introduced i n r o l l i n g and a l s o r e t a r d s r a p i d g r a i n growth. However when t h i s a l l o y was c o l d r o l l e d , i t cracked due to continuous p r e c i p i t a t i o n of second phase at the g r a i n boundaries. This attempt a l s o had t o be abandoned. The f i n a l s u c c e s s f u l technique i n v o l v e d r e t a i n i n g the high temperature deformation s t r u c t u r e and suppressing r e c r y s t a l l i z a t i o n by quenching immediately a f t e r hot r o l l i n g . F i g . 2(a) shows the s t r u c t u r e r e t a i n e d a f t e r hot r o l l i n g . Although t h i s micrograph does not show i t very c l e a r l y , the beta g r a i n s i n t h i s c o n d i t i o n are l a r g e and elongated, c h a r a c t e r i s t i c of the r o l l e d s t r u c t u r e . A second phase i s p r e c i p i t a t e d at the g r a i n boundaries and w i t h i n the g r a i n s . T h i s second phase p r e c i p i t a t e i s s i m i l a r to that observed i n specimens annealed at 750°C , which suggests that the p r e c i p i t a t e s were formed due to the temperature drop during r o l l i n g . These p r e c i p i t a t e s have been i d e n t i f i e d by K l i e r et a l 2 1 as y2t an A l - r i c h i ntermediate phase w i t h a s t r u c t u r e s i m i l a r to 7-brass. I t forms by hyper e u t e c t o i d transformation from the 0-phase at temperatures around 700°C even f o r annealing times as short as 5s. Oshima et a l 1 7 have a l s o reported the presence of the y2 p a r t i c l e s i n t h e i r CuAlNi a l l o y s . (a) (b) (c) ( d) Figure 2 - Micrographs showing the microstructure of a l l o y T l , a) Structure before r e c r y s t a l l i z a t i o n b) Same str u c t u r e i n d e t a i l . R e c r y s t a l l i z e d structure a f t e r c) 10s. d) 15s. e) 20s. Note 7 2 slowly disappearing and g r a i n growth taking place. Magnifications: a) xl25, b)-e) x475. 15 When t h i s s t r u c t u r e was h e a t e d t o 8 0 0 ° C , r e c r y s t a l l i z a t i o n a n d g r a i n g r o w t h t o o k p l a c e . H o w e v e r g r a i n g r o w t h was r e t a r d e d by t h e y2 p r e c i p i t a t e s p r e s e n t a n d f i n e g r a i n s i z e s i n t h e r a n g e l 5 - 4 0 y m w e r e o b t a i n e d f o r a n n e a l i n g t i m e s o f 1 0 - I 5 s . H o w e v e r , i n t h i s g r a i n s i z e r a n g e , y2 p r e c i p i t a t e s w e r e s t i l l p r e s e n t due t o i n c o m p l e t e d i s s o l u t i o n . H o w e v e r , a s shown i n f i g . 2 ( c ) , t h e 7 2 p r e c i p i t a t e s a r e p r e s e n t a s f i n e s p h e r i c a l p a r t i c l e s d i s t r i b u t e d t h r o u g h o u t t h e m i c r o s t r u c t u r e a n d w o u l d n o t be e x p e c t e d t o h a v e a h a r m f u l e f f e c t on t h e m e c h a n i c a l p r o p e r t i e s o f t h e a l l o y . A s a n n e a l i n g t i m e p r o g r e s s e s , a s shown i n f i g . 2 ( c ) - ( e ) , s i m u l t a n e o u s , g r a i n g r o w t h a n d p r e c i p i t a t e d i s s o l u t i o n t a k e s p l a c e a n d b e y o n d a n n e a l i n g t i m e s o f 2 0 s , s i n g l e p h a s e b e t a g r a i n s a r e o b t a i n e d a n d t h e g r a i n s i z e i s more t h a n lOO/im . The t y p i c a l r n i c r o s t r u c t u r e s a f t e r c o m p l e t i o n o f r e c r y s t a l l i z a t i o n a n d p r e c i p i t a t e d i s s o l u t i o n a r e shown i n f i g . 3 ( a ) a n d ( b ) . A t t h i s s t a g e , t h e m i c r o s t r u c t u r a l d i f f e r e n c e s b e t w e e n t h e C u A l N i a n d C u A l N i - T i a l l o y s c a n be s e e n . C u A l N i h a s a s i n g l e /3 -phase s t r u c t u r e , w h e r e a s C u A l N i - T i c o n t a i n s s p h e r i c a l s e c o n d p h a s e p a r t i c l e s w i t h a s i z e r a n g e o f 2-5Mm d i s t r i b u t e d u n i f o r m l y t h r o u g h o u t t h e b e t a g r a i n s a n d a l o n g t h e g r a i n b o u n d a r i e s . T h e v o l u m e f r a c t i o n o f t h e s e c o n d p h a s e i s a r o u n d 2 . 5 % . M i c r o p r o b e a n a l y s i s o f t h e s e p a r t i c l e s s h o w e d t h a t t h e c o m p o s i t i o n o f t h e s e p a r t i c l e s i s a p p r o x i m a t e l y 4 0 % C u - 2 5 % A l -2 0 % T i - l 5 % N i . When t h i s c o m p o s i t i o n i s c o m p a r e d w i t h t h e F i g u r e 3 - M i c r o g r a p h s showing m i c r o s t r u c t u r a l d i f f e r e n c e s i n CuAINi (A2) and C u A l N i - T i (T2). Note the p r e s e n c e of x-phase p a r t i c l e s i n C u A l N i - T i a l l o y s , x95 17 matrix composition of 85%Cu-13%Al-2%Ni-0.01%Ti, i t i s seen that almost a l l of the t i t a n i u m added i s present i n the second phase p a r t i c l e s and a l s o that a d i s p r o p o r t i o n a t e amount of n i c k e l i s a l s o present. The c r y s t a l s t r u c t u r e of the p a r t i c l e s i s not known and c o u l d not be found by x-ray d i f f r a c t i o n techniques because of the small volume f r a c t i o n of the p r e c i p i t a t e s p r e s e n t . F o l l o w i n g the nomenclature of Sugimoto et a l 1 2 , they are i d e n t i f i e d i n t h i s t h e s i s as x-phase p a r t i c l e s . For s o l u t i o n treatment times up to 2 minutes, a s a l t bath at 800°C was used to minimize h e a t i n g time. For longer times, a furnace was used since t h i s gave l e s s c o r r o s i o n than the s a l t bath. The s o l u t i o n t r e a t e d specimens were water quenched and then s t r e s s r e l i e v e d at 100-125°C f o r 5 min. to remove any quenching s t r e s s e s . A f t e r heat treatment, the t e n s i l e specimens were mechanically p o l i s h e d to remove any surface contamination and to o b t a i n a smooth s u r f a c e . The surfaces and edges were examined m e t a l l o g r a h i c a l l y f o r quenching c r a c k s . Attempts at e l e c t r o p o l i s h i n g were u n s u c c e s s f u l , p a r t i c u l a r l y i n C u A l N i - T i , due to the presence of second phase p a r t i c l e s which caused surf a c e p i t t i n g . 2.4 Measurement Of Transformation Temperature The Ms temperature was determined by o p t i c a l microscopy. For determining Ms of the a l l o y s having a transformation temperature below room temperature, a w e l l p o l i s h e d specimen was placed i n a shallow copper c o n t a i n e r which was immersed i n an a l c o h o l bath. The a l c o h o l bath was then slowly cooled by the 1 8 a d d i t i o n of c h i l l e d a l c o h o l at -100°C i n s m a l l q u a n t i t i e s t o ob t a i n a c o o l i n g r a t e of approximately 5°C/min. For a l l o y s having Ms above room temperature, the specimen was p l a c e d i n a water bath and the temperature of the bath was s l o w l y r a i s e d by a d d i t i o n of hot water. A lOx microscope was mounted d i r e c t l y above the specimen. The formation of m a r t e n s i t e at Ms was revealed by rumpling produced on the p o l i s h e d s u r f a c e of the spec imen. 2.5 T e n s i l e And Recovery Tests And Fractography T e n s i l e t e s t s were performed on a f l o o r model I n s t r o n t e n s i l e t e s t i n g machine at a s t r a i n r a t e of 1.4X10"*S" 1. Conventional f l a t t e n s i l e t e s t g r i p s were used f o r t e s t i n g at room temperature. A 25mm-l0% extensometer was c l i p p e d on the specimens being t e s t e d to o b t a i n accurate measurements of s t r a i n . T e n s i l e t e s t s at temperatures d i f f e r e n t from ambient were c a r r i e d out by immersing the specimen e i t h e r i n hot water or c h i l l e d a l c o h o l . I t was not p o s s i b l e to use an extensometer i n these environments, so s t r a i n measurements were made d i r e c t l y from the crosshead movement of the machine w i t h c o r r e c t i o n s being made f o r machine c o m p l i a n c e . 3 3 19 2.6 Fatigue Tests F a t i g u e t e s t s were c a r r i e d out on an MTS c l o s e d loop t e s t machine. Specimens used f o r f a t i g u e t e s t i n g were s i m i l a r t o those used f o r t e n s i l e and recovery t e s t s . I t was necessary to t e s t the specimens i n t e n s i l e l o a d i n g , s i n c e they buckled i n compression. Previous e x p e r i e n c e 1 0 had shown th a t experiments u s i n g s t r a i n and stroke c o n t r o l were u n s u c c e s s f u l . Thus the t e s t s were c a r r i e d out under load c o n t r o l , w i t h the specimen being c y c l e d between a minimum s t r e s s of 15MPa and a maximum s t r e s s of 280MPa. Tests were c a r r i e d out at a frequency of 5Hz. However, accurate s t r a i n measurements were taken at r e g u l a r i n t e r v a l s d u r i n g the t e s t s at a reduced frequency of 0.2Hz using an extensometer c l i p p e d to the specimen. 20 I I I . RESULTS AND DISCUSSION  3.1 Grain Growth Fig.4 shows the change i n g r a i n diameter w i t h i n c r e a s i n g s o l u t i o n treatment time at 800°C f o r a l l o y A l . As can be seen, the data f o l l o w s a p a r a b o l i c curve of the form D=kt 2 where D i s the g r a i n diameter and t i s the isothermal annealing time, f o r much of the annealing . This r e l a t i o n i s the i d e a l g r a i n growth law. However, most p o l y c r y s t a l l i n e metals f o l l o w an e m p i r i c a l equation of the form D=kt n, where n i s the g r a i n -growth exponent and has a value s m a l l e r than 0.5. In f i g 4, the g r a i n diameter values show d e v i a t i o n from the p a r a b o l i c law at i n i t i a l g r a i n s i z e values up to 0.2mm. A l s o , there i s d e v i a t i o n from the p a r a b o l i c growth law at g r a i n s i z e s above 0.8mm and there appears to be a l i m i t i n g g r a i n s i z e of 0.95mm beyond which g r a i n growth has more or l e s s stopped. Attempts at f i t t i n g the g r a i n s i z e vs. s o l u t i o n treatment time data f o r a l l o y T l to a p a r a b o l i c growth law were not s u c c e s s f u l . The g r a i n growth was much slower than i n the case of a l l o y A l and the l i m i t i n g g r a i n s i z e was c l o s e to 0.15mm. A l o g - l o g p l o t of the data f o r a l l o y s A l and T1 i s shown i n f i g . 5 . For a l l o y A l , the values i n the g r a i n s i z e range of 0.2 to 0.8mm f i t q u i t e c l o s e l y to a s t r a i g h t l i n e on the l o g - l o g p l o t , the slope of which i s 0.44, c l o s e to the t h e o r e t i c a l value of 0.5. However, g r a i n s i z e s i n the range 0.02 to 0.2mm de v i a t e s i g n i f i c a n t l y from t h i s power law and g r a i n growth seems to be 21 f a s t e r than p r e d i c t e d . As d i s c u s s e d i n Chapter I I , the a l l o y s are two phase (/J • yt) d u r i n g the i n i t i a l stages of growth up t o 20s. Hence, du r i n g most of t h i s r a p i d g r a i n growth p e r i o d , d i s s o l u t i o n i s t a k i n g p l a c e as w e l l as g r a i n growth. I n i t i a l l y , the second phase p a r t i c l e s are l a r g e and block g r a i n growth. However, as they d i s s o l v e , g r a i n growth can occur p r o g r e s s i v e l y more e a s i l y . T h i s may p o s s i b l y e x p l a i n t h i s i n i t i a l r a p i d g r a i n growth r e g i o n . However, i t should be mentioned t h a t other work* on CuAlNi has reported r a p i d g r a i n growth i n the s i n g l e phase c o n d i t i o n , so t h i s suggestion may not be c o r r e c t . The d e v i a t i o n from the power law at g r a i n s i z e s above 0.8mm and a l i m i t i n g g r a i n s i z e of 0.95mm can be e x p l a i n e d by the well-known 's h e e t - t h i c k n e s s ' e f f e c t . B u r k e 2 2 showed t h a t g r a i n growth tended to zero as the g r a i n s i z e approached the l i m i t i n g g r a i n diameter Doo , which i s s m a l l e r than but comparable to the specimen t h i c k n e s s , the g r a i n growth r a t e dD/dt being p r o p o r t i o n a l to (D~ 1 - D^j, ). This p r e d i c t e d l i m i t i n g g r a i n diameter i s c l o s e t o the a c t u a l l i m i t i n g g r a i n diameter f o r a l l o y A1 of 0.93mm, the specimen t h i c k n e s s being 1mm to 1.2mm. Fig . 5 a l s o confirms that the is o t h e r m a l g r a i n growth data f o r a l l o y T l a l s o obeys a power law, but the g r a i n growth exponent has a lower value of n=0.22, i n d i c a t i n g slower g r a i n growth than s i n g l e phase CuAlNi. I 1 i r— 12000 15000 18000 21000 24000 27000 SOLUTION TREATMENT TIME (S ) 3000 6000 9000 30000 Figure A - V a r i a t i o n of grain s i z e with increasing s o l u t i o n treatment time at 800 °C for a l l o y s A l and T l . 10 M (/) Z < OC o 0.1 0.01 Legend A Al X T1 f>=0.44 m o _ 1000 10000 SOLUTION TREATMENT TIME (S ) ' i i i 100000 1000000 Figure 5 - Log-Log plot of grain size vs. solution treatment time at 800 °C for alloys Al and T l . 24 T h i s g r a i n growth r e t a r d a t i o n i s due t o the T i - r i c h second phase p a r t i c l e s present. This i s a consequence of the w e l l known Zener e f f e c t 2 3 which shows that when a g r a i n boundary encounters a p r e c i p i t a t e p a r t i c l e , the g r a i n boundary area i s reduced by an area equal to the c r o s s - s e c t i o n area of the p a r t i c l e . As the g r a i n boundary t r i e s to move past the p a r t i c l e , a r e s t r a i n i n g f o r c e i s exerted by the p a r t i c l e on the g r a i n boundary, and i s given by 2irrocos0sin0 where o = s p e c i f i c surface energy of the g r a i n boundary r = r a d i u s of the second phase p a r t i c l e and 9 = angle which depends upon the p o s i t i o n of the boundary r e l a t i v e to the p a r t i c l e ( f i g . 6 ) . The isothermal g r a i n growth data f o r Tl a l s o shows s i g n i f i c a n t d e v i a t i o n from the power law i n the i n i t i a l g r a i n s i z e range of 0.02-0.1mm, s i m i l a r to a l l o y A1. The l i m i t i n g g r a i n s i z e at long times i s c l o s e to 0.18mm and i s much sm a l l e r than that of a l l o y A l . This lower l i m i t i n g g r a i n s i z e i s a l s o e x p l a i n e d by the Zener E f f e c t . The second phase p a r t i c l e s put an upper l i m i t on the g r a i n s i z e . This i s because a f t e r the g r a i n boundary has trapped a c e r t a i n number of p a r t i c l e s , the s u r f a c e t e n s i o n f o r c e , which i s small due to the lack of s u f f i c i e n t c u r v a t u r e , cannot overcome the r e t a r d i n g force of the second phase p a r t i c l e s . The l i m i t i n g g r a i n s i z e i s given by F i g u r e 6 - Schematic i l l u s t r a t i o n showing i n t e r a c t i o n between a g r a i n boundary and a second-phase p a r t i c l e . 26 4 r R « . — 3 f where r = Radius of the s p h e r i c a l second-phase p a r t i c l e s R = Radius of the average g r a i n f = Volume f r a c t i o n of the p a r t i c l e s T h i s r e l a t i o n s h i p assumes s p h e r i c a l uniform s i z e p a r t i c l e s and uniform d i s t r i b u t i o n of the p a r t i c l e s . These assumptions are approximately s a t i s f i e d by a l l o y T1 (See F i g . 3 ( b ) ) . Taking a volume f r a c t i o n of 2.5% and an average second phase p a r t i c l e diameter of 3um , the l i m i t i n g g r a i n diameter i s 0.16 mm, which i s c l o s e to the experimental l i m i t i n g g r a i n diameter of 0.18 mm. This confirms our p r e l i m i n a r y c o n c l u s i o n that the g r a i n r e f i n i n g mechanism i n CuAlNi-Ti a l l o y s i s due to i n h i b i t i o n of g r a i n boundary m i g r a t i o n . 3.2 Ms Determination Table 2 g i v e s Ms temperatures f o r a number of a l l o y s i n a range of g r a i n s i z e s . In a l l o y s A1 and T1, the melt composition was approximately the same. However, due to the presence of 0.5% t i t a n i u m i n a l l o y T1, second phase T i - r i c h p a r t i c l e s were formed, which changed the ^-matrix composition r e l a t i v e to that of a l l o y A1. 27 As shown i n Table 2, t h i s change i n 0-matrix composition i s r e f l e c t e d i n a s l i g h t l y higher Ms temperature f o r Tl compared to Al . The e f f e c t of g r a i n s i z e on Ms temperature i s a l s o shown i n Table 2. I t can be seen that these o b s e r v a t i o n s are c o n s i s t e n t w i t h those of Dvorak and Hawbolt 2* on CuZnSn p s e u d o e l a s t i c a l l o y s , where lowering the Ms temperature i s a t t r i b u t e d t o i n c r e a s i n g g r a i n c o n s t r a i n t with d e c r e a s i n g g r a i n s i z e . T a b l e 2 - V a r i a t i o n of Ms w i t h t i t a n i u m a d d i t i o n and g r a i n s i z e A l l o y G r a i n S i z e Ms (mm) ( °C) a) E f f e c t of Composition A1 0.125 68 T 1 0.105 75 b) E f f e c t of g r a i n s i z e A1 0.035 58 0.08 60 0.1 65 0.12 68 0.2 68 0.56 70 0.73 70 T l 0.039 68 0.105 76 0.15 76 29 3.3 T e n s i l e Tests 3.3.1 E f f e c t Of Grain S i z e On T e n s i l e P r o p e r t i e s T e n s i l e t e s t s to f r a c t u r e were performed on both p s e u d o e l a s t i c and s t r a i n memory a l l o y s at v a r y i n g g r a i n s i z e s . As d i s c u s s e d i n the previous s e c t i o n , the l i m i t i n g g r a i n s i z e i n the CuAlNi-Ti a l l o y s (T1 and T2) was 0.18mm, w h i l s t i n CuAlNi a l l o y s (A1 and A2) i t was c l o s e to the specimen sheet t h i c k n e s s of 1 mm. F i g . 7 ( a ) - ( c ) show some t y p i c a l s t r e s s s t r a i n curves f o r a l l o y A l , which i s i n the m a r t e n s i t i c s t a t e . The curves show s i g n i f i c a n t changes with g r a i n s i z e . At the two l a r g e r g r a i n s i z e s , there are three p o r t i o n s t o the s t r e s s s t r a i n curve w i t h an i n i t i a l region of higher s l o p e , then a p l a t e a u of lower slope before the f i n a l region of h i g h slope l e a d i n g to f r a c t u r e . At f i n e g r a i n s i z e s , only the f i r s t two p o r t i o n s of the s t r e s s -s t r a i n curve are v i s i b l e and f r a c t u r e had occurred before the region of high slope. This i s reasonable i f one looks at the t r a n s i t i o n s t r e s s between p a r t s 1 and 2, which i n c r e a s e s as g r a i n s i z e decreases and the slope of stage 2 which i n c r e a s e s as the g r a i n s i z e decreases. The t r a n s i t i o n s t r e s s between stages 2 and 3 becomes very l a r g e at f i n e g r a i n s i z e and f r a c t u r e i n t e r v e n e s before i t i s reached. 30 Fig.7(d) and (e) show s t r e s s - s t r a i n curves f o r a l l o y T1 having Ms s i m i l a r to a l l o y Al but c o n t a i n i n g t i t a n i u m . These curves can be compared with f i g . 7 ( b ) and (c) r e s p e c t i v e l y , which have s i m i l a r g r a i n s i z e s . T h i s comparison c l e a r l y shows that the s t r e s s - s t r a i n behaviour of a l l o y Tl i s s i m i l a r t o that of a l l o y A l w i t h values of t r a n s i t i o n s t r e s s being comparable. A l a r g e r g r a i n s i z e could not be obtained i n a l l o y T l , so i t was not p o s s i b l e to produce a curve correspond to f i g . 7 ( a ) . However from comparison of f i g . 7 ( b ) and (c) w i t h f i g . 7 ( d ) and ( e ) , we can make the p r e l i m i n a r y c o n c l u s i o n that the presence of x-phase p a r t i c l e s i n a l l o y T1 has l i t t l e e f f e c t on the t e n s i l e behaviour of the specimens. Fig.8 shows some t y p i c a l s t r e s s - s t r a i n curves f o r a l l o y s A2 and T2 which have Ms temperatures below room temperature and are thus being t e s t e d i n the p s e u d o e l a s t i c c o n d i t i o n . The curves are very s i m i l a r to those seen i n f i g . 7 for a l l o y s i n the m a r t e n s i t i c s t a t e . Specimens wi t h l a r g e r g r a i n s i z e show three p a r t s i n the s t r e s s - s t r a i n curves, w h i l s t the curve f o r f i n e g r a i n s i z e has only two p a r t s . A d d i t i o n of t i t a n i u m [Fig.8(d) and (e)] had l i t t l e e f f e c t on the s t r e s s - s t r a i n curves, although again coarse g r a i n s i z e s could not be produced. F i g u r e 7 - S t r e s s - s t r a i n curves at v a r y i n g g r a i n s i z e s f o r a l l o y s A l and T l . soo 700-4 Figure 7 (continued) - S t r a i n s t r a i n curves at varying grain s i z e s f6r a l l o y s A l and T l . Figure 8 - S t r e s s - s t r a i n curves at varying grain s i z e s for a l l o y s A2 and T2. to Figure 8 (continued) - S t r e s s - s t r a i n curves at varying grain s i z e s f o r a l l o y s A2 and T2. OJ NJ 0) 33 A schematic s t r e s s - s t r a i n curve i s shown i n F i g . 9 . Remember that i n the f i n e s t g r a i n s i z e specimens the t h i r d part of the curve does not appear s i n c e i t i s above the f r a c t u r e s t r e s s . The main parameters i n the curve a r e : o, = the e x t r a p o l a t e d t r a n s i t i o n s t r e s s , do/de = the slope of the p l a t e a u r e g i o n , and 0 * and = the s t r e s s and s t r a i n to f r a c t u r e . These parameters were measured i n a l a r g e number of t e s t s and are p l o t t e d i n f i g s . 1 0 to 13 a g a i n s t a g r a i n s i z e parameter. Dvorak and Hawbolt 2* have suggested that the mechanical p r o p e r t i e s of p o l y c r y s t a l l i n e a l l o y s are not c o n t r o l l e d s p e c i f i c a l l y by g r a i n s i z e but r a t h e r by grain s i z e r e l a t i v e to the dimensions of the specimen. In standard f l a t t e n s i l e specimens, the r a t i o of g r a i n s i z e to thickness i s a measure of the degree of g r a i n c o n s t r a i n t . Although r e s u l t s are p l o t t e d as ( g s / t ) i t should be emphasised th a t no s p e c i f i c observations were made of the e f f e c t of v a r i a t i o n i n t h i c k n e s s ; only v a r i a t i o n i n g r a i n s i z e s were s t u d i e d . As has been concluded e a r l i e r , p s e u d o e l a s t i c and m a r t e n s i t i c a l l o y s show s i m i l a r t e n s i l e behaviour with v a r i a t i o n i n g r a i n s i z e , and only the r e s u l t s for the m a r t e n s i t i c a l l o y s A l and T1 w i l l be presented. 3.3.1.1. Extrapolated T r a n s i t i o n S t r e s s , The e x t r a p o l a t e d t r a n s i t i o n s t r e s s o,, sometimes termed the "flow s t r e s s " or "apparent y i e l d s t r e s s " i s the s t r e s s a s s o c i a t e d with the t r a n s i t i o n between the i n i t i a l e l a s t i c and the t r a n s f o r m a t i o n a l e l a s t i c p l u s p l a s t i c response of the s t r e s s - s t r a i n curve. 34 S T R A I N Figure 9 - A schematic s t r e s s - s t r a i n curve 35 For an a l l o y i n the m a r t e n s i t i c s t a t e , 0, i s a s s o c i a t e d w i t h the homogenous growth of favourably o r i e n t e d v a r i a n t s . In p s e u d o e l a s t i c a l l o y s , a- i s the s t r e s s r e q u i r e d f o r the n u c l e 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 . Fig.10(a) shows the v a r i a t i o n of oy w i t h g r a i n s i z e i n a l l o y s A1 and T1. In a l l o y A1, 0- increases from 90 MPa to 200 MPa as the g r a i n s i z e decreases from 0.75 mm to 0.05 mm. There i s some s c a t t e r i n the r e s u l t s but the r a p i d l y i n c r e a s i n g value of a, as the g r a i n s i z e becomes smaller can be c l e a r l y seen. S i m i l a r s c a t t e r i s observed i n previous work i n Cu-based a l l o y s and i s probably due to u n c e r t a i n t y i n the l o c a t i o n of a,. Fig.10(a) suggests an inverse r e l a t i o n between a, and g r a i n s i z e . In f i g . 10(b), a, i s p l o t t e d a g a i n s t (gs/t) 2 . A l i n e a r r e l a t i o n i s observed. Since the specimen t h i c k n e s s i s constant at 1mm, t h i s i n d i c a t e s a H a l l - P e t c h r e l a t i o n s h i p of the form O , o ( a where d=grain diameter. The H a l l - P e t c h type r e l a t i o n s h i p o r i g i n a t e s from the b a r r i e r e f f e c t of g r a i n boundaries to s l i p propagating from g r a i n t o g r a i n and the b u i l d up of s t r e s s c o n c e n t r a t i o n owing to d i s l o c a t i o n p i l e up. I t has been pointed out by Khan and D e l a e y 7 i n t h e i r work on CuAl martensites that the increase i n oy i s due not only to i n c r e a s i n g g r a i n c o n s t r a i n t s but a l s o to a decrease i n martensite p l a t e t h i c k n e s s with decreasing g r a i n s i z e . T h e i r work shows that the H a l l - P e t c h r e l a t i o n holds f o r 36 soo 250 O CL 2 . 200 A 150 100 50 \ X \ X V a Legend A At X Tl D Al-Z B Tl-Z \ x ^ . — A — ^ JOO o Q-250 H 200 150 H 100 50 H Legend A Al X Tt • A1-Z 18 TI-2 1A 1 I t i l l ! 2 3 4 5 6 7 8 (GRAIN SIZE/THICKNESS) (b) OH— 1 r- 1 1 1 1 1 ~ „ „ 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 O.B (GRAIN SIZE/THICKNESS) (a) 10 F i g u r e 10 - V a r i a t i o n of e x t r a p o l a t e d t r a n s i t i o n s t r e s s o-with a ) ( g s / t ) and b) (gs/t) 37 the v a r i a t i o n of o, with 0-grain s i z e as w e l l as the v a r i a t i o n of o, w i t h /J' -martensite p l a t e t h i c k n e s s . The v a l i d i t y of the H a l l - P e t c h r e l a t i o n i s a l s o confirmed by.White et a l 1 1 i n t h e i r work on g r a i n r e f i n e d CuZnAl a l l o y s deformed below M . Data f o r a l l o y Tl i s p l o t t e d i n f i g . 1 0 ( a ) and (b) and l i e s i n the same range as for a l l o y A l . A f u r t h e r c o n f i r m a t i o n i s obtained from f i g . 1 0 ( b ) which shows that data f o r a l l o y T l tends to f o l l o w the l i n e a r r e l a t i o n obtained f o r a l l o y A l . This p o i n t s to the f a c t that the x-phase p a r t i c l e s i n a l l o y T1 have l i t t l e e f f e c t on the s t r e s s r e q u i r e d f o r homogenous growth of m a r tensite or f o r n u c l e a t i o n of s t r a i n induced m a r t e n s i t e . As the volume f r a c t i o n of x-phase p a r t i c l e s i s low (approximately 2.5%), we might expect the second phase p a r t i c l e s to p l a y an i n s i g n i f i c a n t r o l e i n i n i t i a t i n g homogenous growth of m a r t e n s i t i c v a r i a n t s . The values of ay f o r specimens w i t h g r a i n s i z e i n the range l5-40jim , where 7 2-phase i s present as s p h e r i c a l p a r t i c l e s [ F i g . 2 ( c ) ] , are a l s o p l o t t e d i n f i g . 1 0 ( a ) and (b) and are r e f e r r e d to as A1-2 and T1-2 i n the legend. Even these values c l o s e l y f o l l o w the H a l l - P e t c h r e l a t i o n . In t h i s case the volume f r a c t i o n of second phase p a r t i c l e s i s higher (= 8-10%) but there i s s t i l l very l i t t l e e f f e c t on the o, v a l u e s . 38 The 7 2 p r e c i p i t a t e s and T i - r i c h x-phase p r e c i p i t a t e s have a f i n e s p h e r i c a l p a r t i c l e s i z e (2-5mn ) and a uniform d i s t r i b u t i o n i n the m a t r i x , and thus would be expected to have l i t t l e e f f e c t on o, at small volume f r a c t i o n s . We can thus conclude t h a t i t i s the g r a i n s i z e which i s the major c o n t r o l l i n g f a c t o r i n determining the magnitude of o,, and the presence of u n i f o r m l y d i s t r i b u t e d f i n e s p h e r i c a l p a r t i c l e s i n small volume f r a c t i o n s (up to 10%) has very l i t t l e e f f e c t . 3.3.1.2. Slope Of The " p l a t e a u " Region As shown s c h e m a t i c a l l y i n f i g . 9 , the s t r e s s - s t r a i n curve f o r a p o l y c r y s t a l l i n e CuAlNi a l l o y can be d i v i d e d i n t o three p a r t s . The i n i t i a l l i n e a r s e c t i o n e s s e n t i a l l y represents e l a s t i c deformation of the matrix or mar t e n s i t e s t r u c t u r e , depending on Ms. In the present work i t i s found that the slope i n t h i s region i s independent of g r a i n s i z e . The l i n e a r p o r t i o n above the t r a n s i t i o n s t r e s s a, i s the most u s e f u l one f o r i n t e r p r e t a t i o n of martensite deformation behaviour. The s t r a i n increase i s obtained by homogenous growth of favourably o r i e n t e d martensite v a r i a n t s . A lower g r a d i e n t , do/de, means easy, r e l a t i v e l y u n r e s t r i c t e d movement of mart e n s i t e p l a t e s over l a r g e d i s t a n c e s r e s u l t i n g i n a l a r g e s t r a i n increase f o r a sm a l l increment i n s t r e s s . A high g r a d i e n t i n d i c a t e s d i f f i c u l t , r e s t r i c t e d m a r t e n s i t e p l a t e movement over small d i s t a n c e s , g i v i n g r i s e to a sma l l e r s t r a i n i n c r e a s e f o r the same increment i n s t r e s s . 39 Fig.11(a) and (b) shows the v a r i a t i o n of do/de w i t h ( g s / t ) and (gs/t) r e s p e c t i v e l y . The i n c r e a s e i n da/dc w i t h decrease i n (gs/t) f o l l o w s an i n v e r s e p a r a b o l i c r e l a t i o n . S i m i l a r r e s u l t s have been obtained by White et a l ' 1 i n p o l y c r y s t a l l i n e CuZnAl a l l o y s . The e x i s t e n c e of a H a l l - P e t c h type r e l a t i o n i n d i c a t e s t h a t m a r tensite deformation by p l a t e movement and v a r i a n t r e o r i e n t a t i o n becomes more d i f f i c u l t as g r a i n s i z e i s reduced. As g r a i n s i z e i s reduced, the degree of g r a i n c o n s t r a i n t i n c r e a s e s . A l s o with d e c r e a s i n g g r a i n s i z e , average m a r t e n s i t e p l a t e t h i c k n e s s decreases. Consequently a l a r g e r s t r e s s i s r e q u i r e d to overcome the r e s t r a i n i n g f a c t o r s i n h i b i t i n g m a rtensite p l a t e movement. I t should be emphasized 1 1 t h a t t h i s i n c rease i n gradient w i t h d e c r e a s i n g g r a i n s i z e i s u n l i k e the work hardening rate i n p o l y c r y s t a l deformation by d i s l o c a t i o n g l i d e , which i s independent of g r a i n s i z e . Data f o r a l l o y T1 i s a l s o p l o t t e d on f i g . 1 1 ( a ) and(b). This f o l l o w s the same l i n e a r H a l l - P e t c h type r e l a t i o n and i n d i c a t e s that the presence of T i - r i c h x-phase p a r t i c l e s u n i f o r m l y d i s t r i b u t e d i n the matrix do not a f f e c t the m a rtensite deformation behaviour i n any s i g n i f i c a n t way when they are present i n small volume f r a c t i o n (=* 2.5%). The data for the two phase f i n e grain specimens of Al and T1 w i t h g r a i n s i z e i n the range 15-40/im and c o n t a i n i n g 7 2 -phase p r e c i p i t a t e s a l s o c l o s e l y f o l l o w s the i n v e r s e p a r a b o l i c r e l a t i o n s h i p . This again confirms that the presence of 40 o rx o 0.0 0.1 0.2 0.3 0.4 0.5 0.6 GRAIN SIZE/THICKNESS (a) 3 « 5 6 7_ 8 9 (GRAIN SIZE/THICKNESS) , / 2 (b) F i g u r e 11 - V a r i a t i o n of the gra d i e n t da/de w i t h a ) ( g s / t ) and b) (gs/t) 41 s p h e r i c a l 7 2 - p r e c i p i t a t e s and T i - r i c h x-phase p r e c i p i t a t e s u n i f o r m l y d i s t r i b u t e d i n the matrix do not have a major e f f e c t on m a r t e n s i t e growth and r e o r i e n t a t i o n under the i n f l u e n c e of s t r e s s . Though we might expect the presence of second phase p r e c i p i t a t e s to provide some r e t a r d a t i o n and c o n s t r a i n i n g e f f e c t on m a r t e n s i t e p l a t e movement, the e f f e c t i s not pronounced at small volume f r a c t i o n of p r e c i p i t a t e s . I t i s g r a i n s i z e that i s the major c o n t r o l l i n g f a c t o r i n determining m a r t e n s i t e deformation behaviour. 3.3.1.3 o f And e f Fig.12(a) shows the v a r i a t i o n i n a w i t h (gs/t) f o r a l l o y s A1 and T1. The change i n ax i s very l a r g e , v a r y i n g from 930MPa fo r the f i n e s t g r a i n s i z e specimens to 150 MPa f o r l a r g e r g r a i n s i z e specimens. This graph c l e a r l y shows the major advantage of producing very f i n e g r a i n s i z e specimens.At f i n e r g r a i n s i z e s , specimens are much stronger and as w i l l be seen s h o r t l y , they have much higher s t r a i n s to f a i l u r e . The r e s u l t s here are comparable to the af of 800 MPa reported by Duerig et a l 1 6 i n CuAlNi PM a l l o y s of g r a i n s i z e equal to 20 microns. Fig.12(b) shows that the data f o l l o w s a H a l l - P e t c h r e l a t i o n s i m i l a r to that found fo r a, and do/de. This increase i n af with decreasing g r a i n s i z e i s t y p i c a l of most p o l y c r y s t a l l i n e m a t e r i a l s and has been confirmed i n f e r r o u s m a r t e n s i t e 8 as w e l l as nonferous m a r t e n s i t e s 9 . Khan and Daleay 7 a l s o c o n f i r m a l i n e a r r e l a t i o n between a." and d"^ i n CuAlFe m a r t e n s i t e s . 42 o Q-2 1000 900 H 800 700 600 500 400 H J00 200 100 H I * k * \ a, \ \ Legend A Al X Tl D AI-2 8 T1-Z * " — — -A. 1 I 1 I 1 I 1 0.0 0.1 0.2 0.1 0.4 0.5 0.6 0.7 0.8 ( GRAIN SIZE/THICKNESS ) 1000 a 2 900 H 800 700 600 500 400 300 200 100-1 ( a ) Legend A Al X Tl D A1-2 B T1-2 B / / / » x * X X A ' A ' A 0-J 1 1 1 1 1 1 1 i 1 2 3 « 5 6 7 8 9 10 (GRAIN SIZE/THICKNESS)" (b) F i g u r e 1 2 - V a r i a t i o n of a f w i t h a) (gs/t) and b) (gs/ t ) 43 Miyazaki et a l 2 5 have proposed that in CuAlNi a l l o y s , cracks are formed along grain boundaries as the stres s i s concentrated at grain boundaries due to the large e l a s t i c anisotropy. Cracks usually i n i t i a t e at t h r e e - f o l d nodes where st r e s s concentration i s highest. The cracks then propagate along grain boundaries or transgranularly along favorably oriented c r y s t a l planes. At large 0-grain s i z e s , fewer grain boundaries and three-fold nodes are present than at fine grain s i z e s . However, due to the large e l a s t i c anisotropy, high stress concentrations develop at grain boundary nodes in larger grains even at small t e n s i l e stresses. Thus cracks nucleate at nodes at r e l a t i v e l y lower t e n s i l e stresses. The crack propagation i s also r e l a t i v e l y unhindered as larger grain size provides longer intergranular and transgranular paths before the crack propagation mode i s a l t e r e d . In f i n e grain s i z e specimens, higher stress i s required for crack nucleation at grain boundary nodes as stress concentration due to high e l a s t i c anisotropy i s l e s s . Also, the crack propagation path encounters more r e s t r i c t i o n s and has to follow an i r r e g u l a r path along the grain boundaries or across the grains, which requires higher propagation energy. These factors contribute to the higher o r of f i n e grain samples. The values for a l l o y T1 are also plotted in fig . 1 2(a)and (b) and although there i s some scatter, the values are in the same range as those for a l l o y s A1 and more or less follow the same li n e a r Hall-Petch type r e l a t i o n . This indicates that titanium-rich p a r t i c l e s have no s i g n i f i c a n t e f f e c t on the o of 44 the a l l o y . Sugimoto et a l 1 2 have shown that matrix hardness changes only s l i g h t l y from 210 VHN to 230 VHN w i t h 0.5% t i t a n i u m a d d i t i o n s i n CuAINi. As most of the t i t a n i u m i s segregated i n x-phase p a r t i c l e s , there i s e s s e n t i a l l y no s o l i d s o l u t i o n hardening e f f e c t and as the volume f r a c t i o n of x-phase p a r t i c l e s i s low (<* 2.5%) and the p a r t i c l e s i z e i s l a r g e (2-5^m ), the d i s p e r s i o n strengthening e f f e c t i s a l s o s m a l l . Thus there i s no s i g n i f i c a n t strengthening e f f e c t of the t i t a n i u m a d d i t i o n and i t s major c o n t r i b u t i o n i s to the g r a i n r e f i n i n g e f f e c t . The 0 f values of f i n e g r a i n samples of a l l o y s Al and T1 c o n t a i n i n g 7 2 - p r e c i p i t a t e s a l s o f o l l o w the H a l l - P e t c h type r e l a t i o n , as i s evident from f i g . 1 2 ( a ) and ( b ) . The 7 2 -p r e c i p i t a t e s segregated along g r a i n boundaries are known to have a d e l e t e r i o u s e f f e c t on the s t r e n g t h of the a l l o y . However when they are present as s p h e r i c a l p a r t i c l e s d i s t r i b u t e d u n i f o r m l y i n the 0-phase ma t r i x , they do not a f f e c t the s t r e n g t h of the a l l o y s i g n i f i c a n t l y . F i g . 13(a) and (b) show the v a r i a t i o n of f r a c t u r e s t r a i n ef w i t h (gs/t) and (gs/t) f o r a l l o y s A1 and T1. The r e l a t i o n i s an i n v e r s e p a r a b o l i c one, s i m i l a r to the other t e n s i l e parameters. Just as with the other t e n s i l e parameters, the two phase specimens of A1 and T1 c o n t a i n i n g 7 2 p r e c i p i t a t e s f o l l o w the same H a l l - P e t c h r e l a t i o n . I t i s remarkable to note that i n the g r a i n s i z e range l5-40mn , f r a c t u r e s t r a i n s are as high as 7%. S i m i l a r high f r a c t u r e s t r a i n s have been found i n CuAINi PM a l l o y s with g r a i n s i z e of 20^m 1 6. During et a l 1 6 p o i n t out that 45 i \ \ Legend X Tl • AI-2 B Tl-E 2~ - - - -A , 1 1 1 1 1 1 1 I 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 OB (GRAIN SIZE/THICKNESS ) 7H 3H 2^ (a) Legend A Al X Tl • AW ES TI-2 X / x X X / 6* 4 3 " -1 /7 (GRAIN SIZE/THICKNESS) l / z (b) F i g u r e 13 - V a r i a t i o n of e with a) (gs/t) and b) (gs/t)' 46 t h i s i s c l o s e t o a t e n - f o l d i n c r e a s e compared t o t h e d u c t i l i t y of 0.6% i n t h e c a s t c o n d i t i o n . I t may a l s o be s a i d t h a t compared t o l a r g e g r a i n s i z e specimens w i t h ( g s / t ) = 0 . 7 5 , t h e i n c r e a s e i n f r a c t u r e s t r a i n i n f i n e g r a i n s i z e specimens w i t h ( g s / t ) = 0 . 0 ! 5 i s more than 3 - f o l d ( 2% compared w i t h 7 % ) . 47 3.3.2. E f f e c t Of Temperature On T e n s i l e P r o p e r t i e s Fig.14 shows s t r e s s - s t r a i n curves for a l l o y A2 i n the temperature range from -50°C to 22°C . Ms for t h i s a l l o y i s -15°C . Curves for coarse g r a i n s i z e specimens with gs/t = 0.42-0.48 are shown i n fig.14(a) to (d) . For convenience these are designated as A2-L specimens. Curves for f i n e g r a i n s i z e specimens with (gs/t)=0.08-0.06, (A2-F), are shown i n f i g . 1 4 ( e ) (g). The A2-L specimens show a systematic v a r i a t i o n with temperature, with the three-stage curve c h a r a c t e r i s t i c of low temperature g r a d u a l l y becoming a two-stage curve at high temperatures. A2-F specimes show only two-stage curves with v a r y i n g temperatures as di s c u s s e d i n s e c t i o n 3.3.1. The increase of o, with temperature can be seen i n both cases but other trends are l e s s evident. F i g . 1 5 ( a ) - ( c ) show s t r e s s - s t r a i n curves f o r a l l o y T2 specimens with an intermediate g r a i n s i z e range of (gs/t)=0.l3-0.15. These are designated as T2-I specimens. Once again a systematic change i n the curves with temperature can be seen and the three stage curve at -25°C and to a l e s s e r extent at 0°C becomes a two-stage curve at 22°C . The increase i n a, with temperature can again be c l e a r l y seen. To obtain a c l e a r idea of the v a r i a t i o n of t e n s i l e 500 500 I ? 5 STRAIN (7.) (a) a a. 2 1 ? STRAIN (%) ( b ) soo A2-1 (GS/T)=0.420 T«il T«mp.=45*C (d) ure 14 - S t r e s s - s t r a i n curves for A2-L and A2-F at varying temperatures. co Figure 15 - S t r e s s - s t r a i n curves for T2-I specimens at varying temperatures. 50 behaviour of the a l l o y s w i t h temperature the t e n s i l e parameters o, and Of were p l o t t e d together a g a i n s t temperature. F i g s . 1 6 -1 8 show the v a r i a t i o n of o, and Of w i t h temperature f o r A2-F,A2-L and T2-I specimens, r e s p e c t i v e l y . Despite some s c a t t e r , a, c l e a r l y shows a decrease i n magnitude as temperature approaches Ms, and beyond Ms the value of o, remains e s s e n t i a l l y c o n s t a n t . The v a r i a t i o n of o, w i t h temperature has been f u l l y i n v e s t i g a t e d i n both s i n g l e c r y s t a l and p o l y c r y s t a l l i n e s t r a i n memory a l l o y s 2 6 - 2 7 . In CuZn s i n g l e c r y s t a l s , Arneodo and A h l e r s 2 6 have shown that a, decreases l i n e a r l y w i t h temperature and becomes zero at Ms. The v a r i a t i o n of c, wit h temperature can be c a l c u l a t e d approximately from a C l a u s i u s - C l a p y r o n type e q u a t i o n , da,/dT = A H / A e T oV m where AH = heat of tra n s f o r m a t i o n Le = S t r a i n corresponding t o complete t r a n s f o r m a t i o n t o m a r t e n s i t e . T = Temperature at which the matrix and ma r t e n s i t e phases o are i n e q u i l i b r i u m at zero s t r e s s . V = Molar Volume m 51 1000 -80 -60 -40 -20 1 1 f 0 20 40 60 Temperature (°C) 80 F i g u r e 16 - V a r i a t i o n of o, and w i t h temperature f o r A2-F specimens 500-400 H o CL 300 5 200 H too H X Ml I I T— -40 1 - 7 -20 - i r 0 20 40 60 80 100 -100 -80 -60 Temperature (°C) F i g u r e 17 - V a r i a t i o n of oy and a f w i t h temperature f o r A2-L specimens 52 Temperature (°C) F i g u r e 18 - V a r i a t i o n o f o, and of w i t h t e m p e r a t u r e i n T2-I s p e c imens 53 I t was shown subsequently by P a t e l and Cohen 2 8 that although martensite forms spontaneously on c o o l i n g below Ms, a f i n i t e s t r e s s i s needed to overcome any f r i c t i o n f o r c e r e s i s t i n g the m a r t e n s i t e - a u s t e n i t e i n t e r f a c e movement. Thus the p r e d i c t i o n of a zero t r a n s i t i o n s t r e s s , a,, does not h o l d even i n s i n g l e c r y s t a l s . In p o l y c r y s t a l l i n e metals, as shown by Melton and M e r c i e r 2 9 , o, i s even higher due to i n c r e a s i n g g r a i n c o n s t r a i n t s . At temperatures below Ms, a, remains more or l e s s constant w i t h t e m p e r a t u r e 2 9 . In the present work values of are n e a r l y constant at temperatures below Ms, c o n f i r m i n g the p r e d i c t e d behaviour of a, below Ms. Examination of the v a r i a t i o n of w i t h temperature f o r A2-F specimens i n f i g . 1 6 shows that i t i s d i f f i c u l t to make d e f i n i t e c o n c l u s i o n s regarding the v a r i a t i o n of w i t h temperature due to considerable s c a t t e r i n the v a l u e s . A s i m i l a r s c a t t e r has been observed i n experiments on p o l y c r y s t a l l i n e N i T i a l l o y s . 2 9 Despite the s c a t t e r i t can be s a i d w i t h some confidence that there i s an increase i n o^ as the temperature decreases. This may be explained by c o n s i d e r i n g the e f f e c t of martensite growth on the t e n s i l e behaviour of the a l l o y . M iyazaki et a l 2 5 have shown that i n CuAINi specimens which are i n the m a r t e n s i t i c s t a t e , there are many deformation modes a v a i l a b l e such as twinning or boundary movements between v a r i a n t s . Thus the s t r e s s c o n c e n t r a t i o n s at g r a i n boundaries due to high e l a s t i c ansiotropy are e a s i l y r e l a x e d . Consequently higher s t r e s s i s required so that s t r e s s c o n c e n t r a t i o n s at complex g r a i n boundary nodes are high enough to nucleate a 54 c r a c k . They a l s o show that martensite p l a t e s o f t e n impede the crack propagation when they are formed at the crack t i p s and crack propagation along g r a i n boundaries i s e a s i l y d e v i a t e d due to r e l a x a t i o n of s t r e s s c o n c e n t r a t i o n at g r a i n boundaries. Another o b s e r v a t i o n i s that the c r a c k i n g tendency d u r i n g quenching from 800°C i s l e s s when the quenching temperature i s below Ms, as thermal s t r e s s e s d u r i n g quenching are r e l a x e d by m a r t e n s i t e formation before thermal s t r e s s e s reach the f r a c t u r e s t r e s s . I t can be seen from f i g . 1 6 that at higher temperatures a higher o, i s r e q u i r e d f o r n u c l e a t i o n of SIM. At temperatures f a r above Ms, fewer deformation modes are a v a i l a b l e f o r r e l a x a t i o n of s t r e s s c o n c e n t r a t i o n s at g r a i n boundary nodes due the higher a,. Consequently crack n u c l e a t i o n at g r a i n boundary nodes and crack propagation along g r a i n boundaries i s e a s i e r , accounting f o r the lower a r of the specimens. At temperatures nearer Ms, SIM fomation i s e a s i e r due to the lower a, and due t o r e l a x a t i o n at high s t r e s s c o n c e n t r a t i o n regions higher s t r e s s e s are r e q u i r e d before cracks can nucleate and propagate. T h i s i s r e f l e c t e d by a higher of the specimens. Fig.17 shows the v a r i a t i o n of a, and w i t h temperature f o r A2-L specimens. The r e s u l t s are q u i t e s i m i l a r to those of A2-F specimens i n f i g . 1 6 . However, values of o, and a^. are lower due t o the inverse p a r a b o l i c dependence of a, and o on g r a i n s i z e , as explained i n s e c t i o n 3.2. The r e s u l t s show th a t as the temperature decreases, a, decreases and a increases as 55 p r e d i c t e d . I t i s remarkable t o note that at temperatures above 35°C , o, i s higher than the s t r e s s r e q u i r e d t o n u c l e a t e and propagate c r a c k s , r e s u l t i n g i n o^ of the a l l o y being lower than the expected o, v a l u e s . Hence at temperatures w e l l above Ms i n A2-L specimens, t e n s i l e f r a c t u r e occurs even before SIM begins to form. One problem w i t h the r e s u l t s of f i g . 1 7 i s t h a t a, appears to continue to decrease below Ms, whereas i t would be expected to be constant as the s t r e s s f o r m a r t e n s i t e v a r i a n t s to grow should not vary w i t h temperature. I t i s hard to e x p l a i n why A2-F r e s u l t s show a constant s t r e s s and A2-L r e s u l t s show a decrease. Conceivably i t may be due to experimental e r r o r . The general s i m i l a r i t y of r e s u l t s f o r A2-F and A2-L specimens i n d i c a t e s t h a t g r a i n s i z e does not i n f l u e n c e m a r t e n s i t e t r a n s f o r m a t i o n behaviour w i t h changing temperatures but only a f f e c t s the s t r e s s l e v e l s at which the t r a n s f o r m a t i o n s take p l a c e . Fig.18 shows the v a r i a t i o n of a, and o^ w i t h temperature for T2-I specimens. Once again though magnitude ranges of a, and o are d i f f e r e n t than f o r A2-F and A2-L specimens due to f g r a i n s i z e d i f f e r e n c e s , the v a r i a t i o n i s very s i m i l a r to that seen i n f i g s . 1 6 and 17. o, decreases as temperature approaches Ms, and 0^ r i s e s as a, keeps decreasing. I t i s hard t o say from the data whether a, i s constant below Ms as found i n A2-F specimens or decreases somewhat as found i n A2-L. However, as the v a r i a t i o n of a, and o ^ i s s i m i l a r , i t can be s a i d that presence of T i - r i c h x-phase p a r t i c l e s i n T2-I specimens does not a f f e c t the martensite c h a r a c t e r i s t i c s with v a r y i n g temperature. Fig.19 shows the v a r i a t i o n of do/de with temperature f o r specimens A2-F, A2-L and T2-I. The values for the f i n e g r a i n specimens are l a r g e r than f o r coarse g r a i n specimens as discussed i n . s e c t i o n 3.3.1.2. and the v a r i a t i o n with temperature i s much l e s s than t h i s e f f e c t . The gradient do/de remains more or l e s s constant with temperature f o r A2-F specimens and decreases with temperature for A2-L and T2-I specimens. As temperature decreases, o, decreases, and hence formation of SIM i s e a s i e r . I t i s to be expected that do/de would a l s o decrease as martensite deformation becomes e a s i e r as temperature approaches Ms and beyond Ms i t would be more or l e s s constant. A2-L and T2-I specimens show a decrease i n da/de with decreasing temperature but f a i l to show constant da/de below Ms. A2-F specimens show constant da/de at temperatures below Ms but show an increase above Ms. Hence no set of r e s u l t s gives p r e c i s e agreement with expected behaviour. / -80 -60 1 -40 -20 0 20 Temperature (°C) r 40 Fig u r e 19 - V a r i a t i o n of da/de with temperature i n A2-L, A2-F and T2-I specimens 58 3 . 3 . 3 . Fractography 3.3.3.1 E f f e c t Of Grain S i z e Fig.20 shows a t y p i c a l t e n s i l e f r a c t u r e s u r f a c e of a l l o y A1-L ( g s / t = 0.59). The f r a c t u r e mode i s mostly i n t e r g r a n u l a r w i t h a small amount of t r a n s g r a n u l a r f r a c t u r e . As po i n t e d out i n s e c t i o n 3.3.1.3, the cracks nucleate at g r a i n boundary nodes where the s t r e s s c o n c e n t r a t i o n i s h i g h . The g r a i n boundaries provide the e a s i e s t crack propagation path and as g r a i n s i z e i s l a r g e , the g r a i n boundaries are long and r e l a t i v e l y s t r a i g h t . Hence the tendency of the crack to d e v i a t e from the i n t e r g r a n u l a r path i s low. This i s r e f l e c t e d i n a l a r g e l y i n t e r g r a n u l a r f r a c t u r e surface and low o of the specimens. Fig.21 shows the t e n s i l e f r a c t u r e s u r f a c e of an A1-F specimen. The f r a c t u r e i s mostly b r i t t l e t r a n s g r a n u l a r - t y p e with a r i v e r p a t t e r n but a l s o with some i n t e r g r a n u l a r c o n t r i b u t i o n . As with the l a r g e g r a i n s i z e specimens, i t i s q u i t e l i k e l y that t e n s i l e cracks nucleate at g r a i n boundary nodes due to high s t r e s s c o n c e n t r a t i o n s , as i s evident by the presence of some areas of i n t e r g r a n u l a r f r a c t u r e . However, due to f i n e g r a i n s i z e , the g r a i n boundary o r i e n t a t i o n changes over small d i s t a n c e s . Thus the crack propagation path encounters many d e v i a t i o n s and can change from an i n t e r g r a n u l a r path to a favourably o r i e n t e d t r a n s g r a n u l a r path. E v i d e n t l y higher energy i s r e q u i r e d f o r crack propagation and i s r e f l e c t e d i n a high °f * 59 F i g u r e 21 - Fractograph showing t e n s i l e f r a c t u r e s u r f a c e of an A2-F specimen (gs/t=0.076), x40 60 3.3.3.2 E f f e c t Of Titanium A d d i t i o n Fig.22 shows the e f f e c t of t i t a n i u m a d d i t i o n on the f r a c t u r e s u r f a c e . As can be seen, the f r a c t u r e i s mostly m i c r o v o i d coalescence-type d u c t i l e f r a c t u r e . Some areas of t r a n s g r a n u l a r and i n t e r g r a n u l a r b r i t t l e f r a c t u r e are a l s o present. M i c r o v o i d coalescence f r a c t u r e i s a s s o c i a t e d w i t h bulk p l a s t i c deformation i n the a l l o y s . However there i s no evidence of s i g n i f i c a n t p l a s t i c deformation i n the s t r e s s - s t r a i n curves. The shapes of the s t r e s s - s t r a i n curves are s i m i l a r to those of a l l o y A1 [see f i g . 7 ] and o values are a l s o f a i r l y c l o s e , but comparison of fig.21 and 22 shows that the f r a c t u r e modes op e r a t i n g are completely d i f f e r e n t . In the specimens with t i t a n i u m a d d i t i o n s , i t i s f a i r to assume th a t the cracks nucleate at g r a i n boundary nodes where high s t r e s s c o n c e n t r a t i o n s develop. The evidence of cr a c k s along g r a i n boundary nodes i n other p a r t s of the gauge l e n g t h a f t e r the t e n s i l e t e s t supports t h i s assumption. A l s o some areas of i n t e r g r a n u l a r f r a c t u r e can be seen on the f r a c t u r e s u r f a c e . However the crack propagation mode i s d u c t i l e m i c r o v o i d coalescence-type. C l e a r l y there i s some i n f l u e n c e from the x-phase p a r t i c l e s present i n the matrix. I t may be that there i s h i g h l y l o c a l i s e d p l a s t i c deformation around the p a r t i c l e s j u s t ahead of the crack t i p , g i v i n g r i s e t o micr o v o i d coalescence f r a c t u r e i n s t e a d of tr a n s g r a n u l a r f r a c t u r e . I f only l o c a l i s e d p l a s t i c deformation i s o c c u r r i n g , energy absorbed d u r i n g crack propagation w i l l not be markedly d i f f e r e n t from the F i g u r e 22 - Fractograph showing the t e n s i l e f r a c t u r e s u r f a c e of a T1-F specimen (gs/t = 0.08) Test temperature =22°C, x120 Fi g u r e 23 - Fractograph showing the t e n s i l e f r a c t u r e s u r f a c e of a T l - F specimen (gs/t=0.09) Test temperature=-35°C, x120 62 energy absorbed d u r i n g crack propagation by the t r a n s g r a n u l a r mode. Thus we can expect the of the C u A l N i T i a l l o y s t o be not markedly d i f f e r e n t from that of the CuAINi a l l o y s as long as t h e i r (gs/t) r a t i o s are n e a r l y the same. 3.3.3.3 E f f e c t Of Temperature The e f f e c t of the t e s t temperature on the f r a c t u r e s u r f a c e s was i n s i g n i f i c a n t i n a l l o y s A1 and A2 i n both l a r g e and small g r a i n s i z e specimens. The A1-L specimens showed the same mostly i n t e r g r a n u l a r type of f a i l u r e [ f i g . 2 0 ] with changing temperature and A1-F specimens showed l a r g e l y t r a n s g r a n u l a r type f a i l u r e [ f i g . 2 1 ] . As both these f a i l u r e s are of the b r i t t l e type, temperature has no e f f e c t on the f r a c t u r e mode. Fig.23 shows the f r a c t u r e s u r f a c e of a l l o y T l - F t e s t e d at -35°C . There are areas of i n t e r g r a n u l a r b r i t t l e f r a c t u r e and mic r o v o i d coalescence type d u c t i l e f r a c t u r e . I f t h i s i s compared the wit h f r a c t u r e s u r f a c e of a l l o y T l - F at room temperature [ f i g . 2 2 ] , i t can be concluded that the f r a c t u r e mode changes from d u c t i l e to b r i t t l e w i t h decreasing temperature. This change i n f r a c t u r e with d e c r easing temperature i s p o s s i b l y due to i n c r e a s i n g b r i t t l e n e s s of the g r a i n boundaries w i t h decreasing temperature. The crac k s nucleate at g r a i n boundary nodes as discussed before, but do not dev i a t e from the crack propagation path along g r a i n boundaries as b r i t t l e g r a i n boundaries provide an easy propagation mode. 63 Once crac k s d e v i a t e from the i n t e r g r a n u l a r path, crack propagation occurs by microvoid coalescence. T h i s e x p l a i n s the p a r t l y i n t e r g r a n u l a r and p a r t l y d u c t i l e f r a c t u r e mode. This change i n f r a c t u r e mode wi t h d e c r easing temperature i s p a r t i c u l a r l y important i n the view of the f a c t that the a f of the a l l o y shows an increase w i t h decreasing temperature. I t would be expected that i n c r e a s i n g i n t e r g r a n u l a r f r a c t u r e mode should give r i s e to a lower ox . However as d i s c u s s e d i n s e c t i o n 3.3.2, 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 may have some e f f e c t i n r e l i e v i n g s t r e s s c o n c e n t r a t i o n s at g r a i n boundaries and t h i s e f f e c t p o s s i b l y o f f s e t s the e f f e c t of change i n f r a c t u r e mode. 3.3.3.4 E f f e c t Of Second Phase P r e c i p i t a t e s As d i s c u s s e d i n s e c t i o n 2.3, i n the g r a i n s i z e range 15-40/zm , 7 2 p r e c i p i t a t e s are present due to incomplete p r e c i p i t a t e d i s s o l u t i o n . Fig.25 shows the f r a c t u r e of a l l o y A2-F wi t h the presence of second phase p r e c i p i t a t e s (volume f r a c t i o n =* 7%). The f r a c t u r e i s mostly microvoid coalescence type. When t h i s i s compared w i t h A2-F i n the s i n g l e /3-phase c o n d i t i o n ( f i g . 2 4 ) , which shows mostly b r i t t l e t r a n s g r a n u l a r type f r a c t u r e , we can conclude that the change i n f r a c t u r e mode i s c l e a r l y due to the presence of the second phase p a r t i c l e s . Note that there are no x-phase p a r t i c l e s i n t h i s a l l o y . The reason f o r t h i s change i n f r a c t u r e mode would be same as the change due to t i t a n i u m a d d i t i o n s . 64 F i g u r e 24 - Fractograph showing t e n s i l e f r a c t u r e surface of an A2-F specimen (gs/t = 0.1) i n s i n g l e 0-phase c o n d i t i o n , x120 Figure 25 - Fractograph showing t e n s i l e f r a c t u r e surface of an A2-F specimen (gs/t=0.0i7) c o n t a i n i n g y2 p r e c i p i t a t e s , x120 65 3.4 Recovery P r o p e r t i e s  3.4.1. E f f e c t Of Temperature Fig.26 shows some t y p i c a l s t r e s s - s t r a i n curves used t o determine recoverable s t r a i n s i n the temperature range above and below Ms [Ms= 5°C ] f o r T2-I specimens which are i n the " v i r g i n " c o n d i t i o n ( i e not s t r a i n e d p r e v i o u s t o the t e s t ) . These were loaded to 3.5-4% s t r a i n and then unloaded and subsequently heated above Ms to obta i n the t o t a l r e c o v e r a b l e s t r a i n . The recovery obtained occurs p a r t l y d u r i n g unloading ( p s e u d o e l a s t i c i t y ) and p a r t l y d u r i n g heating ( s t r a i n memory e f f e c t ) . The shapes of the s t r e s s - s t r a i n curves change d u r i n g the lo a d i n g stage w i t h v a r y i n g temperature i n a way d i s c u s s e d i n s e c t i o n 3.3.2 , although i n t h i s case, specimens were not s t r a i n e d to f r a c t u r e . On unloading, the mode of s t r a i n recovery changes markedly with temperature. At temperatures above Ms [Fig.26(a) and ( b ) ] , 75-90% of the recovered s t r a i n i s obt a i n e d i n the unloading stage by the p s e u d o - e l a s t i c i t y e f f e c t and the re s t of the s t r a i n i s recovered on he a t i n g . At temperatures below Ms [Fi g . 2 6 ( c ) and ( d ) ] , only a small p o r t i o n of the s t r a i n i s recovered i n the unloading stage and the major p o r t i o n i s recovered on heating by the s t r a i n memory e f f e c t . Fig.26 shows that t o t a l s t r a i n recovery i s more or l e s s constant w i t h change i n temperature. Thus the major recovery mode, whether p s e u d o e l a s t i c or s t r a i n memory, does not have any s i g n i f i c a n t e f f e c t on the t o t a l recovery o b t a i n e d . 66 F i g . 2 7 shows the v a r i a t i o n of r e c o v e r y w i t h t e m p e r a t u r e f o r T2-I specimens i n the t e m p e r a t u r e range -50°C t o 60°C (Ms=5°C). The r e c o v e r y i s the r e c o v e r e d s t r a i n r e l a t i v e t o t h e i n i t i a l l o a d i n g s t r a i n and i s d e f i n e d by T o t a l r e c o v e r e d s t r a i n x 100 %R = I n i t i a l l o a d i n g s t r a i n As e x p e c t e d , t o t a l r e c o v e r y i s more or l e s s c o n s t a n t w i t h t e m p e r a t u r e and l i e s i n t h e range of 85-90%. E i s s e n w a s s e r and B r o w n 3 0 have c o n f i r m e d t h i s i n CuZnSn s i n g l e c r y s t a l and p o l y c r y s t a l l i n e specimens. As l a r g e g r a i n s i z e specimens were used, t h e y o b t a i n e d t o t a l r e c o v e r i e s c l o s e t o 100%. I n T2-F s pecimens, p e r c e n t r e c o v e r y i s l o w e r t h a n 100%, but t h i s i s a g r a i n s i z e e f f e c t r a t h e r tha-n a t e m p e r a t u r e e f f e c t . (CS/T)=0.I4 T * - l 2 5 A i 0 1 2 ' 5 * STRAIN (%) STRAIN (X) Figure 26 - S t r e s s - s t r a i n curves of T2-I specimens at varying temperatures showing recovered s t r a i n s on unloading and heating. 120 110 H > O (J UJ U J U CC 100 H 90 80 70 60 -70 T2-1 M S = 5"C Ms -50 -30 -10 10 30 TEMPERATURE ( °C) Figure 27 Variation of percent recovery with temperature for T2-I specimen 50 70 CD 69 3.4.2 E f f e c t Of Grain S i z e To determine the e f f e c t of g r a i n s i z e on recovery, a constant i n i t i a l l o ading s t r a i n of 2% was chosen and percent recovery was c a l c u l a t e d from s t r a i n recovered on unloading and h e a t i n g . I t was not p o s s i b l e t o choose an i n i t i a l l o a d i n g s t r a i n greater than 2% as the l a r g e g r a i n s i z e specimens would have f a i l e d . Once again, v i r g i n specimens were used t o e l i m i n a t e the e f f e c t of s t r a i n hardening due t o previous l o a d i n g . Fig.28 shows the v a r i a t i o n of recovery w i t h decreasing (gs/t) f o r a l l o y s A1 and T1. As (gs/t) decreases from 0.7 to 0.035 i n a l l o y A l , percent recovery decreases from 94% t o 84%. As Dvorak and Hawbolt 2* have pointed out, the amount of non-recoverable s t r a i n increases w i t h decreasing ( g s / t ) , s i n c e due to i n c r e a s i n g g r a i n c o n s t r a i n t more p l a s t i c deformation takes p l a c e along with martensite growth because of the higher s t r e s s e s i n v o l v e d . In a l l o y A1 there i s a decrease i n recovery w i t h i n c r e a s i n g g r a i n c o n s t r a i n t , but percent recovery at (gs/t)= 0.035 i s s t i l l around 85%, which means that the d e t e r i o r a t i o n i s not p a r t i c u l a r l y s i g n i f i c a n t . White et a l 1 1 have obtained lower recovery v a l u e s , around 75-80%, f o r the same (gs/t) range i n CuZnAl a l l o y s . This e f f e c t i s p o s s i b l y due to the higher d u c t i l i t y of CuZnAl compared with CuAINi, whereby more p l a s t i c deformation during loading c o n t r i b u t e s to a lower non-recoverable s t r a i n , consequently lowering the recovery obtained. 120 n o H 100 H 90 H 80 -\ 70 A X . ^ ^ E E^" A Legend A Al X Tl • A1-2 12 T1-2 60 0.0 0.1 0.2 0.3 0.4 0.5 0.6 (GRAIN SIZE/THICKNESS) 0.7 Figur e 28 - V a r i a t i o n of percent recovery w i t h g r a i n s i z e at constant a p p l i e d s t r a i n of 2% 71 The values of r e c o v e r i e s at v a r y i n g (gs/t) f o r a l l o y T1 are a l s o p l o t t e d i n f i g . 2 8 . The recovery values l i e i n the same range as those f o r a l l o y A l . This i n d i c a t e s t h a t the presence of x-phase p a r t i c l e s i n volume f r a c t i o n s of 2.5% have no e f f e c t on recovery p r o p e r t i e s of the a l l o y . Enami et a l 1 3 have a l s o r e p o r t e d e x c e l l e n t recovery o b t a i n a b l e i n CuZnAl a l l o y s w i t h vanadium a d d i t i o n s and they c o n f i r m that the presence of second phase p a r t i c l e s do not s i g n i f i c a n t l y a f f e c t recovery p r o p e r t i e s . Fig.28 shows that A1 and T1 specimens c o n t a i n i n g y2 p r e c i p i t a t e s a l s o show good recovery v a l u e s , comparable to those of s i n g l e 0-phase a l l o y s . Though the t o t a l volume f r a c t i o n of second phase p a r t i c l e s i s up to 10% and g r a i n s i z e i s as f i n e as 15-40ym , recovery p r o p e r t i e s are p o t e n t i a l l y u s e f u l f o r p r a c t i c a l purposes. 3.4.3. E f f e c t Of I n c r e a s i n g S t r a i n Fig.29 shows some of the s t r e s s s t r a i n curves used to determine the e f f e c t of i n c r e a s i n g i n i t i a l s t r a i n on recovery p r o p e r t i e s of a l l o y T2-F. In each t e s t , v i r g i n specimens were used to e l i m i n a t e the s t r a i n hardening e f f e c t due t o c y c l i c l o a d i n g . Curves i n f i g . 2 9 i n d i c a t e that as i n i t i a l l o a d i n g s t r a i n i n c r e a s e s , the s t r a i n recovered decreases which consequently means that the percent recovery i s lower. Fig.30 shows the e f f e c t of i n c r e a s i n g s t r a i n on recovery obtained i n a l l o y T2 specimens w i t h constant ( g s / t ) . A comparison of the r e s u l t s f o r s i n g l e /3-phase a l l o y specimens and 800 Figure 29 - Stress-strain curves at increasing applied strains of a)1.5%, b)4% and c)6.5% in T2 specimens. to 73 specimens c o n t a i n i n g y2 (T2-2) i s a l s o shown. Note th a t i n s i n g l e 0-phase a l l o y s , the l o a d i n g s t r a i n c ould not be higher than 4.5% as the f r a c t u r e s t r a i n was c l o s e to 4.5%. However i n two-phase a l l o y s , the loa d i n g s t r a i n c o u l d be as high as 6.5% as the f r a c t u r e s t r a i n was c l o s e to 7% due to t h e i r .f i n e r g r a i n s i z e . Fig.30 i n d i c a t e s that recovery s t e a d i l y decreases as s t r a i n i n c r e a s e s . however even at 6.5% a p p l i e d s t r a i n , recovery i s s t i l l 72%. Enami et a l 1 3 have a l s o shown e x c e l l e n t s t r a i n memory r e c o v e r i e s up to 6% a p p l i e d s t r a i n i n CuZnAl a l l o y s g r a i n r e f i n e d by vanadium a d d i t i o n s . The decrease i n percent recovery w i t h i n c r e a s i n g a p p l i e d s t r a i n i s probably due to the i n c r e a s i n g amount of p l a s t i c deformation t a k i n g p l a c e . As pointed out by Dvorak and Hawbolt 2* , more p l a s t i c deformation by normal s l i p takes place along w i t h m a r t e n s i t e deformation at f i n e g r a i n s i z e s due to the high degree of g r a i n c o n s t r a i n t , thus c o n t r i b u t i n g to higher non-recoverable s t r a i n . Comparison of percent recovery v a l u e s of s i n g l e 0-phase specimens and two-phase specimens wi t h y2 p r e c i p i t a t e s i n fi g . 3 0 shows that at l e a s t up to 4.5% a p p l i e d s t r a i n , percent r e c o v e r i e s are more or l e s s the same. This once again confirms that the presence of x-phase and 7 2-phase p r e c i p i t a t e s does not a f f e c t recovery p r o p e r t i e s . 74 120 no H IOO H Legend A T2-I X 12-1 £ 90 > o O UJ cn 80 x^ . -A o 70 H 60 H 50 H 40 7 -I r 0 1 5 3 4 PERCENT STRAIN 8 F i g u r e 30 - V a r i a t i o n of percent recovery w i t h i n c r e a s i n g a p p l i e d s t r a i n i n T2 specimens 75 3.5 Fatigue P r o p e r t i e s 3.5.1. E f f e c t Of Grain S i z e On Fatigue L i f e Fig.31 shows a t y p i c a l v a r i a t i o n i n s t r a i n on c y c l i n g i n a T2 specimen with (gs/t)=0.12 under load c o n t r o l c o n d i t i o n s . There i s a gradual decrease i n s t r a i n / c y c l e as the t e s t c o n t i n u e s . In the i n i t i a l stages of c y c l i n g , the r a t e of decrease i n s t r a i n i s r a p i d , and a f t e r the f i r s t few c y c l e s the s t r a i n i s more or l e s s steady and remains constant u n t i l f a i l u r e . This phenomenon on c y c l i n g has been observed by Brown 1 0 i n p s e u d o e l a s t i c /3-CuZnSn, and i s due to the accumulation of non-recoverable s t r a i n i n the f i r s t few c y c l e s . With i n c r e a s i n g c y c l e s , non-recoverable s t r a i n i n each c y c l e p r o g r e s s i v e l y decreases and a steady value of s t r a i n per c y c l e i s reached. The s l i g h t increase i n s t r a i n at the end of the t e s t i s p o s s i b l y due to the presence of cracks and t h e i r propagation j u s t before f a i l u r e . Specimens of v a r i o u s a l l o y s w i t h a range of (gs/t) values were t e s t e d under s i m i l a r load c y c l i n g c o n d i t i o n s . The r e s u l t s are t a b u l a t e d i n Appendix I . This shows that the i n i t i a l s t r a i n s were i n the range of 0.50-0.82% f o r a l l specimens. Fig.32 shows the dependence of f a t i g u e l i f e on (gs/t) f o r a l l the a l l o y s s t u d i e d . For a l l o y A2, there i s no s i g n i f i c a n t change i n f a t i g u e l i f e as (gs/t) v a r i e s from 0.65 to 0.065. The f a t i g u e l i f e l i e s i n the range of 3000-7000 c y c l e s , which i s inadequate for most p r a c t i c a l loading a p p l i c a t i o n s 3 . 76 1 0.9-0.8-0.4-0.3-0 2 - | I I I I M i l I I I I I M i l I I I I I I I 11 I I I I ' I I I M i l t 1 10 100 1000 10000 100000 N (CYCLES) Figure 31 - A t y p i c a l v a r i a t i o n of s t r a i n with c y c l i n g i n T2 specimens 77 R e s u l t s f o r p s e u d o e l a s t i c a l l o y s T2 and T3 are a l s o p l o t t e d on f i g . 3 2 . The f a t i g u e l i f e l i e s i n the range of 3000-5000 c y c l e s f o r (gs/t) values i n the range 0.15 to 0.06. The f a t i g u e l i f e i s i n the same range as that of a l l o y A2 and suggests t h a t the t i t a n i u m a d d i t i o n has no n o t i c a b l e e f f e c t on f a t i g u e l i f e and the values are s t i l l very poor. The f a t i g u e t e s t r e s u l t s f o r a l l o y s A l and T1, i n the s t r a i n memory s t a t e at room temperature, are a l s o p l o t t e d i n f i g . 3 2 . The f a t i g u e l i f e i s the range of 2000-5000 c y c l e s . There i s again no s i g n i f i c a n t change i n f a t i g u e l i f e w i t h v a r i a t i o n i n ( g s / t ) . Oshima and Y o s h i d a 3 1 have concluded i n t h e i r work on p o l y c r y s t a l l i n e a l l o y s that the f a t i g u e l i f e i s higher i n the m a r t e n s i t i c s t a t e than i n the p s e u d o e l a s t i c s t a t e . However the increase i n f a t i g u e l i f e i s not very l a r g e . Comparison of p s e u d o e l a s t i c a l l o y s (A2,T2,T3) w i t h m a r t e n s i t i c a l l o y s (A1,T1) i n f i g . 3 2 shows no s i g n i f i c a n t d i f f e r e n c e i n f a t i g u e l i f e of the a l l o y s . As pointed out by Brown 1 0 i n p o l y c r y s t a l l i n e /3-CuZnSn, the f a t i g u e l i f e i s p r i m a r i l y a f u n c t i o n of the maximum s t r e s s i n v o l v e d i n a t e s t . The present r e s u l t s i n CuAlNi and CuAl N i - T i a l l o y s f u l l y agree w i t h t h i s c o n c l u s i o n . 1000000 3 J* 10000-J 00 U J o o X I3< i 1 1 1 1 i i 1 1 i 1 0 0.1 0.2 0.3 0.4 0.5 C.6 0.7 0.8 0.9 1 GRAIN SIZE/THICKNESS Figure 32 - V a r i a t i o n of number of c y c l e s to f a i l u r e (N) with varying g r a i n s i z e i n A1, A2, T1, T2 and T3 a l l o y s 79 A l l o y s T2 and T3 which have v e r y f i n e g r a i n s i z e (15-40 m i c r o n s ) and a r e two phase s t r u c u r e s show much b e t t e r f a t i g u e v a l u e s . T h e i r f a t i g u e l i f e i s ** 50,000 c y c l e s w h i c h i s a p p r o x i m a t e l y t e n t i m e s l a r g e r t h a n t h e v a l u e s f o r o t h e r s p e c i m e n s . A f a t i g u e l i f e of 50,000 c y c l e s a t a s t r e s s of 280 MPa and a s t r a i n of =* 0.7% i s q u i t e a r e a s o n a b l e v a l u e and i n d i c a t e s t h a t t h e s e a l l o y s may be of p r a c t i c a l v a l u e i n c y c l i c a p p l i c a t i o n s . These specimens have a much h i g h e r o t h a n the o t h e r specimens t e s t e d . I t i s g e n e r a l l y c o n s i d e r e d t h a t f a t i g u e l i f e i s c o n t r o l l e d by the f a t i g u e r a t i o w h i c h i s t h e s t r e s s r e l a t i v e t o t h e °^ 3** Thus t h e s e specimens would be e x p e c t e d t o have a much l a r g e r f a t i g u e l i f e . 3.5.2 F r a c t o g r a p h y F i g . 3 3 shows a t y p i c a l f a t i g u e f r a c t u r e s u r f a c e of a l l o y A2-F. A p a r t of the f a t i g u e f r a c t u r e has o c c u r r e d i n t e r g r a n u l a r l y and r e s t i s t r a n s g r a n u l a r . F a t i g u e s t r i a t i o n s were p r e s e n t i n the i n t e r g r a n u l a r r e g i o n , as can be seen i n f i g . 3 4 . S i m i l a r f a t i g u e s t r i a t i o n s were o b s e r v e d i n t h e i n t e r g r a n u l a r a r e a i n o t h e r a l l o y s s t u d i e d . They were never o b s e r v e d i n t h e t r a n s g r a n u l a r a r e a s . O b s e r v a t i o n s of B r o w n 1 0 show t h a t t h e f a t i g u e c r a c k n u c l e a t i o n b e g i n s a t t h r e e g r a i n b o u n d a r i e s which a r e a r e a s of h i g h s t r e s s c o n c e n t r a t i o n . On c y c l i n g , t h e s e c r a c k s s l o w l y p r o p a g a t e a l o n g g r a i n b o u n d a r i e s and e v e n t u a l l y l i n k up. S i m i l a r e f f e c t s have been o b s e r v e d by Yang e t a l 3 2 i n /3-CuAlNi specimens. In t h e p r e s e n t work a l s o , i t seems t h a t i n a l l o y s A2-F f a t i g u e n u c l e a t i o n and p r o p a g a t i o n 80 F i g u r e 34 - Fractograph showing f a t i g u e s t r i a t i o n s i n the i n t e r g r a n u l a r region of f r a c t u r e surface i n A2, x900 81 occurs i n t e r g r a n u l a r l y and a f t e r the crack has propagated a t h r e s h o l d d i s t a n c e , the o v e r l o a d f r a c t u r e occurs t r a n s g r a n u l a r l y . Fig.35 shows the f a t i g u e f r a c t u r e s u r f a c e of a l l o y T2-I. A p a r t of the f r a c t u r e i s i n t e r g r a n u l a r and the r e s t of i t i s m i c r o v o i d coalescence. I t seems that f a t i g u e crack n u c l e a t i o n and propagation occurs i n t e r g r a n u l a r l y , s i m i l a r to the A2-F specimens, as i s evident from the i n t e r g r a n u l a r r e g i o n of the f r a c t u r e s u r f a c e s . The o v e r l o a d f r a c t u r e a f t e r the crack propagates a t h r e s h o l d d i s t a n c e i s of the m i c r o v o i d coalescence type, s i m i l a r to t e n s i l e f r a c t u r e , because of the presence of x-phase p a r t i c l e s . Fig.36 shows the f a t i g u e f r a c t u r e surface of a l l o y T2-F c o n t a i n i n g 7 2 ~ P r e c i p i t a t e s . The f r a c t u r e appears to be completely microvoid coalescence. There i s no evidence of any i n t e r g r a n u l a r f r a c t u r e , even at higher m a g n i f i c a t i o n [ f i g . 3 6 ( b ) ] . A l s o note that the surface topography i s r e l a t i v e l y f l a t when compared with a l l o y T2-I ( F i g . 3 3 ) . As d i s c u s s e d e a r l i e r , these specimens were loaded to only 30% of t h e i r f r a c t u r e s t r e s s . The s t r e s s c o n c e n t r a t i o n at the g r a i n nodes i s r e l a t i v e l y low and i s presumably not enough to cause i n t e r g r a n u l a r c r a c k i n g to develop. 82 F i g u r e 35 - Fractograph showing f a t i g u e f r a c t u r e s u r f a c e of T2 specimen (gs/t=0.!5), x40 (a) Fig u r e 36 - Fractographs showing f a t i g u e surface of T2 specimens (gs/t=0.0l5) c o n t a i n i n g y2 p r e c i p i t a t e s , M a g n i f i c a t i o n s : a) x40, b) x200 84 As di s c u s s e d i n s e c t i o n 3 . 3 . 3 , these a l l o y s show l o c a l i s e d d u c t i l i t y . Thus i t appears that a f a t i g u e crack s t a r t s due t o l o c a l i s e d p l a s t i c deformation. This crack propagates s l o w l y due to the l o c a l i s e d p l a s t i c deformation at the crack t i p . F i n a l l y o v e r l o a d f r a c t u r e occurs but t h i s i s a l s o of the mi c r o v o i d coalescence type. Thus the e n t i r e f r a c t u r e s u r f a c e appears to be due to p l a s t i c deformation. I t might be expected that some f a t i g u e s t r i a t i o n s would be v i s i b l e . However the g r a i n s i z e i s so s m a l l that they might be masked. Thus the good f a t i g u e l i f e of these specimens i s due to t h e i r high a and d u c t i l e f r a c t u r e mode compared wi t h large g r a i n s i z e specimens. 85 IV. CONCLUSIONS i ) A d d i t i o n of 0.5% T i to 0-CuAlNi a l l o y s produced f i n e r c a s t s t r u c t u r e which c o n t r i b u t e d to b e t t e r f o r m a b i l i t y of the m a t e r i a l . A d d i t i o n of t i t a n i u m gave r i s e t o T i - r i c h x-phase s p h e r i c a l p a r t i c l e s u n i f o r m l y d i s t r i b u t e d i n the ^-matrix which were e f f e c t i v e i n g r a i n growth r e t a r d a t i o n . i i ) A g r a i n s i z e as small as 15/um has been obtained by c o n t r o l l e d s o l u t i o n treatment, although y2 p r e c i p i t a t e s are present i n the matrix due to incomplete p r e c i p i t a t e d i s s o l u t i o n . i i i ) T e n s i l e t e s t s showed that the parameters oy, do/de, and e-f were dependent on g r a i n s i z e , governed by H a l l - P e t c h type r e l a t i o n s . S i g n i f i c a n t improvement i n s t r e n g t h of the a l l o y s i s obtained by g r a i n refinement. F r a c t u r e s t r a i n s up t o 7% were obtained at the f i n e s t g r a i n s i z e of 15/am . i v ) Presence of x-phase or 7 2-phase p a r t i c l e s has no s i g n i f i c a n t e f f e c t on the t e n s i l e p r o p e r t i e s of the a l l o y s . v) The g r a i n refinement d i d not a f f e c t the s t r a i n memory p r o p e r t i e s of the a l l o y s t o any la r g e e xtent. At 6.5% a p p l i e d s t r a i n , a recovery of 75-80% was obtained. Presence of second phase p a r t i c l e s (x and 7 2) d i d not a f f e c t recovery of the a l l o y s i g n i f i c a n t l y . v i ) The f a t i g u e l i f e of the a l l o y was governed by the maximum t e n s i l e s t r e s s a p p l i e d . Grain refinement d i d not a f f e c t f a t i g u e l i f e s i g n i f i c a n t l y . However at f i n e s t g r a i n s i z e s i n 66 the range 15-40MIH , s i g n i f i c a n t improvement i n f a t i g u e l i f e (about 10 times) was observed, p o s s i b l y due t o low maximum s t r e s s / a r a t i o and d u c t i l e f r a c t u r e mode. 87 REFERENCES 1. L. Dalaey, R.V. Krishnan, H. Tas and H. Warlimont: J . Mat. S c i . , 9, (1974), 1521, 1536, 1545. 2. T. S a b u r i , S. Nenno and CM. Wayman: Proc. Conf. ICOMAT-79, Cambridge, USA, (1979), 619. 3. CM. Wayman: J o u r n a l of Metals, 32 ( 1980), 129 4. L.M. Schetky: S c i e n t i f i c American, Nov. 1979, 74 5. J . P e r k i n s : Metals Forum, 4, (1981), 153. 6. K. Otsuka, H. Sakamoto and K. Shimizu: Acta Met., 2_7 (1979), 585. 7. A.Q. Khan and L. Delaey: Z. Metalkunde, 60, (1969), 949. 8. A.Q. Khan and L. Delaey: S c r i p t a Met., 4, (1970), 981. 9. A.H. Kasberg and D.T. Mack: Trans. A.I.M.E., 191, (1951), 903. 10. L.C. Brown: Met. Trans, , 13A, (1982), 25. 11. S.M. White, J.M. Cook and W.M. Stobbs: Proc. Conf. ICOMAT-82, Ed. by L.Delaey and M. Chandrasekaran, Leuven, Belgium, (1982), C4-779. 12. K. Sugimoto, K. Kamei, H. Matsumoto, S. Komatsu and T. Sugimoto: Proc. Conf. ICOMAT-82, Ed. by L.Delaey and M. Chandrasekaran, Leuven, Belgium, (1982), C4-761. 13. K. Enami, V. Takimoto, and S. Nenno: Proc. Conf. ICOMAT-82, Ed. by L.Delaey and M. Chandrasekaran, Leuven, Belgium, ( 1 982), C4-773. 14. Y . I k a i , K. Murakami and K. Mishima: Proc. Conf. ICOMAT-82, Ed. by L Delaey and M. Chandrasekaran, Leuven, Belgium, (1982), C4-785. 15. J . Jassen, F. Willems, B. V e r e l s t , J . Maertens and L. Dalaey: Proc. Conf. ICOMAT-82, Ed. by L. Delaey and M. Chandrasekaran, Leuven, Belgium, (1982), C4-809. 16. T. W. Duerig, J . Albrecht and G.H. G r e s s i n g e r : J o u r n a l of M e tals, 34, (1982), 14. 17. R. Oshima, M. Tanimoto, T. Oka, F.E. • F u j i t a , Y. Hanadate, T. Hamada and M. M i y a g i : Proc. Conf. ICOMAT-82, Ed. by L. Delaey and M. Chandrasekaran, Leuven, 66 Belgium, (1982), C4-749. 18. J.V. Wood: Proc. Conf. ICOMAT-82, Ed. by L. Delaey and M. Chandrasekaran, Leuven, Belgium, (1982), C4-755. 19. H. Matsumoto, T. Tanaka, T. I r i z a w a , T. Sugimoto, K. Sugimoto and K. Kamei: J . Japan Copper Research A s s o c i a t i o n , _H, (1976), 92. 20. K. Kamei, H. Matsumoto, K. Sugimoto, T. Sugimoto and T. I r i z a w a : J . Japan Copper Research A s s o c i a t i o n , 19, (1981), 201. 21. E.P. K l i e r and S.M. Grymko: Trans. A.I.M.E., 186 (1949), 613. 22. J.E. Burke: Trans. A.I.M.E., 184 (1948), 2472. 23. P. Hallman and M. H i l l e r t : Scand. J . M e t a l l u r g y , 4, (1975),211. 24. I . Dvorak and E.B. Hawbolt: Met. Trans., 6A, (1975), 95. 25. S. M i y a z a k i , K. Otsuka, H. Sakamoto and K. Shimizu: Trans. Japan I n s t . Metals, 4 (1981), 224. 26. W. Arneodo and M. A h l e r s : Acta Met., 22, (1974), 1475. 27. N. Nakamishi, T. Mo r i , S. Miura, Y. Murakami and S. Kochi: P h i l . Mag., J_8, ( 1973), 277. 28. J.R. P a t e l and M. Cohen: Acta Met., J_, ( 1 953), 531 . 29. K.N. Melton and 0. Me r c i e r : Acta Met., 29, (1981), 393. 30. J.D. Eissenwasser and L.C. Brown: Met. Trans, 3, (1972), 1359. 31. R. Oshima and Y. Yoshida: Proc. Conf. ICOMAT-82, Ed. by L.Delaey and M. Chandrasekaran, Leuven, Belgium, (1982), C4-803. 32. N.Y.C. Yang, C. L a i r d and D.P. Pope: Met. Trans., 8_A, (1977), 955. 33. K. O i s h i and L.C. Brown: Met. Trans, 2, (1971), 1971. 34. F. B o r i k , W.M. Justusson and V.F. Zackay: Trans. A.S.M., 56, (1948), 327. APPENDIX A - RESULTS OF FATIGUE TESTS Spec imen D e s i g n a t i o n Cycles to (g s / t ) % s t r a i n f a i l u r e ( i n steady s t a t e ) A2-F1 7100 0.084 -A2-F2 1630 0.105 -A2-F3 4850 0.096 0.626 A2-F4 3610 0.291 0.71 A2-F5 3210 0.11 0.58 A2-F6 3450 0.583 0.692 A2-F7 2890 0.624 0.722 A2-F8 3560 0.341 0.622 T2-F1 4530 0.11 0.66 T2-F2 1610 0.095 -T2-F3 3810 0.084 0.64 T2-21 45780 0.016 0.618 T2-22 51840 0.024 0.59 T3-F1 4270 0.084 0 . 8 1 T3-F2 5730 0.12 0.67 T3-21 53000 (Unf a i l e d ) 0.016 0.62 T3-22 48720 0.024 0.58 T3-23 43670 0.024 0.61 T1-F1 4270 0.062 0.54 T1 -F2 5280 0. 105 0.57 T1-21 41 190 0.024 0.51 A1-F1 3940 0. 126 0.52 A1-F2 4640 0.08 0.54 A1-F3 3810 0.468 0.586 A1 -F4 1870 0.638 0.61 

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