Pseudoelasticity and the strain-memory effect in an Ag-45 at. pct Cd alloy
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THEeffect
of applied s t r e s s on m a r t e n s i t i c t r a n s f o r m a t i o n s is well e s t a b l i s h e d ) The application of an u n i a x i a l t e n s i l e s t r e s s can i n c r e a s e the t r a n s f o r m a t i o n t e m p e r a t u r e , the change in this t e m p e r a t u r e being a function of the heat of t r a n s f o r m a t i o n and the v o l u m e change a s s o c i a t e d with the change p a r e n t to m a r t e n s i t e . 2 If the t e m p e r a t u r e is f a v o r a b l e , the m a r t e n s i t e thus f o r m e d r e v e r t s to the p a r e n t phase on r e l e a s e of the s t r e s s giving r i s e to the well known p s e u d o e l a s t i c effect. T h i s effect was f i r s t o b s e r v e d in A u - C d a l l o y s ; 3 and indeed A u - C d is p r o b a b l y the prototype s y s t e m for nonferrous martensitic transformations. Considerable a m o u n t s of p s e u d o e l a s t i c s t r a i n have also been obs e r v e d in In-T1, 2 Cu-A1-Ni, 4'5 C u - Z n - S n ,8'7 and C u - Z n - S i 8 a l l o y s . The effect is m o r e s p e c t a c u l a r in s i n g l e c r y s t a l s p e c i m e n s c o m p a r e d to p o l y c r y s t a l l i n e ones. Another effect r e l a t e d to s t r e s s - i n d u c e d m a r t e n s i t e (SIM) f o r m a t i o n is the s t r a i n - m e m o r y effect in which a s p e c i m e n that is d e f o r m e d at low t e m p e r a t u r e s (below Ms) r e t u r n s to its i n i t i a l shape on heating above Af. The s t r a i n - m e m o r y effect has been o b s e r v e d in A u - C d , 8 In-T1, 9 T i - N i , 1~ T i - N b , H , C u - Z n - S n ,7 Cu-A1Ni, 5'~2 and C u - Z n 13 a l l o y s . The d e f o r m a t i o n at low t e m p e r a t u r e r e s u l t s in a r e a r r a n g e m e n t of the m a r t e n s i t i c p l a t e s , so as to a c c o m m o d a t e the t e n s i l e s t r a i n . A l m o s t a l l a l l o y s which exhibit s t r a i n - m e m o r y effect have twinned m a r t e n s i t i e s t r u c t u r e s : The r e o r i e n t a t i o n , t h e r e f o r e , r e f e r s to the p r e f e r e n t i a l growth of one twin o r i e n t a t i o n over the other. The p r e s e n t i n v e s t i g a t i o n was u n d e r t a k e n to study SIM f o r m a t i o n in Ag-45 at. pct Cd a l l o y s . Both s i n g l e and p o l y c r y s t a l l i n e s p e c i m e n s w e r e e x a m i n e d . T e n s i l e t e s t s w e r e c a r r i e d out over a wide t e m p e r a t u r e r a n g e and d i r e c t o b s e r v a t i o n of the s t r e s s - i n d u c e d m a r t e n site was m a d e . The s t r a i n - m e m o r y e f f e c t w a s e x a m i n e d in single c r y s t a l s p e c i m e n s . A method was developed to c a l c u l a t e the a m o u n t of p s e u d o e l a s t i c s t r a i n that a c R. V. KRISHNANis Research Assistant, Department Metaalkunde, Katholieke Universiteit, Leuven, Belgium. L.C. BROWNis Associate Professor, Department of Metallurgy,University of British Columbia, Vancouver, Canada. This paper is based upon a the
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