Deformation behavior of NiTi-based alloys
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O, MERCIER
The i n t e r m e t a l l i c compound N i T i is w e l l known f o r its shape m e m o r y p r o p e r t i e s , which have b e e n shown to be a c o n s e q u e n c e of its m a r t e n s i t e t r a n s f o r m a t i o n . It has been d e m o n s t r a t e d I that in the alloy s y s t e m (Nil-xCUx)Ti, the flNiTi phase is r e t a i n e d at 800~ f o r v a l u e s of x up to 0.7 and it has b e e n shown ~ that t h e s e a l l o y s a l s o exhibit a s i m i l a r m a r t e n s i t e t r a n s f o r m a t i o n to N i T i a l l o y s . T h e s e N i T i - b a s e d a l l o y s when d e f o r m e d in the m a r t e n s i t e condition show c h a r a c t e r i s t i c s t r e s s s t r a i n c u r v e s s i m i l a r to that i l l u s t r a t e d in F i g . 1, which f o r the p u r p o s e of the p r e s e n t d i s c u s s i o n is a r b i t r a r i l y divided into t h r e e s t a g e s , m a r k e d on the f i g u r e . M o h a m e d and W a s h b u r n 3,4 have p r o v i d e d e v i d e n c e which s u g g e s t s that Stage I r e s u l t s . f r o m a c o m b i n a t i o n of a s t r e s s induced growth of one m a r t e n s i t e o r i e n t a t i o n at the e x p e n s e of an adjacent, u n f a v o r a b l y o r i e n t e d one; a s t r e s s induced r e o r i e n t a t i o n of e x i s t i n g m a r t e n s i t e and a s t r e s s induced twin boundary m i g r a t i o n within a m a r t e n s i t e v a r i a n t . S i m i l a r p r o c e s s e s have b e e n o b s e r v e d to occur in CuA1Ni. 5 None of t h e s e d e f o r m a t i o n m e c h a n i s m s n e c e s s a r i l y c r e a t e s i g n i f i c a n t n u m b e r s of latt i c e d e f e c t s and c o n s e q u e n t l y on h e a t i n g and r e v e r t i n g to the high t e m p e r a t u r e phase, the o r i g i n a l shape is recovered. A s i m i l a r s t r e s s - s t r a i n curve for single crystals of CuZnSi has been r e p o r t e d ~ and a s e c o n d l i n e a r stage has b e e n o b s e r v e d 7'8 in CuA1Ni single c r y s t a l s . It was s u g g e s t e d 7,s that this was the r e s u l t of an e l a s t i c t w i n n i n g - d e t w i n n i n g p r o c e s s since a l m o s t i d e n t i c a l s l o p e s a r e obtained f o r loading and unloading. T h i s has led s o m e a u t h o r s %9 to a s c r i b e the Stage II p a r t of the s t r e s s - s t r a i n c u r v e of N i T i p o l y c r y s t a l s to a s i m i l a r e l a s t i c p r o c e s s . T h e r e is g e n e r a l a g r e e m e n t h o w e v e r 4'%1~ that Stage HI c o r r e s p o n d s the o n s e t of n o r m a l p l a s t i c flow. It is the p u r p o s e of the p r e s ent w o r k to c l a r i f y the o p e r a t i n g d e f o r m a t i o n m e c h a n i s m s in N i T i b a s e a l l o y s with p a r t i c u l a r e m p h a s i s on Stage II, and to show that n o n e l a s t i c p r o c e s s e s o c c u r within this r e g i o n . E x p e r i m e n t a l . The m a t e r i a l used was a 46 wt pct Ti, 47 wt pct Ni 7 wt pct Cu alloy, p r e p a r e d by v
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