Characteristics of the martensitic transformation in TiNi and the memory effect
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A n u m b e r o f i n v e s t i g a t i o n s 1-S o n t h e p h a s e t r a n s f o r m a t i o n o c c u r r i n g n e a r r o o m t e m p e r a t u r e in n e a r l y e q u i a t o m i c T i - N i a l l o y s h a v e b e e n r e p o r t e d . T h i s lowtemperature p h a s e transformation has been c a l l e d m a r t e n s i t i c , s i n c e it t a k e s p l a c e n e a r r o o m t e m p e r a t u r e w h e r e diffusion is negligible. N e v e r t h e l e s s , t h e most important characteristic* of the martensitic *Other
characteristics have been reported in a previous paper.6
t r a n s f o r m a t i o n , t h e s u r f a c e r e l i e f e f f e c t , h a s not y e t been o b s e r v e d unambiguously.* F u r t h e r m o r e , t h e or*Mottled structures observed by Wasilewski et al. 4 may correspond to the surface relief effect. However, this was not explicitly s t a t e d . d e r o f t h i s t r a n s f o r m a t i o n d o e s not s e e m t o be w e l l established. W a s i l e w s k i et a l . 4 r e p o r t e d that t h e t r a n s i t i o n i s f i r s t o r d e r , w h i l e B e r m a n e! a l . 7 r e p o r t e d i t t o be o f a h i g h e r o r d e r , b o t h r e p o r t s b e i n g b a s e d o n specific heat measurements. T h e p u r p o s e o f t h e p r e s e n t p a p e r is to p r e s e n t dir e c t e v i d e n c e o f a s u r f a c e r e l i e f e f f e c t i n the p r e s e n t a l l o y , w h i c h is a m a n i f e s t a t i o n o f t h e m a r t e n s i t i e n a t u r e o f t h e t r a n s f o r m a t i o n , a n d also to p o i n t out t h e thermoelastic n a t u r e of the martensitic t r a n s f o r m a tion. T h e o r d e r of the transformation has been d e t e r m i n e d by X - r a y m e a s u r e m e n t s d u r i n g t h e t r a n s f o r m a t i o n . B a s e d o n t h e s e a n d o t h e r observations 6 t h e origin of t h e u n i q u e m e m o r y e f f e c t i n t h i s a l l o y i s c o n s i d e r e d . 1) E X P E R I M E N T A L
METHODS
T h e a l l o y w a s p r e p a r e d f r o m 9 9 . 8 wt p c t s p o n g e t i t a n i u m a n d 99.9 p c t e l e c t r o l y t i c n i c k e l by a r c m e l t i n g in a n a r g o n a t m o s p h e r e . S e v e r a l r e m e l t s were made t o i n s u r e h o m o g e n e i t y . The a l l o y c o m p o s i t i o n as det e r m i n e d by c h e m i c a l a n a l y s i s w a s T i - 4 9 . 7 5 a t . p c t N i , w i t h a n o x y g e n c o n t e n t o f 430 p p m . T h e a l l o y i n g o t s w e r e hot r o l l e d a t 9 0 0 ° C i n t o s h e e t s 1.5 m m t h i c k t h i c k , w h i c h were cut i n t o s p e c i m e n s i z e s , polished w i t h m e t a l l o g r a p h i c p a p e r , a n d s u b j e c t e d to t h e following heat treatments. For optical microscopic observation: 1 0 0 0 ° C f o r 22 h r f o l l o w e d by a r a p i d w a t e r quench; K. OTSUKA is Research Associate, Institute of Scientific and Industrial Research, Osaka University,Osaka, Japan. T . SAWAMURA formerly Graduate Student, Osaka University, is now with t
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