Homogeneity range and the martensitic transformation in TiNi
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ensitic transformation t a k e s place at -60°C, independent of the overall composition. Q u e n c h i n g f r o m 600°C and a b o v e raises the transition t e m p e r a t u r e in titanium-rich, and l o w e r s it m a r k e d l y in nickel-rich compositions. T h e resulting martensite structure in the latter is also believed to c h a n g e with increasing the prior anneal temperature. T h e o b s e r vations reported previously on the transformation behavior must be considered as obtained on nonequilibrium, metastable materials,which m a y account for the discrepancies in the obs e r v e d property variation. C l o s e control of the processing variables is n e c e s s a r y to e n s u r e reproducibility, particularly if nonequllibrium transformation behavior is desired.
T H E r e m a r k a b l e b e h a v i o r of t h e i n t e r m e t a l l i c c o m p o u n d T i N i w a s f i r s t r e p o r t e d by B u e h l e r a n d W i l e y . 1 ' 2 T h e y f o u n d it t o be d u c t i l e a n d i m p a c t r e s i s t a n t ; t o e x h i b i t u n i q u e d a m p i n g p r o p e r t i e s ; a n d t o be q u e n c h - h a r d enable at s o m e compositions. T h e "mechanical m e m o r y " e f f e c t w a s r e p o r t e d by B u e h l e r ,3 w h o o b s e r v e d that s p e c i m e n s d e f o r m e d a t r o o m t e m p e r a t u r e t e n d p a r tially to r e c o v e r t h e i r o r i g i n a l s h a p e o n f i r s t h e a t i n g to m o d e r a t e (~100°C) t e m p e r a t u r e s . A l l t h e s e a n o m a l i e s w e r e i n i t i a l l y r a t i o n a l i z e d i n t e r m s o f the d e c o m p o s i t i o n of T i N i t o T i 2 N i a n d T i N i s , in a g r e e m e n t w i t h t h e e q u i l i b r i u m d i a g r a m s p r o p o s e d e a r l i e r .4 ' s X - r a y d i f fraction d a t a interpreted a s confirming this view were presented. 6 F u r t h e r m o r e , the decomposition of the " m e t a s t a b l e " T i N i s t r u c t u r e a t a p p r o x i m a t e l y 50°C w a s s t a t e d t o be s t r e s s - s e n s i t i v e , o n e o r the o t h e r o f of the a b o v e compounds b e i n g predominantly f o r m e d depending on whether the applied s t r e s s w a s tensile or c o m p r e s s i v e .3 T h e e x i s t e n c e o f a diffusionless t r a n s f o r m a t i o n in T i N i w a s f i r s t s u g g e s t e d by P u r d y a n d P a r r . 7 S u b s e q u e n t i n v e s t i g a t i o n s s-18 h a v e n o w e s t a b l i s h e d b e y o n d d o u b t that the s t r u c t u r e t r a n s i t i o n o c c u r r i n g i n t h e v i c i n i t y o f r o o m t e m p e r a t u r e is m a r t e n s i t i c a n d c a n n o t i n v o l v e d i f f u s i o n . A wide d i s a g r e e m e n t , h o w e v e r , e x i s t s b e t w e e n t h e o b s e r v a t i o n s r e p o r t e d by d i f f e r e n t i n v e s t i g a t o r s on t h e t r a n s f o r m a t i o n b e h a v i o r . T h i s i s i l l u s t r a t e d by t h e d i f f e r e n t e f f e c t s o f t h e c o m p o s i t i o n o n t r a n s f o r m a t i
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