The effects of applied stress on the martensitic transformation in TiNi
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THE d e f o r m a t i o n
behavior of the compound T i N i has b e e n r e p o r t e d by a n u m b e r o f i n v e s t i g a t o r s , 1-4 t h e u n expectedly low yield s t r e n g t h a n d high ductility h a v i n g been a s c r i b e d t o the martensitic transformation w h i c h o c c u r s i n t h e v i c i n i t y of r o o m t e m p e r a t u r e . H o w e v e r , no d i r e c t r e l a t i o n s h i p h a s b e e n e s t a b l i s h e d . F u r t h e r m o r e , the rationalization of the experimental behavior o n t h e b a s i s of " s t r a i n m a r t e n s i t e " f o r m a t i o n c a n n o t a c c o u n t f o r a l l of the r e p o r t e d o b s e r v a t i o n s . S p e c i f i c a l l y , it c a n n o t a p p l y to t h e r e p o r t e d l o w y i e l d s t r e n g t h a n d high ductility o f t h e m a t e r i a l b e l o w t h e t r a n s i t i o n t e m p e r a t u r e r a n g e .5 T h e recently reported observations of anelastic rec o v e r y in " p l a s t i c a l l y " d e f o r m e d T i N i , 8 a n d o f t h e v a r i a t i o n in t h e s t r e s s - s t r a i n b e h a v i o r a c r o s s t h e martensitic transformation r a n g e of temperature 7 w e r e i n t e r p r e t e d a s due t o t h e e f f e c t s o f a p p l i e d s t r e s s o n t h e t r a n s f o r m a t i o n . T h e anomalously high fatigue s t r e n g t h o f T i N i " w a s s i m i l a r l y i n t e r p r e t e d a s due t o t h e r a p i d w o r k - h a r d e n i n g of t h i s m a t e r i a l t h r o u g h repeated s t r e s s - a s s i s t e d transformation. 9 S u c h a n o m a l o u s b e h a v i o r i s not u n i q u e l y c h a r a c t e r i s t i c of TiNi. Anelastic recovery on the r e m o v a l of s t r e s s h a s b e e n p r e v i o u s l y o b s e r v e d i n In-T1,1°'11 A u C d , ~2'~3 a n d C u - A 1 - N i , ~4 a n d r e f e r r e d t o a s " s u p e r elasticity",14 or ferroelasticity".~s T h e mechanism p r o p o s e d to e x p l a i n t h e l a t t e r r e q u i r e s t h e m a t e r i a l to exist initially in a finely t w i n n e d , m a r t e n s i t i c s t r u c t u r e ; t h e d e f o r m a t i o n a n d r e c o v e r y t a k e p l a c e by t h e g r o w t h o f o n e o f t h e twin o r i e n t a t i o n s at t h e expense o f t h e o t h e r . ~°'n T h i s e x p l a n a t i o n c a n n o t a p p l y to t h e f o r m e r , o r t o t h e r e p o r t e d b e h a v i o r of T i N i , 6 w h e r e t h e i n i t i a l s t r u c t u r e is h i g h - t e m p e r a t u r e ~ . T h e " s h a p e m e m o r y " e f f e c t is c o m m o n l y conside r e d as r e l a t e d to t h e t r a n s f o r m a t i o n . H e r e a s p e c i R. J. WASILEWSKI, on leave from Battelle Memorial Institute, Columbus Laboratories, is currently Staff Associate, EngineeringMaterials, National Science Foundation, Washington, D. C. Manuscript submitted January 2 6 , 1971.
METALLURGICAL
TRANSACTIONS
m e n , a p p a r e n t l y s e v e r e l y d e f o r m e d w h i l e in t h e m a r t e n s i t i c s t r u c t u r e , r e t u r
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