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C. LING

AND

ROY

KAPLOW

The " p r e 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 of the B2 p h a s e in N i T i was studied u si n g e l e c t r i c a l r e s i s t a n c e m e a s u r e m e n t , X - r a y d i f f r a c t i o n , and s h a p e - c h a n g e e f f e c t s . Th e d e g r e e of r h o m b o h e d r a l d i s t o r t i o n of that t r a n s f o r m a t i o n and the e l e c t r i c a l r e s i s t a n c e change show s i m i l a r t e m p e r a t u r e d e p e n d e n c e . In p a r t i c u l a r , when TR >> M s, t h i s e n c o m p a s s e s a n a r r o w t e m p e r a t u r e r e g i o n of r a p i d i n c r e a s e j u s t b el o w TR, f o l l o w e d by an e x t e n d e d r a n g e of l e v e l l i n g off. T h e o n s e t of the 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 is not n e c e s s a r i l y p r e c e e d e d by the r h o m b o h e d r a l d i s t o r t i o n , and the l a t t e r p r o c e e d s c o n t i n u o u s l y below T R and p e r s i s t s t h r o u g h o u t the m a r t e n s i t e t r a n s f o r m a t i o n r a n g e . Thus t h r e e kinds of p h as e t r a n s f o r m a t i o n can o c c u r : B2 ~- R, B2 ~ M, R ~ M; B2 ~- R is not a p r e c u r s o r y p h e n o m e n o n . O b s e r v a t i o n s on the t w o - w a y m e m o r y (TWM) and i t s c o r r e l a t i o n to r e s i s t a n c e c h a n g e s in N i T i l e a d s to the c o n c l u s i o n that when T R > M s , the TWM is c o m p r i s e d of two s t a g e s . On cooling, the B2 ~ R t r a n s f o r m a t i o n is r e s p o n s i b l e f o r the i n i t i a l stage, which is t h e r m a l l y r e v e r s i b l e without h y s t e r e s i s . T h e s e c o n d s t a g e of the TWM, which is due to m a r t e n s i t e f o r m a t i o n , is r e v e r s i b l e in sh ap e change, but with a t h e r m a l h y s t e r e s i s . It is a l s o s u g g e s t e d that on heating, the R ~ B2 t r a n s f o r m a t i o n can c o n t r i bute to the p r i m a r y shape m e m o r y e f f e c t in N i T i .

I. INTRODUCTION

It

is g e n e r a l l y r e c o g n i z e d that the shape m e m o r y and r e l a t e d e f f e c t s (see F i g . 1) in ~3-alloys of the noble m e t a l s a r e a s s o c i a t e d with the i n t e r a c t i o n of s t r e s s with the 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 . 1 M o s t of t h e s e a l l o y s have a CsC1 or FesA1 s t r u c t u r e at high t e m p e r a t u r e and a l l of t h e m t r a n s f o r m m a r t e n s i t i c a l l y to a low t e m p e r a t u r e p h a s e with a l o w e r s y m m e t r y . A s e c o n d g ro u p of a l l o y s which e x h i b i t the shape m e m o r y e f f e c t a r e b i n a r y a l l o y s of t r a n s i t i o n m e t a l s , e the m o s t e x t e n s i v e l y studied of which is N i T i . While the high t e m p e r a t u r e p h a s e in N i T i is a l s o c o n s i d e r e d to be the CsC1 (B2) type, z-4 and the m a r t e n s i t e a m o n o c l i n i c d i s t o r t i o n of the B19 s t r u c t u r e , ~ t h e r e is an i