The reversible martensite transformation in iron-platinum alloys near Fe 3 Pt
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A s e r i e s of i r o n - p l a t i n u m alloys c o n t a i n i n g 25 or 27 at. pet p l a t i n u m and with o r d e r i n g of the r - p h a s e v a r y i n g f r o m s u b s t a n t i a l d i s o r d e r to n e a r l y complete o r d e r have b e e n t h e r m a l l y cycled b e t w e e n 25~ and - l g 6 ~ The k i n e t i c s of the y ~ a t r a n s f o r m a t i o n s , the h y s t e r e s i s r e v e a l e d b y e l e c t r i c a l r e s i s t a n c e / t e m p e r a t u r e plots, the t h e r m o e l a s t i c growth and the r e a p p e a r a n c e of an i d e n t i c a l m i c r o s t r u c t u r e a f t e r t h e r m a l c y c l i n g (the m i c r o s t r u c t u r a l m e m o r y effect) w e r e studied as a function of the o r d e r i n g of the y - p h a s e . T h e r m o e l a s t i c growth does not a p p e a r to be affected by changes in the d e g r e e of o r d e r of p a r t i a l l y o r d e r e d s p e c i m e n s but the m i c r o s t r u c t u r a l m e m o r y was i m p e r f e c t in the m o s t highly o r d e r e d s p e c i m e n e x a m i n e d . In a g r e e m e n t with e a r l i e r o b s e r v a t i o n s by Dunne and Wayman, s the d i f f e r e n c e b e t w e e n the As and Ms t e m p e r a t u r e s and the h y s t e r e s i s d e c r e a s e m a r k e d l y a s the o r d e r is i n c r e a s e d . It is shown b y t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y that i n a l l but the m o s t h i g h l y - o r d e r e d s p e c i m e n s the a ~ r t r a n s f o r m a t i o n p r o d u c e s p l a t e s of a u s t e n i t e with a high d e n s i t y of d i s l o c a t i o n s . T h e s e plates a r e s e p a r a t e d f r o m the s u r r o u n d i n g u n t r a n s f o r m e d p a r e n t a u s t e n i t e b y a r r a y s of d i s l o c a t i o n loops l y i n g in the i n t e r f a c e s b e t w e e n u n t r a n s f o r m e d p a r e n t a u s t e n i t e and the o r i g i n a l m a r t e n s i t e p l a t e s . All the d i s l o c a t i o n s have a B u r g e r s v e c t o r d i r e c t i o n which is the s a m e as that of the u s u a l s l i p d i s l o c a t i o n in a u s t e n ire. Such d i s l o c a t i o n s l y i n g in a habit plane m u s t be s e s s i l e . In the w e l l - o r d e r e d s p e c i m e n d i s l o c a t i o n p a i r s , t y p i c a l of glide d i s l o c a t i o n s in a c r y s t a l with l o n g - r a n g e o r d e r , w e r e f o r m e d in the a u s t e n i t e f o r m e d b y the r e v e r s e t r a n s f o r m a t i o n . T h e s e d i s l o c a t i o n s w e r e s e g r e g a t e d into r o u g h l y p l a t e - l i k e c l u s t e r s , but the n u m b e r of c l u s t e r s in unit v o l u m e was a p p r e c i a b l y l e s s than the n u m b e r of o r i g i n a l p l a t e s of m a r t e n s i t e . In this case, no a r r a y s of s e s s i l e loops of d i s l o c a t i o n s m a r k the l o c a t i o n s of the o r i g i n a l m a r t e n s i t e - a u s t e n i t e i n t e r f a c e . R is deduced f r o m
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