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P A S C O V E R A N D S. V. R A D C L I F F E

T h e thermodynamic approach t o the understanding of martensitic transformations can provide i n s i g h t into t h e m e c h a n i s m o f t r a n s f o r m a t i o n , s e r v e a s a g e n e r a l i z a t i o n t e c h n i q u e a m o n g t h e alloying s y s t e m s , a n d as a f r a m e w o r k f o r t h e synthesis o f d a t a o n kinetic, s t r u c t u r a l , c r y s t a l l o g r a p h i c , a n d o t h e r a s p e c t s o f t h e t r a n s f o r m a t i o n . T h e q u a l i t y a n d u t i l i t y of e x i s t i n g t h e r m o c h e m i c a l m o d e l s and data a r e examined. T h e approach r e q u i r e s long, unverifiable extrapolation o f t h e t h e r m o d y n a m i c f u n c t i o n s a n d m u s t be u s e d w i t h c i r c u m s p e c t i o n . O n t h e o t h e r h a n d , m u c h that i s k n o w n i s d e r i v e d f r o m a t h e r m o d y n a m i c v i e w p o i n t . T h e p r e s e n t a u t h o r s a r e n o t h o p e f u l of g r e a t b r e a k t h r o u g h s i n t h e q u a l i t y o f t h e t h e r m o d y n a m i c d a t a . M o r e s i g n i f i c a n t , it a p p e a r s , w o u l d be a t t e m p t s t o f o r m a l l y c a s t t h e a r e a s o f c r y s t a l l o g r a p h i c a n a l y s i s ( t r a n s f o r m a t i o n s t r a i n e n e r g y ) a n d t h e l i k e into t h e t h e r m o d y n a m i c f r a m e w o r k .

T H E a p p l i c a t i o n o f t h e r m o d y n a m i c s to t h e understandi n g of 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 h a s b e e n e x t r e m e l y p r o d u c t i v e o v e r t h e l a s t s e v e r a l d e c a d e s in m o v i n g t o w a r d s t h e g e n e r a l i z a t i o n of e f f e c t s i n s e v e r a l i r o n a l loy s y s t e m s . A p a r t i c u l a r s t r e n g t h of t h e a p p r o a c h a r i s e s f r o m t h e f a c t that v a r i a t i o n s i n t h e s t r u c t u r a l m o r p h o l o g y a r e r e f l e c t e d in t h e e n e r g e t i c s o f t h e t r a n s f o r m a t i o n , a l t h o u g h r e f i n i n g t h e m e t h o d to a p o i n t w h e r e t h e m o r e s u b t l e c h a n g e s c a n be c l e a r l y i d e n t i f i e d is s t i l l d i f f i c u l t . T h e i n t e n t o f t h i s p a p e r is to provide g e n e r a l description of the thermodynamic app r o a c h by e x a m i n i n g s e v e r a l of t h e m o r e i n s t r u c t i v e e x a m p l e s o f i t s use a n d critically reviewing t h e s t a t e of the thermodynamic calculations themselves. T h e m e t h o d is s c h e m a t i c a l l y d e s c r i b e d in Fig. 1. S e v e r a l specific characteristics of the martensite t r a n s f o r m a t i o n h a v e t o be c o n s i d e r e d i n a p p l y i n g t h e r m o d y n a m i c s . P e r h a p s t h e m o s t i m p o r t a n t is t h e fact t h a t the e q u i l i b r i u m i n v o l v e d i n t h i s t y p e o f p h a s e t r a n s f o r m a t i o n i s a m e t a s t a b l e o n e a n d by d e f i n i t i o n w i l l n e v e r