Microstructural aspects of martensite transformation in coarse-grained Fe-31.1 pct Ni-0.02 pct C
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E X P E R I M E N T A L s t u d i e s and t h e o r e t i c a l c o n s i d e r a t i o n s of 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 l e a d to the conc l u s i o n that the m i c r o s t r u c t u r a l a s p e c t s of the r e a c t i o n a r e of f o r e m o s t i m p o r t a n c e to the d e v e l o p m e n t of a k i n e t i c s m o d e l . ~'~ F i s h e r et a # d e r i v e d an e q u a t i o n f o r the t r a n s f o r m a t i o n c u r v e b y a s s u m i n g that m a r t e n s i t e f o r m s u n i f o r m l y and t r a n s f o r m s a f i x e d f r a c t i o n of the m a t r i x . On the o t h e r hand, M a g e e 2 p r o p o s e d that the h e t e r o g e n e o u s c h a r a c t e r of the t r a n s f o r m a t i o n should l e a d to the c o n s t a n c y of the m e a n p l a t e v o l u m e . M o r e r e c e n t l y , Chen and W i n c h e l l s inv e s t i g a t e d the a r r a n g e m e n t s of m a r t e n s i t e p l a t e s in a 01 type tool s t e e l and p r o p o s e d t h a t the r a n d o m p l a t e r a d i u s could be r e l a t e d to the half of the m e a n f r e e d i s t a n c e in the a u s t e n i t e . P l a t e c l u s t e r i n g was inv o k e d to e x p l a i n the d e v i a t i o n of the d a t a f r o m the model. T h e p u r p o s e of t h i s c o m m u n i c a t i o n is to d e s c r i b e r e s u l t s of a s t u d y u n d e r t a k e n to a n a l y z e the m i c r o s t r u c t u r a l a s p e c t s of the " a t h e r m a l " m a r t e n s i t e t r a n s f o r m a t i o n in a F e - N i - C a l l o y in the light of the available models. The alloy was chosen for experim e n t a l c o n v e n i e n c e . The extent of t r a n s f o r m a t i o n is s i m p l e to c o n t r o l and the m i c r o s t r u c t u r e i s w e l l d e f i n a b l e up to a s i g n i f i c a n t f r a c t i o n t r a n s f o r m e d (7 5 pct). C o n s i d e r a t i o n of the influence of the a u s t e n i t e g r a i n s i z e upon the d i s t r i b u t i o n of the m a r t e n s i t e p l a t e s in the m a t e r i a l 4 i n d i c a t e d t h a t a c o a r s e r g r a i n should be u s e d to a p p r o a c h the " u n i f o r m i t y " c o n d i t i o n i m p l i c i t in the m o d e l s p r o p o s e d by F i s h e r et al 1 and by Chen and W i n c h e l l . 3
tion p r o p o s e d in R e f . 3:
T - v~ s~
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w h e r e V v is t h e v o l u m e f r a c t i o n of m a r t e n s i t e and Svm the m e a n d e n s i t y of m a r t e n s i t e m i d p l a n e in the m a t e r i a l . T h i s choice w a s m o t i v a t e d by the i r r e g u l a r p l a t e p r o f i l e which r e q u i r e s too much o p e r a t o r j u d g m e n t a s to what t h i c k n e s s v a l u e should be u s e d to obtain tvia Fullman's equation. M a r t e n s i t e v o l u m e f r a c t i o n , Vv, and the d e n s i t y of i n t e r n a l b o u n d a r i e s w e r e d e t e r m i n e d b y the m e t h o d s of " p o i n t - c o u n t i n g " and " l i n e a r i n t e r c e p t " , r e s p e ct i v e l y . D e t e
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