Kinetics of strain-induced martensitic nucleation
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T H E d e v e l o p m e n t of h i g h - s t r e n g t h T R I P s t e e l s 1 which achieve high w o r k - h a r d e n i n g r a t e s through d e f o r m a t i o n - i n d u c e d m a r t e n s i t i c n u c l e a t i o n has r e g e n e r a t e d i n t e r e s t in the k i n e t i c s of such t r a n s f o r m a t i o n s . This i n c l u d e s both the c o u r s e of the m a r t e n s i t i c t r a n s f o r mation as a function of p l a s t i c s t r a i n and the a s s o c i ated t e m p e r a t u r e dependence. The r a p i d v a r i a t i o n of the t r a n s f o r m a t i o n k i n e t i c s with t e m p e r a t u r e n e a r the a m b i e n t d e t r a c t s f r o m the potential u s e f u l n e s s of T R I P s t e e l s b e c a u s e even m o d e r a t e s t r a i n r a t e s can r e s u l t in sufficient adiabatic h e a t i n g to inhibit the t r a n s f o r m a t i o n and t h e r e b y cause u n s t a b l e p l a s t i c flow. 2 E m p i r i c a l r e l a t i o n s for the 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 vs p l a s t i c s t r a i n have b e e n proposed, s'4 but a t h e o r e t i c a l b a s i s is s t i l l lacking. An e x t e n s i v e i n v e s t i g a t i o n of the s t r a i n - i n d u c e d t r a n s f o r m a t i o n b e h a v i o r of 304 s t a i n l e s s s t e e l s by Angel 3 has shown that the t r a n s f o r m a t i o n c u r v e (amount of m a r t e n s i t e as a f u n c tion of p l a s t i c s t r a i n at c o n s t a n t t e m p e r a t u r e ) is s i g m o i d a l in shape, g e n e r a l l y r e a c h i n g s a t u r a t i o n below 100 pct. He also p r o p o s e d an e m p i r i c a l e x p r e s s i o n c o n s i s t e n t with the s i g m o i d a l shape, but it did not comply with the o b s e r v e d s a t u r a t i o n below 100 pct. On the other hand, G e r b e r i c h 4 has used a p a r a b o l i c r e l a t i o n s h i p to fit c o r r e s p o n d i n g data obtained on T R I P s t e e l s . However, the c r i t i c a l d i f f e r e n c e b e t w e e n s i g moidal and p a r a b o l i c shapes o c c u r s at r e l a t i v e l y s m a l l s t r a i n s , and t l i s r e g i m e can be equivocal in the case of T R I P s t e e l s due to i n i t i a l i n h o m o g e n e o u s d e f o r m a t i o n (Lfiders band f o r m a t i o n ) . M e a s u r e m e n t s on s t e e l s which d e f o r m h o m o g e n e o u s l y appear to s u p p o r t the s i g m o i d a l shape, s'6 T h e o r e t i c a l t r e a t m e n t of the t r a n s f o r m a t i o n k i n e t i c s G. B. OLSON is Research Associate, Department of Materials Science and Engineering,and MORRIS COHEN is Ford Professor of Materials Science and Engineering,Massachusetts Institute of Technology, Cambridge, Mass. 02139. Manuscript submitted July 1, 1974. METALLURGICALT R A N S A C T I O N S
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vs p l a s t i c s t r a i n f i r s t r e q u i r e s a model for the m e c h a n i s m of s t r a i n - i n d u c e d n u c l e a t i o n . We d i s t i n g u i s h b e tween s t r e s s - a s s i s t e d and s
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