Kinetics of the martensite transformation in athermal Fe-C-Ni-Cr alloys
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G. S. ANSELL
An i n v e s t i g a t i o n was conducted to study 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 k i n e t i c s of a t h e r m a l F e - C - N i - C r a l l o y s . The t r a n s f o r m a t i o n r a t e was found to be dependent on alloy c o m p o s i t i o n and was a function of i) the r a t e of change of c h e m i c a l f r e e e n e r g y with t e m p e r a t u r e , and ii) the e n e r g y expended in d e f o r m i n g the a u s t e n i t e s u r r o u n d i n g a growing m a r t e n s i t e e m b r y o in o r d e r to a c c o m m o d a t e the t r a n s f o r m a t i o n a l v o l u m e and s h e a r s t r a i n s . I n c r e a s i n g e i t h e r of the two f a c t o r s l e a d s to a n i n c r e a s e in t r a n s f o r m a t i o n r a t e . Also, it was found that the magnitude of the c o n t r i b u t i o n of the d e f o r m a t i o n e n e r g y to the o v e r a l l n o n c h e m i c a l f r e e e n e r g y a s s o c i a t e d with the t r a n s f o r m a t i o n depends on the alloy s y s t e m and specific c o m p o s i t i o n .
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a u s t e n i t e t r a n s f o r m s to m a r t e n s i t e in s t e e l s , the t r a n s f o r m a t i o n does not i n i t i a t e at the t e m p e r a t u r e w h e r e the c h e m i c a l f r e e e n e r g y ( z ~ C ~ ~ ' = F ~ , - F~) is z e r o , but r a t h e r at a lower t e m p e r a t u r e (M s t e m p e r a t u r e ) where z k F ~ ~ ' has a negative v a l u e . T h i s additional d r i v i n g f o r c e is n e c e s s a r y to o v e r c o m e the r e s i s t a n c e to the phase t r a n s f o r m a t i o n caused by nonc h e m i c a l f a c t o r s ( A F ~ ' ~ ~ '). The c h e m i c a l f r e e e n e r g y involved with the t r a n s f o r m a t i o n has been well studied in a n u m b e r of alloy s y s t e m s , x'~ but d i f f e r i n g viewpoints e x i s t r e g a r d i n g the f a c t o r s which c o n t r i b u t e to the n o n c h e m i c a l f r e e e n e r g y t e r m . Schoen 4 and Magee 5 e s t i m a t e d that the m e c h a n i c a l r e s i s t a n c e of a u s t e n i t e to s h e a r d u r i n g i n t e r f a c e p r o p a g a t i o n accounted for about 50 to 90 pct of the n o n c h e m i c a l f r e e e n e r g y . B r e i n a n and A n s e l l 6 s u g g e s t e d that in addition to the s h e a r r e s t r a i n t , the v o l u m e e x p a n s i o n 7,s which a c c o m p a n i e s the t r a n s f o r m a t i o n r e q u i r e s c o n s i d e r a b l e e l a s t i c or p l a s t i c def o r m a t i o n of the s u r r o u n d i n g a u s t e n i t e in o r d e r to a c c o m m o d a t e the v o l u m e s t r a i n , and the e n e r g y n e c e s s a r y for this d e f o r m a t i o n could r e p r e s e n t a s u b s t a n t i a l p a r t of the n o n c h e m i c a l f r e e e n e r g y . B r e i n a n 9 att e m p t e d to d e t e r m i n e the magnitude of this d e f o r m a t i o n e n e r g y by u s i n g a m o d e l which d i r e c t l y i n c o r p o r a t e d the v o l u m e change. His model, whic
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