A re-examination of the thermodynamics of the proeutectoid ferrite transformation in Fe-C alloys
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b e e n over a decade s i n c e A a r o n s o n , D o m i a n and P o u n d (ADP) I e m p l o y e d s e v e r a l models of i n t e r s t i t i a l solid s o l u t i o n s in t h e i r study of the t h e r m o d y n a m i c s of the p r o e u t e c t o i d f e r r i t e r e a c t i o n in F e - C a l l o y s . At that t i m e f o r m a l i s m s due to Z e n e r , 2 Kaufman, Radcliffe and Cohen (KRC), 3,4 D a r k e n and Smith, 5 and L e c h e r 6 and F o w l e r and Guggenheim (LFG) 7 were a n a l y z e d c o m p a r a t i v e l y ; the e x p e r i m e n t a l data for the a c t i v i t i e s of c a r b o n in a u s t e n i t e and in f e r r i t e w e r e l i m i t e d to the e x c e p t i o n a l l y well r e g a r d e d CO/COz data of R. P . Smith. 8 T h e s e data, however , a r e adequately c o m p l e t e at only t h r e e t e m p e r a t u r e s . The p r e s e n t i n v e s t i g a t i o n , like that of ADP, was m o t i v a t e d in p a r t by the need to p r e d i c t t h e r m o d y n a m i c p r o p e r t i e s of the p r o e u t e c t o i d f e r r i t e r e a c t i o n in r a n g e s of c o m p o s i t i o n and t e m p e r a t u r e where e q u i l i b r i u m data cannot be obtained. Such t h e r m o d y n a m i c i n f o r m a t i o n c o n s t i t u t e s a f u n d a m e n t a l r e q u i r e m e n t for the i n t e r p r e t a t i o n of t r a n s f o r m a t i o n k i n e t i c s s t u d i e s p e r f o r m e d in t h e s e t e m p e r a t u r e - c o m p o s i t i o n r a n g e s . It is also intended to e s t a b l i s h a b a s i s for future s t u d i e s of the t h e r m o d y n a m i c s and k i n e t i c s of the p r o e u t e c t o i d f e r r i t e r e a c t i o n in F e - C - X s y s t e m s . The s t a t i s t i c a l t h e r m o d y n a m i c s of i n t e r s t i t i a l solid s o l u t i o n s has b e e n an a r e a of much t h e o r e t i c a l activity s i n c e the work of ADP, p a r t i c u l a r l y on the p a r t of R. B. M c L e l l a n and c o w o r k e r s . 9-18 Of the m a n y m o d e l s
c o n s i d e r e d by M e L e l l a n for binary' i n t e r s t i t i a l solid s o l u t i o n s , the widely used f i r s t o r d e r q u a s i c h e m i c a l t r e a t m e n t developed by M c L e l l a n and Dunn (MD) 1~ is the most r i g o r o u s a p p r o a c h which they s u b s e q u e n t l y extended to e n c o m p a s s the s t a t i s t i c a l t h e r m o d y n a m i c s of F e - C - X alloys and hence is the one which will be e m p l o y e d h e r e . C o n c u r r e n t l y with these t h e o r e t i c a l studies, the b a s e of data on the a c t i v i t y of C in ~ and in ~ has b e e n m a r k e d l y expanded in the last decade. The data of B a n - y a , Elliott and C h i p m a n (BEC) 1%2~on the activity of c a r b o n in a u s t e n i t e , a7, of Dunn and M c L e l i a n 15 on the a c t i v i t y of c a r b o n i n f e r r i t e , an, and of Lobo and G e i g e r 21'22 on both a~ and aa now provide e x t e n s i v e c o v e r a g e of both the 7 and the a
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