The thermodynamic properties of carbon in body-centered cubic iron
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T H E t h e r m o d y n a m i c p r o p e r t i e s of bcc i r o n - c a r b o n solid s o l u t i o n s have been i n v e s t i g a t e d m a n y t i m e s in the past. Q u a n t i t i e s such a s the heat of s o l u t i o n of c a r bon in bcc i r o n ( f e r r i t e ) , the heat of s o l u t i o n of c e m e n tite (Fe3C) in f e r r i t e , the p a r t i a l enthalpy of a c a r b o n atom in f e r r i t e , and m o r e r e c e n t l y , the n o n c o n f i g u r a t i o n a l c o n t r i b u t i o n s to the e n t r o p y due to a c a r b o n atom in solution have b e e n studied with v a r i o u s t e c h n i q u e s and with widely v a r y i n g s u c c e s s . The p u r p o s e of the p r e s e n t study w a s to i n v e s t i g a t e the t e m p e r a t u r e d e pendence of the c a r b o n s o l u b i l i t y in f e r r i t e in e q u i l i b r i u m with gas m i x t u r e s of v a r y i n g c a r b o n p o t e n t i a l to r e m o v e s o m e of the existing c o n f u s i o n about the t h e r m o d y n a m i c p r o p e r t i e s of c a r b o n in f e r r i t e . The e x p e r i m e n t a l p r o c e d u r e used involved e q u i l i b r a t i n g p u r e i r o n foils at a c o n s t a n t t e m p e r a t u r e and p r e s s u r e with m i x t u r e s of m e t h a n e and h y d r o g e n held at a fixed CH4/H2 r a t i o . The c a r b o n c o n t e n t of the e q u i l i b r a t e d s a m p l e s w a s d e t e r m i n e d by c o m b u s t i o n a n a l y s i s . Since the c h e m i c a l p o t e n t i a l of the c a r b o n in the gas is known at a l l t e m p e r a t u r e s and gas c o m p o s i t i o n s , a closed s o l u b i l i t y equation could be u s e d to d e r i v e the p a r t i a l enthalpy and p a r t i a l e x c e s s e n t r o p y of a c a r b o n atom in the solid f r o m the e x p e r i m e n t a l data. Since the m a x i m u m s o l u b i l i t y of c a r b o n in f e r r i t e at the eutectoid t e m p e r a t u r e is a r o u n d 0.02 wt pct, ext r e m e l y s m a l l a m o u n t s of c a r b o n d i s s o l v e d in the solid m u s t be m e a s u r e d . Two m e t h o d s of d e t e r m i n i n g the c a r b o n c o n c e n t r a t i o n in i r o n a r e a v a i l a b l e - - i n t e r n a l f r i c t i o n t e c h n i q u e s and c o m b u s t i o n a n a l y s i s . The c a r bon a n a l y s e s of the e q u i l i b r a t e d s a m p l e s obtained in this study w e r e m a d e u s i n g a c o m m e r c i a l l y a v a i l a b l e low c a r b o n a n a l y z e r u t i l i z i n g a c o m b u s t i o n t e c h n i q u e . A d i r e c t m e a s u r e m e n t of c a r b o n content i n d e p e n d e n t of other c a r b o n d e t e r m i n a t i o n s was felt to be a b e t t e r method of a n a l y s i s than the i n d i r e c t i n t e r n a l f r i c t i o n measurements. The g a s - s o l i d e q u i l i b r i u m m e a s u r e m e n t s w e r e made o v e r as l a r g e a t e m p e r a t u r e r a n g e as p o s s i b l e in o r d e r WILLIAM W. DUNN, formerly Graduate Student, W
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