Thermodynamics of the Fe-Mn-C system from solid state EMF measurements
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BENZ
The p a r t i a l m o l a r f r e e e n e r g y of m i x i n g of Mn, GM, in a u s t e n i t e and in v a r i o u s Mn-C and F e - M n - C phase m i x t u r e s was m e a s u r e d at t e m p e r a t u r e s of 690 to 1060~ with solid C a F 2 e l e c t r o l y t e g a l v a n i c c e l l s . The r e s u l t s for the F e - M n s y s t e m can be r e p r e s e n t e d by GM = RT
In yz - (15000 - 7.3 T ) y ~ + 5 0 0 J
w h e r e T denotes t e m p e r a t u r e in K and yl and Y2 a r e the F e - and M n - t o - m e t a l atom r a t i o s , r e s p e c t i v e l y . The c o u r s e of O ~ in its dependence on the c a r b o n atom f r a c t i o n in the M n - C s y s t e m shows a low s t a b i l i t y of MntsC 4 and MnnC with r e s p e c t to d e c o m p o s i t i o n into n e i g h b o r i n g c a r b i d e s . P a r t i a l m o l a r free e n e r g i e s of m i x i n g of Fe and Mn in the m e t a l ( a u s t e n ite o r cr-Mn (ยง F e ) ) - c a r b i d e (M3C , M23C6, or e - M 4 C ) phase m i x t u r e s a r e given by GM (metal + c a r b i d e ) = R T In y , + (1240 - 1.8 T) - (8220 - 3.75 T)y~ and GM (metal + c a r b i d e ) = R T In Y2 + (1220 - 3.5 T) - (8220 - 3.75 T ) y ~ , 2200 J, (3'2(V) -- 0 to 1.0), w h e r e Yi denote the / - t o - m e t a l atom r a t i o s (i = 1, 2, 3) in the c a r b i d e - s a t u r a t e d m e t a l phase. The p a r t i a l m o l a r f r e e e n e r g y of m i x i n g of c a r b o n , G M, in the s a m e t w o - p h a s e r e g i o n s was calculated. The r e s u l t s for the a u s t e n l t e - c e m e n t i t e , t w o - p h a s e , r e g i o n c o n f i r m published gas c a r b u r i z a t f o n data but tend to be m o r e negative in value and have a g r e a t e r c u r v a t u r e tn dependence on Mn content. The c u r v a t u r e shows that i n t e r a c t i o n t e r m s higher than f i r s t o r d e r m u s t be used to r e p r e s e n t GM over the c o m p o s i t i o n r a n g e Y2 = 0 to 0.545. E v a l u a t i o n of G M o v e r closed paths of i n t e g r a t i o n in the t e r n a r y s y s t e m d e m o n s t r a t e s a s e l f - c o n s i s tency of the phase r e l a t i o n s with the emf data. The d e r i v e d free e n e r g y charge for the, r e a c t i o n of m i x i n g of the b i n a r y c e m e n t i t e compounds, Yl FeCl/s + Y2 MnCl/s = MCvs, where M denotes the m i x t u r e (Fe, Mn), is m o r e negative than that of an ideal solution and its yz dependence is a s y m m e t r i c being m o r e negative in M n - r i c h than in F e - R i c h c e m e n t i t e . T H E f r e e e n e r g y of f o r m a t i o n of c e m e n t i t e , M3C ,* as *Asin {he previouspublication,~metalsin ternarycompoundsare represented by M,Thus,(Mn,Fe)23C6is denotedMr (Mn,Fe)lsC, by MIsC4,and so forth. The componentsare numberedFe, 1; Mn, 2; C, 3. a function of Mn c o n t e n t was r e c e n t l y r e p o r t e d by Benz, Elliot-t, and C h i p m a n ' b a s e d upon o b s e r v e d phase b o u n d a r y c o m p o s i t i o n s and c a r b o n a c t i v i t y in a u s t e n i t
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