Thermodynamics of liquid Fe-C solutions
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CHIPMAN
The activity of c a r b o n in liquid i r o n has b e e n d e t e r m i n e d at low c o n c e n t r a t i o n s by m e a n s of the CO-COd e q u i l i b r i u m and at high c o n c e n t r a t i o n s by the s o l u b i l i t y of graphite. At i n t e r m e d i ate c o n c e n t r a t i o n s t h e r e is a b r o a d r e g i o n in which m e a s u r e m e n t s have not b e e n made b e c a u s e of t h e r m a l i n s t a b i l i t y of gas m i x t u r e s high in CO. B e c a u s e of this p h e n o m e n o n it is s u g g e s t e d that the r e p o r t e d a c t i v i t y coefficients at low c o n c e n t r a t i o n s m a y be in e r r o r and a r e only u s e ful in d e t e r m i n i n g a l i m i t i n g value at i n f i n i t e dilution. At t e m p e r a t u r e s below the m e l t i n g point of i r o n , activity coefficients at i n t e r m e d i a t e c o n c e n t r a t i o n s along the liquidus line a r e c a l c u lated from r e c e n t m e a s u r e m e n t s of the a c t i v i t y in a u s t e n i t e and of the solidus and liquidus l i n e s of the p h a s e d i a g r a m . The r e s u l t s a g r e e f a i r l y well with the data of R i c h a r d s o n and Dennis at h i g h e r t e m p e r a t u r e s . S e v e r a l i n t e r p o l a t i o n f o r m u l a e a r e i n v e s t i g a t e d . Equations a r e p r e s e n t e d which d e s c r i b e 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 the b i n a r y liquid s o l u t i o n at all c o m p o s i t i o n s b e t w e e n the eutectic and 1760~ .['RESENT knowledge of the activity of c a r b o n in liquid i r o n s o l u t i o n s r e s t s v e r y h e a v i l y on the e x p e r i m e n t a l r e s u l t s of R i c h a r d s o n and Dennis ~ who studied the e q u i l i b r i u m of dilute solutions in the r e a c t i o n :
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Their results were confirmed by Rist and Chipman z who plotted both sets of data as log p~o/(PCO2. NC) against 1 -Ar~Fe. They assumed a linear relation extending from infinite dilution up to graphite saturation at which point accurate information is available from thermochemical and solubility data. Their Fig. 6 is reproduced in Fig. 1 along with certain additions which are discussed below. When the phase d i a g r a m is a c c u r a t e l y known and the a c t i v i t y in the solid p h a s e has been e s t a b l i s h e d , that of p o i n t s along the liquidus line is also fixed. The t r i a n gles in Fig. 1 r e p r e s e n t the b e s t e s t i m a t e s a v a i l a b l e at the t i m e (1956). They s e e m e d r a t h e r i n s e c u r e howe v e r and the l i n e s for lower t e m p e r a t u r e s w e r e d r a w n n e a r l y p a r a l l e l to the t h r e e h i g h - t e m p e r a t u r e l i n e s in such a way as to make a c o n s i s t e n t f a m i l y . The c r o s s e s which have b e e n added r e p r e s e n t a complete r e c a l c u l a tion utilizing the solidus line of Benz and Elliott,3 the liquidus of Buckley and Hume-Rothery,4 and activity data for the austenite solution of Ba
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