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T H E t h e r m o d y n a m i c d a t a of the F e - S and Co-S l i q uids w e r e s u c c e s s f u l l y a n a l y z e d by S h a r m a and Chang ~'2 in t e r m s of an a s s o c i a t e d solution m o d e l p o s t u l a t i n g the e x i s t e n c e of ' F e S ' and 'COS' s p e c i e s in the liquid p h a s e in a d d i t i o n to the m e t a l and s u l f u r c o m p o n e n t e l e m e n t s . U s i n g this m o d e l f o r the liquid p h a s e s and a s t a t i s t i c a l t h e r m o d y n a m i c m o d e l for p y r r h o t i t e and c o b a l t m o n o s u l f i d e , the p h a s e d i a g r a m s of F e - S and Co-S w e r e c a l c u l a t e d . The c a l c u l a t e d p h a s e d i a g r a m s w e r e in e x c e l l e n t a g r e e m e n t with the e x p e r i m e n t a l data, accounting for the r a n g e s of h o m o g e n e i t y of p y r r h o t i t e and c o b a l t monosuHide at a l l t e m p e r a t u r e s . The o b j e c t i v e s of the p r e s e n t study a r e 1) to a n a l y z e the t h e r m o d y n a m i c d a t a of the Ni-S liquid p h a s e using the s a m e a s s o c i a t e d s o l u t i o n m o d e l and 2) to c a l c u l a t e the N i - S p h a s e d i a g r a m using this m o d e l f o r the liquid p h a s e , a s t a t i s t i c a l t h e r m o d y n a m i c m o d e l for the n i c k e l m o n o s u l f i d e p h a s e and a p p r o p r i a t e t h e r m o d y n a m i c equations of s t a t e for o t h e r i n t e r m e d i a t e p h a s e s in t h i s s y s t e m . T h i s type of a n a l y s i s w i l l p r o d u c e a c o n s i s t e n t s e t of t h e r m o d y n a m i c v a l u e s f o r a l l the p h a s e s involved and the phase diagram data. 1. THERMODYNAMIC MODELS FOR THE VARIOUS PHASES 1.1 The L i q u i d P h a s e 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 N i - S liquid w e r e d e t e r m i n e d by M e y e r , W a r n e r , Rao, and Kellogg, ~ N a g a m o r i and I n g r a h a m , 4 and R o s e n q v i s t s a s a function of c o m p o s i t i o n , f r o m p u r e Ni to ~ 42 a t . p c t S, R. C. SHARMA, formerly Post-Doctoral Research Associate, University of Wisconsin-Milwaukee is now Lecturer, Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur, India. Y. A. CHANG is Professor of Materials Engineering, Materials Department, College of Engineering and Applied Science and Associate Dean for Research of the Graduate School, University of Wisconsin-Milwaukee, Milwaukee, WI 53201. Manuscript submitted March 22, 1979. METALLURGICAL TRANSACTIONS B

o v e r the t e m p e r a t u r e i n t e r v a l , 1073 to 1873 K. M e y e r et al c o r r e l a t e d t h e s e d a t a in t e r m s of a m a t h e m a t i c equation a s s u m i n g that the e q u i l i b r i u m c o n s t a n t b e t w e e n 1/2 S2(g) and S (sol) m a y be r e p r e s e n t e d a s a l i n e a r function of r e c i p r o c a l t e m p e r a t u r e , i.e., log K = A + B / T , and that log ~/S m a y be r e p r e s e n t e d a s a p o w e r s e r i e s in c o m p o s i