Thermodynamics and phase relationships of transition metal-sulfur systems: Part III. Thermodynamic properties of the Fe-
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THE F e - S b i n a r y is
c h a r a c t e r i z e d by a c o n g r u e n t m e l t i n g phase, p y r r h o t i t e , at ~50 at. pet S, a n e u t e c t i c between F e and p y r r h o t i t e and a m o n o t e c t i c b e t w e e n p y r r h o t i t e and S. 1'2 The liquidus in e q u i l i b r i u m with F e at about 20 at. pct S is r a t h e r flat, s u g g e s t i n g a t e n d e n c y toward i m m i s c i b i l i t y in the liquid. P y r r h o tire has an o r d e r e d NiAs s t r u c t u r e (B81) and e x i s t s over a r a t h e r l a r g e range of h o m o g e n e i t y . The g e n e r a l f e a t u r e s of the phase r e l a t i o n s h i p s in the F e - S b i n a r y s u g g e s t s a s t a b l e liquid s o l u t i o n a r o u n d 50 at. pct S and r e l a t i v e l y weak solutions on e i t h e r side of the e q u a - a t o m i c c o m p o s i t i o n . The a c t i v i t y data of s u l f u r in the liquid phase i n c r e a s e s m o n o t o n o u s l y f r o m p u r e F e to about 50 at. pet S and then i n c r e a s e s r a p i d l y with c o m p o s i t i o n , s i m i l a r to that of a component in a highly s t a b l e solid phase, a-7 The D a r k e n S e x c e s s s t a b i l i t y f u n c t i o n of the liquid phase shows a p r o n o u n c e d m a x i m u m while the e l e c t r i c conductivity shows a m i n i m u m n e a r 50 at. pet S. 9 T h i s s u g g e s t s the p o s s i b l e exi s t e n c e of ' F e S ' s p e c i e s in the liquid phase. A s a t i s f a c t o r y t h e r m o d y n a m i c d e s c r i p t i o n of the F e - S liquid phase m u s t take into c o n s i d e r a t i o n the e x i s t e n c e of these s p e c i e s . H i l l e r t and S t a f f a n s s o n 1~ r e c e n t l y p r e s e n t e d a t h e r m o d y n a m i c a n a l y s i s of the F e - F e S s y s tem, t r e a t i n g FeS as a pure c o m p o n e n t . T h e y n e glected the e x i s t e n c e of the r a n g e of h o m o g e n e i t y of the solid phase, p y r r h o t i t e , and the p o s s i b l e e x i s t e n c e of s u l f u r s p e c i e s in the F e - F e S liquid p h a s e . Although they had b e e n able to c a l c u l a t e the F e - F e S p h a s e d i a g r a m in r e a s o n a b l e a c c o r d with the e x p e r i m e n t a l diag r a m , they were unable to d e s c r i b e the t h e r m o d y n a m i c R. C. SHARMA, formerly Post-Doctoral Research Associate, Materials Department, College of Engineeringand Applied Science, University of Wisconsin-Milwaukee,is now Lecturer, Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur, India. Y. A. CHANG is Professor, Materials Department, College of Engineering and Applied Science, and Associate Dean of Research, Graduate School, University of Wisconsin-Milwaukee,Milwaukee, Wl 53201. Manuscript submitted April 10, 1978. METALLURGICALTRANSACTIONS B
p r o p e r t i e s of the liquid phase n e a r 50 at. pet 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 d e s c r i b e the t h e r m o d y n a m i c b e h a v i o r of th
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