Application of thermodynamics of heterogeneous phase equilibria to the construction of the Fe-Si-O ternary phase diagram
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A N D W. O. P H I L B R O O K
B a s e d on the t h e r m o d y n a m i c p r i n c i p l e s of p h a s e r u l e and h e t e r o g e n e o u s e q u i l i b r i a , the t e r n a r y p h a s e d i a g r a m f o r the s y s t e m F e - S i O in the r a n g e of t e m p e r a t u r e s f r o m 1730 to 1200~ h a s b e e n c o n s t r u c t e d with m o r e a t t e n t i o n p a i d to t e m p e r a t u r e s above 1515~ and t o w a r d s the i r o n c o r n e r of the d i a g r a m . The s y s t e m is c h a r a c t e r i z e d b y a r e g i o n of t h r e e l i q u i d s above 1730~ which r e d u c e s to t w o - l i q u i d s r e g i o n s at l o w e r t e m p e r a t u r e s . S e v e r a l t h r e e - p h a s e and f o u r - p h a s e r e a c t i o n s o c c u r in t h i s s y s t e m on cooling, at v a r i o u s t e m p e r a t u r e s . N e a r the i r o n c o r n e r a e u t e c t i c r e a c t i o n o c c u r s a t a fixed t e m p e r a t u r e in which d e l t a i r o n c o - p r e c i p i t a t e s with c r i s t o b a l i t e f r o m liquid i r o n containing s i l i c o n and oxygen. At l o w e r t e m p e r a t u r e s the s y s t e m i s c h a r a c t e r i z e d b y an i n v a r i a n f r e a c t i o n involving liquid i r o n , d e l t a i r o n , c r i s t o b a l i t e and liquid oxide. F o r oxygen r i c h m e l t s , s o l i d i f i c a t i o n t e r m i n a t e s on the F e - O b i n a r y with the m o n o t e c t i c p r e c i p i t a t i o n of d e l t a i r o n and liquid oxide.
DESPITE the significance of the iron-rich side of the Fe-Si-O system in deoxidation, stainless steelmaking, and ferroalloy production, very little information is available regarding the existence and stability of phases present at various temperatures in the entire phase field of this system. Based on data from the literature and the thermodynamic principles of phase rule and heterogeneous equilibrium, the ternary phase diagram for this system is qualitatively constructed here. Experimental evidence and thermodynamic criteria are used to postulate critical temperatures and compositions at which phase transitions occur. Due to l a c k of a d e q u a t e e x p e r i m e n t a l d a t a , t h i s p h a s e d i a g r a m is q u a l i t a t i v e in n a t u r e , although an a t t e m p t h a s been m a d e to c o n f o r m to the q u a n t i t a t i v e d a t a w h e r e p o s s i b l e . B e c a u s e of the e x t r e m e l y l i m i t e d s o l u b i l i t y of oxygen in I r o n - s i l i c o n a l l o y s , the p h a s e b o u n d a r y s h o w ing the l i m i t of oxygen s o l u b i l i t y in liquid m e t a l is a c t u a l l y v e r y n e a r the F e - S i a x i s ; h o w e v e r , f o r the s a k e of c l a r i t y t h i s r e g i o n is shown e x a g g e r a t e d in the e n suing f i g u r e s . This l i m i t a t i o n in the c o n s t r u c t i o n of the p h a s e d i a g r a m s should be b o r n e in mind. APPLICATION OF THERMODYNAMICS OF HETEROGENEOUS PHASE EQUILIBRIA The Fe-O binary system shows a miscibi
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