Thermodynamics of dilute bcc Fe-Si alloys
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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 s i l i c o n in the a - p h a s e of the F e - S i s y s t e m in the r e g i o n 0.028 < xsi < 0.084 and 1100 < ~ < 1370 h a s b e e m m e a s u r e d b y the e m f c e l l Mo, Si(s) I SiO~-Li20 ] (Si-Fe)(s), Mo. The results are expressed in the form log ~Si = 1.19 - 7070/T + [-6.30 + 18,300/T]xsi
T H E a - F e p h a s e (bcc) in the F e - S i s y s t e m is s t a b l e o v e r a r a n g e of c o m p o s i t i o n e x t e n d i n g a s high a s 13 wt pct Si. The e q u i l i b r i u m of this p h a s e with the c l o s e l y r e l a t e d a 1 and a z p h a s e s , both b c c , and the 7 - p h a s e (fcc) a r e shown in the p h a s e d i a g r a m , i Cons i d e r a b l e i n f o r m a t i o n i s a v a i l a b l e on 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 liquid p h a s e ~,2,3 and for c e r t a i n of the n e a r l y s t o i c h i o m e t r i c s o l i d s , 1 but the only r e p o r t e d d a t a on the a - F e p h a s e a r e t h o s e of Sakao, Kubo and Ishino. 4 T h e s e a u t h o r s e q u i l i b r a t e d a l l o y s p e c i m e n s in the r a n g e of 1400 to 1470~ with a g a s s t r e a m c o n t a i n i n g SIC), H 2 and H~O and o b t a i n e d v a l ues of the e x c e s s p a r t i a l m o l a r f r e e e n e r g y of Si. This p a p e r r e p o r t s m e a s u r e m e n t s of the a c t i v i t y of s i l i c o n in s o l i d F e - S i a l l o y s which w e r e o b t a i n e d b y the u s e of the e l e c t r o c h e m i c a l c e l l . (-)
(+)
Mo, Si (s)
Si a§
(si sat. Ag nq.)
(sio. sat.
S i - F e (s), Mo
L i 2 0 - S i O 2 liq.) When o p e r a t e d r e v e r s i b l y and with t h e p a s s a g e of 4 e l e c t r o c h e m i c a l e q u i v a l e n t s of e l e c t r o n s f r o m left to r i g h t in the e x t e r n a l e l e c t r i c a l c i r c u i t , the p o t e n t i a l of the c e l l i s r e l a t e d to the c h e m i c a l p o t e n t i a l and a c t i v i t y of s i l i c o n , /ZSi and a s i , r e s p e c t i v e l y , in the a l l o y on the r i g h t r e l a t i v e to that of p u r e s o l i d s i l i c o n /~Si, o on the left: PSi -- ~t~i = --43e = RT In aSi
METALLURGICAL TRANSACTIONS A
Mo LEAD (I)
(1)
QUARTZ QUILL (2)
[i]
is Faraday's constant and e is the reversible potential of the cell. The experiments were run in conjunction, and concurrently, with measurements of the activity of s i l i con in silver alloys that were reported recently,s The experimental apparatus was that described earlier and the cell arrangement was similar except that the Fe-Si cathodes were made of thin sheet and were formed into the shape shown in Fig. 1. The molybdenum lead was welded to the strip of alloy before the strip was formed into the shape shown. Each lead was protected from the electrolyte and furnace atmosphere by a long quartz quill. Alloys containing up to approximately 4.5 wt pct silicon could be cast and rolled into strips suitable for use in the cell. Alloys with higher concentrations of silicon were too br
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