On the thermochemistry of solid nickel-chromium alloys
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On the Thermochemistry of Solid Nickel-Chromium Alloys M. VIKRAMRAO AND E. FLORES-MAGON L. T H E R E h a v e b e e n n u m e r o u s i n v e s t i g a t i o n s of t h e C r Ni p h a s e d i a g r a m a s d e t a i l e d in H a n s e n I a n d t h e two s u p p l e m e n t s . 2'3 T h e e q u i l i b r i a o n t h e C r - r i c h s i d e h a v e continually been in considerable controversy. Recently, P u g l i e s e a n d F i t t e r e r 4 h a v e a t t e m p t e d to r e s o l v e t h e c o n t r o v e r s y w i t h t h e a i d of m e a s u r e d a c t i v i t i e s ( u s i n g a solid electrolyte technique) and an analytical calcul a t i o n . T h e p u r p o s e of t h i s n o t e i s , in p a r t , t o p o i n t o u t that, although they conducted a very thorough investigaM. VIKRAM RAO is Post-doctoral Research Associate, Stanford University, Stanford, Calif. 94305. E. FLORES-MAGON L., formerly a Graduate Student at Stanford University, is Scientific Investigator, Centro de Investlgacion de Materials, Universidad de Mexico, Culdad Universitario, Mexico 20, D.F Manuscript submitted June 30, 1972 METALLURGICAL TRANSACTIONS
tion, their results should be viewed with some caution. T h e t e m p e r a t u r e d e p e n d e n c i e s of t h e P u g l i e s e a n d F i t t e r e r a c t i v i t y c u r v e s a r e in s e r i o u s c o n t r a d i c t i o n w i t h t h e r e l i a b l e h e a t s of m i x i n g m e a s u r e d b y D e n c h . 5 T h i s s u g g e s t e d t h e v a l u e of a n a t t e m p t a t u n d e r s t a n d i n g why an apparently sound technique was producing such inconsistencies. Pugliese and Fitterer employed a half-cell technique in p r e f e r e n c e t o u s i n g t h e f u l l c e l l a s , f o r e x a m p l e , w a s done by Fruehan ~ for the same system for liquid alloys. S o m e a s p e c t s of t h e p o s s i b l e e r r o r s i n s o d o i n g a r e m e n t i o n e d i n t h e d i s c u s s i o n of t h e p a p e r b y C h a t t e r j i . 7 It s h a l l b e d e m o n s t r a t e d b e l o w t h a t , i n s y s t e m s s u c h a s t h e s e , t h e p o s s i b l e e r r o r s i n t r o d u c e d b y t h e u s e of a t w o - s t e p p r o c e d u r e m a k e it v e r y d i f f i c u l t to o b t a i n r e liable activity data, and that a full cell technique ought to b e e m p l o y e d a s a n a l t e r n a t i v e . T h e a c t i v i t y of C r m a y b e c a l c u l a t e d u s i n g t h e e q u a tion log
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w h e r e F i s t h e F a r a d a y ' s c o n s t a n t , E i s t h e s u m of t h e c e l l e m f ' s i n t h e t w o h a l f - c e l l s , a n d AG].Cr203 is the s t a n d a r d f r e e e n e r g y of f o r m a t i o n of t h e o x i d e a s m e a s u r e d in t h e f i r s t h a l f - c e l l . Pugliese and Fitterer draw a broken line (Fig. 4 in Ref. 4) r e p r e s e n t i n g a n a c c e p t a b l e c o m p r o m i s e b e t w e e n t h e i r a n d o t h e r AG] v a l u e s and, p r e s u n ably, u s e t
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