Multiphase diffusion in Ag-Zn alloys
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Multiphase diffusion was studied in A g - Z n alloys at 600~ with v a p o r - s o l i d diffusion couples. The e x p e r i m e n t s were p e r f o r m e d with a /~(bcc) alloy a s v a p o r - s o u r c e m a t e r i a l and with diffusion d i s k s (fcc) of e i t h e r pure s i l v e r or an ~ alloy. The diffused couples were studied m e t a l l o g r a p h i c a l l y and by e l e c t r o n m i c r o p r o b e a n a l y s i s . I n t e r d i f f u s i o n c o e f f i c i e n t s for each of the p h a s e s were c a l c u l a t e d from data on the e x p a n s i o n of the couple and the motion of the ~/~ i n t e r f a c e on the b a s i s of new r e l a t i o n s developed from an a n a l y s i s of two-phase v a p o r - s o l i d couples. Also, the c o m p o s i t i o n s at the ~/~ i n t e r f a c e differed little from those d e t e r m i n e d from an e q u i l i b r a t e d two-phase alloy. / H E p r o b l e m of diffusion in b i n a r y m e t a l l i c s y s t e m s involving two or m o r e phases p r e s e n t s s e v e r a l i n t e r e s t i n g a s p e c t s for study. Among these a r e the kind of i n t e r f a c e that develops between the n e i g h b o r i n g p h a s e s , the c o n c e n t r a t i o n s of the p h a s e s at the i n t e r face, and the development of the diffusion zone with t i m e . T h e s e v a r i o u s a s p e c t s of m u l t i p h a s e diffusion a r e i n t e r r e l a t e d through the diffusion b e h a v i o r in the individual p h a s e s and the i n t e r f a c e r e a c t i o n s at the interphase boundaries. The i n t e r f a c e s developed in b i n a r y , m u l t i p h a s e couples a r e n o r m a l l y p l a n a r ; hence, the diffusion p r o b lem can be a n a l y z e d for the d e v e l o p m e n t of phases and c o n c e n t r a t i o n p r o f i l e s . The a n a l y s i s of the gene r a l solution of the diffusion equation for m u l t i p h a s e couples has b e e n d i s c u s s e d by K i r k a l d y , ~ C a s t l e m a n n , ~ Kidson, s and B o r i s o v et al. 4 on the b a s i s of p~rabolic m o v e m e n t s of i n t e r p h a s e b o u n d a r i e s . Most of these a n a l y s e s t'2'4 r e f e r to s o l i d - s o l i d couples and utilize diffusion coefficients a s s u m e d independent of composition for each phase. It is the p u r p o s e of this paper to a n a l y z e t w o - p h a s e , b i n a r y v a p o r - s o l i d couples with c o n s t a n t diffusion coefficients and to develop e x p r e s s i o n s r e l a t i n g the coefficients with the i n t e r f a c e motion and the e x p a n s i o n of the couple. T h e s e e x p r e s s i o n s a r e e m p l o y e d in the a n a l y s i s of t w o - p h a s e , v a p o r - s o l i d couples in the A g - Z n s y s t e m i n v e s t i g a t e d at 600~ with chips of a /3(bcc) alloy used as a v a p o r - s o u r c e in contact with an t~(fcc) diffusion disk. The e x p e r i m e n t a l i n v e s t i g a tion of the couples i n c l u d e s a study of the motion of the /3/~ i n t e r f a c e and a c
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