Diffusion studies in Ag-Zn alloys
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M. A. DAYANANDA,
AND
R. E. GRACE
Diffusion s t u d i e s w e r e m a d e in the a l p h a and b e t a p h a s e s of the s i l v e r - z i n c s y s t e m at 600~ with v a p o r - s o l i d and s o l i d - s o l i d diffusion c o u p l e s in o r d e r to d e t e r m i n e i n t r i n s i c diffusion c o e f f i c i e n t s and 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 . I n t r i n s i c diffusion c o e f f i c i e n t s , D Zn and DAg, d e t e r m i n e d e x p e r i m e n t a l l y f o r both ~ and fl a l l o y s , w e r e c o m p a r e d with t h o s e p r e d i c t e d f r o m the m o d e l s of D a r k e n , Manning, and D a y a n a n d a with the aid of t h e r m o d y n a m i c and s i n g l e c r y s t a l t r a c e r diffusion data. The e x p e r i m e n t a l v a l u e s of Dzn w e r e c l o s e to t h o s e p r e d i c t e d b y the m o d e l s of D a r k e n and Manning, while the e x p e r i m e n t a l DAg v a l u e s w e r e h i g h e r than t h o s e d e t e r m i n e d b y both m o d e l s . The e x p e r i m e n t a l v a l u e s of the r a t i o , D Z n / D A g , w e r e found to be l o w e r than t h o s e p r e d i c t e d b y e i t h e r D a r k e n ' s o r M a n n i n g ' s r e l a t i o n s . Also, D a y a n a n d a ' s m o d e l was found to be i n a p p l i c a b l e with the a v a i l a b l e t r a c e r diffusion data. This d i s a g r e e m e n t is c o n s i d e r e d to r e f l e c t the i n a d e q u a c y of the s i n g l e c r y s t a l t r a c e r d i f f u s i v i t i e s in the a n a l y s i s of i n t r i n s i c diffusion in p o l y c r y s t a l l i n e A g - Z n alloys. relationship derived originally by Darken ~between i n t r i n s i c diffusion c o e f f i c i e n t s and t r a c e r diffus i o n c o e f f i c i e n t s h a s b e e n found to be i n a d e q u a t e in the e a s e of m a n y b i n a r y s y s t e m s w h e r e l a r g e K i r k e n d a l l s h i f t s a r e o b s e r v e d . The need to u s e D a r k e n ' s r e l a t i o n s with a p p r o p r i a t e m o d i f i c a t i o n s to include v a c a n c y wind e f f e c t s was o r i g i n a l l y pointed out b y Manning, z and m o d i f i e d e x p r e s s i o n s with v a c a n c y wind c o r r e c t i o n t e r m s w e r e s u g g e s t e d b y Manning s and Dayananda. 4 I o r i o , Dayananda, and G r a c e s a p p l i e d the v a r i o u s e x p r e s s i o n s in an a n a l y s i s of the e x p e r i m e n t a l i n t r i n s i c diffusion d a t a f o r a A g - C d a l l o y s and showed that the v a c a n c y wind c o n t r i b u t i o n s to i n t r i n s i c diffusion in t h e s e a l l o y s can be a p p r e c i a b l e . T h e y r e p o r t e d that the m o d i f i e d r e l a t i o n s of Manning and D a y a n a n d a i n eluding v a c a n c y wind e f f e c t s w e r e s a t i s f a c t o r y in the p r e d i c t i o n of the i n t r i n s i c d i f f u s i v i t y r a t i o , DCd/DAg , f r o m t r a c e r d i f f u s i o n d a t a , b u t not for the e s t i m a t i o n of the i n d
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