Diffusion in ternary Ag-Zn-Cd solid solutions
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CARLSON, M . A . DAYANANDA, AND R. E. GRACE
Diffusion in t h e A g - Z n - C d s y s t e m w a s i n v e s t i g a t e d at 600~ with s e m i - i n f i n i t e , v a p o r - s o l i d c o u p l e s in o r d e r to e v a l u a t 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 in the a and fl p h a s e s . I n t r i n s i c d i f f u s i o n c o e f f i c i e n t s w e r e d e t e r m i n e d at s e v e r a l c o m p o s i t i o n p o i n t s in t h e a - p h a s e r e g i o n a l o n g c o n c e n t r a t i o n l i n e s of 11 a t . pct Zn, 18 a t . p c t Zn, 5 a t . pct Cd, and 9 a t . p c t Cd. 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 in the a p h a s e w e r e d e t e r m i n e d at s e v e r a l p o i n t s of i n t e r s e c t i o n of v a r i o u s c o m p o s i t i o n p a t h s ; t h e s e c o m p o s i t i o n p o i n t s of i n t e r s e c t i o n w e r e along l i n e s of c o n s t a n t z i n c - t o - c a d m i u m c o n c e n t r a t i o n r a t i o s of 3.8, 1.8, and 1.2. A l l t w o - p h a s e diffusion c o u p l e s d e v e l o p e d p l a n a r ~//a i n t e r f a c e s . I n t r i n s i c c o e f f i c i e n t s w e r e c o m p u t e d at t h r e e c o m p o s i t i o n p o i n t s in the/3 r e g i o n c o r r e s p o n d i n g to a s i l v e r c o n c e n t r a t i o n of a p p r o x i m a t e l y 55 a t . p c t . I n t r i n s i c c o e f f i c i e n t s f o r / 3 a l l o y s a r e two to t h r e e o r d e r s of m a g n i t u d e l a r g e r t h a n t h o s e f o r a a l l o y s . S I N C E m o s t m e t a l l i c a l l o y s of c o m m e r c i a l i n t e r e s t contain t h r e e o r m o r e c o m p o n e n t s , a study of t e r n a r y diffusion i s useful in gaining insight into t h e g e n e r a l p r o b l e m of m u l t i c o m p o n e n t diffusion. F i n i t e , ~ s e m i infinite,2,3 and i n f i n i t e 1'4-8 diffusion c o u p l e s h a v e b e e n e m p l o y e d to i n v e s t i g a t e diffusion in t e r n a r y s o l i d s o l u t i o n s . Of p a r t i c u l a r i n t e r e s t to t h e p r e s e n t study a r e t h o s e i n v e s t i g a t i o n s m a d e with infinite 6-s and s e m i infinite 2'3 c o u p l e s f o r t h e d e t e r m i n a t i o n of c o n c e n t r a t i o n - d e p e n d e n t 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 and i n t r i n s i c diffusion c o e f f i c i e n t s in t e r n a r y s y s t e m s . V a p o r - s o l i d c o u p l e s can b e s u c c e s s f u l l y e m p l o y e d in s t u d i e s of t e r n a r y diffusion p r o v i d e d that: 1) the v a p o r p r e s s u r e of t h e s o l v e n t i s n e g l i g i b l e in c o m p a r i son to t h o s e of the s o l u t e s s u p p l i e d f r o m t h e v a p o r p h a s e , and 2) the vap~or p r e s s u r e s of the s o l u t e s a r e high enough to m a i n t a i n a c o n s t a n t c o m p o s i t i o n a t t h e v a p o r - s o l i d i n t e r f a c e d u r i n g the c o u r s e of d i f f u s i o n . In a d
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