Reaction diffusion and associated nonequilibrium effects in the Au-Co system
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AND
G. W. POWELL
I n t e r d i f f u s i o n in t w o - p h a s e Au-Co c o u p l e s was i n v e s t i g a t e d using m e t a l l o g r a p h i c and e l e c t r o n m i c r o p r o b e a n a l y s e s . D e v i a t i o n s f r o m l o c a l e q u i l i b r i u m at the moving a - a i n t e r f a c e w e r e o b s e r v e d . T h e s e d e v i a t i o n s w e r e s i g n i f i c a n t for s h o r t (1 to 2 h d u r a t i o n ) a n n e a l s and, a s the a n n e a l i n g t i m e i n c r e a s e d , the i n t e r f a c e c o m p o s i t i o n s a p p r o a c h e d the e q u i l i b r i u m v a l u e s . The 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 (D) and i n t r i n s i c d i f f u s i v i t i e s (DAu and DCo ) in the a - A u t e r m i n a l s o l i d s o l u t i o n at 950 ~ and 900~ w e r e d e t e r m i n e d . T H E m o t i o n of an i n t e r p h a s e b o u n d a r y under i s o b a r i c , i s o t h e r m a l c o n d i t i o n s r e q u i r e s a d r i v i n g f o r c e . It h a s b e e n shown b y E i f e r t et al. that e x p e r i m e n t a l l y s i g n i f i c a n t d e v i a t i o n s f r o m l o c a l e q u i l i b r i u m o c c u r at a m o v ing a(fcc)-/3(bcc) b o u n d a r y in Cu:Cu-12 pc t Al diffusion c o u p l e s . D e v i a t i o n s f r o m l o c a l e q u i l i b r i u m at a moving i n t e r p h a s e b o u n d a r y a l s o have b e e n o b s e r v e d b y N i s h i z a w a and Chiba a in c~(bcc)-y(fcc), C r : F e - 8 p c t C r c o u p l e s ; the i n t e r f a c e c o m p o s i t i o n s d i f f e r e d f r o m the p h a s e d i a g r a m v a l u e s b y 1 to 3 p c t C r . The m a g n i t u d e of the c o m p o s i t i o n a l d e v i a t i o n s should d e p e n d in p a r t on the n a t u r e of the r e a c t i o n o c c u r r i n g at the moving i n t e r f a c e . The two m o s t c o m m o n t y p e s of i n t e r f a c e s are: 1) Type I; C~A-C~B, an i n t e r f a c e f o r m e d b y two p h a s e s of the s a m e c r y s t a l s t r u c t u r e but with d i f f e r e n t c h e m i c a l c o m p o s i t i o n s ; and 2) Type II; c~-fl, an i n t e r f a c e f o r m e d b y two p h a s e s which have d i f f e r e n t c r y s t a l s t r u c t u r e s and d i f f e r e n t chemical compositions. Although the n o n e q u i l i b r i u m c o m p o s i t i o n s o b s e r v e d b y E i f e r t et al. and N i s h i z a w a and Chiba at Type II (~-/3) i n t e r f a c e s can be e x p l a i n e d in t e r m s of the d r i v ing f o r c e r e q u i r e d f o r the l a t t i c e t r a n s f o r m a t i o n , o t h e r f a c t o r s a l s o a r e c a p a b l e of c a u s i n g n o n e q u i l i b r i u m e f f e c t s at moving i n t e r f a c e s . Should t h e y o c c u r , c o m p o s i t i o n a l d e v i a t i o n s a t a Type I (t~A-OtB) i n t e r f a c e , which d o e s not involve a l a t t i c e t r a n s f o r m a t i o n , m a y be the r e s u l t of n o n e q u i l i b r i u m v a c a n c y c o n c e n t r a t i o n s , an i n c r e a s e in the d i s l o c a t i o n
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