The role of diffusion in determining the controlling creep mechanisms in Al-Zn solid-solutions: Part I
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W. D. NIX
T h i s is the f i r s t of a two p a r t p a p e r in which the r o l e of diffusion in d e t e r m i n i n g the r a t e c o n t r o l l i n g c r e e p m e c h a n i s m s in b i n a r y s o l i d s o l u t i o n s is i n v e s t i g a t e d . P a r t I d e a l s with the a p p r o p r i a t e d i f f u s i o n c o e f f i c i e n t s for c l i m b and glide c o n t r o l l e d c r e e p , while P a r t II i n v o l v e s a c o m p a r i s o n of the c l i m b and glide c r e e p equations with e x p e r i m e n t , The f i r s t p a r t b e g i n s with a r e v i e w of a n u m b e r of diffusion c o e f f i c i e n t s that have b e e n p r o p o s e d for c r e e p in b i n a r y s o l i d s o l u t i o n s . We show that the diffusion c o e f f i c i e n t f o r c l i m b is Dcl = D~D~/(D~{X2 + D~XI) while that f o r glide is Dg l = (D~X2 + D~XI) (1 + d l n Y i / d l n Xi), Two o t h e r diffusion c o e f f i c i e n t s that have been u s e d in the l i t e r a t u r e a r e shown to b e i n c o r r e c t . To e x p e r i m e n t a l l y v e r i f y t h e t h e o r e t i c a l d i f f u s i o n c o e f f i c i e n t s , the c r e e p b e h a v i o r of A1-Zn a l p h a s o l i d s o l u t i o n s was s t u d i e d o v e r a c o m p o s i t i o n r a n g e of 0 to 60 at. p c t Zn. V a l u e s of D~], D*Zn and the t h e r m o d y n a m i c f a c t o r (1 + d l n Y i / d i n X i) a r e t a k e n f r o m the l i t e r a t u r e and u s e d to c a l c u l a t e Dcl and Dg I a s a function of c o m p o s i tion at 360 and 410~ It i s found that the t h e r m o d y n a m i c f a c t o r s t r o n g l y a f f e c t s the diffusion c o e f f i c i e n t Dg l f o r g l i d e - c o n t r o l l e d c r e e p . When t h e s e c u r v e s f o r Dcl and Dg 1 a r e c o m p a r e d with e x p e r i m e n t a l c r e e p d a t a for the A1-Zn s y s t e m , Dg 1 is found to e x p l a i n the d e c r e a s e in c r e e p r a t e o b s e r v e d a t t h e m i s c i b i l i t y gap c o m p o s i t i o n .
I. INTRODUCTION
MUCH
w o r k has b e e n done in an e f f o r t to d e t e r m i n e the a p p r o p r i a t e diffusion c o e f f i c i e n t s f o r c r e e p in b i n a r y s o l i d s o l u t i o n s . W h i l e s o m e p r o g r e s s has b e e n m a d e , confusion about the t h e o r e t i c a l l y a p p r o p r i a t e diff u s i o n c o e f f i c i e n t continues to e x i s t . S o m e of t h i s conf u s i o n m a y be due to m i s i n t e r p r e t a t i o n s of H e r r i n g ' s t r e a t m e n t 1 of m u l t i c o m p o n e n t d i f f u s i o n a l c r e e p , D e t e r m i n a t i o n of the a p p r o p r i a t e diffusion c o e f f i c i ent is c o m p l i c a t e d b y t h r e e p r o b l e m s . F i r s t t h e r e i s t h e p r e s e n c e of two s p e c i e s . T h i s i m m e d i a t e l y r a i s e s the q u e s t i o n a s to w h e t h e r the diffusion c o e f f i c i e n t should be that f o r the solute, s o l v e n t o r s o m e c o m b i n a tion t h e r e o f . If a v a c a n c
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