Altering the time cycle of heat treatment by quenched-ln vacancies
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k . ~ O N S I D E R A B L E s u p e r s a t u r a t i o n of both s o l u t e a t o m s a n d v a c a n c i e s o c c u r s , a t t e n d a n t to h e a t t r e a t m e n t , w h e n a p r e c i p i t a t i o n - h a r d e n a b l e a l l o y is rapidly q u e n c h e d f r o m t h e s i n g l e to t h e t w o - p h a s e r e g i o n o f t h e e q u i l i b rium d i a g r a m . F o r t h e s o l u t e this supersaturation c a n a m o u n t to a f a c t o r o f t e n o r m o r e , while f o r v a c a n c i e s , s u p e r s a t u r a t i o n r a t i o s o f p e r h a p s t e n o r d e r s of m a g n i t u d e c a n be o b t a i n e d . Of n o t a b l e i n t e r e s t i s t h e f a c t t h a t on a g i n g following a q u e n c h from the solution t e m p e r a t u r e ( q u e n c h a g i n g ) , t h e s o l u t e s u p e r s a t u r a t i o n c a n be r e l i e v e d o n l y a l o n g w i t h a c o n c o m i t a n t r e l i e f in t h e s u p e r s a t u r a t i o n of v a c a n c i e s . F u r t h e r m o r e , i f t h e r e is a n o n t r i v i a l e n e r g y o f i n t e r a c t i o n b e t w e e n s o l u t e a n d v a c a n c y , i n s u b s t i t u t i o n a l d i f f u s i o n , the s u p e r s a t u r a tion-relieving p r o c e s s e s b e c o m e coupled. In such a situation, t h e solute a n d vacancy can join t o form what c a n be v i e w e d a s a t h i r d s p e c i e s w h i c h a c t s t o t r a n s port solute d u r i n g low-temperature precipitation r e a c t i o n s . It i s t h i s a p p a r e n t a s s o c i a t i o n b e t w e e n s o l u t e a n d v a c a n c y that h a s o n c e a g a i n a t t r a c t e d c o n s i d e r a b l e i n t e r e s t i n t h e k i n e t i c s of age h a r d e n i n g . T h o u g h it is probable t h a t quenched-in v a c a n c i e s c a n i n f l u e n c e a wide r a n g e o f m e t a l l u r g i c a l p r o c e s s e s , we s h a l l deal here exclusively with G - P (Guinier-Preston) z o n e s b e c a u s e of t h e a b u n d a n c e o f i n f o r m a t i o n o n the b e h a v i o r o f v a c a n c i e s in z o n e - f o r m i n g a l l o y s . A l s o , m a n y of t h e s e c o n c e p t s a n d f i n d i n g s c a n be r e a d i l y e x t e n d e d to o t h e r d i f f u s i o n - c o n t r o l l e d p r o c e s s e s . T h e m a i n g o a l of t h i s p a p e r w i l l be a n e v a l u a t i o n of t h e m o r e r e c e n t a p p r o a c h e s t o the k i n e t i c s o f z o n e f o r m a t i o n a s a f f e c t e d by q u e n c h e d - i n v a c a n c i e s . S e v e r a l r e v i e w s contain extensive background and references f o r the r e a d e r who w i s h e s t o e x a m i n e the e a r l i e r w o r k .1-3 T h e l i t e r a t u r e i n t h i s f i e l d i s so v o l u m i n o u s t h a t a c o m p l e t e l i s t i n g o f a l l r e f e r e n c e s w o u l d be n o t only m o s t d i f f i c u l t but a l s o q u i t e r e d u n d a n t . It i s h o p e d t h a t t h i s a r t i c l e w i l l c o n v e y a s e n s e o f the s t a t e of t h e field.
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