Application of heterogeneous nucleation theory to precipitate nucleation at GP zones
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G. W. LORIMER,
T. L. BARTEL,
The a c c e l e r a t e d n u c l e a t i o n of p r e c i p i t a t e s at GP zones is e x p l a i n e d using h e t e r o g e n e o u s n u c l e a t i o n t h e o r y . N u c l e a t i o n at zone : m a t r i x b o u n d a r i e s is e n c o u r a g e d by s e v e r a l f a c t o r s : 1) the c h e m i c a l i n t e r r a c i a l e n e r g y of zone : m a t r i x b o u n d a r i e s can s i g n i f i c a n t l y d e c r e a s e the i n t e r f a c i a l energy b a r r i e r to nucleation; 2) d e s t r u c t i o n of q u e n c h e d - i n e x c e s s v a c a n c i e s at i n c o h e r e n t p o r t i o n s of the n u c l e u s s u r f a c e may make the change in the v o l u m e free e n e r g y s i g n i f i c a n t l y m o r e negative; 3) the c r y s t a l s t r u c t u r e s of the zone and m a t r i x a r e i d e n t i c a l and p a r a l l e l which p e r m i t s the n u c l e u s to b e faceted in both p h a s e s . Some additional a s s i s t a n c e to n u c l e a t i o n at GP zones is p r o v i d e d by: 4) the a c c e l e r a t e d diffusivity r e s u l t i n g f r o m the p r e s e n c e of e x c e s s v a c a n c i e s and 5) the l a r g e a r e a of zone : m a t r i x b o u n d a r y p e r unit volume of m a t r i x . T h e s e f a c t o r s can m o r e than c o m p e n s a t e for the d e c r e a s e d solute s u p e r s a t u r a t i o n due to the f o r m a t i o n of GP zones and p r o v i d e an explanation for the enhanced n u c l e a t i o n of p r e c i p i t a t e s in the p r e s e n c e of GP z o n e s .
I. INTRODUCTION NUCLEATION of precipitates at GP zones has proved to be an excellent means of obtaining a fine and uniform dispersion of precipitates. This phenomenon has been observed in a number of aluminum alloys, including AI-Zn, I-3 A1-Cu, s'4 AI-Zn-Mg 5-7 and AI-MgSi, 8'9 has been confirmed in c~ Fe-Au I~ and probably also occurs in ~ Fe-Cu. 11,12. The catalysis of pre*GP zones are taken to be solute-rich regions in the matrix phase. Pashley and coworkers ' ' refer to clusters instead of zones. We accept the conclusion of Kelly and Nicholsont3 that the difference between the two is one of nomenclature. 2 8 9
c i p i t a t e n u c l e a t i o n at GP z o n e s is of p a r t i c u l a r i n t e r e s t b e c a u s e , as m a y be a p p r e c i a t e d f r o m F i g . 1, when p r e c i p i t a t e n u c l e a t i o n o c c u r s below the GP zone s o l v u s the m a t r i x s u p e r s a t u r a t i o n is l e s s than it would be in the a b s e n c e of z o n e s and the r a t e of p r e cipitate n u c l e a t i o n would t h e r e f o r e be expected to decrease. One theory of p r e c i p i t a t e n u c l e a t i o n at GP z o n e s is that of L o r i m e r and Nicholson 4-6 who p r o p o s e d that z o n e s whose size equals o r exceeds that of the c r i t i cal n u c l e u s at a given t e m p e r a t u r e act as p r e c i p i t a t e n u c l e i . P a s h l e y and c o w o r k e r s 2'8'9 have c o n s i d e r e d the p r o b l e m of p r e c i
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