Nucleation of a second solid phase along dislocations
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IT is widely known that
n u c l e a t i o n of s o l i d - s o l i d t r a n s f o r m a t i o n s o c c u r s h e t e r o g e n e o u s l y on lattice defects due to the p o s s i b i l i t y of r e l e a s i n g t h e i r e x c e s s f r e e e n e r g y a s well a s r e d u c i n g the total i n t e r f a c e f r e e e n e r g y r e q u i r e d . In p a r t i c u l a r , the f o r m a t i o n of a new phase o c c u r s r e a d i l y in the e l a s t i c field s u r r o u n d i n g a d i s l o c a tion. In fact, the o b s e r v a t i o n of tiny s t r i n g s of s e c o n d phase n u c l e i along d i s l o c a t i o n s , 1'2 l o w - a n g l e b o u n d a r i e s , 3'4 and stacking faults 5'6 has been r e p o r t e d . A l s o , T h o m a s and Whelan 7 have o b s e r v e d that u n d e r c e r t a i n conditions n u c l e a t i o n o c c u r r e d p r e f e r e n t i a l l y on d i s l o c a t i o n s of the edge o r i e n t a t i o n . The v e r y few q u a n t i t a tive e x p e r i m e n t a l s t u d i e s on this p h e n o m e n o n , have been p r e s e n t e d in r e c e n t r e v i e w s f l 'g A t h e o r e t i c a l t r e a t m e n t of n u c l e a t i o n on d i s l o c a t i o n s was p r e s e n t e d by Cahn '~ in 1957. He a s s u m e d that the e l a s t i c e n e r g y of the d i s l o c a t i o n e n c l o s e d in the v o l u m e of the e m b r y o was r e l e a s e d a n d p r e d i c t e d the e x i s t e n c e of m e t a s t a b l e c y l i n d r i c a l e m b r y o s s u r r o u n d i n g all d i s l o c a t i o n s in s u p e r s a t u r a t e d s y s t e m s . However, he did not t r e a t the core e n e r g y of the d i s l o c a t i o n or the e l a s t i c b o u n d a r y conditions at the i n t e r f a c e between e m b r y o and m a t r i x . Lyubov and Solovyev n extended C a h n ' s t r e a t m e n t for c o h e r e n t n u c l e i and s u g g e s t e d that the catalytic effect of d i s l o c a t i o n s was due to the p r e s e n c e of solute a t m o s p h e r e s which r e d u c e d the a m o u n t of solute r e q u i r e c by the s e c o n d phase. A a r o n s o n et al. ~2 c l a r i f i e d this point by e x p l a i n i n g that the f o r m a t i o n of C o t t r e l l a t m o s p h e r e s is a c o n s e q u e n c e of the t e n d e n c y to e q u i l i b r a t e the c h e m ical p o t e n t i a l of solute a t o m s in the s y s t e m a n d t h e r e f o r e the actual d r i v i n g free e n e r g y for n u c l e a t i o n i s not affected; it was f u r t h e r pointed out that the n u c l e a t i o n p r o c e s s o c c u r s f a s t e r b e c a u s e of m o r e r a p i d t r a n s p o r t
of solute to the developing e m b r y o s . D o l l i n s 13 r e c e n t l y p r e s e n t e d a t r e a t m e n t for n u c l e i f o r m i n g not on a d i s location line but on its s u r r o u n d i n g s . However, a s will e m e r g e in the following, it is thought that m o s t of the catalytic effect of d i s l o c a t i o n s a r i s e s f r o m the c o r e . The p u r p o s e
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