Precipitation at interphase boundaries
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M. R. PLICHTA,
G. W. FRANTI,
AND
K. C. RUSSELL
T h r e e p r o b l e m s in p r e c i p i t a t i o n at i n t e r p h a s e b o u n d a r i e s a r e e x a m i n e d . 1) The c l a s s i t a r g u m e n t as to the p a r t i c u l a r p h a s e in which such a p r e c i p i t a t e n u c l e a t e s is shown to be i r r e l e v a n t ; except in a s p e c i a l s i t u a t i o n , the c r i t i c a l n u c l e u s m u s t n o r m a l l y p e n e t r a t e both p h a s e s f o r m i n g the i n t e r p h a s e b o u n d a r y . 2) The r e l a t i v e p e n e t r a t i o n s into the two p h a s e s achieved d u r i n g growth can be v e r y d i f f e r e n t than those expected d u r i n g n u c l e a tion; hence, deductions about the n u c l e a t i o n p r o c e s s b a s e d upon o b s e r v a t i o n s on growth m o r p h o l o g i e s can be quite m i s l e a d i n g . 3) The o b s e r v a t i o n s of Honeycombe and o t h e r s that the n u c l e a t i o n of c a r b i d e s at a u s t e n i t e : f e r r i t e b o u n d a r i e s o c c u r s p r e d o m i n a n t l y at the low e n e r g y , i m m o b i l e b r o a d faces of ledges r a t h e r than at the higher e n e r g y , mobile r i s e r s of ledges a r e accounted for t h e o r e t i c a l l y on the b a s i s of the high v e l o c i t i e s of the r i s e r s p r e v e n t i n g n u c l e a t i o n . E x a m p l e c a l c u l a t i o n s on a T i - N i alloy indicate that p r e cipitation at the r i s e r s of ledges may b e c o m e p o s s i b l e in s u b s t i t u t i o n a l s y s t e m s , but only at lower homologous t e m p e r a t u r e s , and if the m i g r a t i o n of these b o u n d a r i e s is s t i l l cont r o l l e d by v o l u m e diffusion while n u c l e a t i o n is c o n t r o l l e d by i n t e r r a c i a l diffusion. I N T E R P H A S E b o u n d a r y p r e c i p i t a t i o n is an i m p o r t a n t p r o c e s s d u r i n g the b a i n i t e r e a c t i o n in f e r r o u s ~'2 and n o n f e r r o u s 3 eutectoid d e c o m p o s i t i o n and d u r i n g the f o r m a t i o n of a s e q u e n c e of p r e c i p i t a t e p h a s e s in a l loys u n d e r g o i n g aging r e a c t i o n s . '5 The n u c l e a t i o n a s p e c t of this p r o c e s s , o r i g i n a l l y t r e a t e d by Gibbs, 6 a l b e i t for liquids, was r e c e n t l y r e e x a m i n e d for solids in the p r e s e n c e of f a c e t i n g 7 and has b e e n studied in s o m e detail for the s p e c i a l case of p r e c i p i t a t e nucleation at a m a t r i x : GP zone boundary.8 Continuing c o n t r o v e r s i e s about c a r b i d e p r e c i p i t a t i o n d u r i n g the b a i n i t e r e a c t i o n in steel, 2 p a r a l l e l s t u d i e s of the e q u i v a l e n t p r o c e s s d u r i n g eutectoid d e c o m p o s i t i o n in T i - b a s e alloys 9'1~ and the finely detailed e l e c t r o n m i c r o s c o p i c s t u d i e s of Honeycombe and c o w o r k e r s n on i n t e r p h a s e b o u n d a r y p r e c i p i t a t i o n in F
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