Thickening kinetics of proeutectoid ferrite plates in Fe-C alloys
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to a g e n e r a l t h e o r y of solid state p r e cipitate morphology, Z'2 the b r o a d faces of p r e c i p i t a t e p l a t e s should f o r m a fully or p a r t i a l l y c o h e r e n t i n t e r face capable of p r o v i d i n g a b a r r i e r to growth in the n o r m a l d i r e c t i o n thus r e q u i r i n g that growth o c c u r by m e a n s of the ledge m e c h a n i s m . Both the i n t e r r a c i a l s t r u c t u r e and growth k i n e t i c s a s p e c t s of this p r e d i c tion have b e e n t e s t e d s u c c e s s f u l l y in a v a r i e t y of alloy s y s t e m s . 3-7 In each of the s y s t e m s so far e x a m i n e d the atomic m a t c h i n g a c r o s s the b r o a d faces of plates has r a n g e d f r o m quite good (misfit a c c o m m o d a t i n g i n t e r r a c i a l d i s l o c a t i o n s ) to v i r t u a l l y p e r f e c t (fully coh e r e n t ) . The c o h e r e n t r e g i o n s in t h e s e i n t e r f a c e s off e r a b a r r i e r to growth b e c a u s e of the kinetic i n f e a s i b i l i t y of changing the s t r u c t u r e of the m a t r i x plane abutting the i n t e r f a c e an atom at a t i m e without the aid of an effectively d i s o r d e r e d s t r u c t u r e . Often within the c o h e r e n t r e g i o n s s u b s t i t u t i o n a l atoms m u s t t e m p o r a r i l y occupy i n t e r s t i t i a l site in o r d e r to a c c o m p l i s h this change, z On the o t h e r hand, p l a t e s f o r m e d d u r i n g the p r o e u tectoid f e r r i t e t r a n s f o r m a t i o n r e p r e s e n t a different s i t u a t i o n . Mehl, B a r r e t t and Smith 8 concluded m a n y y e a r s ago that the b r o a d faces of f e r r i t e plates a r e d e s c r i b e d c r y s t a l l o g r a p h i c a l l y as {ii0}~ II{iii}7 and (111)c ~ II (110),/. The atomic m a t c h i n g a c r o s s this i n t e r f a c e is good only in l i m i t e d a r e a s . Noting, however, that the d e v e l o p m e n t f r o m g r a i n b o u n d a r y a l l o t r i o m o r p h s of W i d m a n s t a t t e n sideplafes o r i e n t e d p a r a l l e l to the s a m e m a t r i x plane c o n s t i t u t e s good e v i d e n c e s for the p r e s e n c e of an effective b a r r i e r to growth in the d i r e c t i o n n o r m a l to the b r o a d faces, it was p r o posed that t h e s e faces have a m i s f i t d i s l o c a t i o n s t r u c t u r e . z,9 An objection was r a i s e d to this view on the ground that the l a t t i c e m i s m a t c h involved is too l a r g e to s u p p o r t a d i s l o c a t i o n s t r u c t u r e ) ~ Even with the adv e n t of t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y the d i r e c t K. R. KINSMAN is with the Metallurgy Department, Scientific Research Staff, Ford Motor Company, Dearborn, Mich., 48121. E. EICHEN, formerly with the Scientific Research Staff, is now Graphics and Laboratory Services Manager, General Services Division, Ford Motor Company, Dearborn, Mich., 48121. H. I. AARONSON, formerly with the Scientific Resear
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