The Nature of the Barrier to Growth at Partially Coherent FCC:BCC Boundaries
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Fig. 1--(a) A t o m i c c o n f i g u r a t i o n s of t h e {111}fee a n d t h e {110}bee p l a n e s , s u p e r i m p o s e d in a e e o r d a n e e w i t h a N i s h i -
yama-Wasserman orientation relationsMp.1 (b) Superlattiee of "good fit" regions formed by {111}feell {llO}bee in a Nishiyama-Wasserman relationship is shown by diamonds outlined with solid lines; dashed lines outline "good f i t " r e gions in the parallel, immediately adjacent atomic planes. 1 l a t t i c e s at c e r t a i n boundary o r i e n t a t i o n s , the i n t e r r a cial s t r u c t u r e at these o r i e n t a t i o n s is one of full coherency, i n t e r r u p t e d p e r i o d i c a l l y (if n e c e s s a r y ) by m i s f i t d i s l o c a t i o n s . The b a r r i e r to growth which this type of s t r u c t u r e p r o v i d e s * o r i g i n a t e s in the coherent *The existence of the barrier is demonstrated both by the morphological observation that this boundary orientation becomes the dominant one during the evolution of plates 7 and by the growth kinetics finding that these boundaries normally migrate more slowly than volume diffusion-control would allow, and in some instances demonstrably by means of the ledge mechanism, a
r e g i o n s . 8 D i s p l a c e m e n t of these r e g i o n s in the n o r m a l d i r e c t i o n r e q u i r e s that substitutional atoms t e m p o r a r ily occupy i n t e r s t i t i a l s i t e s when the c r y s t a l s t r u c t u r e of the p r e c i p i t a t e differs f r o m that of the m a t r i x . This p r o c e s s is e n e r g e t i c a l l y infeasible in a r e a s o n a b l y close packed c r y s t a l lattice, e However, an explanation of this type would s e e m i n a p p r o p r i a t e , Fig. 1 suggests, for the b a r r i e r to growth provided by p a r t i a l l y coherent fcc:bcc boundaries. The incoherent r e g i o n s of the boundary would a p p e a r to be mobile, and thus to be able r e p e a t e d l y to grow around and cut off the coherent r e g i o n s as s c h e m a t i c a l l y i l l u s t r a t e d in Fig. 2. Howe v e r , growth ledges have been o b s e r v e d on p l a n a r f a c e t s on Cr c r y s t a l s , 1 indicating the p r e s e n c e of a significant b a r r i e r to m i g r a t i o n - - a s does the development of such facets p e r s e during growth. Extensive m e a s u r e m e n t s of the thickening kinetics of p r o e u t e c VOLUME 5, JUNE 1 9 7 4 - 1 5 0 3
Cu (tcr
Cr (b.c.c.) Fig. 2--Growth of disordered regions around "good fit" areas.
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Fig. 3--Misfit dislocations at the centers of the regions between the diamonds of Fig. 1 provide a basis for extension of the essentially coherent regions to encompass nearly the entire interface. toid f e r r i t e sideplates make clear the existence of the b a r r
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