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A N D C. G. R H O D E S

D i r e c t i o n a l l y s o l i d i f i e d c r y s t a l s w e r e grown in which the A1-CuAI~ i n t e r p h a s e b o u n d a r y was u n i f o r m l y m i s o r i e n t e d b y 6-8 deg with r e s p e c t to the u s u a l c r y s t a l l o g r a p h i c r e l a t i o n s i n t e r f a c e II ( l l l ) A 1 II (21I)CuA12. The a r r a y s of f e a t u r e s then found in the b o u n d a r y w e r e shown to be l e d g e s b y 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 . T h e s e l e d g e s could move in the b o u n d a r y p l a n e d u r i n g h e a t i n g - s t a g e e x p e r i m e n t s . The b r o a d f a c e s of the l e d g e s w e r e the l o w - e n e r g y o r i e n t a t i o n s m e n t i o n e d above and w e r e r o t a t e d with r e s p e c t to the m a c r o s c o p i c i n t e r p h a s e b o u n d a r y plane by about 7 deg. F r o m t h e s e o b s e r v a t i o n s , it is c o n c l u d e d that the m i s o r i e n t e d i n t e r p h a s e b o u n d a r i e s in Ai-CuA12 c u r v e d e u t e c t i c c r y s t a l s a r e b e s t d e s c r i b e d in t e r m s of r e g i o n s of good fit s e p a r a t e d b y d i f f u s i o n - g i i d e (DG) l e d g e s .

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i m p o r t a n t f e a t u r e of c o m p o s i t e m a t e r i a l s f o r m e d b y the d i r e c t i o n a l s o l i d i f i c a t i o n of e u t e c t i c - c o m p o s i t i o n a l l o y s is t h e i r l a r g e r a t i o of i n t e r p h a s e i n t e r f a c t a l a r e a to v o l u m e . B e c a u s e the s c a l e of the p h a s e s is u s u a l l y quite s m a l l , t h i s r a t i o m a y be one o r two o r d e r s of m a g nitude l a r g e r than that of c o n v e n t i o n a l l y f a b r i c a t e d c o m p o s i t e s . This l a r g e a m o u n t of i n t e r n a l s u r f a c e a r e a can s i g n i f i c a n t l y affect the d e f o r m a t i o n , c o a r s e n i n g , p h a s e t r a n s f o r m a t i o n s , and n o n m e c h a n i c a l p r o p e r t i e s of the c o m p o s i t e . As a r e s u l t , d e t e r m i n a t i o n of the p r e c i s e n a t u r e of the b o u n d a r i e s in t h e s e m a t e r i a l s is of c o n siderable interest. By analogy with precipitate-matrix interfaces in precipitatlon-hardened materials, eutectic interfaces have often been identified as coherent, semicoherent, or noncoherent. *'a This classification is based on misfit calculations and direct observations of the interfaces by transmission electron microscopy. Most observations of eutectlc interfaces have generally been m a d e on equilibrium structures;that is, the interfaces were thought to be of a relatively low-energy orientation. As a method of deducing the general structure of eutectic interfaces, this procedure is comparable to the observation of those grain boundaries in monolithic materials which are located in the cusps of energy vs orientation plots; the low-energy interfaces are structurally less complex than the high-energy interfaces. Hence, there is presently some un