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i n t e r e s t h a s r e c e n t l y b e e n shown in t h e properties of directionally solidified eutectic all o y s a s c o m p o s i t e m a t e r i a l s . In p a r t i c u l a r , t h e m e c h a n i c a l p r o p e r t i e s of s l o w l y s o l i d i f i e d s p e c i m e n s of t h e e u t e c t i c s A1-A12Cu a n d A 1 - A l s N i h a v e b e e n e x t e n s i v e l y i n v e s t i g a t e d , ~-3 a n d the b e h a v i o r of b o t h a l l o y s h a s b e e n f o u n d t o be s u b s t a n t i a l l y in a g r e e m e n t w i t h the t h e o r y o f idealized c o m p o s i t e behavior, 4 s i n c e t h e dir e c t i o n a l l y s o l i d i f i e d s t r u c t u r e s of b o t h e u t e c t i c s m a y be m a d e t o l i n e up in a n e a r l y p e r f e c t c o m p o s i t e m o r phology,s T h e s e previous investigations have shown that t h e A l s N i f i b e r s i n t h e A1-A13Ni f i b r o u s e u t e c t i c h a d p r o p e r t i e s a p p r o a c h i n g t h o s e o f w h i s k e r s , but t h e v o l u m e f r a c t i o n of t h e f i b e r s w a s i n s u f f i c i e n t t o a c h i e v e h i g h s t r e n g t h s . T h e c o m p o s i t e s t r e n g t h of the l a m e l l a r A1A12Cu e u t e c t i c w a s f o u n d t o be l i m i t e d by t h e l o w a n d v a r i a b l e s t r e n g t h of t h e A12Cu l a m e l l a e . T h e r e h a v e b e e n i n d i c a t i o n s in p r e v i o u s w o r k that i n c r e a s e s i n t h e strength of eutectic composites might r e s u l t from inc r e a s e s in s o l i d i f i c a t i o n r a t e , b e c a u s e o f a b e t t e r p r o b a b i l i t y o f a c h i e v i n g w h i s k e r p r o p e r t i e s in t h e i n t e r m e t a l l i c p h a s e p a r t i c l e s , ~'3'6 but t h i s e f f e c t h a s not b e e n f u l l y e x p l o r e d . O t h e r w o r k on f a b r i c a t e d c o m posite m a t e r i a l s has demonstrated that the m a t r i x p r o p e r t i e s m a y be m o d i f i e d s i g n i f i c a n t l y f r o m t h e i r " b u l k " v a l u e s in a v e r y f i n e c o m p o s i t e , 7 p r o d u c i n g i n c r e a s e d strengthening. On the other hand, an acceptable c o m p o s i t e must also h a v e g o o d f r a c t u r e t o u g h n e s s . Although s o m e m o d e l s are available t o d e s c r i b e the w o r k of f r a c t u r e i n c o m p o s i t e s , 8 l i t t l e e x p e r i m e n t a l w o r k h a s b e e n c a r r i e d o u t on t h e f r a c t u r e t o u g h n e s s of r a p i d l y s o l i d i f i e d e u t e c t i c a l l o y s , a n d i t i s n o t c l e a r how v a r i o u s microstructure p a r a m e t e r s affect t h e f r a c t u r e t o u g h n e s s of a l i g n e d e u t e c t i c s . I n o r d e r to o b t a i n a e u t e c t i c c o m p o s i t e o f n e a r l y W. H. S. LAWSON and H. W. KERR are Graduate Student and Associate Professor, respectively,Materials Science Laboratory, Department of Mechanical Engineering, University of Waterloo, Waterloo, Ontario, Canada. Manuscript su