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T H E m a i n advantage of e u t e c t i c f i b r o u s c o m p o s i t e s for e l e v a t e d t e m p e r a t u r e s t r u c t u r a l use is t h e i r s t r e n g t h at t e m p e r a t u r e . Retention of e l e v a t e d t e m p e r a t u r e s t r e n g t h is p r o m o t e d by the t h e r m o d y n a m i c s t a b i l i t y of the c o n s t i t u e n t p h a s e s . During e l e v a t e d t e m p e r a t u r e e x p o s u r e both the v o l u m e f r a c t i o n and c o m p o s i t i o n of the r e s p e c t i v e p h a s e s r e m a i n e s s e n t i a l l y unchanged. Morphological i n s t a b i l i t y , in the f o r m of i n c r e a s e s in a v e r a g e f i b e r d i a m e t e r a n d / o r d e c r e a s e s in a v e r a g e f i b e r length m a y occur, howe v e r , due to the a s s o c i a t e d f i b e r - m a t r i x i n t e r f a c e s u r face e n e r g y ; such i n s t a b i l i t i e s in t u r n can lead to decreased long-time-elevated-temperature mechanical p r o p e r t i e s . Since a p e r f e c t l y u n i f o r m c o m p o s i t e s t r u c t u r e c a n n o t undergo such m o r p h o l o g i c a l changes, i n s t a b i l i t i e s m u s t be i n i t i a t e d by shape p e r t u r b a t i o n s a n d / o r f i b e r size d i s t r i b u t i o n s . Shape p e r t u r b a t i o n s along the f i b e r length m a y be u n s t a b l e and lead to s p h e r o i d i z a t i o n of f i b e r s . This has b e e n a n a l y z e d t h e o r e t i c a l l y by Cline, ~ and e x p e r i m e n t a l o b s e r v a t i o n s of such s p h e r o i d i z a t i o n have b e e n made in the F e - F e S and Cu-CuS e u t e c t i c s y s t e m s . 2'3 F i b e r t e r m i n a t i o n s or b r a n c h e s , f o r m e d d u r i n g s o l i d i fication, can also lead to m i c r o s t r u c t u r a l c o a r s e n i n g at e l e v a t e d t e m p e r a t u r e s . T e r m i n a t i o n s m i g r a t e in the d i r e c t i o n toward the f i b e r c e n t r o i d and b r a n c h e s m i g r a t e toward the b r a n c h a r m s . As pointed out by S m a r t t et al., 4 longitudinal p r o p a g a t i o n of such faults can then lead to a m o r e r a p i d t w o - d i m e n s i o n a l c o a r s ening. Cline ~ has also t h e o r e t i c a l l y t r e a t e d the c o a r s ening c a u s e d by b r a n c h or t e r m i n a t i o n m i g r a t i o n , and G r a h a m and Kraft 5 have o b s e r v e d that c o a r s e n i n g in the l a m e l l a r A1-CuAI2 eutectic is d i r e c t l y a s s o c i a t e d with t h e i r p r e s e n c e . Two d i m e n s i o n a l Ostwald r i p e n i n g is c a u s e d by a d i s t r i b u t i o n in f i b e r d i a m e t e r s . Such r i p e n i n g has been e x p e r i m e n t a l l y o b s e r v e d in the A1-AlsNi e u t e c t i c by H. B. SMARTTand T. H. COURTNEY are Graduate Research Assistant and Associate Professor, respectively, Department of Mechanical Engineeringand Materials Science Laboratory, University of T e x a s at Austin, Austin, Texas 78712. Manuscript submitted March