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(Aep/2) = K(2Nf) c w h e r e c and K a r e - 0.39 and 0.068 f o r the l a m e l l a r , and - 0.45 and 0.074 f o r the c a s t s t r u c t u r e , r e s p e c t i v e l y . At t h i s t e m p e r a t u r e with d e c r e a s i n g s t r a i n a m p l i t u d e l a m e l l a r m a t e r i a l b e c a m e m o r e r e s i s t a n t to fatigue than a s - c a s t s t r u c t u r e , which has b e e n r e l a t e d to the m o r e e f f i c i e n t d e f l e c t i o n of fatigue c r a c k s b y C r - r i c h l a m e l l a e at l o w e r s t r a i n a m plitudes. 11"'1'

UNIDIRECTIONALLY s o l i d i f i e d e u t e c t i c s a r e r e g a r d e d a s p o t e n t i a l high t e m p e r a t u r e s t r u c t u r a l m a t e r i a l s . T h e y often b e h a v e a s f i b e r - r e i n f o r c e d c o m p o s i t e s and r e m a i n m i c r o s t r u c t u r a l l y s t a b l e at e l e v a t e d t e m p e r a tures. Unidirectional solidification generally results in s i g n i f i c a n t i m p r o v e m e n t s in c r e e p and s t r e s s - r u p t u r e p r o p e r t i e s . *-3 In Ni-51 Cr l a m e l l a r e u t e c t i c , 3 f o r e x a m p l e , m o r e than an o r d e r of m a g n i t u d e inc r e a s e in r u p t u r e t i m e is o b t a i n e d o v e r the c o n v e n tionally cast material. An u n d e r s t a n d i n g of the fatigue b e h a v i o r of the unid i r e c t i o n a l l y s o l i d i f i e d e u t e c t i c s is e q u a l l y i m p o r t a n t if t h e y a r e to be u s e d a s s t r u c t u r a l m e m b e r s . To date only a few fatigue s t u d i e s of e u t e c t i c c o m p o s i t e s have b e e n p e r f o r m e d on F e - F e 2 B 4 and A1-A13Ni5,e f i b r o u s , and A1-CuAla 5 and Ni-NisNb 7 l a m e l l a r e u t e c t i c s . A l though a g e n e r a l i z a t i o n of the c o m p o s i t e e u t e c t i c b e h a v i o r i s not y e t p o s s i b l e , the r e s u l t s s e e m to i n d i c a t e that under low s t r e s s high c y c l e fatigue c o n d i t i o n s c r a c k s a r e u s u a l l y d e f l e c t e d b y the r e i n f o r c i n g f i b e r s and p l a t e l e t s r e s u l t i n g in e n h a n c e m e n t in fatigue p r o p e r t i e s . Under high s t r e s s a m p l i t u d e s , h o w e v e r , f i b e r s o r p l a t e l e t s no l o n g e r offer any r e s i s t a n c e to propagating matrix cracks. The i n v e s t i g a t i o n s c i t e d above have b e e n c o n c e r n e d

p r i m a r i l y with the s t r e s s c o n t r o l l e d t e n s i o n - t e n s i o n fatigue b e h a v i o r . In the p r e s e n t s t u d y the s t r a i n c o n t r o l l e d t e n s i o n - c o m p r e s s i o n fatigue b e h a v i o r of the u n i d i r e c t i o n a l l y s o l i d i f i e d NI-51 C r l a m e l l a r e u t e c t i c i s e x a m i n e d and c o m p a r e d with t h a t of the a s - c a s t m i c r o s t r u c t u r e . I n v e s t i g a t e d a r e the e f f e c t s of t e m p e r a t u r e and s t r a i n a m p l i t u d e on the fatigue h a r d e n i n g / s o f t e n i n