Monotonic and fatigue deformation of Ni-W directionally solidified eutectic
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AND
J. C. WILLIAMS
Unlike m a n y e u t e c t i c c o m p o s i t e s , the N i - W e u t e c t i c e x h i b i t s e x t e n s i v e d u c t i l i t y by s l i p . F u r t h e r m o r e , its p r o p e r t i e s m a y be g r e a t l y v a r i e d by p r o p e r h e a t t r e a t m e n t s . H e r e r e s u l t s of s t u d i e s of d e f o r m a t i o n in both monotonic and fatigue l o a d i n g a r e r e p o r t e d . D u r i n g monotonic d e f o r m a t i o n the f i b e r / m a t r i x i n t e r f a c e a c t s a s a s o u r c e of d i s l o c a t i o n s at low s t r a i n s and an o b s t a c l e to m a t r i x s l i p at h i g h e r s t r a i n s . D e f o r m i n g the q u e n c h e d - p l u s aged e u t e c t i c c a u s e s p l a n a r m a t r i x s l i p , with the r e s u l t that m a t r i x s l i p bands c r e a t e s t r e s s c o n c e n t r a t i o n s in the f i b e r s at low s t r a i n s . The aged e u t e c t i c r e a c h e s g e n e r a l l y h i g h e r s t r e s s l e v e l s for c o m p a r a b l e s t r a i n s than d o e s the a s - q u e n c h e d e u t e c t i c , and the f a i l u r e s t r a i n s d e c r e a s e with i n c r e a s i n g aging t i m e s . F o r the c o m p o s i t e s t e s t e d in fatigue, the aged e u t e c t i c h a s b e t t e r h i g h - s t r e s s fatigue r e s i s t a n c e than the a s - q u e n c h e d m a t e r i a l , but for l o w - s t r e s s , h i g h - c y c l e fatigue t h e i r c y c l e s to f a i l u r e a r e n e a r l y the s a m e . Howe v e r , both c r a c k i n i t i a t i o n and c r a c k p r o p a g a t i o n a r e d i f f e r e n t in the two conditions, so the c o i n c i d e n c e in h i g h - c y c l e fatigue i s p r o b a b l y f o r t u i t o u s . The e f f e c t of m a t r i x s t r e n g t h on c o m p o s i t e p e r f o r m a n c e i s not s i m p l e , s i n c e changes in s t r e n g t h m a y be a c c o m p a n i e d b y a l t e r a t i o n s in s l i p m o d e s and f a i l u r e p r o c e s s e s .
IHE great majority of composites formed by the directional solidificationof eutectic alloys contain a brittle reinforcement phase. Consequently, most studies of mechanical properties of eutectic composites have been directed toward materials having ductility on the order of 1 to 2 pct or less. Because the entire scope of mechanical behavior is compressed into a s m a l l s t r a i n r a n g e , s o m e p r o c e s s e s of i n t e r e s t a r e difficult to r e s o l v e e x p e r i m e n t a l l y . M o r e o v e r , the b r i t t l e n a t u r e of the f i b e r s r e s u l t s in f i b e r f a i l u r e at low s t r a i n s and m a s k s s o m e effects r e l a t e d to f i b e r / m a t r i x i n t e r a c t i o n . The p r e s e n t p a p e r s u m m a r i z e s the r e s u l t s of an i n v e s t i g a t i o n into the d e f o r m a tion b e h a v i o r of a e u t e c t i c c o m p o s i t e which e x h i b i t s e x t e n s i v e d u c t i l i t y b y s l i p in both p h a s e s . Other s t u d i e s of d u c t
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