Creep behavior of Al-Al 3 Ni eutectic composites
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S p e c i m e n s of A1-AlsNi, d i r e c t i o n a l l y solidified at 11 cm p e r h r , w e r e c r e e p - t e s t e d at a single set of conditions to plastic s t r a i n s in the f i r s t , s e c o n d , and t h i r d s t a g e s , and to r u p t u r e . Specim e n s f r o m t h e s e i n t e r r u p t e d t e s t s w e r e s e c t i o n e d and e x a m i n e d by 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 . Due to the s m a l l p l a s t i c s t r a i n s e n c o u n t e r e d in the f i r s t and second s t a g e s of deform a t i o n (~p < 0.3 pct) r e l a t i v e l y little s u b s t r u c t u r e buildup was o b s e r v e d in the m a t r i x of these s p e c i m e n s . In s p e c i m e n s t e s t e d to r u p t u r e , dense d i s l o c a t i o n c e l l s keyed on the w h i s k e r s were found. As a r e s u l t , a t i m e - d e p e n d e n t f r a c t u r e m e c h a n i s m was identified which d i f f e r e d from n o r m a l t e n s i l e f r a c t u r e , and was r e l a t e d to the s u b s t r u c t u r e buildup. The effects of s o l i d i f i c a tion rate on c r e e p r e s i s t a n c e , m a c r o h a r d n e s s , and c o m p r e s s i v e yield s t r e n g t h w e r e a l s o i n v e s tigated and showed that by i n c r e a s i n g the s o l i d i f i c a t i o n r a t e a m o r e c r e e p - r e s i s t a n t , h a r d e r , and s t r o n g e r m a t e r i a l was produced. T h i s has suggested that a d d i t i o n a l s t r e n g t h e n i n g of this m a t e r i a l by a d i s p e r s i o n effect of the w h i s k e r s on the m a t r i x is p o s s i b l e .
WHEN d i r e c t i o n a l l y solidified u n d e r c o n d i t i o n s of p l a n e front growth, eutectic alloys can be f o r m e d with composite m i c r o s t r u c t u r e s a l i g n e d i n the growth d i r e c t i o n . The c r e e p and d e f o r m a t i o n b e h a v i o r of such a eutectic c o m p o s i t e , c o n s i s t i n g of a p p r o x i m a t e l y 10 vol pct of an i n t e r m e t a l l i c phase, AlsNi, u n i f o r m l y d i s p e r s e d within a n a l u m i n u m m a t r i x , has b e e n the s u b j e c t of this study. When t e n s i l e - t e s t e d i n the d i r e c t i o n of a l i g n m e n t , this composite s y s t e m b e h a v e s as a w h i s k e r - r e i n forced m a t e r i a l . 1'2 f u r t h e r m o r e , its m i c r o s t r u c t u r e p o s s e s s e s e x c e l l e n t t h e r m a l s t a b i l i t y u n d e r no load up to 97 pct of the eutectic t e m p e r a t u r e (610~ s and a l s o wheh c r e e p - t e s t e d for a p p r o x i m a t e l y 100 h r at 90 pct of the eutectic t e m p e r a t u r e (552~ 4 T h i s s t a b i l i t y is a t t r i b u t e d to the p r e f e r r e d , l o w - e n e r g y i n t e r f a c e s e s t a b l i s h e d d u r i n g the c o n t r o l l e d growth. 5 At growth r a t e s above which the m o r p h o l o g y b e c o m e s r o d - l i k e , the t e n s i l e s t r e n g t h of the eutectic is r a t h e r i n s e n s i t i v e to the
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