Interface interactions during fabrication of aluminum alloy-alumina fiber composites
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I. INTRODUCTION T H R O U G H O U T t h e i r h i s t o r y , c o m p o s i t e s have evolved u n d e r the idea that a unique, t a i l o r e d set of p r o p e r t i e s in a m a t e r i a l can be obtained f r o m a c o m b i n a t i o n of c o n s t i t u e n t s with ( s o m e t i m e s very) d i s s i m i l a r c h a r a c t e r i s t i c s . One of the m a i n a r e a s of i n t e r e s t has been the d e v e l o p m e n t of f i b e r - r e i n f o r c e d s t r u c t u r a l m e t a l s (e.g. A1, Ni, Ti), in which high s t r e n g t h f i b e r s (e.g. A1203, graphite, SiC) or w h i s k e r s a r e coupled with a ductile m a t r i x to p r o d u c e a m a t e r i a l that e x h i b i t s both p r o p e r t i e s . In these c o m p o s i t e s the p r i m e function of the f i b e r s is that of s u p p o r t i n g m o s t of the applied load, while the r o l e of the m a t r i x is to bind the f i b e r s together and to t r a n s m i t and d i s t r i b u t e the e x t e r n a l loads to the i n d i v i d u a l f i l a m e n t s . Howe v e r , s i n c e the t r a n s f e r of loads r e q u i r e s the e x i s t ence of a bond and hence a c e r t a i n d e g r e e of i n t e r a c tion b e t w e e n the c o n s t i t u e n t s , the i n t e r f a c e b e c o m e s a r e g i o n whose n a t u r e and p r o p e r t i e s a r e c r i t i c a l to the p e r f o r m a n c e of the composite s t r u c t u r e . N o r m a l l y , it is d e s i r a b l e to have a s t r o n g enough i n t e r f a c e which would p e r m i t t r a n s f e r and d i s t r i b u t i o n of load f r o m the m a t r i x to the f i l a m e n t s . S t r o n g i n t e r faces a r e t y p i c a l of s y s t e m s that a r e fully c o m p a t i b l e , that is, the m a t r i x and r e i n f o r c e m e n t f o r m bonds but a r e m u t u a l l y u n r e a c t i v e and i n s o l u b l e . In the i d e a l case the p e r f e c t i n t e r f a c e should be a m e c h a n i c s continuum, i n v o l v i n g c o h e r e n c y of the bond on the
a t o m i c level, and a c h e m i c a l d i s c o n t i n u u m , r e q u i r i n g the a b s e n c e of any i n t e r d i f f u s i o n b e t w e e n the c o n s t i t u ents.~ I n t e r a c t i o n of the p a r e n t p h a s e s may be u n d e s i r a b l e if it leads to the f o r m a t i o n of weak i n t e r f a c e s , s i n c e p r e m a t u r e f a i l u r e at the m a t r i x - f i b e r i n t e r f a c e will l i m i t the load c a r r y i n g a b i l i t y of the c o m p o s i t e . F u r t h e r m o r e , when a finite i n t e r a c t i o n zone is produced, the p r o p e r t i e s of the m a t e r i a l will decay a f t e r the r e a c t i o n exceeds a c r i t i c a l level, and t h e i r final value will depend s o l e l y on the c h a r a c t e r i s t i c s of the c o m pound p r e s e n t at the f i b e r b o u n d a r y . 2 M o r e o v e r , if the i n t e r a c t i o n p r o d u c e s an uneven or rough s u r f a c e on the r e i n f o r c e m e n t , it may have a c a t a s t r o p
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