The aging response of Al-Cu and Al-Cu-Mg directionally solidified eutectics
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THE d i r e c t i o n a l s o l i d i f i c a t i o n of e u t e c t i c s
to p r o d u c e r e i n f o r c e d c o m p o s i t e s has been well d o c u m e n t e d ? - 3 The p o t e n t i a l l y high s t r e n g t h - t o - w e i g h t r a t i o s of d i r e c t i o n a l l y solidified e u t e c t i c s make them a t t r a c t i v e for s t r u c t u r a l a p p l i c a t i o n s w h e r e u n i d i r e c t i o n a l l o a d ing o c c u r s . One a s p e c t of c u r r e n t r e s e a r c h in d i r e c t i o n a l s o l i d i f i c a t i o n is the s e a r c h for h i g h e r - s t r e n g t h e u t e c t i c s . The m a j o r i t y of the eutectic s y s t e m s which have been studied to date have m i c r o s t r u c t u r e s c o n s i s t i n g of b r i t t l e r o d s (or l a m e l l a e ) in a ductile m a t r i x . Some of these ductile m a t r i c e s a r e solid s o l u t i o n s and, as such, m a y r e s p o n d to aging t r e a t m e n t s . P r e c i p i t a t i o n h a r d e n i n g then b e c o m e s a p o t e n t i a l s t r e n g t h e n i n g m e c h a n i s m for d i r e c t i o n a l l y solidified e u t e c t i c s . It has not b e e n e s t a b l i s h e d whether the aging r e s p o n s e (both m i c r o s t r u c t u r e and m e c h a n i c a l p r o p e r t i e s ) of a solid s o l u t i o n phase in a eutectic may be p r e d i c t e d by e x a m i n i n g the aging r e s p o n s e in a dilute alloy of s i m i l a r c o m p o s i t i o n . F u r t h e r m o r e , i t is not c l e a r what the c o m p o s i t i o n of the s o l i d - s o l u tion phase might be, for Spalding e t a l . 4 have shown that the copper c o n t e n t of the a l u m i n u m - r i c h phase in the A1-CuA12 eutectic was c o n s i d e r a b l y l e s s than that p r e d i c t e d by eutectic growth t h e o r i e s . The p u r p o s e of this study is to e x a m i n e the m i c r o s t r u c t u r a l changes in the a l u m i n u m - r i c h solid s o l u tion p h a s e s after aging A1-Cu and A1-Cu-Mg d i r e c t i o n a l l y solidified e u t e c t i c s and to d e t e r m i n e the m e c h a n i c a l p r o p e r t y changes in the A1-Cu e u t e c t i c . The s t r u c t u r e and p r o p e r t i e s of these two s y s t e m s have b e e n r e p o r t e d 2,~'s and the aging study is a c o n t i n u a t i o n of t h e i r c h a r a c t e r i z a t i o n . Aging of dilute A1-Cu and A1-Cu-Mg alloys is r e a s o n a b l y well u n d e r stood, thus p r o v i d i n g a b a s i s for c o m p a r i s o n of aging r e s p o n s e with l i t e r a t u r e r e s u l t s . E X P E R I M E N T A L PROCEDURE The b i n a r y eutectic lies at 66.8 wt pct A1-33.2 wt pct Cu, and the t e r n a r y eutectic l i e s at 60.65 wt pct C. G. RHODES and G, GARMONGare Senior Staff Associate, and Member of Technical Staff, North American Rockwell Science Center, Thousand Oaks, Calif. Manuscript submitted November 5, 1971. METALLURGICALTRANSACTIONS
A1-33.1 wt pct C u - 6 . 2 5 wt pct Mg. Alloy p r e p a r a t i o n and d i r e c t i o n a
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