Influence of nonequilibrium second-phase particles formed during solidification upon the mechanical behavior of an alumi
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H. MULHERIN
AND
HYMAN
ROSENTHAL
The m e c h a n i c a l b e h a v i o r of a wrought high s t r e n g t h a l u m i n u m alloy is e x a m i n e d a s a function of the c o n c e n t r a t i o n of s e c o n d - p h a s e m i c r o c o n s t i t u e n t s . T h e s e s e c o n d - p h a s e p a r t i c l e s a r e located in g r a i n b o u n d a r i e s and i n t e r d e n d r i t i c s i t e s . T h e i r c o n c e n t r a t i o n w a s v a r i e d by t h e r m a l - m e c h a n i c a l p r o c e s s i n g . In the a r e a s of s t r e n g t h s , d u c t i l i t y , c r a c k t o u g h n e s s , fatigue, and s t r e s s c o r r o s i o n c r a c k i n g , e m p i r i c a l r e s u l t s a r e p r e s e n t e d attd d i s c u s s e d . O b s e r v a t i o n s a r e made on c h a n g e s in the a n i s o t r o p i c c h a r a c t e r of t h i s c a t e g o r y of m a t e r i a l with lower c o n c e n t r a t i o n s of second p h a s e s .
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the l i m i t a t i o n of r e a d i l y a v a i l a b l e m a t e r i a l s , only a r e l a t i v e l y s m a l l n u m b e r of alloy s y s t e m s p e r m i t e x t e n s i v e solid s o l u b i l i t y . B e c a u s e of this l i m i t a t i o n , a somewhat u n i q u e condition is a s i n g l e - p h a s e solid s o l u tion s t r e n g t h e n e d m a t e r i a l ( e . g . , 7 0 / 3 0 b r a s s ) . The m o r e u s u a l condition is the p r e s e n c e of one o r m o r e a d d i t i o n a l p h a s e s . The p r e s e n c e of t h e s e s e c o n d a r y p h a s e s a r i s e s through n u m e r o u s m e c h a n i s m s . Some of the m o r e c o m m o n ones a r e n o n e q u i l i b r i u m c o n s t i t u t i o n a l solidification, sol,d-state reactions (precipitate r e a c t i o n s and p h a s e change), and d i r e c t d i s p e r s i o n . This m u l t i p l i c i t y leads to a g r e a t v a r i e t y in c o m p o s i tion, f o r m , and d i s t r i b u t i o n of p a r t i c l e s of such p h a s e s . It is also g e n e r a l l y r e c o g n i z e d that, in a m a t e r i a l with a m e t a l l i c m a t r i x , c o m p l e x m e c h a n i c a l i n t e r a c t i o n s o c c u r between the p h a s e s . T h e r e f o r e , it is r e a s o n a b l e to a n t i c i p a t e that t h e s e c h a r a c t e r i s t i c s w i l l lead, in t u r n , to a g r e a t v a r i e t y of i n f l u e n c e s on m e c h a n i c a l b e h a v i o r . F o r e x a m p l e , at one end of the s p e c t r u m a c o m p l e t e d e g r a d a t i o n of u s e f u l p r o p e r t i e s may r e s u l t f r o m the p r e s e n c e of a b r i t t l e g r a i n b o u n d a r y envelope such as is p r o d u c e d by the p r e s e n c e of m i n u t e q u a n t i t i e s of bismuth in copper, t At the other end of the s p e c t r u m , the d e v e l o p m e n t of a c o h e r e n t i a t r a g r a n u l a r p r e c i p i t a t e may lead to a v e r y b e n e f i c i a l s t r e n g t h e n i n g effect a s in the i n s t a n c e of B e - C u a l l o y s . The e x i
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