The relation between microstructure and toughness in 7075 aluminum alloy
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E l e c t r o n m i c r o s c o p y , f r a c t o g r a p h y , and notched t e a r t e s t s have been used to i n v e s t i g a t e the effects of heat t r e a t m e n t upon the f r a c t u r e b e h a v i o r of aged 7075 a l u m i n u m alloy sheet. T o u g h n e s s , a s m e a s u r e d by crack propagation e n e r g y , d e c r e a s e s as the yield s t r e s s i n c r e a s e s ; the toughness of an overaged s t r u c t u r e is i n f e r i o r to that of an u n d e r a g e d s t r u c t u r e at the s a m e yield s t r e s s . The d e c r e a s e of toughness with i n c r e a s e d aging t i m e is a c c o m panied by a change in f r a c t u r e mode from p r e d o m i n a n t l y t r a n s g r a n u l a r to i n t e r g r a n u l a r . T r a n s g r a n u l a r f r a c t u r e p r o c e e d s by dimple r u p t u r e and is facilitated by c h r o m i u m - r i c h p a r ticles which a r e d i s p e r s e d throughout the m i c r o s t r u c t u r e . I n t e r g r a n u l a r f r a c t u r e p r o c e e d s by the f r a c t u r e of g r a i n boundary p r e c i p i t a t e p a r t i c l e s . The v a r i a t i o n of f r a c t u r e mode with aging t i m e is a t t r i b u t e d to a steady d e c r e a s e of the i n t e r g r a n u l a r f r a c t u r e s t r e s s r e l a t i v e to the t r a n s g r a n u l a r f r a c t u r e s t r e s s , due to i n c r e a s i n g g r a i n b o u n d a r y p a r t i c l e s i z e . A p o s s i b l e explanation of this effect is d i s c u s s e d u s i n g the s t r e s s c o n c e n t r a t i o n due to c o l i n e a r c r a c k a r r a y s a s an analogy. The effects of quenching v a r i a t i o n s and two-step aging a r e d i s c u s s e d . It is shown that, in aged 7075, m i c r o s t r u c t u r a l v a r i a b l e s such a s the width of p r e c i p i t a t e - f r e e zones and the n a t u r e of the m a t r i x p r e c i p i t a t e do not have a c o n t r o l l i n g effect on toughness.
T H E application of high s t r e n g t h a l u m i n u m a l l o y s , p a r t i c u l a r l y the A 1 - Z n - M g - C u - b a s e d 7 0 0 0 - s e r i e s a l l o y s , could be extended if it w e r e not for t h e i r r e l a t i v e l y low f r a c t u r e toughness, and c o n s i d e r a b l e r e s e a r c h has been conducted on the f a c t o r s affecting toughness of these a l l o y s . It is well e s t a b l i s h e d that one m a j o r v a r i a b l e is s t r e n g t h - - s i n c e as s t r e n g t h is i n c r e a s e d there is an a c c o m p a n y i n g d e c r e a s e in t o u g h n e s s ) '2 T h e r e is also evidence that f r a c t u r e is s e n s i t i v e to heat t r e a t m e n t , and p a r t i c u l a r l y that the toughness a s s o c i a t e d with an u n d e r a g e d s t r u c t u r e is s u p e r i o r to that of an o v e r a g e d s t r u c t u r e having the s a m e yield s t r e s s , s-s However, c o n s i d e r a b l e u s e is being made c o m m e r c i a l l y of o v e r aged t e m p e r s , p a r t i c u l a r l y t w o - s t e p ov
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