The role of microstructure in hydrogen-assisted fracture of 7075 aluminum
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W H I L E the r e s i s t a n c e of high s t r e n g t h a l u m i n u m a l l o y s to e n v i r o n m e n t a l l y a s s i s t e d f r a c t u r e long has b e e n a s u b j e c t of c o n s i d e r a b l e p r a c t i c a l c o n c e r n , f u n d a m e n t a l s t u d i e s of t h i s p r o b l e m have u s u a l l y not b e e n s u f f i c i e n t l y d e t a i l e d to identify the u n d e r l y i n g p r o c e s s of f r a c t u r e (for e x a m p l e , w h e t h e r f r a c t u r e is c o n t r o l l e d by anodic d i s s o l u t i o n , h y d r o g e n e m b r i t t l e ment, o r s o m e c o m b i n a t i o n of both). E v e n l e s s u n d e r standing e x i s t s a s to why t h i s type of f r a c t u r e is s e n s i t i v e to m e t a l l u r g i c a l v a r i a b l e s , s u c h a s c o m p o s R i o n , 1-3 m i c r o s t r u c t u r e , 2,s g r a i n s h a p e and t e x t u r e , 2'4,5 and t h e r m a l t r e a t m e n t . 2,3 T h e m i c r o s t r u c t u r e v a r i a b l e is of p a r t i c u l a r i n t e r e s t , s i n c e this is the v a r i a b l e that can be m a n i p u l a t e d in a g i v e n a l l o y o v e r a c o n s i d e r a b l e r a n g e while m a i n t a i n i n g the c o m p o s i t i o n , g r a i n s h a p e , and t e x t u r e d e v e l o p e d f r o m the a l l o y ' s p r o c e s s i n g history. T h e m i c r o s t r u c t u r a l v a r i a b l e that is thought to be of p r i m a r y i m p o r t a n c e in the e n v i r o n m e n t a l f r a c t u r e b e h a v i o r of a l u m i n u m a l l o y s is the c h a r a c t e r (nature and d i s t r i b u t i o n ) of the p r e c i p i t a t e p o p u l a t i o n . 6,7 The influence of p r e c i p i t a t e s both in the g r a i n i n t e r i o r and at g r a i n b o u n d a r i e s can be e x p l a i n e d by two b e h a v i o r f e a t u r e s : i) T h e c h a r a c t e r of t h o s e g r a i n i n t e r i o r p r e c i p i t a t e s which c o n t r o l s l i p mode by w h e t h e r o r not t h e y a r e s h e a r e d d u r i n g d e f o r m a t i o n ; when s h e a r e d , p l a n a r s l i p r e s u l t s , with a c c o m p a n y i n g low r e s i s t a n c e to s t r e s s c o r r o s i o n c r a c k i n g (SCC), while if they a r e not s h e a r e d , s l i p is wavy and SCC r e s i s t a n c e t e n d s to be higher; 6,8 ii) The r e l a t i v e c o v e r a g e of g r a i n bounda r i e s by the i n t e r g r a n u l a r p r e c i p i t a t e s . A n i n c r e a s e d c o v e r a g e has b e e n c o r r e l a t e d with SCC r e s i s t a n c e . %9 U n f o r t u n a t e l y , t h e s e two b e h a v i o r a l f e a t u r e s a r e d i f f i cult to s e p a r a t e e x p e r i m e n t a l l y b e c a u s e t h e r m a l t r e a t m e n t s which change the g r a i n i n t e r i o r p r e c i p i t a t e s a l s o in g e n e r a l a f f e c t g r a i n b o u n d a r y p r e c i p i t a t e s . M e c h a n i s t i c a l l y , it now a p p e a r s 6,7 that b o t h f e a t u r e s p l a y a r o l e in SCC s u s c e p t i b i
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