The relationship of microstructure to monotonic and cyclic straining of two age hardening aluminum alloys
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d i s c o v e r y of age h a r d e n i n g in 19051 h a s b e e n f o l l o w e d by n u m e r o u s i n v e s t i g a t i o n s of the k i n e t i c s , growth, and d i s t r i b u t i o n of t h e p r e c i p i t a t e s with t h e u l t i m a t e goal b e i n g the p r o d u c t i o n of high s t r e n g t h a l l o y s . T h e s e s t u d i e s have p r o d u c e d many a l u m i n u m a l l o y s y s t e m s which have found g r e a t u s a g e in the a i r c r a f t i n d u s t r y w h e r e a high s t r e n g t h to weight r a t i o i s of p a r a m o u n t i m p o r t a n c e . T h e i r a p p l i c a t i o n , howe v e r , has shown that the a l u m i n u m a l l o y s w h i c h show the highest static strength are inherently susceptible to s t r e s s - c o r r o s i o n c r a c k i n g , z'~ and have p o o r f r a c t u r e t o u g h n e s s and f a t i g u e p r o p e r t i e s 5-7 The p r o s p e c t f o r i m p r o v i n g the s t r e s s - c o r r o s i o n r e s i s t a n c e and f r a c t u r e t o u g h n e s s without s a c r i f i c i n g t h e i n i t i a l high s t a t i c s t r e n g t h a p p e a r s p r o m i s i n g , 8'9 and l i m i t e d i m p r o v e m e n t s in fatigue p r o p e r t i e s have b e e n o b t a i n e d by t h e r m o m e c h a n i c a l p r o c e s s i n g , l~ T h e i m p r o v e m e n t s in fatigue r e s i s t a n c e , h o w e v e r , have g e n e r a l l y b e e n the r e s u l t of m e t a U u r g i c a l p r o c e s s i n g changes f o r o t h e r r e a s o n s . ~3 The f a t i g u e r a t i o (fatigue s t r e n g t h / u l t i m a t e t e n s i l e s t r e n g t h ) of high p u r i t y 2XXX and 7XXX s e r i e s a l l o y s i s about 0.3, and t h i s low v a l u e has g e n e r a l l y b e e n a t t r i b u t e d to r e v e r s i o n , 6 d e s t r u c t i o n of p r e c i p i t a t e o r d e r , 14 and o v e r aging, 15 d u r i n g c y c l i c d e f o r m a t i o n . The a c t u a l m e c h a n i s m s a s s o c i a t e d with fatigue in a g e h a r d e n a b l e a l u m i n u m a l l o y s a r e not w e l l u n d e r s t o o d , and the p r o s p e c t s f o r i m p r o v i n g t h e fatigue r e s i s t a n c e would be much b r i g h t e r if the r e l a t i o n s h i p s b e t w e e n c y c l i c d e f o r m a t i o n , c r a c k i n i t i a t i o n , p r o p a g a t i o n , and m i crostructure were established. It was the p u r p o s e of t h i s study to i n v e s t i g a t e the f a t i g u e b e h a v i o r of two 7XXX a l u m i n u m a l l o y s by
T. H. SANDERS, Jr. is Metallurgist, Alcoa Research Laboratory, Aloca Center, PA 15069. E. A. STARKE, Jr. is Professor, School of Chemical Engineering and Metallurgy, Georgia Institute of Technology, Atlanta, GA 30332. This research was supported by the U.S. Air Force Office of Scientific Research (AFSC) under Grant No. AFOSR-742615, Dr. Alan H. Rosenstein, Contract Monitor. Manuscript submitted December 29, 1975 METALLURGICAL TRANSACTIONS A
v a r y i n g the aging t e m p e r a t u r e , t i m e , and c
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