A calorimetric study of fatigue induced microstructural changes in aluminum alloy 7050
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R. J. DeIASI,
AND
P. N. ADLER
D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC) was used to d e t e c t m i c r o s t r u c t u r a l c h a n g e s r e sulting f r o m s t r a i n - c o n t r o l l e d fatigue of a l u m i n u m a l l o y 7050. Tw o s t a r t i n g c o n d i t i o n s w e r e i n v e s t i g a t e d : a G P zone T6X t e m p e r and an o v e r a g e d T73651. T h e c a l o r i m e t r i c s i g n a t u r e of the m i c r o s t r u c t u r e was d e t e r m i n e d f o r s a m p l e s that had b e e n c y c l e d e i t h e r to f a i l u r e or to p r e s e l e c t e d p e r c e n t a g e s of t h e i r e x p e c t e d l i f e t i m e at v a r i o u s s t r a i n a m p l i t u d e s . T h e r m o d y n a m i c and k i n e t i c a n a l y s e s of the c a l o r i m e t r i c r e s u l t s r e v e a l e d a p r o n o u n c e d e f f e c t of p l a s t i c s t r a i n d u r i n g fatigue on the r e a c t i o n e n t h a l p y and r e a c t i o n k i n e t i c s of the GP zone d i s s o l u t i o n p e a k of T6X, and a l e s s e r e f f e c t on the 7' d i s s o l u t i o n p e a k of T73651. No m i c r o s t r u c t u r a l c h a n g e s a f t e r fatigue to f a i l u r e in the n o m i n a l l y e l a s t i c s t r a i n r e g i m e w e r e d e t e c t e d by DSC. Th e c a l o r i m e t r i c r e s u l t s w e r e u n i f o r m t h r o u g h o u t the c r o s s - s e c t i o n of the f a t i g u e s p e c i m e n s . B a s e d upon t h e s e r e s u l t s , it is c o n c l u d e d that a p p r o x i m a t e l y 75 pct of the G P z o n e s i n i t i a l l y p r e s e n t can be a f f e c t e d d u r ing low c y c l e f a t i g u e , and that o v e r a g i n g of the G P zone m i c r o s t r u c t u r e d o e s not o c c u r . T h e r e s u l t s f r o m the T73651 t e m p e r show that low c y c l e fatigue a f f e c t s t h i s o v e r a g e d m i c r o s t r u c t u r e in a d i f f e r e n t m a n n e r . R e v e r s i o n or d i s o r d e r i n g of 7 ' d o e s not o c c u r , but s o m e o v e r a g i n g was d e t e c t e d . It is s u g g e s t e d that the 7 ' p r e c i p i t a t e in t h i s a l l o y is not s h e a r a b l e .
F A T I G U E can modify the p r e c i p i t a t e m i c r o s t r u c t u r e of age h a r d e n i n g a l u m i n u m a l l o y s . T h i s is a m p l y d o c u m e n t e d in the l i t e r a t u r e ~-7 and t~as b e e n r e c e n t l y r e v i e w e d . 8~ D i s l o c a t i o n m o t i o n t h r o u g h m i c r o s t r u c t u r e s containing s h e a r a b l e p r e c i p i t a t e s r e s u l t s in c h a n g e s that have b e e n d e s c r i b e d as r e v e r s i o n , 1-3 o v e r a g i n g , %4 or d i s o r d e r i n g . 5 Since the s t r e n g t h e n i n g p r e c i p i t a t e s a r e u s u al l y s m a l l and v e r y d e n s e l y d i s t r i b u t e d , it is often d i f f i cu l t to c h a r a c t e r i z e the m i c r o s t r u c t u r e a c c u r a t e l y using t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (TEM), p a
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