Microstructural interpretation of work softening in aluminum
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v a r i o u s effects of t e m p e r a t u r e on the p l a s t i c d e f o r m a t i o n of m e t a l s p r e s e n t an i m p o r t a n t p r o b l e m i n the t h e o r y and p r a c t i c e of p h y s i c a l m e t a l l u r g y . A r e m a r k a b l e f e a t u r e of t e m p e r a t u r e - d e p e n d e n t d e f o r m a t i o n of f a c e - c e n t e r e d - c u b i c m e t a l s is work softening. S a m p l e s d e f o r m e d a f t e r a p r e s t r a i n at lower t e m p e r a t u r e u s u a l l y exhibit a s t r e s s - s t r a i n c u r v e with a n i n i t i a l low or even n e g a t i v e slope. That is, the flow s t r e s s may decrease with s t r a i n in c o n t r a s t to the n o r m a l i n c r e a s e a s s o c i a t e d with work h a r d e n i n g . Work softening was identified and studied in a l u m i n u m by C o t t r e l l and Stokes, at t e m p e r a t u r e s b e t w e e n 90 K and 273 K (Ref. 1). S m a l l l o w - t e m p e r a t u r e p r e s t r a i n s p r o d u c e d a lowered slope on s u b s e quent higher t e m p e r a t u r e s t r a i n i n g , while p r e s t r a i n s g r e a t e r than about 5 pct w e r e n e c e s s a r y if a c t u a l work softening was to o c c u r . In g e n e r a l , the g r e a t e r the low t e m p e r a t u r e p r e s t r a i n , the m o r e p r o n o u n c e d was the s u b s e q u e n t s o f t e n i n g effect. However, it was r e d u c e d by a n n e a l i n g at the high t e m p e r a t u r e p r i o r to s t r a i n i n g . T h i s r e s u l t e s t a b l i s h e d that the fall i n s t r e s s is not a strain aging effect. S i m i l a r o b s e r v a t i o n s w e r e made at a l a t e r date in the higher t e m p e r a t u r e r a n g e 273 K to 388 K (Ref. 2). Soon a f t e r these p h e n o m e n a w e r e identified, K e l l y showed that the slip l i n e s which f o r m d u r i n g w o r k softe n i n g a r e i r r e g u l a r and b r o k e n , in a m a n n e r i n d i c a tive of e x t e n s i v e c r o s s s l i p . 3 He s u g g e s t e d that c r o s s rslip p e r m i t s the r e c o m b i n a t i o n of opposite s i g n s c r e w d i s l o c a t i o n s lying on n e i g h b o r i n g slip p l a n e s , and that this p r o c e s s is i m p o r t a n t to the softening effect. MECHANISM OF WORK SOFTENING It i s a g r e e d that w o r k softening is a c o n s e q u e n c e of
thermally-activated structure change. ~-3 The m i c r o s t r u c t u r e a s s o c i a t e d with the low t e m p e r a t u r e p r e s t r a i n is unstable at the h i g h e r t e m p e r a t u r e , p a r t i c u l a r l y if p l a s t i c s t r a i n i s i m p o s e d . The r e s u l t is the r a p i d f o r m a t i o n of a new, m o r e s t a b l e s t r u c t u r e having a s i g n i f i c a n t l y lower s t o r e d e n e r g y of cold w o r k . 4 However, the p r e c i s e n a t u r e of the s t r u c t u r e change has not been e s t a b l i s h e d . U s i n g a p i l e - u p m o d e l of s t r a i n h a r d e n i n g , C o t t r e l l and Sto
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