High temperature work softening yield point phenomena in polycrystalline aluminum
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Work softening yield points have been studied in high purity p o l y c r y s t a l l i n e a l u m i n u m tested in the t e m p e r a t u r e range T2 = 100~ 450~ at constant s t r a i n rate s u b s e q u e n t to p r e s t r a i n i n g in t e n s i o n at a lower t e m p e r a t u r e (TI = room t e m p e r a t u r e or 350 ~ C) or higher s t r a i n r a t e . The yield drops i n c r e a s e with p r e s t r a i n as well a s with the d i f f e r e n c e between the flow s t r e s s e s at the test and p r e s t r a i n t e m p e r a t u r e . R e d u c t i o n s in the yield drop caused by a n n e a l i n g at T2 p r i o r to t e s t i n g indicate that the yield drops a r e the r e s u l t of dynamic r e c o v e r y . It is shown that d y n a m i c r e c o v e r y is a c c e l e r a t e d by c o n c u r r e n t straining. T H E effects of p r e s t r a i n i n g on high t e m p e r a t u r e m e c h a n i c a l p r o p e r t i e s can be studied u n d e r cons t a n t s t r e s s (creep t e s t s ) or c o n s t a n t strairt rate (tensile t e s t s ) conditions. In the case of c r e e p m e a s u r e m e n t s , the " d e l a y e d c r e e p " b e h a v i o r following a s t r e s s r e d u c t i o n from s t e a d y - s t a t e or a f t e r p r e s t r a i n i n g at a higher s t r e s s is convent i o n a l l y studied.l'2 In c o n s t a n t s t r a i n rate t e s t s the effects of p r e s t r a i n i n g a r e studied by e x a m i n i n g the yield drops produced s u b s e q u e n t to p r e s t r a i n i n g at a higher s t r a i n rate or lower t e m p e r a t u r e , as i l l u s t r a t e d in Fig. 1. In the e x p e r i m e n t s r e p o r t e d in this p a p e r , p r e s t r a i n i n g effects a r e studied by m e a s u r i n g the yield d r o p s o b s e r v e d in high t e m p e r a t u r e (T2 = 100 ~ to 450~ c o n s t a n t s t r a i n rate (42) t e n s i l e t e s t s , which a r e c a r r i e d out a f t e r p r e s t r a i n i n g at lower t e m p e r a t u r e s (TI = room t e m p e r a t u r e or 350~ or higher s t r a i n r a t e s (~ 1). Such yield drops have been o b s e r v e d in single c r y s t a l s as well a s in p o l y c r y s t a l l i n e s a m ples p r e s t r a i n e d at t e m p e r a t u r e s as low as - 1 6 3 ~ and as high as 450 ~ C. 3"e Analogous effects have b e e n o b s e r v e d in the c r e e p of a l u m i n u m , 7 and n i c k e l 8'9 p r e s t r a i n e d at room t e m p e r a t u r e . The yield drop and delayed c r e e p phenomena a r e both r e f l e c t i o n s of an i m b a l a n c e in the c o m p e t i t i o n b e t w e e n s t r a i n h a r d e n i n g and r e c o v e r y d u r i n g high t e m p e r a t u r e d e f o r m a t i o n . When d e f o r m a t i o n is allowed to p r o c e e d at a given t e m p e r a t u r e without i n t e r r u p t i o n (no abrupt change in s t r e s s or s t r a i n r a t e ) a d y n a m i c e q u i l i b r i u m d i s t r i b u t i o n of d i s l o c a t i o n s is e s t a b l i s h e d . In the case of n o n s t e a d
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