Experimental study of the origin of strain-rate sensitivity in a common Pb-Sn alloy
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W H E N a c y l i n d r i c a l m e t a l r o d or w i r e i s p l a s t i c a l l y s t r a i n e d in q u a s i s t a t i c t e n s i l e l o a d i n g , the r e s u l t i n g s t r e s s i s often found to depend not only on the s t r a i n i t s e l f but a l s o on the r a t e at which the s t r a i n i s p r o duced. The o r i g i n of t h i s r a t e d e p e n d e n c e , l i k e that of the s t r a i n , i s g e n e r a l l y a d m i t t e d to r e s t u l t i m a t e l y in the m o v e m e n t and i n t e r a c t i o n of d i s l o c a t i o n s and o t h e r i m p e r f e c t i o n s during the s t r a i n i n g but the d e t a i l s of t h i s p r o c e s s a r e not yet fully u n d e r s t o o d . If the d e f o r m a t i o n t e m p e r a t u r e i s s u f f i c i e n t l y low to allow the n e g l e c t of r e c o v e r y e f f e c t s , d i s l o c a t i o n t h e o r y n e v e r t h e l e s s i n d i c a t e s that the flow s t r e s s at any s t a g e of t e n s i l e d e f o r m a t i o n should depend d i r e c t l y on r a t e s e n s i t i v e m e c h a n i s m s a s s o c i a t e d with t h e r m a l l y a s s i s t e d d i s l o c a t i o n m o t i o n s a s w e l l a s on the m o r e c u s t o m a r y r a t e - i n s e n s i t i v e m e c h a n i s m s c o n n e c t e d with s t r a i n h a r d e n i n g . 1'2 On the o t h e r hand, h o w e v e r , when the d e f o r m a t i o n t e m p e r a t u r e i s such that a p p r e c i a b l e r e c o v e r y of the s t r a i n h a r d e n i n g can t a k e p l a c e during the d e f o r m a t i o n , the e n t i r e d i s l o c a t i o n p r o c e s s will c l e a r l y be m o r e c o m p l e x and the flow s t r e s s and i t s r a t e s e n s i t i v i t y m u s t then be r e g a r d e d a s depending a l s o on the r e c o v e r y - i n d u c e d t i m e s c a l e of the s t r a i n h a r d e n i n g . In the s p e c i a l c a s e w h e r e the d e f o r m a t i o n i s c a r r i e d out at a t e m p e r a t u r e c o r r e s p o n d i n g to a s i z a b l e p e r c e n t a g e of the m e l t i n g t e m p e r a t u r e of the s o l i d , the c o n t r i b u t i o n s to the flow s t r e s s f r o m t h e r m a l l y a s s i s t e d d i s l o c a t i o n m o t i o n s m a y , in f a c t , g e n e r a l l y be e x p e c t e d to be n e g l i g i b l e in c o m p a r i s o n with t h o s e a r i s i n g f r o m s t r a i n h a r d e n i n g so that m o s t if not all of the r a t e d e p e n d e n c e o b s e r v e d in t h e s e c i r c u m s t a n c e s m a y t h e r e f o r e b e e x p e c t e d to be a t t r i b u t a b l e d i r e c t l y to r e c o v e r y p r o c e s s e s o c c u r r i n g d u r i n g the deformation. Such a r e c o v e r y concept of m e t a l p l a s t i c i t y h a s long b e e n r e c o g n i z e d in the l i t e r a t u r e of h i g h - t e m p e r a t u r e c o n s t a n t - s t r e s s c r e e p d e f o r m a t i o n s and h a s b e e n c o n s i d e r e d in v a r y i n g d e g r e e s of d e t a i l by B a i l e y
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