On stress-relaxation experiments and their significance under strain-aging conditions
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I, INTRODUCTION S T R E S S - r e l a x a t i o n e x p e r i m e n t s have b e e n e x t e n s i v e l y used to obtain p a r a m e t e r s of m i c r o d y n a m i c a l d e f o r m a t i o n p r o c e s s e s in m e t a l s and a l l o y s . A m o n g t h e s e the m o s t c o m m o n a r e the i n t e r n a l and effective s t r e s s e s , 1-s and the a c t i v a t i o n v o l u m e . 4 The a n a l y s i s of s t r e s s - r e l a x a t i o n e x p e r i m e n t s can be c a r r i e d out along two d i f f e r e n t paths: u s i n g an e m p i r i c a l r e l a t i o n ship b e t w e e n s t r e s s and d i s l o c a t i o n velocity, such a s J o h n s t o n and G i l m a n ' s s (e.g., R e f s . 1 and 2) or u s i n g the t h e o r y of t h e r m a l a c t i v a t i o n p r o c e s s e s (e.g., Ref. 4). T h e s e two a l t e r n a t e r o u t e s a r e d e s c r i b e d by K r a u s z . 6 The c o n v e n t i o n a l methods for the a n a l y s i s of s t r e s s - r e l a x a t i o n r e s u l t s of m e t a l s and a l l o y s a r e b a s e d on one a s s u m p t i o n : that the s u b s t r u c t u r e and b a r r i e r s r e m a i n unchanged d u r i n g the t e s t . The f a i l u r e of one of t h e s e a s s u m p t i o n s would r e n d e r any p a r a m e t e r obtained t h e r e f r o m of doubtful v a l u e . T h i s is cons i d e r e d u p s e t t i n g s i n c e t h e r e a r e e v i d e n c e s in the l i t e r a t u r e that the m o b i l e d i s l o c a t i o n d e n s i t y v a r i e s d u r i n g r e l a x a t i o n . 7-12 Rohde and N o r d s t r o m 7 p r e s e n t e d evidence showing that the d i s l o c a t i o n s u b s t r u c t u r e u n d e r w e n t changes d u r i n g r e l a x a t i o n . Specifically, they o b s e r v e d a d e c r e a s e in m o b i l e d i s l o c a t i o n d e n sity, Pro, d u r i n g r e l a x a t i o n , of the f o r m : Pm = Pm (o)e-OeP r
[1 ]
where 0 is a n i m m o b i l i z a t i o n p a r a m e t e r , s is the p l a s t i c s t r a i n d u r i n g r e l a x a t i o n , and Pm(o) is the m o b i l e d i s l o c a t i o n d e n s i t y at the b e g i n n i n g of the r e l a x a t i o n t r a n s i e n t . Equation [1] could be used to c o r r e c t the c o n v e n t i o n a l m e t h o d s for the a n a l y s i s of r e l a x a t i o n data, if 0 w e r e d e t e r m i n e d . But, a c c o r d i n g to Rohde and N o r d s t r o m v t h e r e is no i n d e p e n d e n t m e a n s of obt a i n i n g 0. R e c e n t l y , Okazaki et al 8 p r e s e n t e d a method f o r the a n a l y s i s of s t r e s s - r e l a x a t i o n r e s u l t s a s s u m i n g a v a r i a b l e d i s l o c a t i o n d e n s i t y . However, t h e i r f u n c t i o n MARC A. MEYERS is Assistant Professor, Department of Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701. J. R. C. GUIMARAESis Professor, Center for Materials Research, Instituto Militar de Engenharia, Rio de Janeiro-R J-Brasil. R. R. AVILLEZ, formerly Graduate Student, Center
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