Relaxation and cyclic hardening of the surface layer of copper
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AND
A. KUMAR
The c o n c e p t that cyclic h a r d e n i n g is p r i m a r i l y s u r f a c e l a y e r d e p e n d e n t is e x a m i n e d and the data s u p p l e m e n t e d by m e a s u r e m e n t s of the a c t i v a t i o n v o l u m e . A c t i v a t i o n v o l u m e m e a s u r e m e n t s w e r e c a r r i e d out on a n n e a l e d OFHC copper b e f o r e and after cyclic h a r d e n ing. As a r e s u l t of c y c l i c h a r d e n i n g , the a c t i v a t i o n v o l u m e d e c r e a s e d m a r k e d l y . The data show that after cyclic h a r d e n i n g and r e m o v a l of the s u r f a c e l a y e r the a c t i v a t i o n v o l u m e is the s a m e as that of the a n n e a l e d s p e c i m e n s d e f o r m e d u n i d i r e c t i o n a l l y to the s a m e s t r a i n . T h i s shows that the d e n s i t y of d i s l o c a t i o n o b s t a c l e s of c y c l i c a l l y work h a r d e n e d s p e c i m e n s i n c r e a s e s only in the s u r f a c e l a y e r . The r a t e of r e l a x a t i o n of the s u r f a c e l a y e r s t r e s s was m e a s u r e d at 297, 340, and 395 K. T h e r e appear to be two d i s t i n c t r e g i o n s i n the r e l a x a t i o n r a t e c u r v e . I n i t i a l l y , the r a t e of r e l a x a t i o n is v e r y r a p i d and after about 10 m i n it d e c r e a s e s by a factor of 20, a p p r o x i m a t e l y . The a p p a r e n t a c t i v a t i o n e n e r g y of the two p o r t i o n s of the r e l a x a t i o n r a t e c u r v e is the s a m e ; 3340 cal p e r m o l .
IN p r e v i o u s work 1'2 it was
r e p o r t e d that for s p e c i m e n s cycled b e t w e e n c o n s t a n t p l a s t i c s t r a i n l i m i t s , the work h a r d e n i n g and work softening was a r e s u l t of a c c o m panying changes in the s u r f a c e l a y e r s t r e s s . After the f i r s t o n e - q u a r t e r c y c l e , the i n t e r i o r of the s p e c i m e n was found to have no f u r t h e r influence on the c y c l i c h a r d e n i n g or softening b e h a v i o r as m e a s u r e d by the change in the s t r e s s . T h i s c o n c e p t of cyclic work h a r d e n i n g is not in a g r e e m e n t with that of other i n v e s t i g a t o r s who used e l e c t r o n m i c r o s c o p e t e c h n i q u e s and p r o p o s e d that the work h a r d e n i n g was due to an i n c r e a s e in d i s l o c a t i o n d e n s i t y in the bulk m a t e r i a l . The l a t t e r c o n c l u s i o n was b a s e d upon the a s s u m p t i o n that the s t r e n g t h i n c r e a s e s with d i s l o c a t i o n d e n s i t y . Although, in g e n e r a l this a p p e a r s to be c o r r e c t , h a r d e n i n g is s t r o n g l y i n fluenced by the e l e m e n t s of the d i s l o c a t i o n s t r u c t u r e such as d i p o l e s , j o g s , k i n k s , n o d e s , and so forth. F u r t h e r , the o b s t a c l e s t r e n g t h of a c l o s e l y packed a r r a y of d i s l o c a t i o n may be l e s s than that expected on the b a s i s of i n d i v
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