Contributions to the theory of jogged screw dislocation glide
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last few y e a r s , t h e r e has been c o n s i d e r a b l e int e r e s t and c r i t i c i s m of the s o - c a l l e d jogged s c r e w m o d e l for s t e a d y - s t a t e c r e e p , which was o r i g i n a l l y p r o p o s e d by Mott b2 and l a t e r r e f o r m u l a t e d by s e v e r a l o t h e r i n v e s t i g a t o r s . 3-s T h i s model is b a s e d on the a s s u m p t i o n that the rate c o n t r o l l i n g p r o c e s s in high t e m p e r a t u r e d e f o r m a t i o n is glide of s c r e w d i s l o c a t i o n s , which a r e impeded by the n o n c o n s e r v a t i v e m o t i o n of jogs located along the d i s l o c a t i o n line. Recently~ t h e r e have b e e n a n u m b e r of c r i t i c i s m s of the m o d e l , 6-~~ which c a s t doubt about its a p p l i c a b i l i t y to s t e a d y - s t a t e c r e e p . T h e s e c r i t i c i s m s r e l a t e both to the s p e c i a l conditions n e e d e d to have the jog d r a g g i n g m e c h a n i s m c o n t r o l the s c r e w d i s l o c a t i o n v e l o c i t y and to the functional form for the s t r e s s dependence of the s c r e w d i s l o c a t i o n glide velocity. In addition, it is now c l e a r that s i g n i f i c a n t i n t e r n a l s t r e s s e s a r e p r o d u c e d d u r i n g high t e m p e r a t u r e c r e e p d e f o r m a t i o n and that r e c o v e r y as well a s glide m u s t be i n c l u d e d in any c o m p r e h e n s i v e view of the c r e e p p r o c e s s . N e v e r t h e l e s s , although the o r i g i n a l s i m p l i f i e d jogged s c r e w m o d e l cannot alone a c c o u n t for the s t e a d y - s t a t e c r e e p p r o p e r t i e s of c r y s t a l s , the m o d e l may yet be a p p l i c a b l e to the glide p a r t of the c r e e p p r o c e s s . F o r e x a m p l e , it i s expected that the jog dragging m e c h a n i s m m a y be the d o m i n a n t p r o c e s s l i m i t i n g d i s l o c a tion glide in pure m e t a l s at high t e m p e r a t u r e s . T h e r e a r e two s e p a r a t e a p p r o a c h e s that have b e e n u s e d to d e t e r m i n e the s t r e s s and t e m p e r a t u r e d e pendence for the v e l o c i t y of jogged s c r e w d i s l o c a t i o n s f l In both a p p r o a c h e s , it has b e e n g e n e r a l l y a s s u m e d that the jogs move n o n c o n s e r v a t i v e l y , the s c r e w d i s l o c a tion s e g m e n t s b e t w e e n the jogs glide v e r y e a s i l y , and in m o s t c a s e s v a c a n c y diffusion along the core of the s c r e w d i s l o c a t i o n is n e g l i g i b l e . The f i r s t a p p r o a c h , which was u s e d by Mott, ~'2 H i r s c h and W a r r i n g t o n , a W. A. COGHLANis Metallurgist, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. W. D. NIX is Associate Professor of Materials Science and Director, Center for Materials Research, Stanford University, Stanford, Calif. Manuscript submitted December 11, 1969. METALLURGICALTRANSACTIONS
and R a y m o n d and
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