Enhancement of the creep resistance of metals
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a r t i c l e s it was shown 1-3 that the s u r f a c e l a y e r f o r m e d a s a r e s u l t of p l a s t i c d e f o r m a t i o n 4 could e x e r t a l a r g e influence on the c r e e p b e h a v i o r of m e t a l s at low t e m p e r a t u r e . T h i s influence a r i s e s f r o m the i n t e r a c t i o n b e t w e e n the e x c e s s d i s l o c a t i o n s in the s u r f a c e l a y e r and the m o b i l e d i s l o c a t i o n s in the i n t e r i o r of the s p e c i m e n . T h i s i n t e r a c t i o n m a y be d e s c r i b e d in t e r m s of a s u r f a c e l a y e r s t r e s s , a s , which a c t s to o p p o s e the m o t i o n of m o b i l e d i s l o c a t i o n s , and thus, d e c r e a s e s the net f o r c e a c t i n g to c a u s e m o t i o n . The v a l u e s for a s m a y be m e a s u r e d by d e t e r m i n i n g the d e c r e a s e in the s t r e s s r e q u i r e d to c a u s e p l a s t i c flow when the s u r f a c e l a y e r i s not p r e s e n t . T h i s s u r f a c e l a y e r has been found to f o r m in s i n g l e c r y s t a l s of A1, Cu, Au, and Cd a s w e l l a s in such p o l y c r y s t a l l i n e m a t e r i a l s a s A1, Cu, Au, Mo, F e , Ti a l l o y s , and A1 a l l o y s of 7075-T6 and 2014-T6. In s o m e m e t a l s 5'6 such a s high p u r i t y A1, Cu, F e , and T i a l l o y s , the s u r f a c e l a y e r s t r e s s can r e l a x v e r y r a pidly at r o o m t e m p e r a t u r e . F o r e x a m p l e , the s u r f a c e l a y e r d i s a p p e a r s c o m p l e t e l y in 4 h in high p u r i t y a l u m i n u m , in 48 h in c o p p e r , and in 1 h in a t i t a n i u m a l l o y (6A1-4V). At t h e s e t e m p e r a t u r e s only the s u r f a c e l a y e r r e l a x e s and the i n t e r i o r i s unaffected. T h i s r e l a x a t i o n b e h a v i o r i s r e f l e c t e d in the v a l u e s for 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 . In m e t a l s , the a c t i v a t i o n e n e r g y for r e c o v e r y i s u s u a l l y in the r a n g e of 20,000 to 30,000 c a b / m o l e . 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 f o r the r e c o v e r y of the s u r f a c e l a y e r s t r e s s f o r c o p p e r i s about 3400 c a l / m o l e . 7 In our i n v e s t i g a t i o n of an a l u m i n u m a l l o y 7075-T6 w h e r e the d i s l o c a t i o n s w e r e s t r o n g l y pinned, no r e l a x a t i o n of the s u r f a c e l a y e r could be d e t e c t e d up to 150~ Above this t e m p e r a t u r e o v e r a g i n g o c c u r s and d i s l o c a t i o n s in both the i n t e r i o r and s u r f a c e l a y e r a r e r e l a x e d . 8 T h e r e l a x a t i o n o r r e d u c t i o n of the s u r f a c e l a y e r s t r e s s d e c r e a s e s the b a c k s t r e s s and would tend to i n c r e a s e the c r e e p r a t e if the n u m b e r of m o b i l e d i s l o c a t i o n s was not d e c r e a s e d . It has been shown, h o w e v e r , 1 that d u r i n g low t e m p e r a t u r e d e f o r m a t i o n th
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