Creep rate enhancement theory for irradiated substitutional solid solution alloys
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p a p e r p r e s e n t s a new theory of c r e e p r a t e e n h a n c e m e n t p r o d u c e d by i r r a d i a t i o n with e n e r g e t i c p a r t i c l e s (such as n e u t r o n s ) . The t h e o r y is applicable only to s u b s t i t u t i o n a l solid s o l u t i o n alloys in which the d i s l o c a t i o n m o t i o n d u r i n g high t e m p e r a t u r e c r e e p is r e g u l a t e d by a diffusion c o n t r o l l e d d i s location d r a g m e c h a n i s m (for h i s t o r i c a l r e a s o n s known a s a m i c r o c r e e p m e c h a n i s m ) . E x a m p l e s of diff u s i o n c o n t r o l l e d m i c r o c r e e p m e c h a n i s m s a r e the C o t t r e l l cloud of i m p u r i t y a t o m s that d r i f t s with a moving d i s l o c a t i o n and the E s h e l b y - S c h o e c k local r e g i o n of s t r e s s induced o r d e r a r o u n d a m o v i n g d i s location line. 1"~ Diffusion c o n t r o l l e d m i c r o c r e e p m e c h a n i s m s give r i s e to a v i s c o u s d a m p i n g f o r c e on a moving d i s l o c a t i o n such that the d i s l o c a t i o n v e l o c i t y is p r o p o r t i o n a l to the f o r c e applied to it. A likely c r i t e r i o n for d e c i d i n g when a m i c r o c r e e p m e c h a n i s m c o n t r o l s the high t e m p e r a t u r e c r e e p r a t e of an alloy is the value of the s t r e s s exponent n of the power law c r e e p equation. ~-7 T h e v a l u e of n at m o d e r ate s t r e s s l e v e l s in p u r e m e t a l s is a p p r o x i m a t e l y n = 4 to 5. In p u r e m e t a l s diffusion c o n t r o l l e d m i c r o c r e e p m e c h a n i s m s , of c o u r s e , cannot o p e r a t e . In s o m e alloy s y s t e m s , but not all, the s t r e s s exponent n dec r e a s e s in v a l u e as the alloy content is i n c r e a s e d and a p p r o a c h e s a value of the o r d e r of n = 3 to 3.5 (Refs. 5 to 7). A c c o r d i n g to t h e o r y 1,5,6,s the power law c r e e p equation has a s t r e s s exponent of n = 3 when the r a t e c o n t r o l l i n g p r o c e s s of c r e e p is a m i c r o c r e e p m e c h a n i s m . T h e decrease~ in the v a l u e of n f r o m about n = 4.5 to about n = 3 upon a l l o y i n g can be explained as c a u s e d by the t a k e o v e r of the r a t e c o n t r o l l i n g c r e e p p r o c e s s by a s t r o n g m i c r o c r e e p m e c h a n i s m f r o m the d i s l o c a t i o n c l i m b o r the d r a g g i n g jog m e c h a n i s m s of the p u r e m e t a l . Up to now no t h e o r y of the e n h a n c e m e n t of the c r e e p r a t e of m e t a l s u n d e r g o i n g i r r a d i a t i o n has c o n s i d e r e d the c a s e i n which the c r e e p r a t e of the u n i r r a d i a t e d m a t e r i a l is c o n t r o l l e d by a m i c r o c r e e p m e c h a n i s m . In this p a p e r a t h e o r y is developed for such m a t e r i a l .
JOHANNES WEERTMANis Consultant, Los Alamos Scientific Laboratory, University of California, Los Alamos, NM 87545; his permanent posit
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