The high-temperature creep behavior of nickel-rich Ni-W solid solutions
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IVEe x p e r i m e n t a l s t u d i e s ~'2 and t h e o r e t i c a l d i s c u s s i o n s 3-5 have i n d i c a t e d that elevated t e m p e r a t u r e c r e e p of p o l y c r y s t a l l i n e m e t a l s and a l l o y s is a t h e r m a l l y - a c t i v a t e d p r o c e s s and may be r e p r e s e n t e d by the g e n e r a l e x p r e s s i o n ,
~s = Af((r, T) exp ( - Q c / R T )
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w h e r e ~s i s the s t e a d y - s t a t e c r e e p r a t e , A i s a cons t a n t dependent on t h e , i n t e r n a l s t r u c t u r e , j~a, T) i s a f u n c t i o n of the applied s t r e s s , ~, and the t e s t t e m p e r a t u r e , T, R i s the gas c o n s t a n t , and Qc is the a c t i v a t i o n energy for'creep. At m o d e r a t e s t r e s s e s , f(a, T) can be a p p r o x i m a t e d by the power law, (alE) n, w h e r e E i s the i s o t r o p i c , p o l y c r y s t a l l i n e u n r e l a x e d e l a s t i c m o d u l u s and n i s a c o n s t a n t for a given m a t e r i a l . F o r p u r e m e t a l s the s t r e s s exponent, n, h a s b e e n o b s e r v e d to have a v a l u e of about 5 o v e r a wide r a n g e of s t r e s s . In c a s e s w h e r e s o l i d s o l u t i o n s have b e e n s t u d i e d over a r a n g e of solute content, the s t r e s s exponent h a s b e e n o b s e r v e d e i t h e r to r e m a i n c o n s t a n t with solute addition or to d e c r e a s e t o w a r d v a l u e s n e a r 3. S t e a d y - s t a t e c r e e p data f o r solid s o l u t i o n s and p u r e m e t a l s which obey a 5 - p o w e r s t r e s s d e p e n d e n c e a r e in q u a l i t a t i v e a g r e e m e n t with t h e o r i e s b a s e d on v a c a n c y diffusion3'4 while a l l o y s exhibiting 3 - p o w e r b e h a v i o r a r e thought to be c o n t r o l l e d by " m i c r o c r e e p " m e c h a n i s m s . 6'7 T h e r e a r e a l s o s e v e r a l alloy s y s t e m s which exhibit a n o m a l o u s b e h a v i o r . 8'9 F o r e x a m p l e , M o n m a , Suto, and Oikawa 9 have r e p o r t e d that the N i - W s y s t e m exhibits a s t r e s s exponent of 5 for p u r e n i c k e l , i n c r e a s e s to 7.2 for Ni-5 pct W and t h e r e a f t e r d e c r e a s e s , r e a c h i n g 3.9 at 25 pct W. Although the t e m p e r a t u r e d e p e n d e n c e of the c r e e p of p u r e m e t a l s has been shown to be identical to that for s e l f - d i f f u s i o n , ~~ the c r e e p b e h a v i o r of s o l i d s o l u W R JOHNSON, formerly Graduate Student, Stanford Univeristy, Stanford, Calif., is now Associate Scientist, Materials Development Department, Gulf General Atomic Inc., San Diego, Calif. C. R. BARRETT and W. D NIX are Associate Professors of Materials Science and Engineering, Stanford University. Manuscript submitted October 30, 1970. METALLURGICALTRANSACTIONS
t i o n s i s l e s s c l e a r l y defined. F o r t h o s e s y s t e m s w h e r e f a i r l y c o m p l e t e c r e e p rate and diffusion data a r e a v a i l able t h e r e a p p e a r s to be a g e n e r a l t r e
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