Creep behavior of Ni-W solid solutions
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Creep Behavior of Ni-W Solid Solutions
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I N i - W OllOys
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F A R G H A L L I A. MOHAMED AND T E R E N C E G. LANGDON
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The c r e e p b e h a v i o r of s o li d solution a l l o y s g e n e r a l l y f a i l s into one of two p o s s i b l e c l a s s e s . Using the d e f i n i t i on s i n t r o d u c e d e l s e w h e r e , ~ c l a s s I a l l o y s exhibit s t e a d y - s t a t e c r e e p r a t e s which v a r y with s t r e s s r a i s e d to a p o w e r c l o s e to 5, w h e r e a s in c l a s s 1"[ a l l o y s the s t r e s s exponent f o r s t e a d y - s t a t e c r e e p is c l o s e to 3. By m a k i n g a s s u m p t i o n s c o n c e r n i n g the n a t u r e of the r a t e - c o n t r o l l i n g d e f o r m a t i o n m e c h a n i s m s in the two alloy c l a s s e s , a c r i t e r i o n was r e c e n t l y d e v e l o p e d to d i s t i n g u i s h b e t w e e n c l a s s I and c l a s s II b e h a v i o r . ~ The o b j e c t of the p r e s e n t c o m m u n i c a t i o n is t w o - f o l d : 1) To apply t h i s c r i t e r i o n to the t h r e e i n v e s t i g a t i o n s r e p o r t e d to date f o r c r e e p of N i - W s o l i d s o l u t i o n s , 2-4 and 2) To e x a m i n e the s u g g e s t i o n , m a d e r e c e n t l y by Oikawa and K a r a s h t m a , 5 that a s t r e s s exponent of 3 is not a s u f f i c i en t c r i t e r i o n to identify c l a s s II b e h a v i o r . F o l l o w i n g the a n a l y s i s p r e s e n t e d e l s e w h e r e , ~ it is a s s u m e d that the m e c h a n i s m of c r e e p in c l a s s I alloys is a d i s l o c a t i o n c l i m b p r o c e s s , and the m e c h a n i s m in c l a s s II a l l o y s is v i s c o u s glide c o n t r o l l e d by the p r e s ence of s o l u t e atom a t m o s p h e r e s . The c r i t e r i o n f o r an a l l o y to exhibit c l a s s II b e h a v i o r is then g i v e n by 1
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DADB (XAD ~ + X B D ~ 4 ) f
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w h e r e D~ and D ~ a r e the t r a c e r d i f f u s i v i t i e s of the A and B a t o m s in the A B alloy, X A and X B a r e the r e s p e c t i v e a t o m i c f r a c t i o n s , and f is a c o r r e l a t i o n f a c t o r c l o s e to 1. F o r glide, Dg is g iv e n by the D a r k e n c h e m i c a l i n t e r d i f f u s i v i t y , 8 defined as
81n f A a
lnX A
w h e r e f A is the a c t i v i t y c o e f f i c i e n t f o r the A s p e c i e s and (8 In f A / ~ l n X A ) -~ 0 f o r dilute s o l i d solutions. FARGHALLI A. MOHAMEDand TERENCE G. LANGDON are Research Associate and Associate Professor, respectively, Department of Materials Science, University of Southern California, Los Angeles, California 90007. Manuscript submitted June 17, 1974. METALLURGICAL TRANSACTIONS A
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Fig. 1--The variation of stacking fault energy with solute concentration in Ni-W alloys9 107
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w h e r e B ~ 8 • 10 ~z, e is the applied s t r e s s ,
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