On the stress exponent and the rate-controlling mechanism of high-temperature creep in some solid solutions
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4s = Aa~ exp ( - Q c / R T )
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h a s b e e n c o n s i d e r e d a s one of the m o s t s i g n i f i c a n t p a r a m e t e r s to c h a r a c t e r i z e h i g h - t e m p e r a t u r e c r e e p b e h a v i o r s in s o l i d s o l u t i o n s , w h e r e 4 s i s s t e a d y - s t a t e c r e e p r a t e s , a c the a p p l i e d ( c r e e p ) s t r e s s , and Qc 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 c r e e p . S h e r b y and B u r k e 1 have c l a s s i f i e d a l l o y s into two g r o u p s a c c o r d ing to t h e i r n - v a l u e s ; one ( c l a s s I) in which n is c l o s e to 3, the o t h e r ( c l a s s ID in which n is about 5 o r m o r e a s in m a n y p u r e m e t a l s . C l a s s I a l l o y s a r e f r e q u e n t l y c o n s i d e r e d a s alloy type m a t e r i a l s in which the c r e e p d e f o r m a t i o n is c o n t r o l l e d b y one of m i c r o c r e e p m e c h a n i s m s f i On the o t h e r hand, c l a s s II a l l o y s a r e c o n s i d e r e d a s p u r e metal type m a t e r i a l s in which a r e c o v e r y p r o c e s s , such a s c l i m b i n g of edge d i s l o c a t i o n s , 3 n o n - c o n s e r v a t i v e m o t i o n of the edge p a r t on j o g g e d s c r e w d i s l o c a t i o n s , 4 o r r e c o v e r y of d i s l o c a t i o n n e t w o r k s , s c o n t r o l s the c r e e p . One of the m o s t s i g n i f i c a n t d i f f e r e n c e s in c h a r a c t e r s of c r e e p b e t w e e n t h e s e two t y p e s is t h e i r e f f e c t i v e s t r e s s d e p e n d e n c e of d i s l o c a t i o n v e l o c i t y . When c r e e p i s c o n t r o l l e d b y one of m i c r o c r e e p m e c h a n i s m s (alloy type), the gliding v e l o c i t y of d i s l o c a t i o n s , v, is p r o p o r t i o n a l to the e f f e c t i v e s t r e s s , a*, i.e., m* is unity in the following equation. {a* ~rn* v = B \--~--/
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w h e r e B and C a r e c o n s t a n t s . On the c o n t r a r y , when c r e e p i s c o n t r o l l e d b y one of HIROSHI OIKAWA and SEIICHI KARASHIMA are Associate Professor and Professor, respectively, Department of Materials Science, Faculty of Engineering, Tohoku University, Sendai 980, Japan. Manuscript submitted September 5, 1973. METALLURGICAL TRANSACTIONS
r e c o v e r y p r o c e s s e s a s in p u r e - m e t a l t y p e m a t e r i a l s , m* is u s u a l l y l a r g e r than unity. F o r e x a m p l e s , the v a l u e of about 3 h a s b e e n r e p o r t e d f o r m * in h i g h t e m p e r a t u r e c r e e p of A1, 8 ~ - F e , 7 and Ni. 8 T h e r e f o r e , t y p e s of c r e e p in a l l o y s m a y b e i d e n t i f i e d b y d e t e r m i n i n g the v a l u e s of m * . I n t e r n a l s t r e s s h a s a l s o b e e n r e g a r d e d a s the o t h e r i m p o r t a n t p a r a m e t e r to identify c r e e p m e c h a n i s m . H o r i u c h i and O t s u k a 9 have s u g g e s t e d that the i n t e r n a l s t r e s s , ~i, in a t y p i c a l m a t e r i a l s of a l l o y t y p e is
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