The role of diffusion in determining the controlling creep mechanisms in Al-Zn solid-solutions: Part II
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o r jogged s c r e w d i s l o c a t i o n motion, ~ have b e e n f o r m u l a t e d f o r the d e s c r i p t i o n of c r e e p of p u r e m e t a l s . E x p e r i m e n t a l i n v e s t i g a t i o n s of c r e e p in p u r e m e t a l s have shown that a b o v e 0.5 T m (Trn = m e l t i n g t e m p e r a t u r e ) the s t e a d y - s t a t e c r e e p r a t e m a y be d e s c r i b e d by the following e m p i r i c a l equation: 8
gcl = K b2
9
[1]
H e r e K i s a c o n s t a n t , Dcl* i s t h e s e l f - d i f f u s i o n c o e f f i *The subscriptcl usedhere and elsewherein the paper refersto dlslocanon climbas the rate lumtingprocess cient, a is the a p p l i e d s t r e s s , E is the d y n a m i c Y o u n g ' s m o d u l u s , y is the s t a c k i n g fault e n e r g y , G i s the s h e a r m o d u l u s , b i s the B u r g e r ' s v e c t o r , n i s the s t r e s s exponent (usually found to b e a p p r o x i m a t e l y 4 to 5 f o r p u r e m e t a l s ) , and m is u s u a l l y about 3. W e e r t m a n 6 h a s d e r i v e d an equation b a s e d on d i s l o c a t i o n c l i m b which y i e l d s an e x p r e s s i o n s i m i l a r to Eq. [1]. F u r t h e r m o r e , a n u m b e r of e x p e r i m e n t s s t r o n g l y s u g g e s t that d i s l o c a t i o n c l i m b i s t h e r a t e l i m i t i n g m e c h a n i s m f o r c r e e p of p u r e m e t a l s . C o n s e q u e n t l y we wilt r e f e r to Eq. [1] a s the s e m i e m p i r i c a l c l i m b equation f o r s t e a d y s t a t e c r e e p . M o d i f i c a t i o n of Eq. [1] is n e c e s s a r y to a d a p t it f o r use in b i n a r y s o l i d - s o l u t i o n s . In p a r t i c u l a r a l l of the p a r a m e t e r s E, :~., G and b m a y b e f u n c t i o n s of c o m p o s i t i o n , In a d d i tion, the a p p r o p r i a t e d i f f u s i o n c o e f f i c i e n t f o r c l i m b c o n t r o l l e d c r e e p is
Dcl
=
* * XD D1D2/( 1 *2 + X2D*I)
[2]
w h e r e D*, D,~ a r e r a d i o a c t i v e t r a c e r d i f f u s i o n coeffic i e n t s and X1, X2 a r e a t o m f r a c t i o n s . The m o d i f i e d f o r m of Eq. [11 i s t h e r e f o r e :
B) G l i d e - C o n t r o l l e d C r e e p in Solid-Solution Alloys W e e r t m a n 3 has t a b u l a t e d five d i f f e r e n t v i s c o u s d r a g VOLUME 8A, OCTOBER 1977-1523
m e c h a n i s m s 9-14 which can be used to d e s c r i b e c r e e p of C l a s s I solid s o l u t i o n s . However, t h e o r i e s b a s e d on the i n t e r a c t i o n of solute atom a t m o s p h e r e s with m o v ing d i s l o c a t i o n s 9,1~ have b e e n the m o s t s u c c e s s f u l in describing creep. The i n t e r a c t i o n of solute a t o m s with m o v i n g d i s l o c a tions has b e e n t r e a t e d by C o t t r e l l and J a s w o n 9 and by F r i e d e l . 1~ T h e r e a r e t h r e e r e a s o n s for c h o o s i n g the C o t t r e l l - J a s w o n model over the f o r m u l a t i o n of F r i e d e l . F i r s t l y , e x p e r i m e n t s by H a r r i
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