Precipitation in the system Al-0.05 wt pct Fe
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in A1-2 pct F e c h i l l - c a s t ingots; it d e c o m p o s e s to FeA1 S after h e a t i n g for a few h o u r s at 550 ~ to 650~ ~~ The e q u i l i b r i u m solid solubility of i r o n i n a l u m i n u m , Fig. 2, was d e t e r m i n e d by E d g a r zz f r o m r e s i s t i v i t y m e a s u r e m e n t s . However, n e a r l y 100 t i m e s m a x i m u m e q u i l i b r i u m solid s o l u b i l i t y of i r o n can be r e t a i n e d i n solution by r a p i d cooling of the melt. ~2 The diffusion of i r o n in a l u m i n u m was f i r s t studied by H i r a n o et al33 by u s i n g r a d i o a c t i v e F e 59, s e c t i o n i n g the s p e c i m e n and m e a s u r i n g the r e s i d u a l activity. T h e t e m p e r a t u r e dependence of the diffusion constant, T a b l e I, was d e s c r i b e d by a v e r y low p r e e x p o n e n t i a l factor and a low activation e n e r g y . B e c a u s e of t h e s e low v a l u e s and b e c a u s e of the r e s t r i c t e d solubility of i r o n i n a l u m i n u m , the a u t h o r s suggested that the m e a s u r e m e n t s w e r e being i n f l u e n c e d by diffusion along d i s l o c a t i o n s . This c o n c l u s i o n was q u e s t i o n e d by Balluffi 14 who argued that l a t t i c e diffusion could not be r u l e d out. More r e c e n t l y , M u r a r k a e t al. is noted that m e a s u r e d diffusion coefficients of V, Cr, Pd, Fe, Co, Ni, Cd, In, and Sn in a l u m i n u m all have low a c t i v a t i o n e n e r g i e s and v e r y low p r e e x p o n e n t i a l f a c t o r s ; while not e x p l i c i t ly r e j e c t i n g diffusion along d i s l o c a t i o n s as r e s p o n s i b l e , they i m p l y that l a t t i c e diffusion o c c u r s and that e l e c t r o n i c i n t e r a c t i o n s between t h e s e s o l u t e s and a l u m i n u m a r e r e s p o n s i b l e for the low v a l u e s . Mortlock IB and T i wari and S h a r m a 17 have a l s o d i s c u s s e d t h e s e r e s u l t s ; the f o r m e r s u g g e s t s that n e i t h e r l a t t i c e diffusion nor diffusion along d i s l o c a t i o n s can a c c o u n t for all the obs e r v a t i o n s , while the l a t t e r m a i n t a i n that diffusion along d i s l o c a t i o n s o c c u r s . Lundy 18 suggested that s o m e of t h e s e r e s u l t s might be explained by a n e a r - s u r f a c e effect, ~ but Anand and A g a r w a l a "-~r e j e c t e d this s u g g e s tion. More r e c e n t m e a s u r e m e n t s of the diffusion k i n e t ics in this s y s t e m have b e e n made by A l e x a n d e r and Slifkin2~ and Hood z2 u s i n g m o d e r n t r a c e r - s e c t i o n i n g techniques. Although t h e r e is a l a r g e d i f f e r e n c e in the v a l u e s they a s s i g n to the f r e q u e n c y factor and a c t i v a tion energy for diffusion, T a b l e I, t h e i r v a l u e s for the diffusion constant at t e m p e r a t u r e s n e a r 600~ a r e in
Table I. Values of D O and A H in the Expression DFe/A ! = D O exp
{-~H/RT}
Do
A H kcal/mol.
R
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