Boundary conditions for diffusion in the pack-aluminizing of nickel
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2
~
Dn (elM - C1p) (C1M-C1B) [1 + D12(C2M-C2B)I (C1M - C1p) 011 ~ i l
= X
D12 (C2M- C2p) ]
e_C~2/4D 11 erfc ( a / 2 D~-D~H)
[15 ]
which, along with Eq. [13], defines the velocity and e q u i l i b r i u m phase d i a g r a m t i e - l i n e c o r r e s p o n d i n g to the p r e c i p i t a t e - m a t r i x i n t e r f a c e . The c o r r e s p o n d i n g e x p r e s s i o n s of P u r d y et al. (Eq. [10]) and S h c h e r b e d i n s k i y and Kondrachenko (Eqs. [11] and [21]) do not have the coefficient m u l t i p l y i n g a / 2 . * - *Theformerconsideredsituaiionsin whichCIp~ 0 (negligiblecarbonsoiubi~-it; in ferrite)and C2M-C2p ~ C2M-C2B (negligiblepartitioningof X).
-bne ~n iniroduce limit [ 3 ] a t a n e a r l i e r Stagetlaan-in the above c a l c u l a t i o n s by s u b s t i t u t i n g Die Dll - De2
_
Die
D12 (1 + /922 ) Dll
[16]
Dll (1-D22//Dll) ~- Du
in Eq. [7]. It i s of i n t e r e s t to a s c e r t a i n the s u p e r s a t u r a t i o n r a n g e for which the e x p r e s s i o n s of P u r d y et al. and S h c h e r b e d i n s k i y and K o n d r a c h e n k o involve the most e r r o r .
Boundary Conditions for Diffusion in the Pack-Aluminizing of Nickel R. SIVAKUMAR, N. B. MENON, AND L. L. SEIGLE
IN o r d e r
to gain f u r t h e r i n f o r m a t i o n about the k i n e t i c s of the p a c k - c e m e n t a t i o n p r o c e s s for the f o r m a t i o n of a l u m i n i d e coatings on heat r e s i s t a n t a l l o y s , a study was made of the s u r f a c e c o m p o s i t i o n s of n i c k e l s p e c i m e n s coated for v a r i o u s lengths of time in a l u m i n i z i n g packs at 2000~ The packs c o n s i s t e d of m i x t u r e s of A1, Ni, NH4F 9H F , and A1203 p o w d e r s which w e r e p r e t r e a t e d or " c o n d i t i o n e d " by heating for 20 h at 2000~ in a p r e p u r i f i e d H2 a t m o s p h e r e (99.99 pct p u r i t y ) . After p r e t r e a t m e n t , polished c y l i n d r i c a l s p e c i m e n s of c o m m e r cial " A " - N i , 1 c m in d i a m e t e r and 1 c m long w e r e i n t r o d u c e d into the p a c k s , heated for the d e s i r e d length of time in H2 at 2000~ r e m o v e d , and cleaned by b r u s h ing and t r e a t m e n t in a c e t o n e u s i n g an u l t r a s o n i c c l e a n e r . The c o m p o s i t i o n s of the s u r f a c e s w e r e obtained by m i c r o p r o b e a n a l y s i s of 10 to 15 spots on the c l e a n e d s u r f a c e s . Weight and d i m e n s i o n a l changes w e r e also m e a s u r e d , and s p e c i m e n s w e r e s e c t i o n e d for m e t a l lographic e x a m i n a t i o n , and d e t e r m i n a t i o n of a l u m i n i d e l a y e r growth r a t e s . Data obtained f r o m two pack c o m p o s i t i o n s a r e given in the following t a b l e . P a c k No. 1 contained 49 wt pct A1203 p o w d e r , 31 wt pct Ni powder, 18 wt pct A1 powder,
R. SIVAKUMARand L. L. SEIGLE are Graduate Student and Professor, Materials Science Department, State University of New York at Stony Brook,
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