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An a d d i t i o n a l p u r p o s e of this study was to develop a g e n e r a l p r o c e d u r e for a n a l y z i n g diffusion in m u l t i component, m u l t i p h a s e s y s t e m s . T h i s a n a l y s i s unifies the d a t a d e r i v e d f r o m the i n d i v i d u a l c o u p l e s and p e r m i t s e x t e n s i o n to o t h e r c o a t i n g / s u b s t r a t e c o m b i n a t i o n s . In addition, the m e t h o d o l o g y d e v e l o p e d is of a g e n e r a l n a t u r e and can be a p p l i e d to o t h e r c o m p l e x r e a c t i o n diffusion p r o b l e m s . Of n e c e s s i t y , many of the d e t a i l s of this study have b e e n d e l e t e d f r o m the following m a n u s c r i p t . F u r t h e r d e t a i l s m a y be found in Ref. 5. BACKGROUND For aluminide coatings the diffusion reaction between the coating and the substrate is more important than oxidation in determining coating life at II00~ 6 Diffusion is also important in the depletion of overlay coatings, but to a lesser degree. This is based on the following qualitative argument. Typically, overlay coatings are about twice as thick as aluminides (0.013 c m f o r o v e r l a y s vs 0.006 c m for a l u m i n i d e s ) and have about half the a l u m i n u m content (12 wt pct for o v e r l a y s v s 25 to 30 wt pct for a l u m i n i d e s ) . If we a s s u m e that r e d u c t i o n of the s u r f a c e a l u m i n u m content by 50 pct c o n s t i t u t e s a l u m i n u m r e s e r v o i r depletion, then coating diffusion life (t) can be c o m p u t e d f r o m the coating t h i c k n e s s (h) and the i n t e r d i f f u s i o n c o e f f i c i e n t (D) by 7 h2 t : -D

[1]

to a f i r s t a p p r o x i m a t i o n . T h i s c r i t e r i o n s e e m s to be reasonable since, according to Pettit,s Ni-AI alloys

ISSN 0360-2133/78/0911-1237500.75/0 9 1978 A M E R I C A N SOCIETY F O R M E T A L S A N D T H E M E T A L L U R G I C A L SOCIETY OF A1ME

VOLUME 9A,SEPTEMBER

1978

1237

b e c o m e s p i n e l f o r m e r s a t a b o u t 14 wt p c t a l u m i n u m . W a l l w o r k a n d H e d 9 f o u n d t h a t b e l o w a b o u t 5 wt p c t aluminum the NiCrAI and CoCrA1 alloys no longer f o r m p r o t e c t i v e A120~ s c a l e s . F r o m E q . [1] i t i s a p p a r e n t t h a t t h e d o u b l e t h i c k n e s s of t h e o v e r l a y s c o u l d r e s u l t in a f o u r - f o l d i n c r e a s e i n l i f e o v e r s i m p l e a l u m i n i d e s , a s s u m i n g e q u a l d i f f u s i o n c o e f f i c i e n t s . In t h e N i - A 1 s y s t e m t h e i n t e r d i f f u s i o n c o e f f i c i e n t is a f u n c t i o n of t h e p h a s e u n d e r c o n s i d e r a t i o n a n d i t s c o m p o s i t i o n . F o r e x a m p l e , J a n s s e n 1~ r e p o r t s t h a t a t l l 0 0 ~ t h e i n t e r d i f f u s i o n c o e f f i c i e n t f o r Ni3A1 i s 3.6 x 10 -1~ c m 2 / s ; f o r t h e s a t u r a t e d t e r m i n a l n i c k e l s o l i d s o l u t i o n D v a r i e s f r o m 2.6 x 10 -1~ c m 2 / s w i t