Chromium diffusion in Ni-20 wt pct
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been s u g g e s t e d I that the s e l e c t i v e oxidation of c h r o m i u m which is o b s e r v e d to o c c u r in t h o r t a t e d N i C r a l l o y s i s brought about by enhanced c h r o m i u m d i f fusion in t h e s e a l l o y s . In s u p p o r t of t h i s h y p o t h e s i s , F l e e t w o o d z and W e n d e r o t t 3 have shown that c h r o m i u m d i f f u s i v i t i e s in Ni-5ThO2 and in N i - 2 0 C r - 0 . 1 C e O 2 , r e s p e c t i v e l y , a r e n e a r l y an o r d e r of m a g n i t u d e f a s t e r than in the s a m e a l l o y s without d i s p e r s o i d . The e n h a n c e d diffusion in t h e s e c a s e s w a s a t t r i b u t e d to the fine g r a i n s u b s t r u c t u r e and a s s o c i a t e d d i s l o c a t i o n d e n s i t y , which i s s t a b i l i z e d by the d i s p e r s o i d p a r t i c l e s . S e l t z e r and W i l c o x , 4 on the o t h e r hand, have d e m o n s t r a t e d that for' a given g r a i n s i z e t h e r e i s no d i f f e r ence in diffusion r a t e s for c h r o m i u m in N i - 2 0 C r and TDNiCr* (Ni-20Cr-2ThO2). The d i f f e r e n c e b e t w e e n t h i s *FansteelMetallurgmalCorporationtrade name. r e s u l t and the e a r l i e r data can be a t t r i b u t e d to the fact that T D N i C r e x h i b i t s v e r y l i t t l e d i s l o c a t i o n s u b s t r u c ture. In o r d e r to f u r t h e r e x a m i n e the a p p a r e n t influence of fine g r a i n s u b s l : r u c t u r e on c h r o m i u m diffusion in d i s p e r s o i d - c o n t a i n i n g a l l o y s , m e a s u r e m e n t s have been m a d e of S1Cr d i f f u s i v i t i e s in N i - 2 0 wt pct C r - 3 vol pct Y203 and Co-21 wt pct C r - 3 vol pct Y2Os p r e p a r e d by m e c h a n i c a l a l l o y i n g . 5 The r a d i o t r a c e r technique u s e d to d e t e r m i n e diffusion c o e f f i c i e n t s at 1323 and 1423 K h a s b e e n d e s c r i b e d in d e t a i l e l s e w h e r e . 4 The " m e c h a n i c a l a l l o y i n g " technique for the p r e p a r a t i o n of a l l o y s s t u d i e d in t h i s i n v e s t i g a t i o n w a s d e s c r i b e d by S t r i n g e r , W r i g h t , W i l c o x , and J a f f e e . 5 B a s i c a l l y , the method i n v o l v e s the s u c c e s s i v e cold w e l d i n g of m e t a l and e n t r a p p e d o x i d e p a r t i c l e s to the b a l l s of an a t t r i t o r g r i n d i n g m i l l . The s u r f a c e l a y e r i s p r o g r e s s i v e l y s m e a r e d out, w h e r e u p o n it c r a c k s f r o m the b a l l and the p r o c e s s i s r e p e a t e d . A f t e r m i l l i n g , the p o w d e r i s given an h o m o g e n i z a t i o n a n n e a l , then cold c o m p a c t e d and s i n t e r e d . The s i n t e r e d b i l l e t s a r e f i n a l l y hot p r e s s e d and hot r o i l e d to a d e s i r e d s h e e t thickness. F i g . l shows m i c r o s t r u c t u r e s of the Ni-20Cr-3YzO3 p a r a l l e l to and t r a n s v e r s e to the p l a n e of the s h e e t , w h i l e F i g .
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