The influence of iron and aluminum on the precipitation of metastable Ni 3 Nb phases in the Ni-Nb system
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S E V E R A L high s t r e n g t h n i c k e l - and i r o n - b a s e a l l o y s a r e r e p o r t e d to be s t r e n g t h e n e d by m e t a s t a b l e p h a s e s b a s e d on t h e c o m p o s i t i o n N s ~ u .
E i s e l s t e i n~ identi-
*For simplicitythe idealizedcompositionNi3Nbwillbe used throughout the text, althoughit is understood that the substitution of other elementsfor nickel and niobiumis basic to the current study. fled the s t r e n g t h e n i n g p h a s e in I n c o n e l 718 as 7 ' having the fcc L12 s t r u c t u r e , with N%A1 b e in g the n i c k e l p r o totype. E i s e l s t e i n l a t e r c o n c l u d e d that the p r e c i p i t a t e p o s s e s s e d a b c t D022 s t r u c t u r e , ~ of which Ni3V was the n i c k e l p r o t o t y p e . O t h e r i n v e s t i g a t o r s s u g g e s t e d that the Ni3Nb p r e c i p i t a t e r e s p o n s i b l e f o r the h a r d e n i n g in i r o n - b a s e a l l o y s was the e q u i l i b r i u m o r t h o r h o m b i c NiaNb /3 p h a s e . 6 K i r m a n and W a r r i n g t o n 4 pointed out that e l e c t r o n d i f f r a c t i o n p a t t e r n s f r o m b c t D022 and fc c L12 p h a s e s , as w e l l as f r o m the ~ p h a s e , could be confused in a n u m b e r of i n s t a n c e s and c o n c l u d e d that the bct D022 p h a s e was the only p r e c i p i t a t e c o n t r i b u t i n g to s t r e n g t h e n i n g in the v a r i o u s a l l o y s that had b e e n studied. R e c e n t l y , s e v e r a l i n v e s t i g a t o r s 2'7'~ d e t e r m i n e d that both D0~2 and L12 p h a s e s w e r e p r e s e n t in I n c o n e l 718. M o r e e n 7 and P a u l o n i s et al. ~ d e m o n s t r a t e d the d i f f i c u l t i e s that a r e e n c o u n t e r e d in m a k i n g s t r u c t u r e d e t e r m i n a t i o n s on the p r e c i p i t a t e s when both a r e p r e s ent in the p a r e n t fcc m a t r i x . P a u l o n i s et al. a l s o pointed out the difficulty in studying m e t a s t a b l e NiaNb p r e c i p i t a t i o n by X - r a y d i f f r a c t i o n m e t h o d s . Although the L12 7 ' a f f o r d s s o m e s t r e n g t h e n i n g , m o s t i n v e s t i g a t o r s have c o n c l u d e d that when both t y p e s of m e t a s t a b l e W. E. QUIST is Research Specialist, The Boeing Co., Seattle, Wash. R. TAGGART and D. H. POLONIS are Professors of Mechanical Engineering, and Metallurgical Engineering, respectively. University of Washington, Seattle, Wash. Manuscript submitted July 13, 1970. METALLURGICAL TRANSACTIONS
Ni3Nb p r e c i p i t a t e a r e p r e s e n t in an alloy, the m a j o r s h a r e of the p r e c i p i t a t i o n h a r d e n i n g is c o n t r i b u t e d by the bct D022 p h a s e , 7'8 h e r e a f t e r r e f e r r e d to as 7 , . 8 Th e r e a s o n s that the 7 ' and 7 " t y p e s of N%Nb c o e x i s t in Inconel 718, and p r o b a b l y in o t h e r s i m i l a r a l l o y s , have not been r e s o l v e d . T h i s l e a d s to an o b s t a c l e in the d e v e l o p m e n t of
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