Effect of iron addition on the precipitation behavior of CoNiCr alloys containing Nb
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M A N Y nickel base s u p e r a l l o y s d e r i v e t h e i r high s t r e n g t h by the p r e s e n c e of a m e t a s t a b l e 7 " - p h a s e whose s t r u c t u r e and morphology has been extensively studied. 1-13 It has been e s t a b l i s h e d that the y H- p h a s e has a DO22 type o r d e r e d bct s t r u c t u r e , with a c/a r a t i o of 2.04. It nucleates homogeneously throughout the austenitic (fcc) m a t r i x and grows coherently into thin d i s c s on {100} m a t r i x planes with the C - a x i s being p e r pendicular to the d i s c s . On overaging, y" t r a n s f o r m s into a stable o r t h o r h o m b i c fl-Ni3Nb phase. Quist et al ~o have o b s e r v e d that in a Ni-12 pct Nb alloy the addition of 3.0 pct Fe p r o m o t e s the p r e c i p i t a t i o n of the y " - p h a s e . Based on the E n g e l - B r e w e r c o r r e l a t i o n they concluded that when s and p e l e c t r o n s a r e considered, the p r e f e r r e d e/a r a t i o for the p r e c i p i t a t i o n of ~"phase is between 2.50 to 2.62. In addition, they suggested that i n t e r f a c i a l s t r a i n and c r y s t a l l o g r a p h i c fact o r s should a l s o be f a v o r a b l e . S i m i l a r conclusions have been a r r i v e d at by Raghavan in Cu-Ni-Nb and Cu-Ni-Nb-A1 a l l o y s . ~%15 In a F e - N i - C o - T a ~6 alloy it was found that the p r e c i p i t a t i o n of ~" not only r e q u i r e s a favorable e/a r a t i o but also a Co/Ni r a t i o of l e s s than 0.45. In a previous study '7 in Co Ni Cr alloys conraining 3 to 8 pct Nb the main p r e c i p i t a t i n g phase was o b s e r v e d to be orthorhombic ~-NiaNb. The p r e s e n t study was initiated to study the effect of the addition of Fe on the p r e c i p i t a t i o n behavior of C o - N i - C r - N b a l l o y s . EXPERIMENTAL TECHNIQUES 650 g melts of three alloys whose composition (in wt pct) is given in Table I, were p r e p a r e d in an induction melting furnace in an argon a t m o s p h e r e using 99.99 pct pure alloying e l e m e n t s except Nb which was 99.95 pct p u r e . The melts were then c a s t into 20 mm d i a m ingots and homogenized for 3 days at 1423 K in an argon a t m o s p h e r e . The ingots were cold swaged to 5 mm r o d s with frequent i n t e r m e d i a t e anneals at 1423 K. These r o d s were used for h a r d n e s s s p e c i m e n s , e x t r a c t i o n r e p l i c a s and for the e l e c t r o l y t i c e x t r a c t i o n
of p r e c i p i t a t e which was c a r r i e d out in a 20 pct H2SO4 + 80 pct water bath at about 2 V. Five mm diam rods were r o l l e d down to 125 ~m s t r i p s for e l e c t r o n m i c r o s copy. Thin foils were p r e p a r e d by an e l e c t r o l y t i c jet polishing technique using a 5 pct p e r c h l o r i c acid-95 pct methanol bath at 233 to 223 K. W h e r e v e r p o s s i b l e , e l e c tron diffraction p a t t e r n s with {100} m a t r i x o r i e n t a t i o n were obtained as this p r o v i d e s an unambiguous identification of y " - p r e c i p i t a t e
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