Effect of aluminum addition on the precipitation behavior of a Co-Ni-Cr base superalloy containing Nb
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p o l i s h i n g t e c h n i q u e u s i n g 10 pct p e r c h l o r i c a c i d - 9 0 pet m e t h a n o l bath at 233 to 223 K. All h e a t - t r e a t m e n t s w e r e c a r r i e d out with s p e c i m e n s s e a l e d in a r g o n f i l l e d V y c o r c a p s u l e s followed b y a f a s t quench in i c e d b r i n e . RESULTS The specimens were given a final solution treatment at 1473 K for one hour and quenched into iced brine. The as-quenched structure was austenitic (FCC). Occasionally some undissolved particles were also observed. The lattice parameter of the as-quenched specimen was determined by the X-ray powder technique and was calculated to be 3.558A. The solution treated specimens were aged between 873 and 1073 K for various lengths of time. The aging kinetics were followed by the hardness measurements and the variation in hardness with aging time, at different temperatures, are shown in Fig. I. It is seen that the alloy exhibits normal aging behavior i.e., as the temperature of aging is r e d u c e d , the t i m e r e q u i r e d to a t t a i n the p e a k h a r d n e s s , and the p e a k h a r d n e s s v a l u e a r e i n creased. The s t r u c t u r a l changes on aging, w e r e followed m a i n l y b y e l e c t r o n m i c r o s c o p y . The m a j o r p r e c i p i t a t i o n r e a c t i o n at a l l the aging t e m p e r a t u r e s was the f o r m a t i o n of ~' p h a s e which h a s an o r d e r e d CusAu t y p e of FCC s t r u c t u r e . The l a t t i c e p a r a m e t e r of the electrolytically extracted y' precipitate, as determined b y the X - r a y p o w d e r technique was 3.6034. A v e r y l i m i t e d a m o u n t of c a r b i d e p r e c i p i t a t e within the g r a i n s a s w e l l a s at the g r a i n b o u n d a r i e s , and e p h a s e w e r e a l s o o b s e r v e d . The p r e c i p i t a t i o n and growth of t h e s e p h a s e s is d e s c r i b e d b e l o w . P R E C I P I T A T I O N O F y' P H A S E The T' particles in this alloy nucleate homogeneously throughout the matrix and are coherent with the parent lattice. In the early stages of aging their presence gives rise to a modulated matrix structure and superlattice spots in the electron diffraction pattern. The earliest stage that the individual y' particles could be clearly seen was after aging two hours at 1073 K. A dark field micrograph taken with the (100) superlattice spot of 7' is shown in Fig. 2. The particle size is about VOLUME 6A, JANUARY 1975-213
1004 and t h e y s e e m to be of m i x e d shape i . e . , cubic as well a s s p h e r i c a l . In c e r t a i n a r e a s , t h e y s e e m to be a l i g n i n g t h e m s e l v e s in (100) d i r e c t i o n s . As aging is continued the v ' p a r t i c l e s develop a well defined cubic s h a p e and align t h e m s e l v e s along (100) d i r e c t i o n s (Fig. 3). The growth c h a r a c t e r i s t i c s of ~' p r e c i p i t a t e p a r t i c l e s w e r e a l s o s t u d i e d . The change in v o l u m e f r a c t i o n of ~' was d e t e r
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