The development of a pseudoeutectic nickel-based composite alloy

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re N have e e m e t h o d s by C O N V E N TtIh eO Abeen L Lt h rY which n i c k e l - b a s e alloys, for elevated t e m p e r a t u r e s e r v i c e , have been s t r e n g t h e n e d . These a r e solid s o l u tion s t r e n g t h e n i n g , p r e c i p i t a t i o n s t r e n g t h e n i n g ( p r i m a r ily by ~'), and c a r b i d e d i s p e r s i o n s t r e n g t h e n i n g . A c o m m o n p r o b l e m in c o n v e n t i o n a l s u p e r a l l o y s is that a f t e r long t e r m , elevated t e m p e r a t u r e s e r v i c e , c h a n g e s in the d i s t r i b u t i o n of the v a r i o u s e l e m e n t s p r e s e n t occ u r , and these changes f r e q u e n t l y l i m i t the life of c o m p o n e n t s made from the alloys. The changes which occ u r involve the c o a r s e n i n g of the y ' p r e c i p i t a t e , c h a n g e s in the composition and d i s t r i b u t i o n of the c a r b i d e s and in s o m e i n s t a n c e s f o r m a t i o n of a phase~ Such p h e n o m e n a have r e c e i v e d d e t a i l e d attention 1-3 and m a n y dev e l o p m e n t s have been made in alloy c o m p o s i t i o n and t r e a t m e n t to m i n i m i z e the r e s u l t i n g d e l e t e r i o u s effects while allowing the o v e r a l l p r o p e r t i e s of the alloys to be i m p r o v e d . In the work d e s c r i b e d in this paper a somewhat diff e r e n t approach to the d e v e l o p m e n t of n i c k e l - b a s e a l loys has been taken. W h e r e a s the c o n v e n t i o n a l a l l o y s a r e e s s e n t i a l l y a n i c k e l - b a s e phase s t r e n g t h e n e d by the t h r e e l i s t e d m e c h a n i s m s , in the p r e s e n t a p p r o a c h the m a t e r i a l is also s t r e n g t h e n e d by the p r e s e n c e of a m a s s i v e second phase and thus i s b a s i c a l l y a c o m p o s ite. The morphology of this second phase is rod like in the a s - c a s t form and it can be aligned by the u s e of d i r e c t i o n a l s o l i d i f i c a t i o n s t e c h n i q u e s J '5 This a l i g n m e n t m a y lead to an i m p r o v e m e n t in the m e c h a n i c a l p r o p e r t i e s of the alloy. 6- s Due to this additional s t r e n g t h e n i n g m e c h a n i s m it is p o s s i b l e to e l i m i n a t e c a r b o n f r o m the n i c k e l - b a s e alloy and thus e l i m i n a t e one of the p r o b l e m s of s t r u c t u r a l stability. The b a s i c s y s t e m chosen was N i - C r - B e . The object was to take advantage of the Ni-NiBe eutectic (of the b i n a r y N i - B e system) which should lend i t s e l f to cont r o l l e d solidification p r a c t i c e s . However, a f u n d a m e n tal p r o b l e m with NiBe is that while it is of quite high m e l t i n g point (~ 1470~ it e x i s t s over a quite wide c o m p o s i t i o n a l range. This factor coupled with the s i m ple c r y s t a l s t r u c t u r e (CsC1 type) r e s u l t in an i n t e r m e t a l l i c compound of r e l a t i v e l y low s t r e n g t h as c o m YUAN-SHOU SHEN is wit