Creep behavior of Ni-Cr lamellar eutectic alloy
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T H E d e v e l o p m e n t of n i c k e l - b a s e d alloys for e l e v a t e d t e m p e r a t u r e s t r u c t u r a l a p p l i c a t i o n s has s p u r r e d the i n t e r e s t in c r e e p and c r e e p - r u p t u r e b e h a v i o r of these a l l o y s . On one hand n u m e r o u s i n v e s t i g a t o r s have c o v e r e d the e n t i r e r a n g e f r o m nickel ~-7 through v a r i ous N i - C r a l l o y s 7-'~ to c h r o m i u m , 11 to cite s e v e r a l . But on the other, v e r y few i n v e s t i g a t o r s have cons i d e r e d the c r e e p b e h a v i o r of composite m a t e r i a l s and s p e c i f i c a l l y l a m e l l a r e u t e c t i c s . They have s t u d i e d c r e e p in t u n g s t e n f i b e r r e i n f o r c e d s i l v e r ~2 and in the l a m e l i a r e u t e c t i c s y s t e m s Ni-NiaNb 13 and N i - N i a T i ; 14 s o m e s t r e s s r u p t u r e data on a l u m i n u m b a s e d e u t e c t i c s 15,1B has been p u b l i s h e d . When we set out to i n v e s t i g a t e the c r e e p b e h a v i o r of the N i - C r l a m e l l a r e u t e c t i c alloy, we soon r e a l i z e d that the s y s t e m is quite complex, in that it i n c l u d e s x~ the following: N i - C r and C r - N i rich solid s o l u t i o n s ; a fine d i s p e r s i o n of oxides and o r i e n t e d n i c k e l - r i c h p r e c i p i t a t e s in the c h r o m i u m phase; and a c e l l u l a r p r e c i p i t a t i o n in the n i c k e l - r i c h phase; all c o m b i n e d in an oriented lamellar structure. It is t h e r e f o r e a p p r o p r i a t e to s u m m a r i z e the known m a j o r f a c t o r s that d e t e r m i n e high t e m p e r a t u r e c r e e p d e f o r m a t i o n and that m a y r e l a t e to the p r e s e n t s y s t e m . a) F r o m the i n v e s t i g a t i o n of the t e n s i l e p r o p e r t i e s of the l a m e l l a r eutectic alloy, Kossowsky et al., 'a concluded that the s t r e n g t h of the alloy could be r a t i o n a l i z e d by the s i m p l e r u l e of m i x t u r e s . T h i s i m p l i e s a c o m p l e t e load t r a n s f e r a c r o s s the N i - C r e u t e c t i c l a m e l l a r i n t e r f a c e and i s o s t r a i n e l o n g a t i o n s ~9 d u r i n g d e f o r m a t i o n of the whole c o m p o s i t e . b) Yields s t r e n g t h s , c r e e p s t r e n g t h s , and e l a s t i c m o d u l i of c a s t N i - C r a l l o y s a r e s t r o n g f u n c t i o n s of c o m p o s i t i o n f l ~ F o r e x a m p l e , at 625~ the d y n a m i c m o d u l u s of e l a s t i c i t y for a c a s t 35 C r - 6 5 Ni a l l o y is of the o r d e r of 18 x 106 psi; it d e c r e a s e s to 12 x 106 psi for a 50 C r - 5 0 Ni alloy; while the 0.2 pct yield s t r e n g t h i n c r e a s e s f r o m 17,000 to 48,000 psi. c) The effect of s t a c k i n g fault e n e r g y on s t e a d y state c r e e p r a t e is quite a p p r e c i a b l e . T h i s h a s been o r i g i n a l l y p r o p o s e d by Sherby and c o w o r k e r s 21'~2 and RAM KOSSOWSKYis Se
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