Microstructure and fatigue of a Ni,Cr, Al-TaC eutectic composite
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N. S. STOLOFF
The N i , 1 0 C r , 5 A 1 - T a C a l i g n e d e u t e c t i c has b e e n heat t r e a t e d s u b s e q u e n t to s o l i d i f i c a t i o n to p r o d u c e s e v e r a l m i c r o s t r u c t u r a l c o n d i t i o n s . High c y c l e fatigue t e s t s have b e e n c o n ducted at 25 and 825~ while c r e e p t e s t s w e r e p e r f o r m e d at 825 and 950~ Experim e n t a l v a r i a b l e s in the fatigue t e s t s w e r e s t r e s s l e v e l , t e s t f r e q u e n c y , and e n v i r o n m e n t . P o s t s o l i d i f i c a t i o n heat t r e a t m e n t s c a u s e d s u b s t a n t i a l i m p r o v e m e n t s in fatigue p r o p e r t i e s at 825~ but had l i t t l e effect at r o o m t e m p e r a t u r e . E l e v a t e d t e m p e r a t u r e fatigue l i v e s w e r e s u b s t a n t i a l l y r e d u c e d at low t e s t f r e q u e n c i e s in vacuum, a c c o m p a n i e d by a shift f r o m e x t e r n a l to i n t e r n a l c r a c k n u c l e a t i o n s i t e s . D i s l o c a t i o n s u b s t r u c t u r e s p r o d u c e d in high t e m p e r a t u r e fatigue t e s t s r e s e m b l e d t h o s e of c r e e p s p e c i m e n s .
E U T E C T I C c o m p o s i t e s , s t r e n g t h e n e d by r e f r a c t o r y c a r b i d e f i b e r s , r e p r e s e d t an i m p o r t a n t new c l a s s of high t e m p e r a t u r e a l l o y s for g a s t u r b i n e s e r v i c e . Ni, C r , A 1 - T a C a l l o y s (Nitac) a r e d u c t i l e and s t r o n g at r o o m t e m p e r a t u r e , 1 while r e v e a l i n g e x c e l l e n t c r e e p r e s i s t a n c e at e l e v a t e d t e m p e r a t u r e s . 2 F a t i g u e p r o p e r t i e s of t h e s e a l l o y s , in p a r t i c u l a r , a p p e a r to be o u t standing u n d e r both high c y c l e , s t r e s s c o n t r o l c o n d i tions 3 and low c y c l e , s t r a i n c o n t r o l c o n d i t i o n s . 4 F u r t h e r i m p r o v e m e n t s in r o o m t e m p e r a t u r e fatigue r e s i s t a n c e of the c l o s e l y r e l a t e d Co, C r , N i - T a C (Cotac) a l l o y s have r e c e n t l y b e e n r e p o r t e d to r e s u l t f r o m p o s t - s o l i d i f i c a t i o n heat t r e a t m e n t s , 3 although the e f fect d i s a p p e a r e d at 825~ 6 T h i s p a p e r d e a l s with the influence of c h a n g e s in y'(Ni3A1) d i s t r i b u t i o n p r o d u c e d by p o s t s o l i d i f i c a t i o n heat t r e a t m e n t , on fatigue r e s i s t a n c e and c r a c k p a t h s of a s i m p l e Ni,IOCr,5A1-TaC e u t e c t i c a l l o y . T e s t v a r i a b l e s include t e m p e r a t u r e , e n v i r o n m e n t and t e s t f r e q u e n c y . C r e e p p r o p e r t i e s a l s o have b e e n d e t e r m i n e d to aid in the a n a l y s i s of e l e v a t e d t e m p e r a t u r e fatigue p r o p e r t i e s in t e r m s of m o d e l s of c r e e p - f a t i g u e i n t e r a c t i o n s . E X P E R I M E N T A L PROCEDURE M a s t e r ingots of c o m p o s i t i o n 69 wt p c t Ni
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