Creep-rupture in powder metallurgical nickel-base superalloys at intermediate temperatures
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AND
M. J. BLACKBURN
T o gain i n s i g h t into the f a c t o r s which c o n t r o l the c r e e p - r u p t u r e p r o p e r t i e s of powder m e t a l l u r g i c a l n i c k e l - b a s e s u p e r a l l o y s at i n t e r m e d i a t e t e m p e r a t u r e s (650 to 775~ a c o m p a r a t i v e study was conducted on the a l l o y s A F l l 5 , modified M A R - M 4 3 2 (B6) and modified IN100 (MERL76). C r e e p - r u p t u r e p r o p e r t i e s in t h e s e a l l o y s were c h a r a c t e r i z e d o in t e r m s of the s t r e s s and t e m p e r a t u r e d e p e n d e n c e of the s e c o n d a r y c r e e p r a t e , ~S, and r u p t u r e t i m e , t R . Within the l i m i t e d s t r e s s r a n g e s used, the s t r e s s d e p e n d e n c e of both cS and tR at 704~ can be r e p r e s e n t e d by power laws ES = Ccrn and tR = M ~ - P ; w h e r e C, M, n, and p a r e c o n s t a n t s . The s t r e s s exponents n and p a r e a p p r o x i m a t e l y equal for both A F l l 5 and B6 with v a l u e s of 16 and 7, r e s p e c t i v e l y . In the c a s e of M E R L 7 6 , ,~ and p a r e d i f f e r e n t , with v a l u e s of 15 and 5, r e s p e c t i v e l y . The a p p a r e n t a c t i v a t i o n e n e r g i e s , Q, a r e 700, 370 and 520 KJ mo1-1 for A F l l 5 , B6 and MERL76, r e s p e c t i v e l y . F o r t h e s e a l l o y s , long c r e e p - r u p t u r e l i v e s a r e a s s o c i a t e d with l a r g e v a l u e s of n and Q. The s i g n i f i c a n t d i f f e r e n c e s in n and Q v a l u e s b e t w e e n A F l 1 5 and B6 were r e l a t e d to c r e e p r e c o v e r y p r o c e s s e s for which the l a t t i c e m i s f i t b e t w e e n the g a m m a and the g a m m a p r i m e was identified to be an i m p o r t a n t p a r a m e t e r . However, the u n e q u a l n and p v a l u e s in MERL76 c o m p a r e d with those in A F l l 5 mad B6, were t r a c e d to d i f f e r e n c e s in f r a c t u r e mode. F a i l u r e s in A F l l 5 and B6 were i n i t i a t e d at c a r b i d e p a r t i c l e s at g r a i n b o u n d a r i e s . In c o n t r a s t , f r a c t u r e in MERL76 was i n i t i a t e d at g r a i n b o u n d a r y t r i p l e j u n c t i o n s . The r u p t u r e l i v e s of A F l l 5 and B6 can be modeled r e a s o n a b l y well by the growth of c a v i t i e s d u r i n g s e c o n d a r y c r e e p and p r o p a g a t i o n of a s u r f a c e - n u c l e a t e d c r a c k d u r i n g the t e r t i a r y creep.
INTRODUCTION I N an e a r l i e r p a p e r 1 some of the r e q u i r e m e n t s for t u r b i n e c o m p o n e n t s that o p e r a t e at i n t e r m e d i a t e t e m p e r a t u r e s (650 to 775~ were d e s c r i b e d , and the need for good c r e e p and s t r e s s - r u p t u r e c a p a b i l i t y was noted. The p a p e r a l s o d e s c r i b e d a n alloy d e v e l o p m e n t p r o g r a m on n i c k e l - b a s e s u p e r a l l o y s i n the powder f o r m which had the goal of achieving s u p e r i o r t e n s i l e and c r e e
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