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The t h r e e n i c k e l - b a s e s u p e r a l l o y s B-1900, TRW-NASA VIA, and Ren~ 80 w e r e s t u d i e d util i z i n g m e t a l l o g r a p h i c and r e s i d u e a n a l y s i s t e c h n i q u e s in conjuction with m e c h a n i c a l p r o p e r t y t e s t s to d e t e r m i n e the effect of t h e r m a l e x p o s u r e on the m i c r o s t r u c t u r e and m e c h a n i c a l p r o p e r t i e s . E x p o s u r e t i m e s of 10, 100, 1000, and 5000 h at t e m p e r a t u r e s f r o m 1400 to 2000~ (760 to 1093~ w e r e e v a l u a t e d . F o u r m i n o r p h a s e s - - M C , M6C, M23C~, and M3B 2 plus g a m m a - p r i m e w e r e o b s e r v e d in the g a m m a m a t r i x of t h e s e a l l o y s . Significant v a r i a tions in the m e c h a n i c a l p r o p e r t i e s w e r e o b s e r v e d to o c c u r with t h e r m a l e x p o s u r e . M i c r o s t r u c t u r a l e v a l u a t i o n i n d i c a t e d that t h e s e v a r i a t i o n s in p r o p e r t i e s w e r e due p r i m a r i l y to g a m m a - p r i m e a g g l o m e r a t i o n o r r i p e n i n g . P e r t u r b a t i o n s noted in a n u m b e r of the m e c h a n i c a l p r o p e r t y v s e x p o s u r e t e m p e r a t u r e c u r v e s in the 1500 to 1900~ (816 to 1038~ t e m p e r a t u r e r a n g e a p p e a r e d to be due to the p r e c i p i t a t i o n and growth of M6C a n d / o r M2zC6 carbides.

C O N C E R N h a s been growing in r e c e n t y e a r s o v e r the effect of e x t e n d e d s e r v i c e t i m e s upon the p e r f o r m a n c e of g a s t u r b i n e c o m p o n e n t s , p a r t i c u l a r l y t h o s e l o c a t e d tn the hot s t a g e s . The m a t e r i a l s p r e d o m i n a n t l y e m p l o y e d In t h e s e a r e a s a r e the n i c k e l - b a s e s u p e r a l l o y s . Studies (e.g., Refs. 1 through 11) have i n d i c a t e d that s i g n i f i c a n t v a r i a t i o n s in both the m i c r o s t r u c t u r e and the m e c h a n i c a l p r o p e r t i e s can o c c u r in such m a t e r i a l s under e x p o s u r e conditions s i m i l a r to t h o s e e x p e r i e n c e d in g a s t u r b i n e s e r v i c e . F o r e x a m p l e , the MC c a r b i d e which i s one of the m o s t abundant of the m i n o r p h a s e s found in n i c k e l - b a s e a l l o y s (e.g., c a r b i d e s , b o r i d e s , s i g m a , mu, Laves) is s t a b l e for s h o r t t i m e s to t e m p e r a t u r e s above 2000~ (1093~ However, a f t e r long e x p o s u r e t i m e s in the t e m p e r a t u r e r a n g e f r o m about 1600 to 2000~ (871 to 1093~ it will d e c o m p o s e into M6C a n d / o r M23C8 p l u s g a m m a - p r i m e . Long e x p o s u r e t i m e s at high t e m p e r a t u r e will a l s o r e s u l t in the a g g l o m e r a tion of the g a m m a - p r i m e p h a s e which is the p r i n c i p a l n i c k e l - b a s e s t r e n g t h e n e r . Other m o r p h o l o g i c a l c h a n g e s which have b e e n o b s e r v e d to o c c u r under such cond