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a p r e v i o u s study, 1 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 the m e c h a n i c a l p r o p e r t i e s of n i c k e l - b a s e s u p e r a l l o y s in the c o n v e n t i o n a l equiaxed condition was i n v e s t i g a t e d . The alloys studied i n c l u d e d two widely e m p l o y e d c a s t m a t e r i a l s , B-1900 and Ren6 80, and a p r o m i s i n g high t e m p e r a t u r e cast m a t e r i a l with an i m p r o v e m e n t of about 20~ ( l l ~ in use t e m p e r a t u r e over c u r r e n t c o m m e r c i a l a l l o y s , TRW-NASA VIA. The a l l o y s were s e l e c t e d to cover the g a m u t of the t h r e e p r i n c i p a l m i n o r p h a s e s g e n e r a l l y identified in n i c k e l - b a s e m a t e r i a l s with B-1900 b e i n g a s t r o n g M6C f o r m e r , TRW-NASA VIA b e i n g p r e d o m i n a t e l y a MC f o r m e r and Ren6 80 b e i n g a s t r o n g M23C6 f o r m e r . The r e s u l t s indicated that the v a r i a t i o n s o b s e r v e d in the p r o p e r t i e s of the m a t e r i a l s with t h e r m a l e x p o s u r e were 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 or r i p e n i n g . In addition, p e r t u r b a t i o n s o b s e r v e d for a n u m b e r of the p r o p e r t y r e s u l t s in the 1500 to 1900~ (816 to 1038~ t e m p e r a t u r e e x p o s 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 of M6C a n d / o r M23C6 c a r b i d e s . Changes in the MC type c a r b i d e , however, did not g e n e r a l l y a p p e a r to have any a p p r e c i a b l e d i r e c t effect upon the p r o p e r t i e s . The p u r p o s e of this study was to extend the work on the effect of t h e r m a l e x p o s u r e to one of the m a t e r i a l s c a s t b y a second t e c h n i q u e , 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 (DS). DS was s e l e c t e d b e c a u s e of the r a p i d l y r i s i n g i n t e r e s t in the u t i l i z a t i o n of DS c o m p o n e n t s in j e t e n g i n e s . This i n t e r e s t has r e s u l t e d f r o m work 2-4 which has shown that such c o m p o n e n t s can have s i g n i f i c a n t l y g r e a t e r t h e r m a l fatigue and c r e e p r e s i s t a n c e and b e t t e r H. E. COLLINS is Section Manager, MaterialsTechnology, TRW Inc., Cleveland,Ohio 44117. Manuscript submitted March 4, 1974. METALLURGICALTRANSACTIONSA

s t r e s s r u p t u r e l i v e s than t h e i r equiaxed c o u n t e r p a r t s . The m a t e r i a l chosen was TRW-NASA VIA which p r e v i ous work at TRW s had shown produced the o p t i m u m DS p r o p e r t i e s for a c o n v e n t i o n a l n i c k e l - b a s e alloy c o m p o s i t i o n . The t h e r m a l e x p o s u r e conditions evaluated i n cluded an e x p o s u r e t i m e of 1000 h at t e m p e r a t u r e s f r o m 1500 to 1900~ (816 to 1038~ in the u n s t r e s s e d condition. I. E X P E R I M E N T A L PROCEDURES A c o m m e r c i a l heat of v a c u u m indu