The partitioning of refactory metal elements in hafnium-modified cast nickel- base superalloys
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a p r e v i o u s p a p e r by Dunn, S p o n s e l l e r , and Dahl, 1 p r e l i m i n a r y r e s u l t s of the e f f e c t of h a f n i u m additions 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 of the c a s t n i c k e l - b a s e s u p e r a l l o y s B-1900 and Alloy 713 LC w e r e p r e s e n t e d . It was noted that the p r o p e r t i e s of the h a f n i u m - f r e e b a s e a l l o y s d e t e r m i n e d on specimens machined from actual cast turbine components w e r e s i g n i f i c a n t l y l o w e r than the s a m e p r o p e r t i e s d e t e r m i n e d on c a s t - t o - s i z e t e s t b a r s . ~-S Howe v e r , the addition of 1.3 to 1.5 pct Hf to B-1900 and Alloy 713 LC p r o d u c e s p r o p e r t i e s within the t u r b i n e c o m p o n e n t s that a r e e q u i v a l e n t to the c a s t - t o - s i z e t e s t b a r s fo r the u n m o d i f i e d a l l o y s . 4'~ Studies 4-9 a i m e d at e s t a b l i s h i n g the m e c h a n i s m by which hafnium i n c r e a s e s the s t r e n g t h , r u p t u r e life, and d u c t i l i t y of c a s t n i c k e l - b a s e s u p e r a l l o y s have noted that the h af n iu m addition m a r k e d l y c h a ng es the p r i m a r y MC c a r b i d e s and the p h a s e d i s t r i b u t i o n in g r a i n b o u n d a r i e s . In p a r t i c u l a r , the m o r p h o l o g y of the MC c a r b i d e s is a l t e r e d f r o m a s c r i p t - t y p e m o r phology to a n g u l a r , d i s c r e t e p a r t i c l e s , and the g r a i n b o u n d a r i e s a r e a l t e r e d f r o m p l a n a r to convoluted b o u n d a r i e s f i l l e d with y ' . (See Lund 4 f o r B-1900 + 1.5 pct Hf and Kotval et a l . s f o r Alloy 713 LC + 1.5 pct Hf.) An e x a m p l e of the change in c a r b i d e and g r a i n b o u n d a r y m o r p h o l o g y obtained by adding h a f nium to B-1900 is shown in Fig. 1. It has a l s o b e e n o b s e r v e d that, by adding 2.0 pct Hf to d i r e c t i o n a l l y s o l i d i f i e d M a r - M 2 0 0 , 7 the M23C6 c a r b i d e s in the g r a i n b o u n d a r i e s a r e c o n v e r t e d f r o m a continuous n e t w o r k to d i s c r e t e p a r t i c l e s s u r r o u n d e d by y ' . In addition to o b s e r v a b l e c h a n g e s in the m i c r o s t r u c t u r e , X - r a y d i f f r a c t i o n a n a l y s i s of the MC c a r b i d e s e x t r a c t e d f r o m h e a t - t r e a t e d (1080~ h plus 900~ h) B-1900 + 1.5 pct Hf r e v e a l e d at l e a s t four d i f f e r e n t l a t t i c e p a r a m e t e r s for the p h a s e r a n g ing f r o m 4.399 to 4.594/k, 4 i n s t e a d of the s i n g l e p a r a m e t e r of 4.368/], d e t e r m i n e d for the MC c a r b i d e in B-1900. The s p e c i f i c X - r a y data a r e s u m m a r i z e d in J. M. DAHL and W. F. DANESI are Research Supervisor, Research Laboratory, and Manager, High Temperature Alloy Development, respectively, Climax Molybdenum Company of Michigan, Ann Arbor, Mich. 48106, a subsidiary of American Metal C
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