Estimation of gamma phase composition in nickel-base superalloys
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L. DRESHFIELD
The g a m m a - g a m m a p r i m e region of the N i - A I - C r - T I s y s t e m at 750~ has been a n a l y z e d through the use of phase r u l e p r i n c i p l e s and a n a l y t i c g e o m e t r y to allow e s t i m a t i o n of g a m m a phase c o m p o s i t i o n when the c o m p o s i t i o n of a two-phase q u a t e r n a r y alloy is known. By application of c u r r e n t l y used phase computation methods with the above c a l c u l a t i o n , the g a m m a phase c o m p o s i t i o n of s e v e r a l n i c k e l - b a s e s u p e r a l l o y s was e s t i m a t e d and found (except for tungsten) to be in good a g r e e m e n t with c o m p o s i t i o n s for these alloys as reported in the l i t e r a t u r e .
T H E n i c k e l - b a s e s u p e r a l t o y s used in gas t u r b i n e s a r e placed in s e r v i c e in a m e t a s t a b t e condition. A decade ago when gas t u r b i n e s were designed for h u n d r e d s of h o u r s instead of the thousands of hours r e q u i r e d today, the lack of m e t a l l u r g i c a l s t a b i l i t y of these alloys was of little c o n s e q u e n c e . In the past s e v e r a l y e a r s , howe v e r , s e v e r a l i n v e s t i g a t o r s have o b s e r v e d that a few of the c o m m e r c i a l n i c K e l - b a s e s u p e r a t l o y s will p r e cipitate s i g m a phase and o t h e r r e l a t e d i n t e r m e t a l l i c compounds when subjected to prolonged e x p o s u r e s at elevated t e m p e r a t u r e s . In s o m e a l l o y s , loss of s t r e s s r u p t u r e life and a d e c r e a s e in low t e m p e r a t u r e ductility have been c o r r e l a t e d with the p r e c i p i t a t i o n of the int e r m e t a l l i c compounds. To predict this type of i n s t a b i l i t y and to be able to adjust composition d u r i n g m e l t i n g to avoid u n s t a b l e c o m p o s i t i o n s in these a l l o y s , s e v e r a l c a l c u l a t i o n s have been proposed. T h e s e c a l c u l a t i o n s r e l a t e the s t a b i l i t y of the alloy to the c h e m i s t r y of the g a m m a (fcc matrLx) phase which is e s t i m a t e d from the melt c h e m i s t r y . '" B e c a u s e the c a l c u l a t i o n s in Refs. 1 and 2 a r b i t r a r i l y set a l u m i n u m at z e r o a c a l c u l a t i o n based on the N I - C r - A I phase d i a g r a m was proposed in Ref. 3. This work is an extension of Ref. 3 to a q u a t e r n a r y system. This paper will d e m o n s t r a t e one approach for s i m plifying a two-phase region of a q u a t e r n a r y phase d i a g r a m so that it can be e a s i l y a n a l y z e d g e o m e t r i c a l l y . Specifically, a method is developed to allow the e s t i m a tion of the composition of the g a m m a phase given the c o m p o s i t i o n of a two-phase (gamma plus g a m m a p r i m e ) alloy in the N I - A t - C r - T i s y s t e m . This e s t i m a tion method is then applied to n i c k e l - b a s e s u p e r a l l o y s and the r e s u l t s of the e s t i m a t i o
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