The role of hafnium in modifying the microstructure of cast nickel-base superalloys
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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 , used in t u r b i n e h a r d w a r e , a r e typically c o m p o s e d of a p p r o x i m a t e l y 60 vol. pct of a V' phase c o h e r e n t l y p r e c i p i t a t e d in an fcc m a t r i x , together with eutectic p h a s e s and one o r m o r e c a r b i d e p h a s e s . Since t h e s e alloys cannot be heat t r e a t e d , i . e . one cannot s o l u t i o n h e a t - t r e a t and r e p r e c i p i t a t e the y ' p h a s e , t h e i r p r o p e r t i e s a r e g o v e r n e d by the a s - c a s t m i c r o s t r u c t u r e . The l a t t e r has b e c o m e i n c r e a s i n g l y c o m p l e x in r e s p o n s e to the c o n t i n u e d need for i m p r o v e m e n t s in e l e v a t e d - t e m p e r a t u r e s t r e n g t h . U n f o r t u n a t e l y , c o n c o m i t a n t with g a i n s in high t e m p e r a t u r e s t r e n g t h , t h e r e has been an a s s o ciated penalty in that the a m b i e n t and i n t e r m e d i a t e temperat~xre ductility of c a s t s u p e r a l l o y s is u s u a l l y poor. R e c e n t l y , however, it has been shown ~'z that a d ditions of hafnium to n i c k e l - b a s e s u p e r a l l o y s can lead to s u b s t a n t i a l i m p r o v e m e n t in both d u c t i l i t y and high t e m p e r a t u r e c r e e p p r o p e r t i e s . This p a p e r p r e s e n t s r e s u l t s f r o m a study u n d e r t a k e n to define the r o l e of hafnium in modifying the s t r u c t u r e and s t r u c t u r e - d e p e n d e n t p r o p e r t i e s of n i c k e l - b a s e a l l o y s .
E X P E R I M E N T A L PROCEDURE The e x p e r i m e n t s r e p o r t e d h e r e were c a r r i e d out on the c o m m e r c i a l alloy ALLOY 713-LC and the h a f n i u m - m o d i f i e d a l l o y MM-004 kindly p r o v i d e d by the M a r t i n M e t a l s C o m p a n y in the f o r m of i n v e s t m e n t - c a s t t e s t b a r s . The c o m p o s i t i o n s of t h e s e alloys a r e shown in T a b l e I. The e l e c t r o l y t e u s e d for etching the s p e c i m e n s p r i o r to r e p l i c a t i o n , and for optical e x a m i n a t i o n , was a 6:8:30 m i x t u r e of H F , HNOa, and g l y c e r o l , r e s p e c t i v e l y . Thin foils w e r e p r e p a r e d by t w i n - j e t e l e c t r o polishing of 3 m m d i a m disc s p e c i m e n s in an e t h a n o l 10 pct p e r c h l o r i c acid bath at 32~ All o b s e r v a t i o n s P. S. KOT,VAL,formerly with Research Institute for Advanced Studies, Baltimore, Md., is now Senior Staff Scientist, Union Carbide Research Institute, Tarrytown, N.Y.J.D. VENABLES is a Senior Research Scientist with Research Institute for Advanced Studies, R. W. CALDER was formerly with Research Institute for Advanced Studies. Manuscript submitted August 9, 1971. METALLURGICALTRANSACTIONS
Table I. Alloy Compositions
Composition in WtPct 713-LC MM-O04
Cr
Mo
Al
T1
Cb
Zr
B
C
Hf
Ni
12.5 4.1 12.5 4.1
6.1 6.1
0.8 0.8
2.0 0.10 0.12 0.08 2.0 0.10 0.12 0,08 1.5
Bal Bal
w e r e
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