Cast microstructure of Inconel 713C and its dependence on solidification variables
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T. Z. KATTAMIS,
AND
J. E. MORRAL
The dependence of c a s t m i c r o s t r u c t u r e of Inconel 713C on solidification v a r i a b l e s was i n v e s t i g a t e d over a wide r a n g e of local cooling r a t e s , E, and t h e r m a l g r a d i e n t s in the liquid at the s o l i d - l i q u i d i n t e r f a c e , G. The shape of MC c a r b i d e p a r t i c l e s was found to depend g r e a t l y on: 1) the G/R r a t i o at the s o l i d - l i q u i d i n t e r f a c e , where R is growth r a t e , through the effect of this r a t i o on the s o l i d p h a s e , Yg, growth morphology. Under p l a n a r front growth conditions the c a r b i d e p a r t i c l e s were octahedral, under c e l l u l a r growth conditions they were p l a t e like, elongated along the c e l l u l a r growth d i r e c t i o n , and under d e n d r i t i c growth conditions they w e r e i r r e g u l a r l y shaped; 2) the local cooling rate, ~, when ~ was dendritic, with a t r a n s i t i o n f r o m o c t a h e d r a l to d e n d r i t i c with i n c r e a s i n g e. The size of MC c a r b i d e p a r t i c l e s was found to be c o n t r o l l e d by c o a r s e n i n g and to become finer with i n c r e a s i n g e. In this a l loy the c o m p o s i t i o n of the MC c a r b i d e was e s t a b l i s h e d as (Nbo.63Tio.31MOo.o6)C and was p r a c t i c a l l y independent of local cooling r a t e . Other o b s e r v a t i o n s were that the p r e c i p i t a t i o n of y ' and the f o r m a t i o n of n o n e q u i l i b r i u m eutectics, such as MC-y, ~ - ~ ' or M C - ~ - y ' w e r e s u p p r e s s e d at s p l a t - c o o l i n g r a t e s . Also, m i c r o s e g r e g a t i o n of all alloying e l e m e n t s with the exception of a l u m i n u m was n o r m a l , with c o n c e n t r a t i o n i n c r e a s i n g from the d e n d r i t e c e n t e r - l i n e to the d e n d r i t e a r m boundary. A l u m i n u m behaved in the opposite m a n n e r . Within the cooling r a t e r a n g e used h e r e i n , this v a r i a b l e had only a slight effect on m i c r o segregation. N I C K E L - B A S E s u p e r a l l o y s f o r m the m a j o r v o l u m e of high t e m p e r a t u r e m a t e r i a l s used in g a s - t u r b i n e s , b e c a u s e they c o m b i n e high s t r e n g t h with high oxidation r e s i s t a n c e at elevated t e m p e r a t u r e s . The purpose of this i n v e s t i g a t i o n was to a n a l y z e the solidification b e h a v i o r of a n i c k e l - b a s e s u p e r a l l o y and e s t a b l i s h the effect which v a r i o u s solidification p a r a m e t e r s , in particular cooling rate and thermal gradient at the solid-liquid interface, have on microstructure. Inconel 713C was selected for this investigation. The nominal composition of this alloy by wt pct is: 0.12 C, 12.5 Cr, 4.2 Mo, 2.0 (Nb +Ta), 0.8 Ti, 6.1 A1, 0.012 B, 0.10 Zr and balance nickel. The cast microstructure of Inconel 713C normally consists of coherent ~' precipitate in a y dendritic matrix, with carbide phase dis
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