Microstructural instabilities in an industrial gas turbine engine vane
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n i c k e l - b a s e s u p e r a l l o y s e n t e r s e r v i c e in the hot s e c t i o n s of a i r c r a f t and i n d u s t r i a l g a s t u r b i n e e n g i n e s in a m e t a s t a b l e condition. D u r i n g long t i m e o p e r a t i o n a t high t e m p e r a t u r e s t h e s e m a t e r i a l s h o m o g e n i z e and v a r i o u s m i c r o s t r u c t u r a l c h a n g e s o c c u r . Changes can o c c u r in the v o l u m e f r a c t i o n and m o r p h o l o g y of the s t r e n g t h e n i n g p r e c i p i t a t e g a m m a p r i m e (Ni3(A1, Ti)), p r i m a r y MC c a r b i d e s d e g e n e r a t e to f o r m v a r i o u s s e c o n d a r y c a r b i d e s such a s M23C6, and t o p o l o g i c a l l y c l o s e p a c k e d (TCP) i n t e r m e t a l l i c p h a s e s s u c h a s s i g m a p h a s e can p r e c i p i t a t e in the austenitic T matrix phase. These microstructural c h a n g e s can have a m a r k e d , and often d r a s t i c , effect on the m e c h a n i c a l p r o p e r t i e s of the a l l o y s . In the i n t e r e s t s of engine e f f i c i e n c y and e c o n o m y it i s i m p o r t a n t that hot s e c t i o n c o m p o n e n t s o p e r a t e s a f e l y and r e l i a b l y f o r long t i m e s u n d e r e x t r e m e cond i t i o n s of t e m p e r a t u r e and s t r e s s . It i s i m p o r t a n t t h e r e f o r e t h a t c h a n g e s o c c u r r i n g in engine c o m p o n e n t s a r e u n d e r s t o o d and that t h e o r e t i c a l m o d e l s a r e a v a i l a b l e to a l l o w t h e s e c h a n g e s to b e p r e d i c t e d . In t h i s p a p e r the m i c r o s t r u c t u r a l c h a n g e s o c c u r r i n g in a c a s t U d i m e t 500 t u r b i n e vane f r o m an i n d u s t r i a l g a s t u r b i n e engine a r e e x a m i n e d , and it i s shown that t h e s e c h a n g e s can b e p r e d i c t e d m o r e a c c u r a t e l y u s i n g a modified Phacomp (phase computation) procedure. T H E O R E T I C A L BACKGROUND The microstructural examinations, described later, showed that instabilities had occurred in the austenitic 7 matrix with the precipitation of sigma phase and the degeneration of the MC carbide. Thus it was possible to compare the experimental observations with predictions of phase stability obtained by the available numerical procedures. It is well established I-5 that sigma phase precipitation generally occurs when the 7 matrix reaches a W. WALLACEand J. M. TRENOUTH are Head and Associate Research Officer, respechvely, Materials Science Section, Structures and Materials Laboratory, National Aeronautical Estabhshment, National Research Council of Canada, Ottawa, Ontario. J. D. DAW is Metallurgical Engineer, Turbine and Generator Dwision, Westinghouse Canada Limited, Hamdton, Ontario. Manuscript submitted July t5, 1975. METALLURGICAL TRANSACTIONS A
c r i t i c a l e l e c t r o n - v a c a n c y c o n c e n t r a t i o n r a n g e . If the c o m p o s i t i o n of 7 i s known i t s a v e r a g e e l e c t r o n - v
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