SIGMA-SAFE: A phase diagram approach to the sigma phase problem in ni base superalloys
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S UPERALLOYS a r e known to have a t e n d e n c y to p r e c i p i t a t e the s i g m a p h a s e at e l e v a t e d t e m p e r a t u r e s . 1 B e c a u s e the p r e s e n c e of the s i g m a p h a s e in s u p e r a l l o y s i s d e l e t e r i o u s , much e f f o r t h a s b e e n d e v o t e d to p r e d i c t i o n of i t s a p p e a r a n c e . 2 A p r o c e d u r e , known by the n a m e PHACOMP, i s p r e s e n t l y in use to c o n t r o l the c o m p o s i t i o n of s u p e r a l l o y s . Indeed, s p e c i f i c a t i o n s of e l e c t r o n v a c a n c y n u m b e r , N v , a s c o m p u t e d by a PHACOMP (or equivalent) p r o c e d u r e g o v e r n the a c c e p t a n c e of s u p e r a l l o y h e a t s by a i r c r a f t engine manufacturer s. The b a s i s of the PHACOMP p r o c e d u r e is the a s s u m p t i o n that s i g m a p h a s e will p r e c i p i t a t e in a g a m m a p h a s e (fcc) when the e l e c t r o n v a c a n c y n u m b e r (as e v a l u a t e d a c c o r d i n g to a r e c i p e of the p a r t i c u l a r p r o c e d u r e ) of this g a m m a p h a s e e x c e e d s a c r i t i c a l v a l u e , i.e. the ~ p h a s e b e c o m e s s u p e r s a t u r a t e d with r e s p e c t to p r e c i p i t a t i o n of a when the c r i t i c a l e l e c t r o n v a c a n c y n u m b e r is e x c e e d e d . T h e a v a i l a b l e e m p i r i c a l e v i d e n c e a p p e a r s to s u p p o r t t h i s concept, although m o d i f i c a t i o n s to the b a s i c concept have been r e q u i r e d to f o r c e the p r o c e d u r e to fit the o b s e r v a t i o n s . Much of t h i s e v i d e n c e i s b a s e d upon 103 to 104 h of e q u i l i b r a tion at about 875~ T h i s t i m e l i m i t i s (or was) cons i s t e n t with a i r c r a f t engine life e x p e c t a t i o n . R e c e n t l y , a i r c r a f t engine gas t u r b i n e s have been used to g e n e r a t e s u p p l e m e n t a r y p o w e r in e l e c t r i c i t y g e n e r a t i n g s t a t i o n s . F o r t h i s and o t h e r t y p e s of e l e c t r i c i t y g e n e r a t i o n u s e it i s d e s i r e d that the gas t u r b i n e s have a life in e x c e s s of l 0 S h. T h i s fact h a s l e d to the n e e d to d e v e l o p a s i g m a p h a s e p r e c i p i t a t i o n r a t ing that i s r e p r e s e n t a t i v e of the e q u i l i b r i u m s t a t e c o r r e s p o n d i n g to the s u p e r a l l o y c o m p o s i t i o n . The fact that the rate of p r e c i p i t a t i o n of the s i g m a p h a s e is e x t r e m e l y slow i s a d d i t i o n a l j u s t i f i c a t i o n f o r this m i s s i o n . T h u s , the p r e s e n t p r o g r a m was d e v e l o p e d a s a r e s p o n s e to t h i s a i m . It i s a p p a r e n t that e q u i l i b r i u m p h a s e d i a g r a m s , if a v a i l a b l e , a r e the b e s t m e a n s of p r o v i d i n g a m e a s u r e E. S. MACHLIN and JOEY SHAO are Professor and Graduate Student, respectively, H. Krumb School of Mines, Columbia University, New
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