Effect of heat treatment on the structure and properties of extruded P/M alloy 718
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718 i s a v e r s a t i l e high s t r e n g t h , w e l d a b l e wrought n i c k e l - b a s e a l l o y which was d e v e l o p e d f o r use at t e m p e r a t u r e s up to about 704~ The p h y s i c a l m e t a l l u r g y of a l l o y 718 h a s been the s u b j e c t of a n u m b e r of i n v e s t i g a t i o n s . A c o n s i d e r a b l e amount of w o r k was done i n i t i a l l y to d e t e r m i n e the p h a s e s p r e c i p i t a t e d d u r i n g a g i n g ) "4 It i s now f i r m l y e s t a b l i s h e d that h a r d e n i n g is due to the f o r m a t i o n of the m e t a s t a b l e , d i s c - s h a p e d 7". 5,6 T h i s p h a s e , which i s c o h e r e n t with the m a t r i x , has an o r d e r e d body c e n t e r e d t e t r a g o n a l s t r u c t u r e (D022 type s u p e r l a t t i c e ) with a c/a r a t i o of about 2.044. T h e 7" p h a s e f o r m s on the cube p l a n e s of the m a t r i x s u c h that: and (001)~/,,/./, (001!y [100]y,, / / [ 1 0 0 ] 7 . Upon p r o l o n g e d aging the y" p h a s e i s r e p l a c e d by the e q u i l i b r i u m o r t h o r h o m b i c 5 phase having the s a m e c h e m i c a l c o m p o s i t i o n (NisCb). The 6 p h a s e m a y f o r m e i t h e r by a c e l l u l a r p r e c i p i t a t i o n at g r a i n b o u n d a r i e s o r a s i n t r a g r a n u l a r p l a t e s . Although the m a j o r s t r e n g t h e n i n g p h a s e in a l l o y 718 i s the ~" p h a s e , the w o r k of P a u l o n i s et al5 r e v e a l e d that the o r d e r e d f a c e c e n t e r e d cubic 7' p h a s e a l s o f o r m s , but in s m a l l e r quantities. T h e e x c e p t i o n a l l y high s t r e n g t h of a l l o y 718 has e n a b l e d the a l l o y to gain u t i l i z a t i o n in gas t u r b i n e e n g i n e s a s d i s c s , b l a d e s and v a r i o u s s t r u c t u r a l s h a p e s . F o r the l a t t e r a p p l i c a t i o n , r o l l i n g to c l o s e t o l e r a n c e i s a c o m m o n f a b r i c a t i o n m e t h o d although l i m i t e d to relatively simple shapes. Extrusion offers greater f l e x i b i l i t y but the high s t r e n g t h of the m a t e r i a l l i m i t s the e x t r u s i o n r a t i o that can be e m p l o y e d and r e s t r i c t s the s i z e of the e x t r u d e d p a r t . The advent of p o w d e r m e t a l l u r g y (P/M) t e c h n o l o g y h a s a d d e d a new d i m e n s i o n to the m a n u f a c t u r e of s u p e r a l l o y s h a p e s . D e v e l o p m e n t s such a s the " f i l l e d b i l l e t " technique 7 e n a b l e c o m p l e x s h a p e s having quite thin c r o s s s e c t i o n s to be e x t r u d e d . At the p r e s e n t time there is still very little information regarding the p r o p e r t i e s of e x t r u d e d P/M s u p e r a l l o y p r o d u c t s . T h e p r e s e n t i n v e s t i g a t i o n was u n d e r t a k e n to b r o a d e n the knowledge of P/M a l l o y p r o p e r t i e s by a study of the effect of heat t r e a t m e n t on the s t r u c t u r e and p r o
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