An analytical electron microscopy study of the high temperature carbide formed in a V-5 Ti-C alloy
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I. INTRODUCTION
RAPIDLY
growing i n t e r e s t in high t e m p e r a t u r e m a t e r i a l s had led to an i n c r e a s i n g a m o u n t of work on v a n a d i u m and o t h e r r e f r a c t o r y m e t a l - i n t e r s t i t i a l s y s tems.~-~~ One of the m a j o r p r o b l e m s in t h e i r u s e f u l application has b e e n the c o n t r o l of i n t e r s t i t i a l e l e m e n t s such as c a r b o n , oxygen and n i t r o g e n . F o r this r e a s o n the d e t e r m i n a t i o n of phase r e l a t i o n s h i p s and p r e c i p i t a t i o n p r o c e s s e s in such m a t e r i a l s has b e e n an a r e a of s p e c i a l c o n c e r n . R e f r a c t o r y m e t a l - i n t e r s t i t i a l s y s t e m s exhibit a g r e a t v a r i e t y of s e c o n d p h a s e s depending upon c o m p o s i t i o n and heat t r e a t m e n t . B e c a u s e of the n u m e r o u s p h a s e s p o s s i b l e , p r e c i p i t a t i o n p r o c e s s e s n o r m a l l y involve m o r e than one stage with the f o r m a t i o n of s e v e r a l i n t e r m e d i a t e s t r u c t u r e s p r e c e e d i n g the final e q u i l i b r i u m p h a s e . M u l t i - s t a g e p r e c i p i t a t i o n has been r e p o r t e d in a l l o y s such as Ta-O, 9 Nb-O,11 Nb-O-N,12 Fe-N, 13 Fe-C, 14,1s Nb-C ,7 and V -C .8 P r e c i p i t a t i o n in refractory metal-carbon s y s t e m s has b e e n r e l a t i v e l y n e g l e c t e d in c o m p a r i s o n to oxygen and n i t r o g e n c o m b i n a t i o n s . The p r i m a r y r e a s o n is the l a r g e d i f f e r e n c e in solubility; the s o l u b i l i t y for oxygen and n i t r o g e n is n o r m a l l y 102 to 103 t i m e s g r e a t e r than for c a r b o n . However, solid s o l u t i o n s of m o d e r a t e c o n c e n t r a t i o n s can be a c h i e v e d with a s u i t a b l e quenching method. E m p l o y i n g such a t e c h n i q u e , D i e r c k s and W e r t 8 studied p r e c i p i t a t i o n in V-C a l l o y s and found it to be at l e a s t a t w o - s t a g e p r o c e s s . The c a r b o n was o b s e r v e d to p r e c i p i t a t e i n i t i a l l y as v e r y finely d i s p e r s e d c a r b i d e s v i s i b l e in the t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e (TEM) t h r o u g h s t r u c t u r e f a c t o r c o n t r a s t . With i n c r e a s i n g aging, the p r e c i p i t a t e s
c o a r s e n , and the c a r b i d e s a p p e a r a s thin, c o h e r e n t p l a t e l e t s on {310} p l a n e s . T h i s c o h e r e n t p r e c i p i t a t e p o s s e s s e s a b o d y - c e n t e r e d cubic s t r u c t u r e c l o s e l y r e l a t e d to that of the m a t r i x . As aging t i m e is i n c r e a s e d , the second p h a s e p a r t i c l e s thicken, lose c o h e r e n c e and t r a n s f o r m to the e q u i l i b r i u m h e x a g o n a l V2C p h a s e . The r a p i d o v e r a g i n g in the b i n a r y v a n a d i u m - c a r b o n s y s t e m (on the o r d e r of 4 h at t e m p e r a t u r e s above
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