Precipitation of carbides in deformed vanadium
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P R E C I P I T A T I O N f r o m s u p e r s a t u r a t e d solid solution is a s t r u c t u r e s e n s i t i v e p r o c e s s . C r y s t a l l o g r a p h i c d e fect s t r u c t u r e s m a y s t r o n g l y affect the r a t e of solute p r e c i p i t a t i o n , the p r e c i p i t a t e morphology, and the m e c h a n i c a l p r o p e r t i e s . Such defect s t r u c t u r e s can be i n t r o d u c e d b y quenching, x-' t r a c e i m p u r i t y addition, s-8 heavy p a r t i c l e b o m b a r d m e n t , 9-11 and p l a s t i c d e f o r m a tion. 12-14 In this p a p e r we r e p o r t the effect of p r i o r p l a s t i c d e f o r m a t i o n on the p r e c i p i t a t i o n of c a r b i d e s in vanadium. The b i n a r y s y s t e m s F e - C , F e - N , and A1-Cu have r e c e i v e d the m o s t e x t e n s i v e study of p r e c i p i t a t i o n p h e n o m e n a e i t h e r in the a s - q u e n c h e d state ( n o r m a l aging) o r in the d e f o r m e d state ( s t r a i n aging). In the Al-Cu system several metastable phases exist before the s t a b l e A1Cu2 is f o r m e d . The p r e c i p i t a t i o n s e q u e n c e in this s y s t e m is the following: solid solution - - G. P. zone ~ 0" ~ 0' ~ 0. The p r e c i p i t a t i o n r a t e for aging at t e m p e r a t u r e s higher than 150~ is a c c e l e r a t e d by d e f o r m a t i o n . 12'15'16 This a c c e l e r a t i o n i n c r e a s e s with the a m o u n t of d e f o r m a t i o n . Aging at (or above) this temperature produces initially formed precipitates of the m o r e i n c o h e r e n t t r a n s i t i o n phase 0' or even of the s t a b l e phase 0. However, for aging t e m p e r a t u r e s below 100~ only the G. P. zone is o b s e r v e d in r e a s o n able aging t i m e s and the p r e c i p i t a t i o n r a t e is not a f fected b y d e f o r m a t i o n . 17 Aging in s y s t e m s such as F e - C and F e - N is s i m i l a r to that in the s u b s t i t u t i o n a l alloys of A1 in s o m e r e s p e c t s . However, p r i o r d e f o r m a t i o n in t h e s e i n t e r s t i t i a l s y s t e m s always a c c e l e r a t e s the p r e c i p i t a t i o n rate, at l e a s t in the t e m p e r a t u r e r a n g e 30~ to 200~ 14'18'21 In t h e s e two s y s t e m s , an o r d e r of m a g n i t u d e i n c r e a s e in p r e c i p i t a t i o n r a t e can e a s i l y be achieved. We have shown in a p r e v i o u s study that the p r e c i p i tation of c a r b i d e s in v a n a d i u m is a c o m p l e x m u l t i s t a g e p r o c e s s . 2~ F o u r h a r d e n i n g stages a r e c l e a r l y shown in the aging c u r v e s ; an i n i t i a l i n c r e a s e to a plateau followed b y t h r e e d i s t i n c t a g i n g peaks. Unfortunately, we have been unable to d i f f e r e n t i a t e a m o n g those m e t a s t a b l e c a r b i d e p r e c i p i t a t e s which e x i s t at t h r e e aging peaks b y 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 y , though we a r e able to v e r i f y the e x i s
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