Precipitation kinetics in Nb(Cb)-N alloys
- PDF / 1,552,445 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 81 Downloads / 217 Views
Niobium w i r e s were doped with 0.2 wt pct N and then d i r e c t l y aged at t e m p e r a t u r e s f r o m 300 ~ to 650~ for v a r i o u s lengths of t i m e . M e t a l l o g r a p h i c e x a m i n a t i o n r e v e a l e d a product phase at s u b g r a i n b o u n d a r i e s which exhibited l i n e a r growth for each r e a c t i o n t e m p e r a t u r e . An a c t i v a tion e n e r g y of 5.6 kcal per mole for the p r o c e s s was obtained. Although the r e a c t i o n was thought to be d i s c o n t i n u o u s p r e c i p i t a t i o n , additional i n f o r m a t i o n from e l e c t r o n m i c r o s c o p y , i n t e r n a l f r i c t i o n , and h a r d n e s s m e a s u r e m e n t s indicated that both p r e c i p i t a t i o n and a c o a r s e n i n g p r o c e s s were o c c u r r i n g s i m u l t a n e o u s l y
IN r e c e n t
y e a r s t h e r e has b e e n c o n s i d e r a b l e i n t e r e s t in r e f r a c t o r y m e t a l a l l o y s b e c a u s e of t h e i r high t e m p e r a t u r e a p p l i c a t i o n s . Many of the p h y s i c a l and t h e r m o d y n a m i c p r o p e r t i e s of the bcc m e t a l s such as n i o b i u m and t a n t a l u m containing i n t e r s t i t i a l s o l u t e s of c a r b o n , oxygen, or n i t r o g e n a r e p r e s e n t e d in the l i t e r a t u r e . 1-4 Study of the k i n e t i c s of p r e c i p i t a t i o n in these s y s t e m s has just begun, s-7 The p r e s e n t work cons i d e r s the Nb-N s y s t e m and e x a m i n e s p r e c i p i t a t e m o r phology and growth r a t e s in o r d e r to d e r i v e a growth r a t e law and an a c t i v a t i o n e n e r g y . F r o m these facts c o n c l u s i o n s can be made about the p r e c i p i t a t i o n m e c h anism. In work done by de L a m o t t e , Huang, and A l t s t e t t e r s i n the Nb-N s y s t e m , a p r e c i p i t a t e was o b s e r v e d to grow inward from the s u b b o u n d a r i e s in addition to continuous p r e c i p i t a t i o n within the s u b g r a i n s . This s u b b o u n d a r y p r e c i p i t a t i o n effect, thought to be d i s c o n t i n uous p r e c i p i t a t i o n , was not found in all s p e c i m e n s , but only under specific (but somewhat unknown) conditions of n i t r o g e n c o n c e n t r a t i o n and cooling r a t e . D i s c o n t i n uous p r e c i p i t a t i o n , a l s o known as c e l l u l a r p r e c i p i t a t i o n , is the nucleation and coupled growth of n i t r i d e and depleted m a t r i x into s u p e r s a t u r a t e d m a t r i x . The i n t e r face s e p a r a t i n g the p a r e n t and product would be expected to move at a c o n s t a n t rate in a s i m p l e d i s c o n t i n u o u s p r e c i p i t a t i o n r e a c t i o n . 8 In c o n t r a s t to t h i s , continuous p r e c i p i t a t i o n , studied in Nb-N a l l o y s by Gebhardt e t a l . , 7 o c c u r s s i m u l t a n e o u s l y throughout the e n t i r e s p e c i m e n with a p a r a b o l i c r a t e of i n t e r f a c e motion.
Data Loading...
