Precipitation from supersaturated Nb-C solid solutions
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e l e m e n t c a r b o n e x e r t s a s t r o n g influence on the m e c h a n i c a l p r o p e r t i e s of m a n y m e t a l s . One of its m a n i f e s t a t i o n s is the Snoek effect which a r i s e s f r o m the s t r e s s induced o r d e r i n g of c a r b o n in the bcc s y s t e m s . Using this effect, Powers and Doyle found that the Shock peak of c a r b o n in Nb, Ta, and V d e c r e a s e d v e r y r a p i d l y d u r i n g aging c o m p a r e d to the other i n t e r s t i rials oxygen and n i t r o g e n . l - 3 They a t t r i b u t e d this phen o m e n o n to r a p i d p r e c i p i t a t i o n of c a r b i d e s , but did not study the r e a c t i o n in detail. S e v e r a l s t u d i e s were l a t e r r e p o r t e d in which m e t a l l o g r a p h y and h a r d n e s s m e a s u r e m e n t s w e r e used to i n v e s t i g a t e the morphology and d i s t r i b u t i o n of the c a r b i d e s . 4-6 Kinetic studies w e r e not p o s s i b l e s i n c e the alloys w e r e not quenched r a p i d l y enough f r o m the solution t e m p e r a t u r e to p r o vide r e p r o d u c i b l e solid s o l u t i o n s . This p a p e r r e p o r t s a s u c c e s s f u l study of both the k i n e t i c s of c a r b i d e p r e cipitation and an e x a m i n a t i o n of the n a t u r e of the fine scale precipitates. The Shock effect is o r d i n a r i l y an ideal tool to follow the d e c o m p o s i t i o n of i n t e r s t i t i a l solid s o l u t i o n s . The o b s e r v a t i o n t i m e r e q u i r e d is too long for the Nb-C solid solutions, however, b e c a u s e of the r a p i d i t y of aging at the t e m p e r a t u r e of o b s e r v a t i o n . Thus we s e lected c h a n g e s in e l e c t r i c a l r e s i s t i v i t y as a next b e s t a l t e r n a t i v e . These m e a s u r e m e n t s were s u p p l e m e n t e d with 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 to yield m o r e insight into the p r o c e s s . EXPERIMENTAL PROCEDURE The s p e c i m e n s used in this study w e r e foils 0.01 in. thick cold r o l l e d f r o m 0.06 in. w i r e s obtained by s w a g ing z1 in. n i o b i u m rod of 99.92 pct p u r i t y . Twelve inch s t r i p s of this foil were d e g a s s e d at 2300~ in a v a c uum b e t t e r than 10 -6 t o r r for about 2 h to r e m o v e i n t e r s t i t i a l oxygen and n i t r o g e n . The r e s i d u a l oxygen and n i t r o g e n content a f t e r such t r e a t m e n t is e s t i m a t e d to be l e s s than 10 at. ppm. 7 The foils w e r e then c a r b u r i z e d to a n o m i n a l c o m p o s i t i o n of 0.3 at. pct C with a c a l c u l a t e d a m o u n t of m e t h a n e gas of r e s e a r c h grade p u r i t y . B e c a u s e of the e x t r e m e l y low s o l u b i l i t y of c a r R. K. VISWANADHAM,formerly Research Assistant, University of Illinoisat Urbana-Champaign,is now with the Department of Materials Science and Engineering,Cornell University, Ithaca, N.Y. 14850. C.A. WERT is Head, Department of Metallurgy an
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