Precipitation of carbon from solid solution in vanadium
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A s t r i k i n g f e a t u r e of s u p e r s a t u r a t e d solid s o l u t i o n s of c a r b o n in V, Nb, and Ta i s the r a p i d d e c o m p o s i t i o n of the solid solutions at r e l a t i v e l y low t e m p e r a t u r e s , 150 ~ to 300~ P r e c i p i t a t i o n of c a r b o n o c c u r s much m o r e r a p i d l y than does c o r r e s p o n d i n g p r e c i p i t a t i o n of n i t r o g e n o r oxygen. N u m e r o u s e v i d e n c e s of this effect e x i s t in m e c h a n i c a l p r o p e r t i e s , but the m o s t s t r i k i n g d i s p l a y of this effect came in the e a r l y s t u d i e s of i n t e r n a l f r i c t i o n in these a l l o y s . 1-4 F r o m t h e s e w o r k s and other l a t e r i n v e s t i g a t i o n s one finds that c a r b o n p r e c i p i t a t e s at l e a s t 100 t i m e s f a s t e r than do n i t r o g e n o r oxygen for the s a m e conditions. The p r e s e n t study was u n d e r t a k e n to e x a m i n e the k i n e t i c s of p r e c i p i t a t i o n of c a r b o n in v a n a d i u m in detail u s i n g i n t e r n a l f r i c t i o n , e l e c t r i c a l r e s i s t i v i t y , and e l e c t r o n m i c r o s c o p y .
I v.8 -
o
~
I I0
2%c
162*C
I Trine
I00
[ I000 rnmules
1
Fig. 1--Decreasein internal friction during aging of a V-C alloy at 162~
EXPERIMENTAL TECHNIQUES AND RESULTS The specimens used for internal friction were wires carburized to a nominal 0.2 at. pct C after vacuum outgassing (at 1450~ in a vacuum of 10-6 torr). To ensure the existence of a solid solution, a quenching technique was developed in which the wires were quenched in liquid metal at 40~ from 1300~ Electron microscopy showed no precipitates observable at a magnification of XI20,000. Internal friction measurements were performed by observing the decay of the carbon Snoek peak at frequencies in the range 0.1 to 5 Hz. The apparatus, an inverted torsional pendulum, has been described earlier.5 By its use, the specimens could be heated rapidly to an appropriate aging temperature (chosen to be that of the Snoek peak of carbon for the particular frequency). Aging could be followed by noting the decay in the height of the Shock peak with time. Heating time to temperature was about I min. An example of the time dependence of disappearance of the Snoek peak during aging of a typical specimen is shown in Fig. I.
D. R. MOSHER is with the Department of Mechanical Engineering, University of Colorado, Boulder, Colo. 80304. D. R. DIERCKS is with the ArgonneNational Laboratory, Argonne, Ill. 60440. C. A. WERT is with the Department of Metallurgyand Mining Engineering,University of Illinois, Urbana, I11.61801. Manuscript submitted May 11, 1972. METALLURGICALTRANSACTIONS
The shape of such c u r v e s has often been shown to fit an equation of the f o r m
1 - w = e x p [--(t/T) n]
[1]
H e r e w i s the f r a c t i o n of c a r b o n p r e c i p i t a t e d at t i m e t, T i s a t i m e c o n s t a n t , and n is a p a r a m e t e r often u s e d to i n f e r the g e o m e t
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