Thermodynamics and phase relations in refractory metal solid solutions containing carbon, nitrogen, and oxygen
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FROMM
AND
HERMANN
JEHN
The r e f r a c t o r y m e t a l s Nb, T a , Mo, and W d i s s o l v e C, N, and O by f o r m i n g i n t e r s t i t i a l s o l i d s o l u t i o n s . The c o n c e n t r a t i o n of t h e s e c o m p o n e n t s can be i n c r e a s e d o r d e c r e a s e d by a n n e a l ing t r e a t m e n t s , depending on the p a r t i a l p r e s s u r e of g a s e s such a s N2, 02, H20, CH4, o r CO and on the t e m p e r a t u r e of t h e s p e c i m e n . New r e s u l t s ha b i n a r y and t e r n a r y s y s t e m s c o m b i n e d with t h o s e o b t a i n e d in the l a s t few y e a r s now p r o v i d e m o s t of the d a t a n e e d e d f o r the t h e r m o c h e m i c a l a n a l y s i s of the s o l i d solution p h a s e s and for t h e e s t a b l i s h m e n t of p - T - c p h a s e d i a g r a m s . The m e c h a n i s m s of the g a s - m e t a l r e a c t i o n s and the g e n e r a l f e a t u r e of the constitution diagrams are similar for all refractory metals. However, marked differences e x i s t b e t w e e n g r o u p Va m e t a l s n i o b i u m and t a n t a l u m and g r o u p Via m e t a l s m o l y b d e n u m and t u n g s t e n in the a b s o l u t e v a l u e s and t e m p e r a t u r e d e p e n d e n c e of the e q u i l i b r i u m p r e s s u r e of g a s e o u s s p e c i e s and t h e r e f o r e a l s o in the a m o u n t of g a s a b s o r b e d . The d a t a now a v a i l a b l e f o r the e s t i m a t i o n of the final content of i n t e r s t i t i a l s in s o l i d s o l u t i o n a f t e r a n n e a l i n g t r e a t m e n t s a r e c o m p i l e d and d i s c u s s e d .
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ten y e a r s e x t e n d e d i n v e s t i g a t i o n s h a v e been p e r f o r m e d on the i n t e r a c t i o n of oxygen, n i t r o g e n , w a t e r v a p o r , and c a r b o n m o n o x i d e with s o l i d s o l u t i o n s of N, C, and O in r e f r a c t o r y m e t a l s at high t e m p e r a t u r e s . T h e s e r e a c t i o n s a r e of p a r t i c u l a r i n t e r e s t , b e c a u s e t h e y a r e r e s p o n s i b l e f o r t h e r e d u c t i o n of i n t e r stitial contamination through vacuum metallurgical p r o c e s s e s . Gas a b s o r p t i o n i s a l s o r e s p o n s i b l e f o r p r o p e r t y c h a n g e s of r e f r a c t o r y m e t a l s when u s e d a s high t e m p e r a t u r e m a t e r i a l s u n d e r r e d u c e d p r e s s u r e . W h e t h e r the c o n c e n t r a t i o n of one o r m o r e of the i n t e r s t i t i a l a t o m s i s i n c r e a s e d o r d e c r e a s e d d e p e n d s on the t e m p e r a t u r e of the s p e c i m e n , the p a r t i a l p r e s s u r e of r e s i d u a l g a s e s in the v a c u u m c h a m b e r , and the c h e m i cal b e h a v i o r of the r e f r a c t o r y m e t a l . In p r a c t i c e usually various reactions take place simultaneously, and often t h e y a r e not i n d e p e n d e n t of each o t h e r , but c l o s e l y coupled. T h e r e f o r e , a c o m
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