A high-temperature thermodynamic investigation of the nb-mo system

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W. L. WORRELL

The t h e r m o d y n a m i c p r o p e r t i e s of s o l i d N b ( C b ) - M o a l l o y s have b e e n d e t e r m i n e d b e t w e e n 1050 and 1300 K using s o l i d s t a t e g a l v a n i c c e l l s with a t h o r i a - b a s e d e l e c t r o l y t e . The a c t i v i t i e s of niobium and m o l y b d e n u m exhibit n e g a t i v e d e v i a t i o n s f r o m R a o u l t ' s law. T h e e x c e s s e n t r o p i e s of m i x i n g a r e v e r y s m a l l and n e g a t i v e , and the e x c e s s i n t e g r a l f r e e e n e r g i e s of m i x i n g a r e a l m o s t equal to the i n t e g r a l e n t h a l p i e s of m i x i n g . The n e g a t i v e e n t r o p i e s of m i x i n g a r e a t t r i b u t e d to a n e g a t i v e v i b r a t i o n a l c o n t r i b u t i o n a n d / o r s h o r t - r a n g e o r d e r i n g .

A knowledge of the t h e r m o d y n a m i c p r o p e r t i e s of r e f r a c t o r y - m e t a l a l l o y s i s e s s e n t i a l for p r e d i c t i n g t h e i r s t a b i l i t y and b e h a v i o r in d i f f e r e n t e n v i r o n m e n t s . In o u r two p r e v i o u s p a p e r s , d e t e r m i n a t i o n s of the hight e m p e r a t u r e t h e r m o d y n a m i c p r o p e r t i e s of T a - M o 1 and T a - W 2 a l l o y s w e r e r e p o r t e d . In t h i s p a p e r , the 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 N b - M o a l l o y s have b e e n measured using a similar electromotive-force technique. T h e r e has b e e n no p r e v i o u s t h e r m o d y n a m i c i n v e s t i g a t i o n of this s y s t e m . N i o b i u m and m o l y b d e n u m f o r m a continuous s e r i e s of s o l i d s o l u t i o n s with the b c c s t r u c t u r e . 3-s No i n t e r m e d i a t e p h a s e s o r s u p e r l a t t i c e s have b e e n r e p o r t e d . 4'5 T h e r o o m t e m p e r a t u r e l a t t i c e p a r a m e t e r s of the NbMo a l l o y s exhibit a s m a l l n e g a t i v e d e v i a t i o n f r o m V e g a r d ' s law. 4-6 In o r d e r to d e t e r m i n e the 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 N b - M o s y s t e m , g a l v a n i c c e l l s of the f o l l o w ing type w e r e u s e d : Nb, NbO/ThO2 (6 wt p c t Y203)/Nb-Mo ( a l l o y ) , S b O [A] e x c e p t for the 0.10 a t o m f r a c t i o n Nb a l l o y . If the c i r cuit of c e l l [A] w e r e c l o s e d , the o v e r a l l c e l l r e a c t i o n would b e : S b ~ Nb (in alloy)

[1]

F o r the 0.10 a t o m f r a c t i o n Nb a l l o y e l e c t r o d e , NbO2 i s the e q u i l i b r i u m oxic'~e, and the o v e r a l l c e l l r e a c t i o n i s : 2Nb + NbO2 ~ Nb (in alloy) + 2NbO

[2]

R e a c t i o n [2] m u s t be added to the following r e a c t i o n to y i e l d r e a c t i o n [1]: 2NbO = NbO2 + Nb

[3]

The s t a n d a r d f r e e e n e r g y change f o r r e a c t i o n [3] i s : 7 AG O (cal) = - 1 4 , 2 5 0 + 4.15 T

[4]

S. C. SINGHAL is Senior Scientist, Metallurgy and Metals Processing Department, Research Laboratories, Westingh