High temperature thermodynamic properties of solid ta-w alloys
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AND
W. L. WORRELL
The p a r t i a l m o l a r and i n t e g r a l m o l a r 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 T a - W a l l o y s have been d e t e r m i n e d between 1050 and 1300 K using 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 in t h i s s y s t e m exhibit n e g a t i v e d e v i a t i o n s f r o m i d e a l i t y , but the m a g n i t u d e of the n e g a t i v e d e v i a t i o n is l e s s than that o b s e r v e d p r e v i o u s l y for the T a - M o a l l o y s . The n e g a t i v e e x c e s s f r e e e n e r g i e s of m i x i n g a r e due m a i n l y to the e n t h a l p i e s of m i x i n g . The o b s e r v e d s m a l l n e g a t i v e e x c e s s e n t r o p i e s of m i x i n g can be a t t r i b u t e d to v i b r a t i o n a l c o n t r i b u t i o n s a n d / o r s h o r t r a n g e o r d e r i n g in t h e s e a l l o y s .
ALLOYS
of Nb, T a , Mo, and W have high m e l t i n g p o i n t s and e x c e l l e n t s t r e n g t h at high t e m p e r a t u r e s . T h e s e a l l o y s a r e useful in a n u m b e r of high t e m p e r a t u r e s p a c e and n u c l e a r a p p l i c a t i o n s . Thus a k n o w l edge of the t h e r m o d y n a m i c s t a b i l i t y of t h e s e r e f r a c t o r y m e t a l a l l o y s is i m p o r t a n t both for t e c h n i c a l a p p l i c a t i o n s and t h e o r e t i c a l i n t e r e s t . The p r e s e n t i n v e s t i g a t i o n of the T a - W s y s t e m w a s u n d e r t a k e n a s p a r t of a g e n e r a l study 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 . E x p e r i m e n t a l r e s u l t s for 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 T a - M o a l l o y s have been r e ported previously? T a n t a l u m and t u n g s t e n f o r m s i n g l e - p h a s e bcc s o l i d s o l u t i o n s o v e r the e n t i r e c o m p o s i t i o n a l r a n g e . 2 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 in t h i s b i n a r y s y s t e m have been r e p o r t e d . 3-s G a l v a n i c - c e l l t e c h n i q u e s using s o l i d - o x i d e e l e c t r o l y t e s w e r e i n t r o d u c e d by Kiukk01a and W a g n e r 6'7 in 1957 and have been u s e d 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 m a n y a l l o y s y s t e m s . B-l~ In m a n y of the p r e v i o u s g a l v a n i c - c e l l s t u d i e s , e i t h e r ZrO2(CaO) o r ZrO2(Y203) w a s the e l e c t r o l y t e . H o w e v e r , a z i r c o n i a - b a s e d e l e c t r o l y t e cannot be u s e d to obtain r e l i a b l e data in c e l l s with t a n t a l u m a l l o y s b e c a u s e of the e x t r e m e l y low oxygen a c t i v i t i e s . In such c e l l s n - t y p e e l e c t r o n i c conductivity and s h o r t - c i r c u t i n g b e c o m e s i g n f f i c a n t J ~''2 To o v e r c o m e t h i s difficulty, W o r r e l l has u s e d s u c c e s s f u l l y
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