Surface reaction controlled oxygen absorption in a Ta-8 W-2 Hf alloy: kinetics and concentration gradients
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O X Y G E N a b s o r p t i o n r a t e s in b c c m e t a l s such as Ta, Nb and s o m e of t h e i r alloys at high t e m p e r a t u r e and low p r e s s u r e w e r e r e v i e w e d by Inouye. t He noted that the a v e r a g e oxygen c o n c e n t r a t i o n in s h e e t s p e c i m e n s was d i r e c t l y p r o p o r t i o n a l to t i m e and oxygen p r e s s u r e and i n v e r s e l y p r o p o r t i o n a l to s p e c i m e n t h i c k n e s s ; no s u r f a c e r e a c t i o n p r o d u c t was o b s e r v e d . These o b s e r v a t i o n s showed that the oxygen a b s o r p t i o n r a t e was i n dependent of the oxygen m o b i l i t y in the m a t r i x . The a b s o r p t i o n r a t e was p r e s u m e d to be s u r f a c e - r e a c t i o n l i m i t e d . No i n f o r m a t i o n was given c o n c e r n i n g the oxygen c o n c e n t r a t i o n p r o f i l e within the s p e c i m e n d u r i n g a b s o r p t i o n , which m u s t be d e p e n d e n t upon the oxygen diffusion coefficient within the m a t r i x . The oxygen a b s o r p t i o n b e h a v i o r of m e t a l s at high t e m p e r a t u r e i n h i g h e r p r e s s u r e oxygen (~ 1 aim) has b e e n t r e a t e d in d e t a i l both t h e o r e t i c a l l y and e x p e r i m e n tally, and was r e v i e w e d r e c e n t l y by Swisher. e In this c a s e the r a t e of a b s o r p t i o n is not s u r f a c e - r e a c t i o n l i m i t e d ; r a t h e r , it is l i m i t e d by the diffusion r a t e of oxygen in one of the solid p h a s e s p r e s e n t . The t h e o r e t ical t r e a t m e n t a s s u m e s that the oxygen c o n c e n t r a t i o n in the m a t r i x at the g a s - m e t a l i n t e r f a c e , or e q u i v a l e n t l y the o x i d e - m e t a l i n t e r f a c e if a c o n t i n u o u s oxide l a y e r is f o r m e d , r i s e s to its m a x i m u m v a l u e in v e r y s h o r t t i m e s a f t e r oxidation s t a r t s and is t i m e i n v a r i a n t t h e r e a f t e r . In this p a p e r we d e s c r i b e the r e l a t i o n s h i p s b e t w e e n oxygen c o n c e n t r a t i o n g r a d i e n t and a b s o r p t i o n r a t e c a l culated f r o m two d i f f e r e n t a n a l y t i c a l m o d e l s a p p r o x i m a t i n g the e x p e r i m e n t a l conditions noted in I n o y u e ' s r e v i e w , and c o m p a r e the r e s u l t s with s o m e e x p e r i m e n tal m e a s u r e m e n t s of oxygen a b s o r p t i o n r a t e and o x y gen c o n c e n t r a t i o n p r o f i l e for a T a - 8 W-2 Hf alloy at high t e m p e r a t u r e and low oxygen p r e s s u r e , w h e r e the a b s o r p t i o n r a t e is s u r f a c e - r e a c t i o n r a t e l i m i t e d . We a s s u m e d that a s u r f a c e r e a c t i o n r a t e l i m i t e d a b s o r p R. W. CARPENTER and C. T. LIU are Research Staff Members, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830. Manuscript submitted July 12, 1974. METALLURGICALTRANSACTIONS A
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