Kinetics of diffusion in the Nb-Al system
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OGURTANI
Diffusion k i n e t i c s w e r e studied by v a r y i n g the t i m e and t e m p e r a t u r e (800 ~ to 1300~ of dipping s o l i d n i o b i u m into m o l t e n a l u m i n u m . It was found by X - r a y d i f f r a c t i o n a n a l y s i s that only the AlaNb phase f o r m s at the s u r f a c e of the n i o b i u m s p e c i m e n and that the t h i c k n e s s of this l a y e r for a given dip t e m p e r a t u r e v a r i e s p a r a b o l i c a l l y with t i m e . The a c t i v a t i o n e n e r g y and the p r e exponential f a c t o r of the diffusion p a r a m e t e r , (5/9) were found to be 36.5 § 0.45 k c a l p e r mole and 2.0 :F 0.34 sq c m p e r s e c , r e s p e c t i v e l y .
O N E of the m o s t e x c i t i n g t e c h n i c a l d e v e l o p m e n t s of the past s e v e r a l y e a r s has b e e n the growth in i m p o r t a n c e of the u n c o m m o n m e t a l s . This i n t e r e s t has extended g e n e r a l l y a l l a c r o s s the P e r i o d i c T a b l e f r o m l i t h i u m and b e r y l l i u m to t h o r i u m and u r a n i u m . Niob i u m has s h a r e d in this enhanced recognit.ion only r e c e n t l y . B e c a u s e of its s c a r c i t y and high c o s t , and the l i m i t e d i n f o r m a t i o n on its p o t e n t i a l a s a s t r u c t u r a l m e t a l , n i o b i u m had b e e n l a r g e l y o v e r l o o k e d except a s f e r r o a l l o y . But now, this s i t u a t i o n is changing. The fact that the oxide of n i o b i u m is r e l a t i v e l y s t a b l e (melting point of 1460~ m a k e s it one of the most oxidation-resistant refractory metals. Neverthel e s s , it oxidizes at r a t e s which a r e f a r beyond a c c e p t a b l e v a l u e s so that s o m e kind of s u r f a c e p r o t e c t i o n for n i o b i u m is a n e c e s s i t y . 1-s Diffusion of a l u m i n u m into the s u r f a c e of n i o b i u m s e e m s to be one of the m o s t p r o m i s i n g m e t h o d s to i n c r e a s e its oxidiaton r e s i s t a n c e . A l u m i n i z e c o a t i n g s (calorizing) a r e known to i n c r e a s e the oxidat4on r e s i s t a n c e of F e , Cu, Co, and N i - b a s e a l l o y s . The a p p l i c a b i l i t y of t h e s e c o a t i n g s is dependent on t i m e and t e m p e r a t u r e of e x p o s u r e of the diffusion m e t a l and the s u b s e q u e n t diffusion p o s t treatment. In o r d e r to d e t e r m i n e the effects of t h e s e v a r i a b l e s on the oxidation r e s i s t a n c e of n i o b i u m and n i o b i u m a l l o y s , a l u m i n u m - d i p p e d coating (Aldico) s t u d i e s were p e r f o r m e d by C a r l s o n . 4 He found a p a r a b o l i c growth r e l a t i o n s h i p for the A13Nb l a y e r t h i c k n e s s and d e t e r m i n e d the rate c o n s t a n t to be 3.02 • 10 -6 sq c m p e r s e c , f o r F a n s t e e l 82 (33 pct T a , 0.75 pct Z r and b a l . Nb) at 980~ He a s s u m e d that this c o n s t a n t equals the diffusivity D (cf. L u s t m a n and Mehl S) and that Do is 0.01
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