Hydrogen transport through annealed and deformed armco iron
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d i f f u s i v i t y and s o l u b i l i t y of h y d r o g e n in f e r r o u s m a t e r i a l s n e a r 300 K a r e of s u b s t a n t i a l i n t e r e s t with r e s p e c t to h y d r o g e n b r i t t l e n e s s . The c h a r a c t e r i s t i c d i s a p p e a r a n c e of h y d r o g e n b r i t t l e n e s s at l o w e r t e m p e r a t u r e s and high s t r a i n r a t e s I i m p l i e s that h y d r o g e n t r a n s p o r t is an e s s e n t i a l s t e p in the b r i t t l e n e s s m e c h a n i s m . 2'3 However, it is difficult to d e v e l o p t h i s i n t e r p r e t a t i o n f u r t h e r , s i n c e a d e t a i l e d c h a r a c t e r i z a t i o n of h y d r o g e n d i f f u s i v i t y and s o l u b i l i t y in the b r i t t l e n e s s t e m p e r a t u r e r a n g e is lacking. Studies n e a r 300 K often s u g g e s t that h y d r o g e n i s t r a p p e d and h i n d e r e d in i t s diffusive m o t i o n b y m i c r o s t r u c t u r a l h e t e r o g e n e i t i e s . A n o m a l o u s v a l u e s of both d i f f u s i v i t y and s o l u b i l i t y a r e f r e q u e n t l y e n c o u n t e r e d , with the d i f f u s i v i t y l e s s than and the s o l u b i l i t y m o r e than the c o r r e s p o n d i n g v a l u e s o b t a i n e d f r o m e x t r a p o l a t i o n of high t e m p e r a t u r e data. T a b l e 14-19 s u m m a r i z e s the m e a s u r e d d i f f u s i v l t i e s of h y d r o g e n in i r o n . A f o r m i d a b l e s c a t t e r in the 298 K d i f f u s i v i t y is a p p a r ent, w h e t h e r m e a s u r e d d i r e c t l y o r e x t r a p o l a t e d f r o m s l i g h t l y h i g h e r t e m p e r a t u r e s , and it is m o s t l y c a u s e d by trapping. It i s s o m e t i m e s o b s e r v e d that h y d r o g e n d e s o r b s with a l o w e r e f f e c t i v e d i f f u s i v i t y than a n t i c i p a t e d f r o m a b s o r p t i o n m e a s u r e m e n t s . F o r e x a m p l e , in T a b l e I the two l o w e s t d i f f u s i v i t i e s m e a s u r e d at 298 K a r e f r o m e v o l u t i o n e x p e r i m e n t s . O r i a n i , z~ u s i n g an a p p r o a c h d e v e l o p e d e a r l i e r f o r i m p u r i t y diffusion in s e m i c o n d u c t o r s , 21 h a s shown that with the a s s u m p t i o n of l o c a l e q u i l i b r i u m b e t w e e n t r a p p e d and d i f f u s i n g p o p u l a t i o n s , the e f f e c t i v e d i f f u s i v i t y b e c o m e s
Deft =DL
CL C L + C T ( 1 - n)
[1]
A. J. KUMNICK and H. H. JOHNSON are, respectively, Post-Doctoral Associate and Professor, Department of Materials Science and Engineering, Cornell University, Ithaca, N.Y. 14850. Manuscript submitted November 5, 1973, METALLURGICAL TRANSACTIONS
D L = lattice
diffusivity
CL = lattice
concentration
CT = t r a p p e d c o n c e n t r a t i o n
n = f r a c t i o n a l o c c u p a n c y of t r a p s T h i s equation is in a c c o r d 2~ with the o b s e r v e d a s y m m e t r y in e f f e c t i v e d i f f u s i v i t i e s b
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