Relation between hydrogen embrittlement and the formation of hydride in the group V transition metals
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T H A T h y d r o g e n c a u s e s e m b r i t t l e m e n t of V, Nb, a n d T a is an i n d i s p u t a b l e fact. 1-16 The i m p o r t a n t , but unanswered, q u e s t i o n i s : By what m e c h a n i s m d o e s h y d r o gen p r o d u c e the p r e c i p i t o u s and c a t a s t r o p h i c l o s s of d u c t i l i t y in t h e s e m e t a l s .9 C o n s i d e r a b l e a t t e n t i o n h a s been given to the effects of h y d r o g e n in n i o b i u m and s o m e effort h a s been d e v o t e d to t a n t a l u m . The c u r r e n t k n o w l e d g e , with the e x ception of one o r two p a p e r s to b e d i s c u s s e d l a t e r , about h y d r o g e n e m b r i t t l e m e n t of n i o b i u m and t a n t a l u m h a s been r e v i e w e d by C h a n d l e r and W a l t e r 13 and w o n ' t be resummarized here. In e a r l i e r w o r k on v a n a d i u m it w a s d e m o n s t r a t e d that: a) a s l i t t l e a s 6 ppmw* (~0.03 at. pct) of h y d r o g e n *ppmw = parts per million by weight.
i s c a p a b l e of c a u s i n g c a t a s t r o p i c l o s s of d u c t i l i t y , 5 b) d u c t i l i t y i s not s e r i o u s l y i m p a i r e d at 77 K when the h y d r o g e n content i s below about 100 ppmw (~0.5 at. pct), c) a s e c o n d p h a s e , h y d r i d e , f o r m s by a s h e a r p r o c e s s n e a r the d u c t i l e to b r i t t l e t r a n s i t i o n t e m p e r a t u r e ( D - B T T ) , d) w h i l e the y i e l d s t r e s s is r a i s e d s o m e what in t h e b r i t t l e r a n g e it i s not sufficient to explain the b r i t t l e n e s s . 6 The a b o v e and s o m e a d d i t i o n a l o b s e r v a t i o n s a r e i l l u s t r a t e d s c h e m a t i c a l l y in F i g . 1. S e v e r a l a s p e c t s of the d e f o r m a t i o n b e h a v i o r of h y d r o gen c h a r g e d v a n a d i u m shown in F i g . 1 d e s e r v e c o m ment. At point A the d u c t i l i t y of h y d r o g e n - f r e e and h y d r o g e n c h a r g e d v a n a d i u m a r e equal (R.T. in t h i s p a r t i c u l a r e x a m p l e ) . Between p o i n t s A and B the d u c t i l i t y of the h y d r o g e n c h a r g e d v a n a d i u m d e c r e a s e s s m o o t h l y while the d u c t i l i t y of the h y d r o g e n - f r e e m a t e r i a l r e m a i n s c o n s t a n t . In t h i s r e g i o n the f r a c t u r e s u r f a c e s r e v e a l a c o m b i n a t i o n of c l e a v a g e f a c e t s and dull f i b r o u s a r e a s with the amount of c l e a v a g e i n c r e a s i n g t o w a r d B. At point B the d u c t i l i t y d r o p s p r e c i p i t o u s l y v i r t u a l l y to z e r o and the f r a c t u r e is e s s e n t i a l l y a l l c l e a v a g e . At t e m p e r a t u r e s s o m e w h a t below point B the s e c o n d p h a s e h y d r i d e f o r m s . The o n s e t of h y d r i d e f o r m a t i o n was d e t e c t e d by o p t i c a l m i c r o s c o p y 1~ on each of the t e n s i l e t e s t s a m p l e s giving a r a n g e of t e m p e r a t u r e s . Below the h y
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