Gaseous hydrogen-induced cracking of Ti-5Al-2.5Sn
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R . ECENT i n v e s t i g a t i o n s have d e m o n s t r a t e d the ext r e m e s u s c e p t i b i l i t y of a - ~ t i t a n i u m a l l o y s to e m b r i t t l e m e n t in g a s e o u s h y d r o g e n e n v i r o n m e n t s . T h i s e m b r i t t l e m e n t r e s u l t s f r o m h y d r o g e n - i n d u c e d c r a c k i n g of the m a t e r i a l at s t r e s s (or s t r e s s - i n t e n s i t y ) l e v e l s c o n s i d e r a b l y below those r e q u i r e d for f a i l u r e in a i r . The d e g r e e of e m b r i t t l e m e n t m e a s u r e d in m o s t e x p e r i m e n t s is r e l a t e d to the r a t e of h y d r o g e n - i n d u c e d c r a c k i n g which, in t u r n , is a function of v a r i o u s e n v i r o n m e n t , s t r e s s , and m a t e r i a l v a r i a b l e s . However, the exact r e l a t i o n s h i p s b e t w e e n t h e s e v a r i a b l e s and the c r a c k growth r a t e have not yet b e e n s u f f i c i e n t l y well defined to allow i d e n t i f i c a t i o n of the c r a c k i n g m e c h a n i s m s . In a r e c e n t p a p e r , Nelson e t al. 1 d e m o n s t r a t e d that the r a t e of h y d r o g e n - i n d u c e d c r a c k growth was v e r y dependent on the m i c r o s t r u c t u r e of the t i t a n i u m alloy. At a h y d r o g e n p r e s s u r e of about 1 a r m (760 t o r r ) a n d at r o o m t e m p e r a t u r e , c r a c k i n g p r o c e e d e d much m o r e r a p i d l y in those m i c r o s t r u c t u r e s c o n t a i n i n g a continuous /3-phase m a t r i x than in those c o n t a i n i n g an a - p h a s e m a t r i x . In a s e p a r a t e study, Nelson 2 a l s o d e m o n s t r a t e d that, at the s a m e t e m p e r a t u r e but at a m u c h lower hyd r o g e n p r e s s u r e , the c r a c k - g r o w t h r a t e was m o r e r a p i d for those m i c r o s t r u c t u r e s having a c o n t i n u o u s a - p h a s e m a t r i x . This r e v e r s a l in b e h a v i o r was due to the fact that the c r a c k - g r o w t h r a t e was p r e s s u r e - d e pendent for the ~ - m a t r i x m a t e r i a l and e s s e n t i a l l y p r e s s u r e - i n d e p e n d e n t for the a - m a t r i x m a t e r i a l . The dec r e a s e in c r a c k - g r o w t h r a t e for the ~ - p h a s e t i t a n i u m with d e c r e a s i n g p r e s s u r e was a c c o m p a n i e d by a change in f r a c t u r e mode f r o m p r e d o m i n a n t l y i n t e r g r a n u l a r at the h i g h e r p r e s s u r e s to p r e d o m i n a n t l y t r a n s g r a n u l a r ( a c r o s s a g r a i n s ) at the lower p r e s s u r e s . The r e s u l t s of t h e s e s t u d i e s w e r e i n t e r p r e t e d in t e r m s of m e c h a n i s m s involving i n t e r g r a n u l a r c r a c k i n g due to h y d r o gen t r a n s p o r t within the ~ - t i t a n i u m l a t t i c e and t r a n s g r a n u l a r c r a c k i n g c a u s e d by an i n t e r a c t i o n involving DELL P. WILLIAMSand HOWARDG. NELSON are Research Scientists, NASA-AmesResearch Cent
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