In situ scanning auger analysis of hydrogen-induced fracture in Ti-6Al-6V-2Sn
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Hydrogen-Induced Fracture in Ti-6AI-6V-2Sn KENNETH A. PETEI~SON, JAMES C. SCHWANEBECK, AND WILLIAM W. GERBERICH F o r the f i r s t t i m e , an i n - s i t u s c a n n i n g Auger a n a l y s i s u n a m b i g u o u s l y identified h y d r o g e n - i n d u c e d c r a c k growth to r e s i d e in the a phase or an i n t e r p h a s e r e g i o n and not at an (~/~ or i n t e r p h a s e / ~ i n t e r f a c e or in the fi phase r e g i o n . This finding was f u r t h e r s u b s t a n t i a t e d by SEM and e l e c t r o n f r a c t o g r a p h y . E s t a b l i s h i n g the c h a r a c t e r of h y d r o g e n - i n d u c e d f r a c t u r e s in a~/,8 alloys is e s s e n t i a l b e c a u s e of the c o m plexity of the p r o c e s s ( e s ) and the n u m b e r of d i v e r s e opinions which exist. In fact, two m a j o r f r a c t u r e m o r phologies have been a s s o c i a t e d with g a s e o u s o r i n t e r n a l hydrogen e m b r i t t l e m e n t along with five p r o p o s e d e m b r i t t l e m e n t m e c h a n i s m s . The d e t a i l s ~-~3 of t h e s e for a//~ alloys as well as s o m e h y d r i d e o b s e r v a t i o n s in all c~-atloys a r e s u m m a r i z e d in T a b l e I and T a b l e II. With r e g a r d s to g a s e o u s hydrogen, ~-4 m o s t a g r e e that t e r r a c e d cleavage is due to h y d r i d e f o r m i n g locally which then p r o c e e d s to f r a c t u r e by a film r u p t u r e m e c h a n i s m . F o r i n t e r n a l hydrogen, t h e r e is a g r e a t e r d i v e r g e n c e of opinion on both the f r a c t u r e i n i t i a t i o n site as well as the m e c h a n i s m w h e r e cohesive s t r e n g t h , c r e e p c r a c k i n g and s t r a i n - i n d u c e d h y d r i d e s have b e e n proposed. 5~ Morphologically, 9-x3 the h y d r i d e i n v e s t i gation by Paton and Spurling~3 cleared up some recent misconceptions since it was previously considered that {0001} and habits near {0001} were disallowed. It was demonstrated that {0001} and {1017}, which is 15 deg from the former, are the most commonhydride habits when the hydrogen concentration is low while {I010} habits were most common at lower aluminum concentrations and/or high hydrogen concentrations. Thus, it is quite possible for strain-induced hydride fracture to be the presiding mechanism either at ~/~3 interfaces or well within the (~ phase. One further complication arises because of an interfacial phase region detected by Rhodes and Williams.~2 Although this region may be morphologically synonymous with "hydride precipitation" as identified by Pittinato and Hanna,'~ Rhodes and Williams report this to be a separate non-Burgers hcp or a fcc phase. Their rationale was that heating stage experiments at 500~ did not dissolve the interphase region, whereas the hydride solvus ranges from 200 to 400~ One could argue that the boundary composition might be very different than what is normally anticipated so that the hydride may be more stable. However, the fact that this interphase region was identified
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