Basal and near- basal hydrides in Ti- 5AI- 2.5Sn
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H Y D R O G E N has long b e e n known to influence the m e c h a n i c a l and f r a c t u r e p r o p e r t i e s of t i t a n i u m a l l o y s and p a r t i c u l a r a t t e n t i o n h a s b e e n p a i d to i t s r o l e in s t r e s s c o r r o s i o n c r a c k i n g (scc) and s u s t a i n e d load c r a c k i n g (slc). Meyn 1 r e p o r t e d that, for h y d r o g e n l e v e l s g r e a t e r than 50 ppm, i n c r e a s i n g h y d r o g e n p r o d u c e d an i n c r e a s e in the r a t e of s l c in T i - 6 A 1 - 4 V and o t h e r ~ + ~3 t i t a n i u m a l l o y s . W i l l i a m s ~ noted the s a m e effect and a l s o found that the f r a c t u r e t o u g h n e s s was s u b s t a n t i a l l y i m p r o v e d by r e d u c i n g the h y d r o g e n content. An i n t r i g u i n g f e a t u r e of the w o r k of Meyn ~ and o t h e r r e p o r t s of s i m i l a r r e s e a r c h is the f r e q u e n c y with which c l e a v a g e is found to o c c u r on b a s a l o r n e a r - b a s a l p l a n e s . B a s a l plane c r a c k i n g a p p e a r s to be a s s o c i a t e d with liquid m e r c u r y e m b r t t t l e m e n t 3 w h e r e a s c l e a v a g e on n e a r - b a s a l p l a n e s can o c c u r t h r o u g h s c c in a wide v a r i e t y of e n v i r o n m e n t s including, for e x a m p l e , d i s tilled water, salt water, dry air, moist air, methanol, m e t h a n e , hexane, c a r b o n t e t r a c h l o r i d e and p o t a s s i u m c h l o r i d e s o l u t i o n . 4"~ T h e s e n e a r - b a s a l p l a n e s a p p e a r to be g e n e r a l l y 14 to 16 d e g f r o m (0001)~, m a k i n g {10i7}~ o r {10i8}a the m o s t c o m m o n l y p r o p o s e d p l a n e s , s e e R e f . 4 f o r e x a m p l e . A c l e a v a g e p l a n e 13 to 14 deg f r o m (0001)o~ has a l s o b e e n d e d u c e d by L a z i o u f r o m the m a c r o s c o p i c f r a c t u r e p l a n e of t e x t u r e d T i - 6 A 1 - 4 V t e s t e d in s a l t w a t e r . 7 C l e a v a g e c r a c k i n g of T i - S A I - I M o - I V was r e p o r t e d to o c c u r on y e t a n o t h e r p l a n e , c l o s e to {1010}c~, when t e s t e d in h y d r o g e n gas, 8 although t h i s h a s s i n c e b e e n a t t r i b u t e d to c r a c k i n g along the a l p h a / b e t a i n t e r f a c e s which w e r e a p p r o x i m a t e l y p a r a l l e l to {10i0}~.9 T h e v a r i e t y of c l e a v a g e p l a n e s e n c o u n t e r e d has n a t u r a l l y l e d to s p e c u l a t i o n c o n c e r n i n g the m e c h a n i s m s r e s p o n s i b l e and W a n h i l l ~~ c o n c l u d e d that h y d r i d e p r e c i p i t a t i o n was u n l i k e l y to be involved s i n c e t h i s had not, at that t i m e , b e e n r e p o r t e d to o c c u r on p l a n e s c l o s e to {1017}c~. In one c a s e t h e s e f a i l u r e s w e r e even a t t r i b u t e d to n o r m a l c r e e p p r o c e s s e s , xl In o r d e r to u n d e r s t a n d t h e s e p h e n o m e n a it i s e v i d e n t that d a t a is r e q u i r e d c
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