The microstructural influence of a 2 wt pct Mo addition to a Ti-1.5 wt pct Ni alloy
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Communications The Microstructural Influence of a 2 Wt Pct Mo Addition to a Ti-1.5 Wt Pct Ni Alloy A~AC.F~R A d d i t i o n s of 1 to 2 wt pet n i c k e l g r e a t l y enhance t i t a n i u m ' s c r e v i c e c o r r o s i o n r e s i s t a n c e . 1-4 T h i s a l l o y ing was developed b e c a u s e e a r l i e r it had been found in high t e m p e r a t u r e t e s t s , that t i t a n i u m is s u s c e p t i b l e to c o r r o s i o n in the r e s t r i c t e d a r e a s and low pH s o l u tions p r e s e n t in a c r e v i c e 5,6 While the addition of n i c k e l s e e m e d to e l i m i n a t e the c r e v i c e c o r r o s i o n p r o b l e m of t i t a n i u m , the m a t e r i a l was difficult to f a b r i c a t e , and t e a r i n g and edge c r a c k i n g o c c u r r e d . L a r g e Ti2Ni p r e c i p i t a t e s a p p e a r e d in a r a n d o m or s e g r e g a t e d a r r a n g e m e n t , d e p e n d i n g on the t h e r m a l h i s t o r y of the m a t e r i a l , 7 and t h e s e could s e r v e as local c o r r o s i o n s i t e s and not offer the p r o t e c t i o n of a widely and finely d i s p e r s e d Ti2Ni phase. Recently, it was r e p o r t e d that the T i - 2 M o - l N i alloy is a good candidate for a c r e v i c e c o r r o s i o n r e s i s t a n t m a t e r i a l . 8 At the s a m e t i m e , the high c o r r o s i o n r e s i s t a n c e and i m p r o v e d m e c h a n i c a l p r o p e r t i e s of a n alloy with the c o m p o s i t i o n , T i - 0 . 3 M o - 0 . 8 N i was r e p o r t e d and this l a t t e r m a t e r i a l is now a p r o d u c t i o n i t e m . 9 T i t a n i u m alloy m i c r o s t r u c t u r e s and phase t r a n s f o r m a t i o n s have been studied by n u m e r o u s i n v e s t i g a t o r s and much of this work has been s u m m a r i z e d in review articles.l~ Until now, no work has shown the m i c r o s t r u c t u r a l changes which o c c u r in the T i - l . 5 wt pct Ni a l l o y by adding a s m a l l a m o u n t of a beta s t a b i l i z e r s u c h as m o l y b d e n u m . R e c e n t l y it was r e p o r t e d that the addition of a s m a l l a m o u n t of m o l y b d e n u m s i g n i f i c a n t l y changes the alloy m i c r o s t r u c t u r e , 12 and these m i c r o s t r u c t u r e s will be shown and d i s c u s s e d in this p a p e r . The t i t a n i u m - r i c h end of the T i - N i b i n a r y phase d i a g r a m zs is s i m i l a r to that of the T i - C u s y s t e m except that a - T i has a s m a l l s o l u b i l i t y (2.1 wt pct) for Cu w h e r e a s the s o l u b i l i t y of Ni i n ot-Ti is n e g l i g i b l e . Both the T i - N i and the T i - C u s y s t e m s in the c o m p o s i tion r a n g e s b e i n g c o n s i d e r e d h e r e , undergo a/3 d e c o m p o s i t i o n r e a c t i o n where/3 ~ ot + i n t e r m e t a l l i c c o m pound (Ti2Ni o r Ti2Cu). T h e s e m i c r o s t r u c t u r a l t r a n s f o r m a t i o n s for T i - C u have b e e n s t u d i e d p r e v i o u s l y and d i s c u s s e d in d e t a
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