Microstructure and phase relations for Ti-Mo-Al alloys
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ALTHOUGH
the phase diagram of the binary Ti-Mo
s y s t e m is w e l l e s t a b l i s h e d , t h e r e e x i s t s c o n s i d e r a b l e c o n t r o v e r s y with r e g a r d to the b i n a r y Ti-A1 s y s t e m . I n v e s t i g a t i o n s by S a g e l e t a l . , 1 A n d e r k o e t a l . , 2 C r o s s l e y and C a r e w , 3 and by R u s s i a n w o r k e r s , a s r e p o r t e d by Kornilov,4 r e v e a l e d m a r k e d d i s c r e p a n c i e s , p a r t i c u l a r l y with r e s p e c t to the p h a s e r e g i o n above 12 at. p e t A I , w h e r e i n d i c a t i o n s of a s u p e r s t r u c t u r e of the DO~g type w e r e noted. W h i l e this p h a s e h a s now b e e n c o n f i r m e d in r e l a t i o n s h i p to the a phase,S the b o u n d a r i e s in the h i g h - t e m p e r a t u r e r e g i o n have not b e e n e l u c i d a t e d a s y e t . Of p a r t i c u l a r t h e o r e t i c a l and p r a c t i c a l t e c h n o l o g i c a l i m p o r t a n c e i s the q u e s t i o n w h e t h e r t h e r e e x i s t s at high t e m p e r a t u r e an e q u i l i b r i u m b e t w e e n the bee ~ p h a s e and the h e x a g o n a l TisA1 phase. In the t e r n a r y T i - M o - A 1 s y s t e m , K e s s l e r 6 d e t e r m i n e d the ~ / ~ + ~ b o u n d a r y with a l u m i n u m content f r o m 2 to 6 wt p c t and m o l y b d e n u m content up to 6 wt p c t . M a r g o l i n e t a l . v and Btihm and Ltlhberg 8 s t u d i e d the t e r n a r y p h a s e d i a g r a m for l a r g e r c o n c e n t r a t i o n r a n g e s . Only Btihm and Ltlhberg pointed out that an o r d e r e d 13 p h a s e with a CsC1 type of s t r u c t u r e e x i s t s . E x t r e m e b r i t t l e n e s s was a s c r i b e d to t h i s o r d e r e d p h a s e . Studies in s i m i l a r s y s t e m s , such a s T i - N b - A 1 , T i - V - A 1 , T i - M o - G a , s u g g e s t e d that t h i s o r d e r e d p h a s e d e r i v e d f r o m the T i - M o s y s t e m and that it b e c o m e s s t a b i l i z e d onty by addition of a l u m i n u m o r g a l l i u m , a None of t h e s e i n v e s t i g a t i o n s on the t e r n a r y a l l o y s took the Ti3A1 p h a s e into account, the e x i s t e n c e of which is now w e l l e s t a b l i s h e d . 5 Thus the e q u i l i b r i u m d i a g r a m in the t i t a n i u m - r i c h c o r n e r of the T i - M o - A 1 s y s t e m given by Ge D h z h i - M i n and P y l a e v a 9 and shown in F i g . l(a) r e c o g n i z e d only the s i m p l e ot and ~3 e q u i l i b rium. The p r e s e n t study was u n d e r t a k e n to c l a r i f y s o m e of the u n c e r t a i n t i e s which e x i s t in the T i - r i c h c o r n e r of the T i - M o - A 1 s y s t e m . EXPERIMENTAL
of A m e r i c a . T h e oxygen content was l e s s than 0.1 p c t . The i n i t i a l and final r o l l i n g t e m p e r a t u r e s w e r e 1090 ~ and 950~ for the a l l o y containing 16 p e t A1 and 1010 ~ and 790~ for the a l l o y with 7 p c t A1. The d e t a i l s of t e m p e r a t u r e s and
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