High-temperature strength and ductility increases of Ti-Mo-Al alloys by step aging
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T H E e x i s t e n c e of a h i g h - t e m p e r a t u r e , t w o - p h a s e field, fi + TisAl, in T i - 7 at. p c t Mo-A1 a l l o y s and i t s p h a s e r e l a t i o n at l o w e r t e m p e r a t u r e to a t h r e e - p h a s e field,/~ + a + TizA1, I-3 s u g g e s t that the m e c h a n i c a l and h e a t - r e s i s t a n t p r o p e r t i e s of t h e s e a l l o y s could be o p t i m i z e d by d e s i g n i n g an a p p r o p r i a t e m i c r o s t r u c t u r e through h e a t t r e a t m e n t . The i m p o r t a n c e of h o m o g e n e o u s d i s p e r s i o n and p a r t i c l e s i z e of the Ti3A1 p r e c i p i t a t e s in c o n t r o l l i n g the s l i p m o d e and t h e r e b y c o n f e r r i n g d u c t i l i t y to the a l l o y s h a s a l r e a d y been d e m o n s t r a t e d . 2 T h e s e f e a t u r e s of the m i c r o s t r u c t u r e will r e p r e s e n t , t h e r e f o r e , n e c e s s a r y ing r e d i e n t s in the d e s i g n of the a l l o y s . Studying T i - 3 5 wt p c t Nb a l l o y s , M e n d i r a t t a , L f i t j e r i n g and W e i s s m a n n 4 have shown that r e m a r k a b l e d u c t i l i t y and s t r e n g t h i n c r e a s e s could be o b t a i n e d b y a p p l i c a t i o n of a s t e p - a g i n g p r o g r m n which was so designed that a microstructure was formed, consisting of p r e c i p i t a t e d p a r t i c l e s of d i f f e r e n t p h a s e s , which upon d e f o r m a t i o n of the a l l o y s a c t i v a t e d d i f f e r e n t m e c h a n i s m s of d i s l o c a t i o n - p a r t i c l e i n t e r a c t i o n . Thus d u c t i l i t y i n c r e a s e s w e r e a c h i e v e d by inducing the d i s l o c a t i o n s to b y p a s s the l a r g e a p a r t i c l e s , p r e c i p i t a t e d at e l e v a t e d t e m p e r a t u r e s , while i n c r e a s e s in y i e l d s t r e n g t h w e r e o b t a i n e d b y the cutting of s m a l l e r p a r t i c l e s p r e c i p i t a t e d at l o w e r t e m p e r a t u r e s . In view of the m a n y p h a s e f i e l d s t h a t e x i s t in the T i - M o - A 1 p h a s e d i a g r a m , 1,3 and p a r t i c u l a r l y in view of the fact that the ~ p h a s e d e c o m p o s e s at l o w e r t e m p e r a t u r e s into Ti3A1 + ~ p a r t i c l e s , m u l t i p l e - s t e p aging programs appeared quite feasible. These may b e so d e s i g n e d that not only s t r e n g t h and d u c t i l i t y i n c r e a s e s can be c o n f e r r e d upon the a l l o y s but a l s o i m p r o v e m e n t s in h i g h - t e m p e r a t u r e s t a b i l i t y . The w o r k d e s c r i b e d in t h i s p a p e r a i m s to a c c o m p l i s h this task. EXPERIMENTAL PROCEDURE T e n s i l e s p e c i m e n s of 3 . 3 - m m d i a m and 2 5 . 4 - m m gage length w e r e p r e p a r e d f r o m 1 . 2 7 - c m r o d s of T i 7 at. p c t M o - A l a l l o y s s u p p l i e d b y the RMI T i t a n i u m M. HIDA, formerly Assistant Research Professor, Materials Research Laboratory, Rutgers University, is now
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