The relationship between microstructure and age hardening response in the metastable beta titanium alloy Ti- 11.5 Mo-6 Z
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T H E R E r e c e n t l y has b e e n r e n e w e d i n t e r e s t in the c l a s s of T i alloys known as fi-alloys, or m o r e p r o p e r l y , m e t a s t a b l e f i - a l l o y s . As a r e s u l t , s e v e r a l new alloys have b e e n developed. 1 The m e t a s t a b l e fi-alloys have n u m e r o u s a d v a n t a g e s , a m o n g t h e s e a r e cold f o r m a b i l i t y , p o t e n t i a l for high s t r e n g t h with good f r a c t u r e t o u g h n e s s , and deep h a r d e n i n g c h a r a c t e r i s t i c s . In this context deep h a r d e n i n g m e a n s that the k i n e t i c s of f o r m a t i o n of the s t r e n g t h e n i n g p r e c i p i t a t e a r e s l u g g i s h enough that u n i f o r m p r o p e r t i e s can be obtained in thick (deep) s e c t i o n s . T h u s t h e s e alloys have the p o t e n t i a l for developing good c o m b i n a t i o n s of s t r e n g t h and toughn e s s for l a r g e c o m p o n e n t s . In addition, they a r e r e a d i l y f a b r i c a t e d into sheet and such sheet e x h i b i t s good d u c t i l i t y in s h e e t f o r m i n g a p p l i c a t i o n s . The c o n s t i t u t i o n of the m e t a s t a b l e f i - a l l o y s is r e l a t i v e l y c o m p l e x and depends both on a l l o y c o m p o s i t i o n and on aging t e m p e r a t u r e . The d e c o m p o s i t i o n of the m e t a s t a b l e fi-phase has r e c e n t l y b e e n r e v i e w e d , 2 thus, we will only give a b r i e f s u m m a r y of the d e c o m p o s i t i o n b e h a v i o r . In addition to two types of s - p h a s e which can f o r m d u r i n g fl-phase d e c o m p o s i t i o n , 3 t h e r e a r e two t r a n s i t i o n p h a s e s : the bcc, s o l u t e - l e a n ~ ' - p h a s e 4 and the hexagonal, s o l u t e - l e a n w-phase. ~-7 The f o r m e r of t h e s e t r a n s i t i o n p h a s e s f o r m s in alloys which a r e m o r e heavily f l - s t a b i l i z e d than those which f o r m co-phase. F u r t h e r , p r o c e s s i n g h i s t o r i e s which r e s u l t in a high d e n s i t y of h e t e r o g e n e o u s n u c l e a t i o n s i t e s such as i n c r e a s e d d i s l o c a t i o n d e n s i t i e s or s u b b e u n d a r i e s p r o m o t e d i r e c t s - p h a s e f o r m a t i o n without the f o r m a F. H. FROES is Group Leader, Air Force Materials Laboratories, Dayton, OH 45433. C. F. YOLTON, J. M. CAPENOS, and M. G. H. WELLS are Research Engineers and Manager, respectively, Crucible Materials Research Center, Colt Industries, Pittsburgh, PA 15230. J. C. WILLIAMSis Professor of Metallurgy and Materials Science, Carnegie-MellonUniversity, Pittsburgh, PA 15213. Manuscript submitted August 11, 1977. METALLURGICAL
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tion of t r a n s i t i o n p r e c i p i t a t e s . It is g e n e r a l l y a c k n o w l edged that co-phase f o r m a t i o n leads to s e v e r e d u c t i l i t y l o s s e s a , g a l t h o u g h it has b e e n shown that p r o p e r c o n t r o l of the co-phase v o l u m e f r a c t i o n can
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