The aging behavior of orthorhombic marten site in Ti-6-2-4-6
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S I M I L A R to m o s t other t i t a n i u m a l l o y s , a r a n g e of t e m p e r a t u r e s e x i s t s for the T i - 6 A 1 - 2 S n - 4 Z r - 6 M o alloy ( 6 - 2 - 4 - 6 i n wt. pct) where /3 t r a n s f o r m s to m a r t e n s i t e on quenching. Unlike o t h e r s y s t e m s it has b e e n r e p o r t e d that the m a r t e n s i t e has an o r t h o r h o m b i c s t r u c t u r e ~ r a t h e r than the u s u a l hexagonal v a r i e t y . Most aging s t u d i e s have b e e n conducted on the hexagonal f o r m of m a r t e n s i t e w h e r e it has been found that b e t a p r e c i p i t a t e s h e t e r o g e n e o u s l y within the m a r t e n s i t e plates.2 W i l l i a m s and H i c k m a n ~ have d i s c u s s e d aging m e c h a n i s m s i n o r t h o r h o m b i c m a r t e n site ( a " ) . They r e p o r t e d that the f i r s t stage c o n s i s t s of the homogeneous p r e c i p i t a t i o n of alpha n e e d l e s i n the a". T h e s e n e e d l e s w e r e o b s e r v e d to c o - e x i s t with h e t e r o g e n e o u s l y p r e c i p i t a t e d p a r t i c l e s of b e t a for aging t i m e s of 150 h at 873 K. B y w a t e r and C h r i s t i a n s have e x a m i n e d the aging p r o c e s s of o r t h o r h o m b i c m a r t e n s i t e f o r m e d i n the T i - T a s y s t e m . They o b s e r v e d that aging o c c u r r e d by p r e c i p i t a t i o n of the b e t a phase. T h e s e s t u d i e s l-s did not a t t e m p t i n any e x t e n s i v e way to c o r r e l a t e the aged s t r u c t u r e with m e c h a n i c a l p r o p e r t i e s n o r did they e x a m i n e the s y s t e m ' s a p p r o a c h to the e q u i l i b r i u m a plus /3 s t r u c t u r e at h i g h e r aging t e m p e r a t u r e s . T h i s i n v e s t i g a t i o n has b e e n c o n c e r n e d with the aging b e h a v i o r of o r t h o r h o m b i c m a r t e n s i t e i n the t e m p e r a t u r e r a n g e of 773 to 1083 K. In this r a n g e the s t r u c t u r e changes f r o m that c h a r a c t e r i s t i c of m a x i m u m s t r e n g t h to o v e r a g e d and g r o s s l y overaged. E X P E R I M E N T A L PROCEDURE The p r i n c i p a l 6 - 2 - 4 - 6 m a t e r i a l i n v e s t i g a t e d was in the f o r m of flat b a r stock which has b e e n p r e v i o u s l y w a r m worked i n the a + fl field. Its c h e m i c a l a n a l y s i s i s shown in T a b l e I. MORRIS YOUNG, ERNEST LEVINE, and HAROLD MARGOLIN are Graduate Student, Associate Professor, and Professor, respectively, Department of Physical and Engineering Metallurgy, Polytechnic Institute of New York, Brooklyn, N. Y. 11201. Manuscript submitted July 16, 1973. METALLURGICAL TRANSACTIONS
The flat b a r stock was s e c t i o n e d , polished and a n a l y z e d in an X - r a y d i f f r a c t o m e t e r with a p u l s e height a n a l y z e r . O r t h o r h o m b i c l a t t i c e p a r a m e t e r v a l u e s were obtained by c a l c u l a t i o n f r o m high index reflections. H a r d n e s s was obtained f r o m e l e c
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