The influence of Si on precipitation phenomena and age hardening of a beta Ti alloy

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use of s i l i c i d e s to s t r e n g t h e n a(hcp)-fl(bcc) and n e a r - a Ti a l l o y s is w e l l e s t a b l i s h e d and h a s been e x a m i n e d by s e v e r a l i n v e s t i g a t o r s 1-4 The f o r m a t i o n of s i l i c i d e p r e c i p i t a t e s in a fi T i a l l o y h a s a l s o been r e p o r t e d ; ~ thus the p o s s i b i l i t y e x i s t s of u s i n g S i - i n duced p r e c i p i t a t i o n h a r d e n i n g a s an a l t e r n a t i v e o r a d d i t i o n a l method (to a - p h a s e f o r m a t i o n ) of age h a r d e n i n g fl T i a l l o y s . In an effort to e x a m i n e such s t r e n g t h e n i n g , p r e v i o u s s t u d i e s 6-8 have e x a m i n e d the m e c h a n i c a l b e h a v i o r of (Ti-V) + Si a l l o y s with a r e l a t i v e l y s t a b l e E - p h a s e (bcc) m a t r i x . In p a r t i c u l a r , the b e h a v i o r of both s i n g l e c r y s t a l s and p o l y c r y s t a l l i n e a l l o y s with a c o m p o s i t i o n of ~ T i - 4 0 a t . pct V-1 at. pct Si show that t h e r e a r e two m a x i m a o r p e a k s in the d e p e n d e n c e of the y i e l d s t r e s s o r h a r d n e s s upon a g i n g t i m e at 570~ in the s o l u t i o n - t r e a t e d condition. T h e p u r p o s e of the p r e s e n t study is to e x a m i n e the effect of Si on the p r e c i p i t a t i o n p h e n o m e n a and r e s u l t i n g age h a r d e n i n g b e h a v i o r in a T i - V - S i a l l o y . In o r d e r to avoid c o m p l i c a t i o n s in the i n t e r p r e t a t i o n of the r e s u l t s , the a l l o y c h o s e n contains sufficient V in the r - p h a s e m a t r i x to p r e v e n t a - p h a s e o r cophase formation. E X P E R I M E N T A L PROCEDURE A T i - 4 0 at. pct V-1 at. p c t Si a l l o y was p r e p a r e d by a r c melting; a f t e r a h o m o g e n i z a t i o n a n n e a l at 1300~ f o r 4 h, s i n g l e c r y s t a l s w e r e grown by e l e c t r o n b e a m zone m e l t i n g . The d e t a i l s of t h i s technique a r e d e s c r i b e d e l s e w h e r e P T h e p o l y c r y s t a l l i n e T i - 4 0 at. p c t V - 1 . 2 5 at. pct Si was a l s o a r c m e l t e d and h o m o g e n i z e d . The m a t e r i a l was s o l u t i o n - t r e a t e d for 1 h at 1000~ and h e l i u m gas quenched, and s u b s e q u e n t aging was p e r f o r m e d at 570~ A l l heat t r e a t m e n t s w e r e p e r S. M. TUOMINEN is presently with Climax Molybdenum Company of Michigan, Ann Arbor, MI. G. W. FRANTI and D. A. KOSS are Postdoctoral Research Associate and Professor, respectively, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931, with which S. M. Tuominen was formerly affiliated. Manuscript submitted April 5, 1976. METALLURGICAL TRANSACTIONSA

f o r m e d in v a c u a of at l e a s t 5 • 10 -6 T o r r . A f t e r a g ing, the s i n g l e c r y s t a l s had an i n t e r s t i t i a l content by weight of about 550 ppm oxygen, 250 ppm n i t r o g e n and 8 p p m h y d r o g e n .