Asymmetrical mechanical behavior of a precipitation hardened beta titanium alloy
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s t r e n g t h e n i n g by s e c o n d p h a s e p a r t i c l e s is u s u a l l y c o m b i n e d with a m a t r i x h a r d e n i n g c o m p o n e n t in m o s t s t r u c t u r a l a l l o y s . In o r d e r to u n d e r s t a n d the f u n d a m e n t a l s of s u c h s t r e n g t h e n i n g , it is n e c e s s a r y to s e p a r a t e and to a n a l y z e i n d i v i d u a l l y the v a r i o u s c o m p o n e n t s to the flow s t r e s s . To do t h i s , the y i e l d s t r e s s of an a l l o y is c o m m o n l y a s s u m e d to be the l i n e a r s u m of a p a r t i c l e h a r d e n i n g c o m p o n e n t and a f r i c tional s t r e s s of the m a t r i x . ~ C o m p u t e r s t u d i e s z have i n d e e d shown t h a t if an a l l o y is h a r d e n e d b y h a r d p a r t i c l e s (such a s l a r g e p r e c i p i t a t e s ) and soft p a r t i c l e s (such a s s o l u t e a t o m s ) then the flow s t r e s s is given by the l i n e a r s u m of h a r d and soft p a r t i c l e flow s t r e s s c o m p o n e n t s . Such an equation d e s c r i b e s the r e l a t i o n s h i p b e t w e e n s o l i d s o l u t i o n h a r d e n i n g and p a r t i c l e h a r d e n i n g in a Cu-SiO 2 a l l o y with Au added, z Howe v e r , in b c c s o l i d s o l u t i o n a l l o y s the f r i c t i o n s t r e s s e s a r e u s u a l l y much l a r g e r and can be v e r y d i f f e r ent in n a t u r e than in fcc a l l o y s (for e x a m p l e , d i s l o c a tion c o r e effects e x i s t in bcc a l l o y s ) . The p u r p o s e of this r e s e a r c h is to e x a m i n e the r e l a t i o n s h i p between h a r d e n i n g due to p r e c i p i t a t e p a r t i c l e s and that due to f r i c t i o n a l e f f e c t s of a b c c m a t r i x . The T i - 4 0 at. pct V a l l o y h a s a r e l a t i v e l y s t a b l e b e t a - p h a s e (bcc) s t r u c t u r e . Single c r y s t a l s of this a l l o y exhibit an a s y m m e t r y of the a c t i v e s l i p plane and of the y i e l d s t r e s s 4 as well a s a s t r o n g t e m p e r a t u r e d e p e n d e n c e of the y i e l d s t r e s s . 5 Such b e h a v i o r m a y b e i n t e r p r e t e d 4 a s i n d i c a t i n g that the d i s l o c a t i o n c o r e s t r u c t u r e i n f l u e n c e s the flow s t r e s s of the T i 40 V a l l o y in a m a n n e r s i m i l a r to that o b s e r v e d in o t h e r b c c m e t a l s and d i l u t e a l l o y s . In the p r e s e n t study, the T i - 4 0 V a l l o y s have been p r e c i p i t a t i o n h a r d e n e d b y the a d d i t i o n of 1.0 at. p c t Si, which r e s u l t s in 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 when s o l u t i o n t r e a t e d m a t e r i a l i s aged. ~ The p r e s e n t s t u d y u t i l i z e s the T i - 4 0 at. p e t V + 1 at. p e t Si a l l o y a s a m o d e l s y s t e m to s t u d y the effect of p r e c i p i t a t e p a r -
t i c l e s on the a s y m m e t r y of the s l i p p l a n e and of the yield stress.
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