• PDF / 932,153 Bytes
• 6 Pages / 612 x 783.28 pts Page_size
• 76 Downloads / 245 Views

DOWNLOAD

REPORT

---

M E C H A N I C A L twinning h a s b e e n o b s e r v e d to p l a y an i m p o r t a n t r o l e in the d e f o r m a t i o n of b e t a s t a b i l i z e d Ti a l l o y s . 1-2 This b e h a v i o r h a s a l s o b e e n r e p o r t e d in the c o m m e r c i a l a l l o y B e t a HI, which h a s the n o m i n a l c o m p o s i t i o n ( T i - l l . 5 p c t Mo-5.5 pct Z r - 4 . 5 p c t Sn). This c h a r a c t e r i s t i c is m o s t c l e a r l y e v i d e n t when the a l l o y is d e f o r m e d in the s o l u t i o n t r e a t e d and quenched c o n d t t i o n J 'a'4 R a c k e t a l 4 r e p o r t that d e f o r m a t i o n b y cold r o l l i n g p r o d u c e d {112} twins, w h e r e a s B l a c k b u r n and F e e n e y x found a l e s s c o m m o n twinning s y s t e m , {332} (113), to be o p e r a t i v e . T h e s e s y s t e m s w e r e d e t e r m i n e d b y s i n g l e s u r f a c e t r a c e a n a l y s e s . R o b e r s o n and A d a i r s in a r e c e n t c o m m u n i c a t i o n have r e p o r t e d that twinning i n f l u e n c e s the s u b s e q u e n t b e h a v i o r of B e t a III titanium alloy. In X - r a y / m e t a l l o g r a p h i c t r a c e a n a l y s i s , one h a s to r e l y on the c r y s t a l l o g r a p h i c o r i e n t a t i o n of s i n g l e c r y s t a l s which h a s b e e n d e t e r m i n e d p r i o r to d e f o r m a t i o n . Thus, t h e r e i s a l w a y s s o m e i n a c c u r a c y in the r e f e r ence d i r e c t i o n of the c r y s t a l s b e c a u s e p r e c e d i n g and c o n c u r r e n t s l i p i n e v i t a b l y c a u s e s r o t a t i o n of the c r y s t a l s a d j a c e n t to the twins. B e c a u s e t r a c e s a r e s o m e t i m e s a s s o c i a t e d with the p r o d u c t s of s t r a i n - i n d u c e d a n d / o r m a r t e n s i t i c t r a n s f o r m a t i o n , it is n e c e s s a r y to r e v e a l the c r y s t a l l o g r a p h i c s t r u c t u r e of such p r o d u c t s b y a d i f f r a c t i o n m e t h o d . However, X - r a y d i f f r a c t i o n is not s u i t a b l e for t h i s p u r p o s e b e c a u s e the d e f o r m a tion p r o d u c t s a r e u s u a l l y s m a l l in s i z e and h e a v i l y d i s t o r t e d . Thus, t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (TEM), coupled with e l e c t r o n d i f f r a c t i o n h a s b e e n e m p l o y e d to s u p p l e m e n t and to s u p p o r t the o p t i c a l t r a c e a n a l y s i s . E X P E R I M E N T A L PROCEDURE Single c r y s t a l o r c o a r s e g r a i n e d s a m p l e s of Beta III titanium alloy were prepared by annealing pre-machined t e n s i l e s p e c i m e n s in a p u r i f i e d d y n a m i c a t m o s p h e r e of h e l i u m at 1400~ for 24 h. The s p e c i m e n s w e r e then quenched in w a t e r , e l e c t r o p o l i s h e d , and e t c h e d to r e v e a l g r a i n b o u n d a r i e s . In m a n y c a s e s the g r a i n b o u n d a r i e s w e r e n o r m a l to the t e n s i l e a x i s and e x t e n d e d