Investigation of the fracture mechanism of Ti-5AI-2.5Sn at cryogenic temperatures
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STONE,
J. R. LOW,
Jr., AND
J. L. SHANNON,
Jr.
F r a c t o g r a p h y and m e t a l l o g r a p h i c s e c t i o n i n g w e r e u s e d to i n v e s t i g a t e the i n f l u en ce of m i c r o s t r u c t u r e on the f r a c t u r e m e c h a n i s m and f r a c t u r e t o u g h n e s s (Kic) of n o r m a l i n t e r s t i t i a l and e x t r a low i n t e r s t i t i a l (ELI) T i - 5 A 1 - 2 . 5 S n at 20 K ( - 4 2 3 ~ and 77 K ( - 3 2 0 ~ P l a t e s of e a c h g r a d e w e r e m i l l a n n e a l e d at 815~ (1500~ followed by e i t h e r a i r o r f u r n a c e cooling. T h e s e v a r i a t i o n s in c o m p o s i t i o n and co o l i n g r a t e r e s u l t e d in d i f f e r e n c e s in the v o l u m e f r a c t i o n and i n t e r n a l s t r u c t u r e of the d i s p e r s e d / 3 p h ase and in the o r d e r i n g of the e m a t r i x . T h e E L I a l l o y s w e r e t o u g h er than the n o r m a l i n t e r s t i t i a l p l a t e s . Kic of the f u r n a c e - c o o l e d ELI plate was 25 pct l o w e r than that of the a i r - c o o l e d ELI m a t e r i a l . V a r i a t i o n s in c o o l i n g r a t e had no i n f l uen ce of Kic of the n o r m a l i n t e r s t i t i a l a l l o y s . F r a c t o g r a p h y showed that a l a r g e p o r t i o n of the f r a c t u r e s u r f a c e s w e r e c o v e r e d with e l o n g a t e d d i m p l e s c o m m o n l y c a l l e d " f l u t e s . " M e t a l l o g r a p h i c s e c t i o n s of s p e c i m e n s d e f o r m e d at 77 K showed that t h e s e f e a t u r e s f o r m at the i n t e r s e c t i o n s of slip bands or d e f o r m a t i o n twins with g r a i n or twin b o u n d a r i e s . O r d e r i n g and h i g h e r i n t e r s t i t i a l l e v e l s i n c r e a s e the l o c a l s t r a i n in slip bands r e s u l t i n g in void n u c l e a t i o n at l o w er m a c r o s c o p i c s t r a i n s and l o w e r Kic v a l u e s . s are N c o m m oIn l y uU s e d in M aerospace T I Ta l l o yA s t r u c t u r e s due to t h e i r high s t r e n g t h - t o - d e n s i t y r a t i o . Th e ~ t i t a n i u m a ll o y Ti-SA1-2.SSn is often used f o r c r y o g e n i c s e r v i c e such as liquid h y d r o g e n s t o r a g e tanks in A m e r i c a n s p a c e c r a f t b e c a u s e of its e x c e l l e n t c o m b i n a t i o n of s m o o t h and notched p r o p e r t i e s . 1 I n v e s t i g a t o r s have studied the m i c r o s t r u c t u r e , 2,3 d e f o r m a tion m o d e s , 2 and m e c h a n i c a l p r o p e r t i e s 1,4,5 of T i - 5A12.5Sn, but v i r t u a l l y no a tt e n ti o n has b e e n paid to d i r e c t o b s e r v a t i o n s of the influence of m i c r o s t r u c t u r e on the f r a c t u r e m e c h a n i s m . T h e p u r p o s e of this i n v e s t i g a t i o n is to identify t h o s e m i c r o s t r u c t u r a l f e a t u r e s which p a r t i c i p a t e in the f r a c t u r e p r o c e s s of T i - 5 A 1 - 2 . 5 S n at c r y o g e n i c t e m p e r a t u r e s . It is hoped that t h e s e r e s
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