The influence of microstructure on low temperature creep of ti-5 al-2.5 sn
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W. THOMPSON
AND
BEN
C. ODEGARD
C r e e p t e s t s w e r e p e r f o r m e d on n o r m a l grade T i - 5 A1-2.5 Sn at 194, 2 9 9 , 3 3 9 , and 422 K. T h r e e p r o c e s s i n g h i s t o r i e s w e r e studied: b a r stock and f o r g i n g s made i n the a and/3 r a n g e s . C r e e p s t r e s s e s r a n g e d f r o m 40 to 90 pct of the 299 K t e n s i l e yield s t r e s s , a y . Not all c o m b i n a t i o n s of s t r e s s and t e m p e r a t u r e gave r e l i a b l e c r e e p data. At 60 and 80 pct e y , the forged m a t e r i a l s w e r e m o r e c r e e p r e s i s t a n t than the b a r stock, while at 90 pct cry all t h r e e m a t e r i a l s w e r e alike. The a p p a r e n t a c t i v a t i o n e n e r g y for c r e e p , about 37 k J / m o l , was about o n e - f o u r t h the e n e r g y for selfdiffusion. A c t i v a t i o n a r e a s w e r e about 10q~2; thus the r a t e - c o n t r o l l i n g p r o c e s s at the s t r e s s e s used was p r o b a b l y the o v e r c o m i n g of i n t e r s t i t i a l o b s t a c l e s . O b s e r v a t i o n s on thin foils showed that the b a r stock and a - f o r g e d m a t e r i a l had equiaxed g r a i n s t r u c t u r e s , while t h e / 3 - f o r g e d m a t e r i a l c o n s i s t e d of m a s s i v e m a r t e n s i t e (or') p l a t e s . F i l m s of r e t a i n e d / 3 a p p e a r e d to be p r e s e n t in m a n y a ' b o u n d a r i e s ; this r e s t r i c t e d slip to i n d i v i d u a l or' p l a t e s . A m i c r o s t r u c t u r a l r a t i o n a l e was c o n s t r u c t e d , s u g g e s t i n g that each m a t e r i a l contained d i s l o c a t i o n s o u r c e s which differed i n e a s e of o p e r a t i o n . The ~3-forged m a t e r i a l was fitted into this r a t i o n a l e by the o b s e r v a t i o n that d i s l o c a t i o n s in a ' b o u n d a r i e s could a p p a r e n t l y act as s o u r c e s . It was concluded that the o b s e r v e d t r a n s i e n t c r e e p had a r i s e n f r o m e x h a u s t i o n of the e a s i e s t s o u r c e s at each s t r e s s level. O V E R the past twenty y e a r s , it has b e e n e s t a b l i s h e d that t i t a n i u m and its alloys a r e s u b j e c t to c r e e p at a m b i e n t t e m p e r a t u r e s (below 0.2Tin), as has b e e n b r i e f l y r e v i e w e d . 1'2 T h i s p h e n o m e n o n i s of c o n s i d e r able t e c h n o l o g i c a l i m p o r t a n c e , s i n c e d e s i g n s t r e s s e s at a m b i e n t t e m p e r a t u r e s a r e often l a r g e f r a c t i o n s of the t e n s i l e yield s t r e s s e y , p a r t i c u l a r l y in tough m a t e rials. A few c r e e p t e s t s have b e e n p e r f o r m e d on T i - 5 A1-2.5 Sn ( T i - 5 - 2 . 5 ) at r o o m t e m p e r a t u r e and at s t r e s s e s >- 85 pct o y , 2'3 but these r e s u l t s w e r e obtained on sheet m a t e r i a l and w e r e for t i m e s
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