Superplastic deformation of Ti-6Al-4V alloy
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AND
A. ROSEN
T h e a l l o y T i - 6 - A 1 - 4 V d e f o r m s s u p e r p l a s t i c a l l y in the t e m p e r a t u r e r a n g e 750 to 950~ T h e m o s t i m p o r t a n t f a c t o r which i s r e s p o n s i b l e f o r s u p e r p l a s t i c b e h a v i o r was found to be the v e r y fine g r a i n s i z e . S t r a i n r a t e h a s no d i r e c t effect on s u p e r p l a s t i c i t y , h o w e v e r when the s t r a i n r a t e is v e r y low ( a p p r o x i m a t e l y 2 • 10 -5 s-Z), p r o l o n g e d e x p o s u r e to high t e m p e r a t u r e c a u s e s g r a i n growth and e a r l y f a i l u r e . The s t r a i n r a t e s e n s i t i v i t y f a c t o r m = 0.5 and 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 AH = 45,000 c a l / m o l e , which i s a p p r o x i m a t e l y the s a m e a s the a c t i v a t i o n e n e r g y f o r g r a i n b o u n d a r y s e l f diffusion of t i t a n i u m . Both v a l u e s a r e independent of s t r a i n r a t e within the r a n g e 10 - 5 - 2.5 • 10 -2 s -z. A l l the e x p e r i m e n t a l p o i n t s f a l l i 2 a s t r a i g h t line when p l o t t e d a s log (4kTd*e/DgbGb 3) vs log (a/G) with a s l o p e n = 1 / m = . T h i s i s in e x c e l l e n t a g r e e m e n t with the t h e o r y of g r a i n b o u n d a r y s l i d i n g a c c o m m o d a t e d by d i s l o c a t i o n motion. S U P E R P L A S T I C I T Y is the p r o p e r t y of c e r t a i n m e t a l s and a l l o y s to be s t r e t c h e d m a n y t i m e s t h e i r i n i t i a l length without f r a c t u r e . T h e p h e n o m e n o n was o b s e r v e d o v e r half a c e n t u r y ago, 1-3 h o w e v e r , the f i r s t s y s t e m a t i c study of the s u p e r p l a s t i c b e h a v i o r of m e t a l s is s l i g h t l y m o r e than ten y e a r s o l d : Since 1964, the a c t i v i t y in t h i s f i e l d h a s b e e n v e r y i n t e n s i v e and h a s b e e n r e p o r t e d in c o m p r e h e n s i v e r e v i e w s , s-9 The m a i n f a c t o r s which influence s u p e r p l a s t i c d e f o r m a t i o n a r e the t e m p e r a t u r e , the g r a i n s i z e and the s t r a i n - r a t e d e p e n d e n c e of the flow s t r e s s . T h e a l l o y T i - 6 A 1 - 4 V finds i n c r e a s i n g a p p l i c a t i o n in a i r and s p a c e i n d u s t r i e s due to i t s high s t r e n g t h , low d e n s i t y and good c o r r o s i o n r e s i s t a n c e . B e c a u s e of i t s high s t r e n g t h and low m o d u l u s , it i s difficult to p r o duce p a r t s of c o m p l e x s h a p e by p l a s t i c d e f o r m a t i o n . M o s t of the p r o b l e m s can b e o v e r c o m e by d e f o r m a t i o n at high t e m p e r a t u r e s s i n c e t h i s a l l o y is known to be s u p e r p l a s t i c in a c e r t a i n t e m p e r a t u r e r a n g e . l ~ T h e p u r p o s e of t h i s i n v e s t i g a t i o n was to s t u d y the f a c t o r s influencing s u p e r p l a s t i c d e f o r m a t i o n of t h i s a l loy, p a r t i c u l a r l
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