Aging induced shape instability in an elastically bent uranium + 7.5 wt pct niobium + 2.5 wt pct zirconium alloy
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a l l o y and unlike 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 s no o b v i ous beginning to the Vo " p h a s e " was a p p a r e n t in that data: 3 n o r w e r e any i n t e r f a c e s evident by o p t i c a l m i c r o s c o p y . In k e e p i n g with this " c o n t i n u o u s n a t u r e " of the V to Vo t r a n s f o r m a t i o n , it w a s a l s o shown that m i n o r c o m p o s i t i o n a l changes and v a r i a t i o n s in quenching r a t e s could c a u s e m e a s u r e a b l e a l t e r a t i o n s in, for e x a m p l e , the l a t t i c e p a r a m e t e r s of t h i s m e t a s t a b l e V0 s t r u c t u r a l s t a t e . 3 G i r a n d - H e r a n d and G u i l t a u m i n 4 sought to c h a r a c t e r i z e the m i c r o s t r u c t u r e s of v - p h a s e d e c o m p o s i t i o n in t h i s a l l o y u t i l i z i n g e l e c t r o n t r a n s m i s s i o n m i c r o s c o p y . In the a s - q u e n c h e d condition only a m o d u l a t e d o r s t r i a t e d s t r u c t u r e was noted, s u g g e s t i n g the p r e s e n c e of a p e r i o d i c s t r a i n f i e l d of o r d e r of t e n s to a few h u n d r e d a n g s t r o m s in d i m e n s i o n . Aging of the quenched a l l o y i n i t i a t e s t r a n s f o r m a t i o n s to o t h e r s t r u c t u r a l s t a t e s . I s o t h e r m a l T T T d i a g r a m s have b e e n p r e s e n t e d by Dean 5 b a s e d on o p t i c a l m i c r o s c o p y and h a r d n e s s m e a s u r e m e n t s , by K a r n o w s k y and Rohde 6 f r o m e l e c t r i c a l r e s i s t i v i t y m e a s u r e m e n t s and by V a n d e r m e e r 3 b a s e d on d i l a t o m e t r y s t u d i e s . F i g . 1 shows when the four s t a g e s of t r a n s f o r m a t i o n that have b e e n d e t e c t e d upon aging b e g i n . The r o o m t e m p e r a t u r e s t r e s s - s t r a i n t e n s i l e b e h a v i o r of t h i s a l l o y a f t e r quenching f r o m the y - p h a s e was i n v e s t i g a t e d in s o m e d e t a i l by O a k e s 7 and A1b r i g h t and Stein. 8 Both of t h e s e s t u d i e s r e v e a l e d the e x i s t e n c e of a p s e u d o e l a s t i c effect at low s t r a i n s . S t r e s s e s of the o r d e r of 172 to 276 M n / m 2 (25 to 40,000 psi) w e r e n e c e s s a r y b e f o r e the p s e u d o e l a s t i c effect b e c a m e e v i d e n t . P e r m a n e n t d e f o r m a t i o n did not m a n i f e s t i t s e l f until s t r e s s e s in the v i c i n i t y of 552 to 828 M a / m 2 (80 to 120,000 psi) w e r e r e a c h e d . T h i s type of s t r e s s - s t r a i n r e s p o n s e r e s e m b l e s b e h a v i o r noted in o t h e r m e t a s t a b l e b c c a l l o y s s u s c e p t i b l e to further phase transformation. Materials displaying s u c h p s e u d o e l a s t i c b e h a v i o r f r e q u e n t l y e x h i b i t the i n t r i g u i n g s h a p e m e m o r y effect (SME). 9 SME in u r a n i u m VOLUME 7A, JUNE 1976-871
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