Low stress and superplastic creep behavior of Zn-22 Pct Al eutectoid alloy
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A. K. S. YU, AND
A. K. MUKHERJEE
The t e m p e r a t u r e , g r a i n s i z e , and s t r e s s d e p e n d e n c e of s t e a d y - s t a t e s t r a i n - r a t e s w e r e i n v e s t i g a t e d for Z n - 2 2 pct A1 eutectoid a l l o y u s i n g double s h e a r type s p e c i m e n s . T e s t s w e r e p e r f o r m e d on s p e c i m e n s of g r a i n size f r o m 1.3 m to 3.7 p m over a r a n g e of t e m p e r a t u r e f r o m 450 to 525 K. The applied s t r e s s e s w e r e in the r a n g e b e t w e e n 10 -1 and 4.0 • 101 M P a with the r e s u l t i n g t r u e s t r a i n - r a t e s r a n g i n g f r o m 10 -7 to ~10-2s -1. The a l l o y exhibited two d i s t i n c t r e g i o n s of c o n s t a n t s t r e s s d e p e n d e n c e with s t r e s s exponents 1 and 2.2. The t r a n s i t i o n s t r e s s e s b e t w e e n t h e s e two r e g i o n s w e r e b e t w e e n 3 • 10 "1 and 7 • 10 -~ M P a . The t r u e a c t i v a t i o n e n e r g i e s a s s o c i a t e d with t h e s e r e g i o n s were found to be 95.9 + 2.1 and 69.9 • 2.1 k J / m o l , r e s p e c t i v e l y . The g r a i n s r e m a i n e d equiaxed following l a r g e d e f o r m a tion, although t h e r e is e v i d e n c e of e x c e s s i v e g r a i n growth and " g r a i n c l u s t e r i n g . " S p e c i m e n s d e f o r m e d in both r e g i o n s showed few or no d i s l o c a t i o n s within the g r a i n s . T h e r e was no e v i d e n c e of s u b g r a i n f o r m a t i o n or d i s l o c a t i o n p i l e - u p at the g r a i n b o u n d a r i e s . It is p o s s i b l e that d i s l o c a t i o n s found d u r i n g d e f o r m a t i o n w e r e a n n i h i l a t e d when s t r e s s was withdrawn or lost d u r i n g thin foil p r e p a r a t i o n . T h e r e was c l e a r e v i d e n c e of p r i m a r y stage in both r e g i o n s . However, it is s u g g e s t e d that w h e r e a s the p r i m a r y stage in R e g i o n II is due to d i s l o c a t i o n m u l t i p l i c a t i o n p r o c e s s , at low s t r e s s e s the p r i m a r y stage in R e gion I may be due to e l a s t i c bowing of the e x i s t i n g d i s l o c a t i o n s in the s t r u c t u r e . At low s t r e s s e s N a b a r r o - H e r r i n g d i f f u s i o n a l c r e e p was found to be the r a t e c o n t r o l l i n g m e c h a n i s m . At i n t e r m e d i a t e s t r e s s e s , s u p e r p l a s t i c c r e e p was found to be the d o m i n a n t m e c h a n i s m . The t r a n s i t i o n b e t w e e n t h e s e two m e c h a n i s m s , a s well as Coble c r e e p and the r a n g e s of m a n i f e s t a t i o n for s u p e r p l a s t i c c r e e p , a r e a n a l y z e d and the i m p o r t a n c e of the p r e e x p o n e n t i a l t e r m A, in the d i m e n s i o n l e s s r e l a t i o n :
and the role of the concurrent
grain growth
are emphasized.
THE phenomenon of micrograin superplasticity has received considerable attention in recent years and has resulted in two major advances: a) several new models hav
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