The kinetics of transformation in Zn-Ai superplastic alloys
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O V E R the last d e c a d e many s t u d i e s have b e e n publ i s h e d on s u p e r p l a s t i c a l l o y s (see R e f . 1 f o r a r e c e n t c o m p r e h e n s i v e r e v i e w ) . Of m a j o r i m p o r t a n c e has b e e n the d o c u m e n t a t i o n that the a b i l i t y of a n a l l o y to d e f o r m s u p e r p l a s t i c a l l y d e p e n d s s t r o n g l y on its m i crostructure. It is well k n o w n that the m i c r o s t r u c t u r e of a n a l l o y d e p e n d s on its t h e r m a l and m e c h a n i c a l h i s t o r y . A c o m m o n m e t h o d of p r e s e n t i n g the r e s u l t s of " t h e r m a l " s t u d i e s is the s o - c a l l e d t i m e - t e m p e r a t u r e t r a n s f o r m a t i o n c u r v e ( i s o t h e r m a l ) . T h e s e c u r v e s r e c o r d the o n s e t of d e t e c t a b l e t r a n s f o r m a t i o n and the c o m p l e t i o n t i m e s of t r a n s f o r m a t i o n on a s e m i l o g t e m p e r a t u r e v s t i m e p l o t . W h e n c o m b i n e d with the a t t e n d a n t m i c r o s t r u c t u r e s , they c a n b e u s e f u l in u n d e r s t a n d i n g the m e c h a n i s m s and k i n e t i c s of n o n i s o t h e r m a l t r a n s f o r mations. T h e a l l o y used in this s t u d y is a e u t e c t o i d Z n - A 1 a l l o y with s m a l l a d d i t i o n s of Qu, Mg and Ca. A t r o o m t e m p e r a t u r e , the a s - r o l l e d s h e e t h a s a t e n s i l e s t r e n g t h of 345 M P a (50 k s i ) .2 T h e s h e e t e x h i b i t s i n c r e a s i n g s u p e r p l a s t i c i t y a s the t e m p e r a t u r e a p p r o a c h e s the e u t e c t o i d t e m p e r a t u r e , 2 7 8 ° C (551 K). T h e s m a l l a m o u n t s of C u , Mg and C a that a r e p r e s e n t in the a l l o y g r e a t l y r e d u c e the k i n e t i c s of the e u t e c t o i d t r a n s f o r m a t i o n , t h e r e b y a l l o w i n g the s t r u c t u r a l a s p e c t s of the low t e m p e r a t u r e r e a c t i o n to b e s t u d i e d in d e t a i l .3 T h e a l l o y e x i s t s as a s o l i d s o l u t i o n o t ' - p h a s e a b o v e the e u t e c t o i d t e m p e r a t u r e , 2 7 8 ° C (551 K). Upon c o o l ing, the o t ' - p h a s e d e c o m p o s e s into t w o - p h a s e s d e s i g n a t e d ot + /3, w h e r e or' a n d ot a r e f c c - p h a s e s with diff e r i n g Z n c o m p o s i t i o n a n d / 3 is the h e x a g o n a l Z n r i c h p h a s e . T h i s r e a c t i o n has b e e n t e r m e d a e u t e c t o i d r e a c t i o n ( m o r e p r o p e r l y a m o n o t e c t o i d r e a c t i o n ) , and c a n give r i s e to a v a r i e t y of m i c r o s t r u c t u r e s , d e p e n d ing on the t e m p e r a t u r e of t r a n s f o r m a t i o n o r d e g r e e of d e f o r m a t i o n d u r i n g the t r a n s f o r m a t i o n .
In this i n v e s t i g a t i o n w e p r e s e n t the t i m e - t e m p e r a t u r e t r a n s f o r m a t i o n c u r v e s f o r the e u t e c t o i d r e a c t i o n in two a l l o y s (
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