Superplasticity in eutectoid steel
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A S r e c e n t l y r e v i e w e d by Johnson* and D a v i e s et a l . , 2 s u p e r p l a s t i c e l o n g a t i o n s can b e g e n e r a t e d by two d i s t i n c t m e a n s : i) m i c r o g r a i n p l a s t i c i t y , and if) t r a n s f o r m a t i o n p l a s t i c i t y . In the f o r m e r , u n c o m m o n l y l a r g e t e n s i l e e x t e n s i b i l i t i e s a r e o b t a i n e d in u l t r a - f i n e g r a i n e d m a t e r i a l s at t e m p e r a t u r e s above o n e - h a l f of the m e l t i n g point, while in the l a t t e r such e l o n g a t i o n s can be g e n e r a t e d by r e p e a t e d t h e r m a l c y c l i n g of a m a t e r i a l through a p h a s e t r a n s f o r m a t i o n while u n d e r load. D e s p i t e n u m e r o u s s t u d i e s of t h e s e two f o r m s , t h e c r i t i cal question of how t h e y r e l a t e to one a n o t h e r h a s r e m a i n e d an e n i g m a . T h e p u r p o s e of the p r e s e n t w o r k i s to a t t e m p t a r e s o l u t i o n% of t h i s p r o b l e m . To e x p l o r e the r e l a t l o n s h , p of t r a n s f o r m a t i o n p l a s t i c i t y to m i c r o g r a i n p l a s t i c i t y , a e u t e c t o i d s t e e l of e x c e p t i o n a l l y fine i n t e r p h a s e ( f e r r i t e - c e m e n t i t e ) s p a c i n g w a s s e l e c t e d with t h e hope that it would e x h i b i t both the m i c r o g r a i n and t r a n s f o r m a t i o n f o r m s of superplasticity. To c h a r a c t e r i z e u n a m b i g u o u s l y any m a n i f e s t a t i o n s of m i c r o g r a i n p l a s t i c i t y which a p p e a r for c o n d i t i o n s w h e r e no p h a s e t r a n s f o r m a t i o n is involved, t h e s t r a i n r a t e s e n s i t i v i t y and t o t a l elongation have b e e n m e a s s u r e d a s a function of s t r a i n r a t e f o r i s o t h e r m a l t e n s i l e s t r a i n i n g of t h i s m a t e r i a l a s a f e r r i t e - c e m e n t i t e a g g r e g a t e j u s t below the e u t e c t o i d t e m p e r a t u r e , A e , , and a s a u s t e n i t e above A e , . A high s t r a i n - r a t e s e n s i t i v i t y of the flow s t r e s s i s the one c h a r a c t e r i s t i c which all m i c r o g r a i n s u p e r p l a s t i c a l l o y s s h a r e . H e n c e , a s Backofen et al. s pointed out, t h e flow s t r e s s i s a b l e to r i s e s o r a p i d l y with s t r a i n r a t e that p l a s t i c i n s t a b i l i t y i s r e s i s t e d w h e r e v e r the s t r a i n r a t e is i n c r e a s e d by i n c i p i e n t necking. T h i s i s e x p r e s s e d by (r = (k~rn)c,T
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w h e r e a i s the flow s t r e s s , ~ i s the s t r a i n r a t e , k i s a c o n s t a n t , and m is the s t r a i n - r a t e s e n s i t i v i t y exponent u s e d to define m i c r o g r a i n p l a s t i c i t y (m > 0.3); t h i s GEORGE R. YODER, formerly Graduate Student, Syracuse University, Syracuse, N. Y., is NRC Postdoctoral Resident Research Associate, Physical Metallurgy Branch, Metallurgy Divisi
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