Unstable flow in martensite and ferrite

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of s t u d i e s of s e r r a t e d flow ( P o r t e v i n - L e C h a t e l i e r effect) in a n n e a l e d a l l o y s of i r o n c o n t a i n i n g c a r b o n and n i t r o g e n . ~-s The p h e n o m e n o n h a s been r e p o r t e d 6 to o c c u r in a l l o y s quenched to m a r t e n site but no q u a n t i t a t i v e s t u d i e s of s e r r a t e d flow in u n t e m p e r e d m a r t e n s i t e a r e known. S e r r a t i o n s in a s t r e s s - s t r a i n c u r v e obtained by a constant s t r a i n - r a t e test indicate a plastic instability and it h a s been shown by Hall I and o t h e r s 3's'~3 that each s e r r a t i o n is p r o d u c e d by the p r o p a g a t i o n of a s e p a r a t e Ltiders band. Detailed d e s c r i p t i o n s of d i f f e r e n t t y p e s of s e r r a t i o n s in the s t r e s s - s t r a i n c u r v e s for f e r r i t e t e s t e d at d i f f e r e n t t e m p e r a t u r e s have a p p e a r e d r e c e n t l y ~-4 but, except for a s u g g e s t i o n by Keh e t a l . 5 that the " j e r k y " c u r v e ( i r r e g u l a r s e r r a t i o n s with s m a l l d r o p s in the load) o b s e r v e d at low t e s t i n g t e m p e r a t u r e s m a y be due to a Snoek effect, no a t t e m p t has b e e n m a d e to explain the d i f f e r e n t types of s e r r a t i o n in t e r m s of the c o n c e p t s of d i s l o c a t i o n d y n a m i c s . T h e r e is s u b s t a n t i a l e v i d e n c e that the p h e n o m e n o n exhibited by f e r r i t i c a l l o y s is a s s o c i a t e d with the p r e s e n c e of c a r b o n and n i t r o g e n ; the two i n t e r s t i t i a l s o l u t e s b e h a v i n g e x a c t l y alike so that the effects p r o d u c e d by them a r e additive, s This is m o s t d e a r l y d e m o n s t r a t e d 7 by the v a l u e of the a c t i v a t i o n e n e r g y o b t a i n e d f r o m a plot of In ~s vs 1 / T s , w h e r e ~s is the s l o w e s t s t r a i n r a t e and T s the lowest t e m p e r a t u r e at which s e r r a t i o n s a r e o b s e r v e d . Values of this a c t i v a tion e n e r g y , ~'8'9 collected by Keh e t a l . , ~ all lie b e t w e e n 18,200 and 20,000 cal m o l e -1. The s m a l l e r e n e r g y i s the a c t i v a t i o n e n e r g y for diffusion of n i t r o g e n in f e r r i t e and the l a r g e r that for the diffusion of c a r b o n . Keh M. J. ROBERTS, formerly Research Associate, Department of Materials Science and Engineering,Comell University, Ithaca, N. Y., is Research Engineer, Homer Research Laboratories, Bethlehem Steel Corp., Bethlehem, Pa. W. S. OWEN, formerly Director, Department of Materials Science and Engineering,Cornell University, is Dean, The Technological Institute, Northweste,rn University, Evanston, Ill. Manuscript submitted May 5, 1969. METALLURGICALTRANSACTIONS

noted that although the m e a s u r e d a c t i v a t i o n e n e r g y was unaffected by d i f f e r e n c e s in the c h e m i c a l c o m p o s i t i o n and the t h e r m a l h