Volume expansion, pressure effects, and the strength differential in as-quenched iron-carbon martensite
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short-range types, deformation-induced austenite t r a n s f o r m a t i o n , and B a u s c h i n g e r e f f e c t s a r i s i n g f r o m the 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 . In a d d i t i o n , R a d c l i f f e and L e s l i e ~z h a v e s u g g e s t e d that a r e l a t i v e l y l a r g e v o l u m e e x p a n s i o n d u r i n g d e f o r m a t i o n of m a r t e n s i t e could l e a d to the SD e f f e c t . T h e s e h y p o t h e s e s have been b r i e f l y r e v i e w e d by H i r t h and Cohen 11 and s o m e h a v e b e e n f u r t h e r d i s c u s s e d by Rau ch and L e s l i e f H i r t h and Cohen ~ h a v e s u g g e s t e d a t h e o r y b a s e d upon the n o n l i n e a r - e l a s t i c i n t e r a c t i o n b e t w e e n d i s l o c a t i o n s and i n t e r s t i t i a l c a r b o n a t o m s , and an e f f e c t i v e m o d u l u s i n t e r p r e t a t i o n h a s b e e n o f f e r e d by P a m p i l l o e t a l . 13-~9 An e x p l a n a t i o n f o r the o b s e r v e d t e m p e r a t u r e d e p e n d e n c e of the SD e f f e c t , b a s e d on a t h e r m a l l y - a c t i v a t e d d o u b l e - k i n k m e c h a n i s m of d i s l o c a t i o n m o t i o n , h a s b e e n p r o p o s e d by F l e t c h e r e t a l . 1~ L e s l i e and S o b e r ~ h a v e shown that the SD e f f e c t c a n n o t be a t t r i b u t e d to f r i c tion e f f e c t s in c o m p r e s s i o n t e s t i n g .
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The '~r expansion hypothesis" is the name that has been given to the idea that the SD effect in ironcarbon martensites results from a volume expansion accompanying plastic deformation. The hypothesis was originally suggested by Radcliffe and Leslie 12 in analogy with apparently similar behavior in glassy polymers, which have both volume changes during deformation ~~ and a s y m m e t r i c y i e l d c r i t e r i a , z~ T o t e s t t h i s h y p o t h e s i s we m e a s u r e d the bulk d e n s i t i e s of a s q u e n c h e d m a r t e n s i t i c AISI 4310, 4320 and 4330 s t e e l s b e f o r e and a f t e r t e n s i l e d e f o r m a t i o n , and of AISI 4320 b e f o r e and a f t e r c o m p r e s s i v e d e f o r m a t i o n . T h e s e m e a s u r e m e n t s show that t h e s e i r o n - c a r b o n m a r t e n s i t e s e x h i b i t an u n u s u a l l y l a r g e v o l u m e change f o r m e t a l s ; h o w e v e r , m u c h of the change can be a t t r i b u t e d to t r a n s f o r m a t i o n of r e t a i n e d a u s t e n i t e . D r u c k e r 2s h as p o s t u l a t e d t h a t the w o r k done on a body by a n y a d d i t i o n a l load d u r i n g p l a s t i c flow m u s t be n o n n e g a t i v e , a notion w h i ch c e r t a i n l y s e e m s p l a u s i b l e f o r w o r k - h a r d e n i n g m a t e r i a l s such as the i r o n - c a r b o n m a r t e n s i t e s . E v e n when the y i e l d function of the m a t e r i a l is not c o m p l e t e l y s p e c i f i e d , it i s p o s s i b l e to VOLUME 6A. DECEMBER 1975 2279
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