Effects of substructure on the mechanical properties of Fe-Ni-Co-C alloys
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i H E effect of m o r p h o l o g y and s u b s t r u c t u r e of e x p e r i m e n t a l l y d e s i g n e d m a r t e n s i t i c s t e e l s on t h e i r s t r e n g t h and t o u g h n e s s has b e e n the topic of long r a n g e r e s e a r c h p r o g r a m . 1-9 The effect of c a r b o n and o t h e r s u b s t i t u t i o n a l a l l o y i n g e l e m e n t s on the t r a n s f o r m a t i o n s u b s t r u c t u r e of m a r t e n s i t e is now w e l l d o c u m e n t e d . 1~ Low c a r b o n m a r t e n s i t e s c o n s i s t m a i n l y of d i s l o c a t e d laths while high c a r b o n m a r t e n s i t e s c o n s i s t of p l a t e s that a r e i n t e r n a l l y twinned. It h a s b e e n c o n v i n c i n g l y d e m o n s t r a t e d that in c a r b o n containing a l l o y s t e e l s , f o r c o m p a r a b l e s t r e n g t h l e v e l s , the t o u g h n e s s v a l u e s of s t e e l s with twinned m a r t e n s i t i c s u b s t r u c t u r e w e r e i n v a r i a b l y l o w e r c o m p a r e d to s t e e l s in which the s u b s t r u c t u r e of m a r t e n s i t e c o n s i s t e d of d i s l o c a t e d l a t h s . M o r e r e c e n t l y , the influence of r e t a i n e d a u s t e n i t e on t o u g h n e s s has a l s o been e m p h a s i z e d . 6,7 S e v e r a l s u g g e s t i o n s have b e e n put f o r w a r d to e x p l a i n the p o o r t o u g h n e s s of twinned m a r t e n s i t e s . M i c r o c r a c k i n g in a c i c u l a r m a r t e n s i t e could be d e t r i m e n t a l to toughness.16 A n i m p o r t a n t f a c t o r could be a c o r r e s p o n d e n c e b e t w e e n the s u b s t r u c t u r e of m a r t e n s i t e and its mode of d e f o r m a t i o n at d i f f e r e n t t e m p e r a t u r e s . It might be e x p e c t e d that m a r t e n s i t e f o r m e d by twinning s h e a r could s u b s e q u e n t l y d e f o r m by twinning r a t h e r than by s l i p thus d e g r a d i n g the t o u g h n e s s . M o r e o v e r , on t e m p e r i n g the m a r t e n s i t e , c a r b i d e s p r e f e r e n t i a l l y n u c l e a t e at the twins f o r m i n g b a r r i e r s to s l i p d i s l o cations. I n v e s t i g a t i o n s by M a g e e and D a v i e s 17 on the mode of d e f o r m a t i o n of lath and l e n t i c u l a r m a r t e n s i t e s showed that the mode of d e f o r m a t i o n d e p e n d e d not only on the m o r p h o l o g y and s u b s t r u c t u r e of the m a r t e n s i t e but a l s o on the c a r b o n content. T h e y d e m o n s t r a t e d that l e n t i c u l a r m a r t e n s i t e d e f o r m e d by twinning when the c a r b o n content e x c e e d e d 0.3 pct while the lath m a r t e n s i t e d e f o r m e d by s l i p at l e a s t up to 0.54 pct C. The p r e s e n t i n v e s t i g a t i o n was p r i m a r i l y a i m e d at i s o l a t i n g the effect of t r a n s f o r m a t i o n twins and p r e f e r e n t i a l twin b o u n d a r y p r e c i p i t a t i o n of c a r b i d e s on the f r a c t u r e t o u g
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