The morphology and substructure of martensite in maraging steels
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T H E morphology of f e r r o u s m a r t e n s i t e s can be c l a s sified into two g e n e r a l types, twinned p l a t e s and d i s l o c a t e d laths. The s u b s t r u c t u r e within a m a r t e n s i t i c r e g i o n is often complex and m a y c o n t a i n a m i x t u r e of d i s l o c a t e d and twinned types. The s t r u c t u r a l c h a r a c t e r i s t i c s of the m a r t e n s i t i c y to ~ ' t r a n s f o r m a t i o n a r e f u r t h e r c o m p l i c a t e d by the r e c e n t o b s e r v a t i o n s that lath m a r t e n s i t e can also contain twinned r e g i o n s . 1'2 In s o m e f e r r o u s alloys c o n t a i n i n g c h r o m i u m , cobalt and m a n g a n e s e a hexagonal 9 m a r t e n s i t e a s s o c i a t e d with the bcc a m a r t e n s i t e i n t r o d u c e s a n o t h e r factor into the v a r i a b l e s which m a y influence the s u b s t r u c t u r e and m o r p h o l o g y of (~ m a r t e n s i t e . N u m e r o u s i n v e s t i g a t i o n s have b e e n devoted to the study of the s t r u c t u r a l c h a r a c t e r i s t i c s of the ~ ' and 9 p h a s e s in s t e e l s c o n t a i n i n g about 18 pct c h r o m i u m and 8 pct n i c k e l . 3-15 It has b e e n shown that the hexagonal e m a r t e n s i t e is f o r m e d s i m u l t a n e o u s l y with the ~ ' phase. 3-~5 Although it is well known that the a m o u n t of t r a n s f o r m a t i o n to e i t h e r ~ ' or 9 is dependent upon the t e m p e r a t u r e and the extent to which any d e f o r m a tion is applied, t h e r e is a c e r t a i n a m o u n t of a m b i g u i t y s u r r o u n d i n g the n u c l e a t i o n m e c h a n i s m o f t h e two p h a s e s . A n u m b e r of w o r k e r s 3-5'7'9'n have taken the point of view that the 9 f o r m s f i r s t and ~ ' n u c l e a t e s within the 9 On the other hand it is a r g u e d ~~ that the 9 f o r m s as a c o n s e q u e n c e of the l a r g e shape d e f o r m a t i o n due to the y - - ~ ' t r a n s f o r m a t i o n . Goldman e t a l , z6 using t h r e e d i f f e r e n t t e c h n i q u e s detected only one t r a n s f o r m a t i o n t e m p e r a t u r e in an F e - 1 5 . 1 pct C r 11.7 pct Ni alloy. They suggested that if 9 was an i n t e r m e d i a t e phase in the y - - ~ ' t r a n s f o r m a t i o n then a s e p a r a t e M s for the y ~ c t r a n s f o r m a t i o n should have b e e n o b s e r v e d . Mangonon and T h o m a s , 's u s i n g e l e c t r o n m i c r o s c o p y to study the f o r m a t i o n of m a r t e n s i t e in type 304 s t a i n l e s s s t e e l s have shown that 9 is a t h e r m o d y n a m i c a l l y stable phase and that ~ ' f o r m a t i o n could only be induced b y p l a s t i c d e f o r m a t i o n s u b s e q u e n t to the f o r m a t i o n of 9 thus s u g g e s t i n g that a t r a n s f o r m a t i o n t e m p e r a t u r e b e t w e e n 9 and ~ ' e x i s t s in F e - C r Ni a l l o y s . Schumann and von F i r s c h ~7 c o n f i r m e d that A. WIRTH is Lecturer, Department of Metall
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