Substructure of ausformed martensite in Fe-Ni and Fe-Ni-C alloys
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AND
C. M. WAYMAN
T h e m a r t e n s i t e s u b s t r u c t u r e a f t e r a u s f o r m i n g has been s t u d i e d f o r two d i f f e r e n t m a r t e n s i t e m o r p h o l o g i e s : p a r t i a l l y twinned, l e n t i c u l a r m a r t e n s i t e ( F e - 3 3 pct Ni, M s = - 105~ and c o m p l e t e l y twinned " t h i n p l a t e " m a r t e n s i t e ( F e - 3 1 p c t N i - 0 . 2 3 p c t C, M s = - 1 7 0 ~ and in both c a s e s a u s f o r m i n g p r o d u c e s a d i s l o c a t i o n c e l l s t r u c t u r e in the a u s t e n i t e which is i n h e r i t e d , without m o d i f i c a t i o n , by the m a r t e n s i t e . In the F e - N i alloy, the d i s l o c a t i o n c e l l s t r u c t u r e is found in both the twinned (near the m i d r i b ) and untwinned ( n e a r the i n t e r f a c e ) r e g i o n s , the l a t t e r a l s o c o n t a i n i n g a r e g u l a r d i s l o c a t i o n n e t w o r k g e n e r a t e d by the t r a n s f o r m a t i o n i t s e l f and which i s u n a l t e r e d by the a u s t e n i t e d i s l o c a t i o n c e l l s t r u c t u r e . S i m i l a r l y , in the F e - N i - C a l l o y , the t r a n s f o r m a t i o n twins a r e u n i m p e d e d by the p r i o r c e l l s t r u c t u r e . T h e s e o b s e r v a t i o n s show that c a r b i d e p r e c i p i t a t i o n d u r i n g a u s f o r m ing is not n e c e s s a r i l y r e q u i r e d to pin the a u s t e n i t e c e l l s t r u c t u r e and that the m a r t e n s i t e - a u s t e n i t e i n t e r f a c e , b a c k e d by e i t h e r twins o r d i s l o c a t i o n s , d o e s not exhibit a " s w e e p i n g " e f f e c t . Although the m a r t e n s i t e t r a n s f o r m a t i o n twins a r e not inhibited b y the a u s f o r m i n g c e l l s t r u c t u r e , they do u n d e r g o a r e f i n e m e n t with i n c r e a s e d a u s f o r m i n g , and it is i n d i c a t e d that the t r a n s f o r m a t i o n twin width in m a r t e n s i t e d e p e n d s on the a u s t e n i t e h a r d n e s s . H o w e v e r , the r e l a t i v e twin widths r e m a i n unchanged, a s e x p e c t e d f r o m the c r y s t a l l o g r a p h i c t h e o r y .
S I N C E the a u s f o r m i n g i s an a t t r a c t i v e t h e r m o m e c h a n i c a l t r e a t m e n t f o r o b t a i n i n g high s t r e n g t h s t e e l s , t h e r e have been n u m e r o u s i n v e s t i g a t i o n s of the m e c h a n i c a l p r o p e r t i e s and the s u b s t r u c t u r e of a u s f o r m e d martensites.~-7 Although v a r i o u s f a c t o r s have b e e n s u g g e s t e d to c a u s e the s t r e n g t h of a u s f o r m e d m a r t e n s i t e , the m o s t i m p o r t a n t of t h e s e have b e e n c o n s i d e r e d to be the i n h e r i t a n c e by the m a r t e n s i t e of d e f e c t s g e n e r a t e d d u r i n g the d e f o r m a t i o n of a u s t e n i t e at e l e v a t e d t e m p e r a t u r e s , t h e s e d e f e c t s b e i n g a) d i s l o c a t i o n t a n g l e s and c e l l
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