The precipitation of epsilon-carbide in twinned martensite
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MURPHY
AND
JOHN
ANTHONY
WHITEMAN
T e m p e r i n g of m a r t e n s i t e has b e e n i n v e s t i g a t e d by m e a n s of thin foil e l e c t r o n m i c r o s c o p y in a high c a r b o n s t e e l , a high n i c k e l s t e e l , and a s i l i c o n steel. E c a r b i d e has b e e n u n a m b i g u o u s l y identified in each s t e e l . It was found that the c a r b i d e was p r e c i p i t a t e d with the Jack o r i e n t a t i o n relationship:
(O001)e l{(011) a' (10i0) e {I( 2 i l ) a, In the s i l i c o n s t e e l the E c a r b i d e p r e c i p i t a t e d in the f o r m of n e e d l e s which g r e w with a [0110]e close to [21i]a,. This growth d i r e c t i o n m i n i m i z e s the s u r f a c e e n e r g y of the n e e d l e s , yet allows growth in a d i r e c t i o n of low m i s m a t c h . T H E t e m p e r i n g of m a r t e n s i t i c s t e e l s i s an aging p r o c e s s which is n o r m a l l y c o n s i d e r e d to o c c u r in t h r e e stages: 1) The d e c o m p o s i t i o n of m a r t e n s i t e and the p r e c i p i tation of E c a r b i d e . 2) The 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 . 3) The t r a n s f o r m a t i o n of E c a r b i d e to c e m e n t i t e . These stages of t e m p e r i n g have all b e e n well r e viewed. ~-3 In the most r e c e n t work on the t e m p e r i n g of m a r t e n s i t e , t h i n - f o i l e l e c t r o n m i c r o s c o p y has b e e n used to i n v e s t i g a t e the p r e c i p i t a t i o n p r o c e s s e s in twinned m a r t e n s i t e . 2,4-s In such m a r t e n s i t e s the i n h o m o g e n e o u s s h e a r that a c c o m p a n i e s the m a r t e n s i t e f o r m a t i o n is twinning on {211} m a r t e n s i t e p l a n e s . C e m e n t i t e with a {211}a, habit plane f o r m s d u r i n g the t e m p e r i n g of twinned m a r t e n s i t e s . Twinning in i r o n on {211} p l a n e s m o v e s the i r o n a t o m s into p o s i t i o n s v e r y close to those occupied by the i r o n a t o m s in c e m e n t i t e , and also p r o v i d e s a l a r g e r site for the c a r b o n atom on the twin plane. The f o r m a t i o n of c e m e n t i t e on the {211} p l a n e s of i r o n was suggested by B a g a r y a t s k i i 7 before it was known that m a r t e n s i t e was i n t e r n a l l y twinned. Recent work s i n d i c a t e s that the {211}~, habit c e m e n t i t e is d i s t o r t e d and has lattice p a r a m e t e r s that differ f r o m those of n o r m a l c e m e n t i t e . T h i s d i s t o r t i o n is probably due to the c a r b i d e b e i n g deficient in c a r b o n , and i n i t i a l l y c o h e r e n t with the m a t r i x . One f u r t h e r o b s e r v a t i o n is that the d i s t o r t e d c e m e n t i t e m a y have the a s y m m e t r i c a l I s a i c h e v 8 o r i e n t a t i o n r e l a t i o n s h i p with the m a t r i x r a t h e r than the s y m m e t r i c a l Baga r y a t s k i i 7 r e l a t i o n s h i p that has p r e v i o u s l y b e e n r e
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