Mechanism of formation of Mo 2 C needles in an Fe-C-Mo alloy
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Mechanism of Formation of Mo2C Needles in an Fe-C-Mo Alloy M. G. HALL, K. R. KINSMAN, AND H. I. AARONSON T H E t h r e e b a s i c m o r p h o l o g i e s in which Mo2C develops when p r e c i p i t a t e d in a s s o c i a t i o n with p r o e u t e c t o i d f e r r i t e in r a t h e r low c a r b o n s t e e l s (up to 0.2 pct C) cont a i n i n g s u b s t a n t i a l a m o u n t s of m o l y b d e n u m (up to 4 pct) have b e e n w e l l - d o c u m e n t e d by R e l a n d e r 1 and by B e r r y , D a v e n p o r t , and H o n e y c o m b e . 2'3 T h e s e m o r p h o l o g i e s a r e a g g r e g a t e s of p a r a l l e l , c l o s e l y s p a c e d f i b e r s ; a p p a r e n t l y a l l o t r i o m o r p h i c p r e c i p i t a t e s f o r m e d at a : 7 boundaries (currently termed "interphase precipitat i o n " ) ; and W i d m a n s ~ t t e n n e e d l e s . O r i e n t a t i o n r e l a t i o n s h i p and habit d i r e c t i o n data i n d i c a t e that the f i b r o u s s t r u c t u r e m a y have n u c l e a t e d in the a u s t e n i t e a n d that the n e e d l e s a r e l i k e l y to have f o r m e d within the f e r r i t e p h a s e . 2'3 No i n f o r m a t i o n has a s yet b e e n r e p o r t e d upon the growth k i n e t i c s of any of t h e s e c a r b i d e s . T h i s c o m m u n i c a t i o n r e p o r t s such r e s u l t s for the n e e d l e m o r p h o l o g y , which is p a r t i c u l a r l y a m e n a b l e to s t u d i e s of t h i s type. T a k e n in c o n j u n c t i o n with m i c r o s t r u c t u r a l o b s e r v a t i o n s , t h e s e r e s u l t s will be s e e n to c o n f i r m in full the c o n c l u s i o n with r e s p e c t to t h i s m o r p h o l o g y d r a w n f r o m the p r i o r c r y s t a l l o g r a p h i c studies. A v a c u u m m e l t e d and c a s t F e - 0 . 1 1 pct C-1.95 pct Mo alloy, h o m o g e n i z e d 3 days at 1300~ was u s e d for this study. F o i l s p e c i m e n s 0.012 cm thick w e r e a u s t e n i t i z e d for 15 rain at 1300~ and then i s o t h e r m a l l y r e a c t e d in the t e m p e r a t u r e r a n g e 600 ~ to 800~ for t i m e s a s long a s 24 h r . S p e c i m e n s w e r e t h i n n e d for t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y in a m e t h a n o l bath cont a i n i n g 1 pct p e r c h l o r i c a c i d and m a i n t a i n e d at -70~ and e x a m i n e d in a P h i l i p s EM 200 e l e c t r o n m i c r o s c o p e . Fig. 1 shows t y p i c a l m i c r o g r a p h s of f e r r i t e at r e l a t i v e l y e a r l y r e a c t i o n t i m e s , d e m o n s t r a t i n g that it i s f r e e of c a r b i d e s . * R a t h e r l a r g e v o l u m e s of f e r r i t e *The ferrite grainsize decreasedand the averagedislocationdensityincreased with decreasingreactiontemperaturethroughoutthe rangestudied. f o r m e d in this m a n n e r , a s s u g g e s t e d by the c o n s i d e r a b l e s e p a r a t i o n s i n t i m e b e t w e e n the T T T - c u r v e s for the i n i t i a t i o n of the p r o e
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