Fine structure in quenched Fe-Al-C steels

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C. M. WAYMAN

The o r i g i n of " e x t r a " s p o t s in e l e c t r o n d i f f r a c t i o n p a t t e r n s taken f r o m r e t a i n e d a u s t e n i t e and m a r t e n s i t e in F e - 7 p c t A1-2 p c t C h a s been i n v e s t i g a t e d . The p r e s e n t i n t e r p r e t a t i o n d i f f e r s f r o m that of p r e v i o u s i n v e s t i g a t o r s . It i s c o n c l u d e d that a high d e n s i t y of fine c a r b i d e p r e c i p i t a t e s ~50A in d i a m e t e r i s r e s p o n s i b l e for the e x t r a r e f l e c t i o n s . The p r e c i p i t a t e s have the p e r o v s k i t e s t r u c t u r e and grow e p i t a x i a l l y in the a u s t e n i t e d u r i n g quenching. When m a r t e n s i t e f o r m s at a l o w e r t e m p e r a t u r e , the p r e c i p i t a t e s a r e t r a n s f o r m e d i n s i t u .

M A R T E N S I T E f o r m e d in a l u m i n u m s t e e l s h a s b e e n the s u b j e c t of a n u m b e r of r e c e n t i n v e s t i g a t i o n s 1-5 and i s known to have an u n u s u a l l y high t e t r a g o n a l i t y . A l though the c r y s t a l l o g r a p h y and m o r p h o l o g y of A l - s t e e l m a r t e n s i t e s have been d e t e r m i n e d by X - r a y d i f f r a c t i o n , o p t i c a l m e t a l l o g r a p h y and 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 , an u n r e s o l v e d p r o b l e m c o n c e r n i n g " e x t r a " s p o t s which a p p e a r in t r a n s m i s s i o n e l e c t r o n d i f f r a c t i o n p a t t e r n s r e m a i n s . N i s h i y a m a e l a l . 4 s u g g e s t e d that the e x t r a r e f l e c t i o n s a p p e a r i n g in the m a r t e n s i t e a r e a c o n s e q u e n c e of the Bain s t r a i n o p e r a t i n g on an a u s t e n i t e having the Cu3Au s u p e r l a t t i c e . M o r e r e c e n t l y T a d a k i e t a l . 5 i n v e s t i g a t e d both the a u s t e n i t e and m a r t e n s i t e in F e - 9 . 7 p c t A l - l . 5 pct C and r e p o r t e d that the a u s t e n i t e c o n s i s t s of a n t i p h a s e d o m a i n s ~120A in d i a m e t e r and that the a n t i p h a s e b o u n d a r i e s a r e of the s e c o n d t y p e . H o w e v e r , the s t r e a k i n g of both f u n d a m e n t a l and s u p e r l a t t i c e r e f l e c t i o n s w a s a t t r i b u t e d to a n t i p h a s e b o u n d a r i e s , and t h i s i n t e r p r e t a t i o n a p p e a r s doubtful s i n c e f u n d a m e n t a l r e f l e c t i o n s should r e m a i n u n a f f e c t e d . F u r t h e r , the e l e c t r o n i c and e l a s t i c i n t e r a c t i o n s b e t w e e n i r o n and a l u m i n u m a t o m s give no i n d i c a t i o n of a s u p e r l a t t i c e at high t e m p e r a t u r e s . On the o t h e r hand the e x t r a s p o t s w e r e left u n e x p l a i n e d by W a t a n a b e and W a y m a n 1,2 who e x p r e s s e d s o m e doubts about the i n t e r p r e t a t i o n s of N i s h i y a m a e t al. In the p r e s e n t w o r k a d e t a i l e d e x