Interstitial atom configurations in stable and metastable Fe-N and Fe-C solid solutions
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ACTIONS A
tensite are initially distributed between t h e octahedral a n d t e t r a h e d r a l i n t e r s t i t i a l sites.IS,14 T h i s i s n o w t h o u g h t t o be a n i n c o r r e c t m o d e l a s c a r b o n a t o m s a r e b e l i e v e d t o o c c u p y o n l y o c t a h e d r a l s i t e s , ~5,16 a t l e a s t in a l l o y s w i t h o u t c e r t a i n s u b s t i t u t i o n a l e l e m e n t s w h i c h m a y a l t e r t h e n a t u r e of t h e m a r t e n s i t i c t r a n s f o r m a t i o n . T h i s p o i n t is d i s c u s s e d f u r t h e r in t h e Summary. S i m i l a r , but l e s s n u m e r o u s , r e p o r t s h a v e b e e n p u b l i s h e d o n t h e i r o n - n i t r o g e n s y s t e m , ~7-~9 but t h e s e do not p r o v i d e a n a d e q u a t e p i c t u r e o f t h e s t r u c t u r e a n d tempering p r o c e s s e s a s s o c i a t e d with iron-nitrogen austenite and martensite. I n t e r p r e t a t i o n of t h e M 6 s s b a u e r p a t t e r n s of i n t e r s t i t i a l a l l o y s o f i r o n r e q u i r e s c o n s i d e r a t i o n of t h e b a s i c atomic arrangements involved. For example, c a r b o n a n d n i t r o g e n a t o m s in f c c a u s t e n i t e o c c u p y t h e o c t a h e d r a l i n t e r s t i t i a l s i t e s a s s h o w n in Fig. l ( a ) . T h e six n e a r e s t n e i g h b o r i r o n a t o m s w h i c h s u r r o u n d the site form a r e g u l a r octahedron and are s y m m e t r i c a l l y d i s p l a c e d w h e n t h e s i t e is filled. I n body c e n t e r e d tetragonal martensite, c a r b o n and nitrogen a t o m s occupy t h e set o f octahedral interstitial s i t e s i n h e r i t e d f r o m t h e a u s t e n i t e by t h e B a i n t r a n s f o r m a t i o n . T h e s e o c t a h e d r a a r e not r e g u l a r , but s h o r t e n e d i n o n e d i r e c t i o n (Fig. l(b)). W h e n occupied, t h e two iron a t o m s w h i c h a r e t h e f i r s t n e a r n e i g h b o r s of t h e i n t e r s t i t i a l s i t e are s e v e r e l y displaced, a l o n g t h e four fold s y m m e t r y a x i s , producing " d i p o l e distortions". T h e four s e c o n d near neighbor and eight t h i r d near neighbor i r o n a t o m s of t h e s i t e a r e a l s o d i s p l a c e d , but t o a lesser extent. 1. E X P E R I M E N T A L
PROCEDURES
T o facilitate the s t u d y of retained austenite, v i r g i n martensite and the s t r u c t u r a l rearrangements w h i c h m a r t e n s i t e experiences at r o o m temperature, alloys with Ms temperatures c l o s e t o or b e l o w room t e m p e r a t u r e w e r e n e e d e d . I r o n 1.86 wt p c t c a r b o n1 a n d i r o n - n i t r o g e n a l l o y s of g r e a t e r t h a n 2.34 wt p c t n i t r o VOLUME 8A, JANUARY 1977-35
g e n 2° s a t i s f y t h i s c r i t e r i a . V i r t u a l l y f u l l y a u s t e n i t i c f l a k e s (0.0005 i n . i n t h i c k n e s s ) of t h e i r o n c a r b o n a l loy w e r e m a d e by C h o o by s p l a t q u e n c h i n g f r o m t h e m
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