The tempering of martensite in an Fe-1.5 pct N alloy
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G. THOMAS
The t e m p e r i n g of Fe 1.5 pct N m a r t e n s i t e has b e e n studied at t e m p e r a t u r e s up to 300~ using X - r a y and e l e c t r o n m i c r o s c o p e t e c h n i q u e s . Stage 1 d e c o m p o s i t i o n o c c u r s below " 270~ by the g e n e r a l p r e c l.p l t a .h o n ,. r e s.e m b.h n g s p m o d a l m o r p h ology, of free ~ " (Fe16N2) l a m e l l a e on {001} habit p l a n e s in both m a t r i x and twin c r y s t a l s of the p a r t i a l l y {112} twinned m a r t e n s i t e p l a t e s . Yet, gaged b y c h a n g e s in the X - r a y s p e c t r u m , the r e a c t i o n is d i s c o n t i n u o u s , the t e t r a g o n a l m a r t e n s i t e doublets d e c a y i n g in i n t e n s i t y without change in their Bragg p o s i t i o n s . The a n o m a l y and the f a i l u r e to detect b y e l e c t r o n m i c r o s c o p y r e g i o n s exhibiting f r a c t i o n a l s t a g e s of the fine s c a l e a ' ~ ~ + ~" r e a c t i o n is a t t r i b u t e d to its o c c u r r e n c e at d i f f e r e n t t i m e s in d i f f e r e n t m a r t e n s i t e (or p a r t s of m a r t e n s i t e ) p l a t e s . It is b e l i e v e d that t r a n s f o r m a t i o n o c c u r s in this m a n n e r b e c a u s e the n u c l e a t i o n of c o h e r e n t ~" p l a t e s is c o n t r o l l e d by the p r e v a i l i n g i n t e r n a l s t r e s s field. Thus the t i m e exponent " n " for the r e a c t i o n d e c a y s f r o m a n o r m a l value b e t w e e n 1 and 0.67 to l e s s than 0.3 as s t r e s s r e l i e f by r e c o v e r y d o m i n a t e s the m o r e p r o t r a c t e d s t a g e s of the r e a c tion. Above 200~ the m o r e stable n i t r i d e y' (Fe4N) f o r m s at an i n c r e a s i n g r a t e as p l a t e s on {012} habit p l a n e s , a c c o m p a n i e d by m a r k e d softening.
ALTHOUGH
t h e r e is a v o l u m i n o u s l i t e r a t u r e on the t e m p e r i n g of F e - C m a r t e n s i t e , 1"~ c o m p a r a t i v e l y little a t t e n t i o n has b e e n given to the b e h a v i o r of the a n a l o gous F e - N a l l o y s . This is s u r p r i s i n g in view of the s t r u c t u r a l s i m p l i c i t y of both the t r a n s i t i o n and the e q u i l i b r i u m n i t r i d e s which f o r m d u r i n g the f i r s t and t h i r d stages r e s p e c t i v e l y of the t e m p e r i n g of F e - N martensite. A tetragonal intermediate precipitate, Fe~6N 2 or ~" with a = 5.72A c = 6.29A, was f i r s t i d e n tified by Jack 8'~ who detected its p r e s e n c e only on t e m p e r i n g below 200~ Above this t e m p e r a t u r e , n i t r o g e n m a r t e n s i t e was p r e s u m e d to d e c o m p o s e d i r e c t l y into f e r r i t e and the fcc n i t r i d e , Fe4N or r ' . The work r e p o r t e d h e r e c o n s i s t s of a m i c r o s t r u c t u r a l study of the p r e c i p i t a t i o n p r o c e s s e s d u r i n g the t e m p e r i n g of a quenched (nominally) 1.5 vet pct N alloy. Such a n i t r o g e n r i c h
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