The M s temperature and morphology of martensite in Fe-31 Pct Ni-0.23 Pct C alloy
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The M s Temperature and Morphology of Martensite in Fe-31 Pct Ni-0.23 Pct C Alloy TADASHI MAKI, SADAMASA SHIMOOKA, A N D IMAO T A M U R A
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have been many investigations o n the morp h o l o g y a n d s u b s t r u c t u r e o f m a r t e n s i t e in F e - N i a l l o y s . 1-6 T h e m a r t e n s i t e s i n F e - N i a l l o y s c o n t a i n i n g a b o u t 29 t o 33 p c t N i a r e l e n t i c u l a r w i t h a m i d - r i b . A s t h e n i c k e l c o n t e n t i n c r e a s e s ( i . e . , a s t h e Ms t e m p e r a t u r e d e c r e a s e s ) , t h e e x t e n t o f t w i n n e d r e g i o n in the lenticular martensite i n c r e a s e s and the austenitem a r t e n s i t e i n t e r f a c e b e c o m e s s t r a i g h t e r , s-6 I n t h e s e p a s t i n v e s t i g a t i o n s , t h e c h a n g e s in m a r t e n s i t e m o r phology have been examined a s a function of c h e m i c a l c o m p o s i t i o n s , a n d t h e s e r e s u l t s seem to h a v e c o n s i d ered that the morphology and s u b s t r u c t u r e of m a r t e n s i t e do not c h a n g e i f t h e c h e m i c a l c o m p o s i t i o n s o f a l l o y s a r e t h e s a m e . I n t h i s p a p e r , it is s h o w n t h a t t h e morphology and s u b s t r u c t u r e of martensite change w i t h t h e Ms t e m p e r a t u r e ( i . e . , t h e f o r m a t i o n t e m p e r a t u r e ) e v e n in t h e s a m e a l l o y . It i s w e l l - k n o w n that t h e Ms t e m p e r a t u r e i s d e p r e s s e d with d e c r e a s i n g austenitizing temperature w i t h o u t a n y c h a n g e s in t h e c h e m i c a l c o m p o s i t i o n o f t h e a u s t e n i t e ( t h e r m a l s t a b i l i z a t i o n of a u s t e n i t e ) . 7 - 9 I n t h i s r e s e a r c h , t h e Ms t e m p e r a t u r e w a s a l t e r e d by the s e v e r a l austenitizing treatments, and the morphology o f martensites f o r m e d at t h e t e m p e r a t u r e s j u s t b e l o w t h e Ms w a s o b s e r v e d . T h e F e - 3 0 . 8 7 pct N i - 0 . 2 3 p c t C (wt p c t ) a l l o y w a s p r e p a r e d by v a c u u m i n d u c t i o n m e l t i n g . T h e s p e c i -
m e n w a s austenitized at v a r i o u s t e m p e r a t u r e s b e t w e e n 900 ° a n d 1200°C f o r 1 h r i n v a c u u m a n d t h e n furnace-cooled. A f t e r the s p e c i m e n s were c h e m i c a l l y p o l i s h e d slightly in o r d e r to r e m o v e t h e s u r f a c e l a y e r , t h e Ms t e m p e r a t u r e o f e a c h s p e c i m e n w a s d e t e r m i n e d by e l e c t r i c r e s i s t i v i t y c h a n g e . S p e c i m e n s w e r e c o o l e d t o t h e t e m p e r a t u r e j u s t b e l o w t h e Ms , and the optical microscopic (etching solution w a s 5 pct nital) and the t r a n s m i s s i o n electron microscopic observations were c a r r i e d o u t . F i g . 1 s h o w s t h e v a r i a t i o n i n t h e Ms t e m p e r a t u r e w i t h t h e a u s t e n i t i z i n g t e m p e r a t u r e . T h e Ms t e m p e r a t u r e in F e - 3 1 N i - 0 . 2 3 C
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