Measurement and interpretation of isothermal martensitic kinetics
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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 s in i r o n - b a s e a l l o y s h a v e r e c e n t l y been c l a s s i f i e d a c c o r d i n g t o t h e obs e r v e d k i n e t i c b e h a v i o r . I n one o f t h e c l a s s i f i c a t i o n s , * t h e t r a n s f o r m a t i o n s f a l l into t h r e e c a t e g o r i e s . T h e f i r s t o f t h e s e is t h e well-known a t h e r m a l t r a n s f o r m a t i o n , w h e r e the p r o g r e s s of t h e r e a c t i o n d e p e n d s m a i n ly o n f a l l i n g t e m p e r a t u r e . T h e s e c o n d g r o u p i n c l u d e s certain F e - N i and F e - N i - C a l l o y s with transformation r a n g e s at s u b z e r o t e m p e r a t u r e s , w h e r e t h e r e a c t i o n is i n i t i a t e d by a n a b r u p t b u r s t . A l s o , i n t h i s t e m p e r a t u r e r a n g e , we h a v e t h e t h i r d c a t e g o r y o f c o m p l e t e l y i s o t h e r m a l b e h a v i o r , a s e x h i b i t e d by F e - N i - M n a n d F e - N i - C r alloys. Another type of classification 2 a s s o c i a t e s the a t h e r mal behavior with dynamic stabilization. In that context, the burst and the completely-isothermal t r a n s f o r m a t i o n s are g r o u p e d t o g e t h e r as " i s o t h e r m a l , " t h e c o m m o n feature between them b e i n g the a b s e n c e of concurrent stabilization. B a s e d on r e c e n t f i n d i n g s , R a g h a v a n a n d C o h e n 3 h a v e redefined t h e transformation behavior at s u b z e r o t e m p e r a t u r e s in t h e f o l l o w i n g m a n n e r . T h e y h a v e n o w s h o w n that a completely isothermal transformation to l e n t i c u l a r m a r t e n s i t e o c c u r s in a n F e - ~ 2 9 N i * a l l o y *Weight percentages are given throughout the text, unless otherwise indicated. within a n a r r o w temperature r a n g e , a n d this i s o t h e r m a l b e h a v i o r is m e a s u r a b l e f r o m t h e v e r y b e g i n n i n g of the t r a n s f o r m a t i o n . O n q u e n c h i n g t h e s a m e a l l o y t o s t i l l l o w e r temperatures, however, transformation d u r i n g c o o l i n g t o t h e t e s t t e m p e r a t u r e c a n n o t be avoided, a n d t h e t e r m " a n i s o t h e r m a l " is a d o p t e d h e r e to d e s c r i b e such r e a c t i o n k i n e t i c s . T h e t r a n s f o r m a t i o n u n d e r s u c h c o n d i t i o n s i s t h e r m a l l y a c t i v a t e d , but t h e a c t i v a t i o n e n e r g i e s f o r n u c l e a t i o n a r e t h e n so l o w that t h e t r a n s f o r m a t i o n r a t e s c a n n o t be m e a s u r e d e x perimentally. In the same F e - N i alloy, b u r s t s with an a u d i b l e c l i c k o c c u r o n q u e n c h i n g w e U into t h i s r a n g e , V. RAGHAVAN and MORRIS COHEN are Research Associate and Ford Professor of Materials Science and Engineering, respectively,Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, Cambridge, Mass. This paper is based on an invite
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