Martensite-to-austenite reverse transformation in Fe-Ni-Co alloys
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T H E f o r w a r d and r e v e r s e m a r t e n s i t i c t r a n s f o r m a tion has b e e n i n v e s t i g a t e d e x t e n s i v e l y in F e - N i b i n a r y a l l o y s . l - 9 M a r t e n s i t e is also known to o c c u r in c e r t a i n F e - N i - C o alloys 1~ where t r a n s f o r m a t i o n is s i m i l a r to that in the F e - N i b i n a r y case. 11 The m a j o r app l i c a t i o n of F e - N i - C o a l l o y s is for s e a l i n g to h a r d b o r o s i l i c a t e g l a s s e s where t h e r m a l e x p a n s i o n c o m p a t i b i l i t y is n e c e s s a r y . Since this is not p o s s i b l e when m a r t e n s i t e i s f o r m e d , c o m m e r c i a l c o m p o s i t i o n s do not u s u a l l y t r a n s f o r m to m a r t e n s i t e at t e m p e r a t u r e s above that of liquid n i t r o g e n . S e v e r a l c h a r a c t e r i s t i c s of F e - N i - C o a l l o y s differ f r o m the b i n a r y F e - N i a l l o y s used in r e v e r s e m a r t e n site t r a n s f o r m a t i o n i n v e s t i g a t i o n s . Both a n s t e n i t i c and m a r t e n s i t i c p h a s e s a r e f e r r o m a g n e t i c with high s a t u r a t i o n and C u r i e t e m p e r a t u r e s s u b s t a n t i a l l y above r o o m t e m p e r a t u r e . 12 C o n v e r s e l y , in F e - N i a l l o y s c o n t a i n i n g about 30 at. pct Ni, f e r r o m a g n e t i c s a t u r a tion induction and C u r i e t e m p e r a t u r e a r e c o n s i d e r a b l y r e d u c e d in the a u s t e n i t e phase. Also, the t h e r m a l e x p a n s i o n coefficient of the F e - N i - C o a u s t e n i t e phase b e l o w its Curie t e m p e r a t u r e is s m a l l , due to the high positive value of s p o n t a n e o u s m a g n e t o s t r i c t i o n , w h e r e a s a n o m a l o u s e x p a n s i o n of the a u s t e n i t e phase in F e - N i alloys used in f o r w a r d and r e v e r s e m a r t e n site t r a n s f o r m a t i o n s t u d i e s i s not d o c u m e n t e d . An inv e s t i g a t i o n of the r e v e r s e m a r t e n s i t e - t o - a u s t e n i t e t r a n s f o r m a t i o n in the t e r n a r y F e - N i - C o a l l o y s is r e ported herein. 1) E X P E R I M E N T A L PROCEDURE S a m p l e s of t h r e e e x p e r i m e n t a l a l l o y s which w e r e known to t r a n s f o r m to m a r t e n s i t e at t e m p e r a t u r e s below room t e m p e r a t u r e were obtained f r o m the Specialty Metal D i v i s i o n of Westinghouse. T h e s e m a t e r i a l s had b e e n a i r induction m e l t e d , forged and g r o u n d to b a r s a p p r o x i m a t e l y ~ in. diam by 4 in. S p e c i m e n s for d i l a t o m e t r y , 0.250 in. diam by 2.00 in., and for c o e r c i v e f o r c e m e a s u r e m e n t s , 0.250 by 1.00 in. were m a c h i n e d from these b a r s . Additional D. A. COLLING is Physical Metallurgist, MagneticsDepartment, Westinghouse Research Laboratories, Pittsburgh, Pa. Manuscript submitted December 15, 1969. METALLURGICALTRANSACTIONS
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