Magnetic investigation of the effect of small additions of cobalt and manganese on the martensite reversal in an Fe-Ni a
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THE m a g n e t i c p r o p e r t i e s
of F e - N i a l l o y s 1-s and the m a r t e n s i t e t r a n s f o r m a t i o n in this s y s t e m 6-9 have r e ceived much a t t e n t i o n in the l i t e r a t u r e . Of p a r t i c u l a r i n t e r e s t in the p r e s e n t study is the F e - 2 9 . 0 at. pct Ni a l l o y and the effect of s m a l l a d d i t i o n s of cobalt o r m a n g a n e s e on the m a g n e t i c p r o p e r t i e s and the r e v e r s e t r a n s f o r m a t i o n t e m p e r a t u r e . The r e v e r s e t r a n s f o r m a tion 1~ r e f e r s to the t r a n s f o r m a t i o n f r o m m a r t e n s i t e to a u s t e n i t e . N u m e r o u s s t u d i e s 12-15 have b e e n m a d e on the m a r t e n s i t e t r a n s f o r m a t i o n in p u r e F e - N i a l l o y s and the c o m p o s i t i o n dependence of the M s t e m p e r a t u r e in t h e s e b i n a r y a l l o y s is well c h a r a c t e r i z e d . The m o s t c o m p l e t e set of data for M s and A s t e m p e r a t u r e is due to K a u f m a n and Cohen.12 T h e change in M s and A s with c o m p o s i t i o n is quite l a r g e for b i n a r y a l l o y s in the r a n g e of F e - 3 0 pct Ni. The A s t e m p e r a t u r e of the r e v e r s e t r a n s f o r m a t i o n has not b e e n d e s c r i b e d as well s i n c e it is v e r y s e n s i t i v e to the p u r i t y of the alloy and the p o s s i b i l i t y of c o m p e t i n g r e a c t i o n s at the e l e v a t e d temperatures. The p u r p o s e of t h i s p a p e r i s to d e s c r i b e a new t e c h nique for d e t e r m i n i n g A s f r o m e x p e r i m e n t a l m a g n e t i zation m e a s u r e m e n t s obtained by s t a n d a r d t e c h n i q u e s . Since the a u s t e n i t e p h a s e for t h e s e a l l o y s i s p a r a m a g n e t i c in the t e m p e r a t u r e r e g i o n of the t r a n s f o r m a t i o n , m a g n e t i c m e a s u r e m e n t s a r e v e r y useful in following the c o u r s e of the t r a n s f o r m a t i o n f r o m f e r r o m a g n e t i c m a r t e n s i t e to p a r a m a g n e t i c a u s t e n i t e with i n c r e a s i n g t e m p e r a t u r e . In m o s t c a s e s , it was found that the m a g n e t i z a t i o n did not change a b r u p t l y at the t r a n s f o r m a t i o n t e m p e r a t u r e . T h e r e f o r e , the A s t e m p e r a t u r e , which would be r e p r e s e n t e d by a d e p a r t u r e f r o m the n o r m a l l o s s of m a g n e t i z a t i o n with i n c r e a s i n g t e m p e r a t u r e , was difficult to d e t e r m i n e d i r e c t l y f r o m the data. In o r d e r to m o r e a c c u r a t e l y l o c a t e A~, u s e was m a d e of the m o l e c u l a r field a p p r o x i m a t i o n , 6 fitting a B r i l l o u i n function to the e x p e r i m e n t a l data. Although
this m o d e l has s e v e r a l l i m i t a t i o n s , it was found that the t h e o r y did give a good r e p r e s e n t a t i o n of the data in the t e m p e r a t u r e region of the m e a s u
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