EFFECT OF MAGNETIC-FIELDS ON ATHERMAL AND ISOTHERMAL MARTENSITIC TRANSFORMATIONS IN FE-NI-MN ALLOYS

The effect of magnetic fields on the athermal martensitic transformati on in an Fe-31.4Ni-0.5Mn alloy (mass%), whose M(s) temperature is abou t 195 K, and on the isothermal one in an Fe-24.9Ni-3.9Mn alloy (mass%) , whose nose temperature is about 153 K, have been examined in order t o clarify the difference between the athermal and isothermal kinetics of martensitic transformations by measuring magnetic field susceptibil ity and magnetization, and by observing optical microstructure, applyi ng pulsed ultra high magnetic fields up to 31 MA/m. As a result, the f ollowing characteristics were found: The austenitic state in the forme r alloy was ferromagnetic, whereas that in the latter alloy was microm agnetic and/or spin glass. Even in the latter alloy, martensitic trans formation was induced instantaneously under pulsed magnetic fields hig her than a critical one over a wide temperature range, as in the forme r alloy. This result suggests that the originally isothermal kinetics of martensitic transformation is changed to the athermal one under hig h magnetic fields. Optical microscopy showed that respective isotherma l martensite plates of the Fe-24.9Ni-3.9Mn alloy under no magnetic fie ld grew gradually during isothermal holding. Moreover, morphology of t he magnetic field-induced martensites of both the alloys was almost th e same as that of thermally-induced ones, irrespective of the formatio n temperature. A thermodynamic calculation for the critical magnetic f ield vs. temperature relation was done for both the alloys by using an equation previously proposed, and the calculated relations were all i n good agreement with the experimental ones in the wide temperature ra nge examined.