Effects of magnetic field on athermal and isothermal martensitic transformations in Fe-Ni-Cr alloys

Effects of magnetic field on the athermal martensitic transformation in an Fe-31.4Ni-0.5Cr alloy (mass%), whose M-s temperature is about 183 K, and on the isothermal one in an Fe-25.0Ni-4.0Cr alloy (mass%), whose nose temperature is about 143 K, have been examined by measuring magnetic field susceptibility and magnetization, and by observing optical microstructure, applying pulsed high magnetic fields up to 31 MA/m. The obtained results were the following: The austenitic state in the Fe-31.4Ni-0.5Cr alloy is ferromagnetic, whereas that in the Fe-25.0Ni-4.0Cr alloy is spin glass. Even in the latter alloy, martensitic transformation is induced instantaneously under pulsed magnetic fields higher than a critical one over a wide temperature range, as in the former alloy. This result suggests that the originally isothermal process of martensitic transformation changes to the athermal one under high magnetic fields. Optical microscopy showed that isothermal martensite plates of the Fe-25.0Ni-4.0Cr alloy under no magnetic field grow gradually during isothermal holding. Moreover, morphology of the magnetic field-induced martensites of both alloys was almost the same as that of thermally-induced ones, irrespective of the formation temperature. A thermodynamic calculation for the critical magnetic field vs temperature relation has been done for both the alloys by using an equation previously proposed, and the calculated relations are all in good agreement with the experimental ones in the wide temperature range examined. The present results are almost the same as those in the study previously examined on the athermal and isothermal martensitic transformations in Fe-Ni-Mn alloys under pulsed high magnetic fields.