Magnetic Field-Induced Martensitic Transformation in an Fe-31.7 at%Ni Alloy

Abstract

The magnetic field-induced martensitic transformation in an Fe-31.7 at%Ni alloy has been studied by means of magnetic measurements and optical and electron microscopies, pulsed magnetic field being applied at the Ultra High Magnetic Field Laboratory, Osaka University. As a result, it is shown that there exists a critical strength of magnetic field to induce the martensitic transformation at a given temperature above M_s. The critical strength increases with increasing temperature difference from M_s, ΔT, and those plotted as a function of ΔT lie on two straight lines bent at a temperature of ΔT=22 K. However, the martensitic transformation does not start in the time when the magnetic field has reached the critical strength, but is delayed. The delay time is dependent on ΔT alone and on both ΔT and H in the temperature ranges above and below ΔT=22 K, respectively. The amount of the magnetic field-induced martensites is almost constant without regard to the maximum strength of applied magnetic field, if ΔT is kept constant, but it increases with decreasing ΔT. Optical and electron microscopy observations show that the morphology of magnetic field-induced martensites is the same as that of thermally induced ones by cooling below M_s, irrespective of ΔT and H, being lenticular and internally twinned. It is suggested from a consideration based on the results that the raise of transformation temperature is not caused by Zeeman effect alone.