Abstract
Magnetic filed-induced martensitic transformations in Fe–Ni–C Invar (Fe-30.2 Ni-0.4C and Fe-30.4 Ni-0.3C (mass%)) and non-Invar (Fe-26.0 Ni-0.4C (mass%)) alloys were studied to clarify the Invar effect on the transformations by means of magnetization measurement and optical microscopy, applying a pulsed ultra-high magnetic field. The morphology of the magnetic field-induced martensites was compared with that of deformation-induced martensites to examine the formation temperature effect on the martensite morphology. As a result, the following are found. A magnetic filed higher than a critical one is needed to induce the martensitic transformations above Ms irrespective as to whether the alloy is Invar or non-Invar. The critical field increases with increasing temperature, and when plotted against the temperature difference (ΔT) from Ms, it lies on a line consisting of two straight parts for the Invar alloys, but on a straight line for the non-Invar alloy. This result and a thermodynamical analysis suggest that the influence of magnetic field on the martensitic transformations in the Invar alloys comprises the Zeeman, high field susceptibility and forced volume magnetostriction effects, while in the non-Invar alloys it comprises the former two effects only. The amount of the magnetic field-induced martensite increases with the maximum strength of magnetic field for all the three alloys, but a little difference is observed among the three alloys in the manner of the increase at the critical magnetic field. The morphology of the magnetic field-induced martensite is the same as that of thermally-induced one in each alloy, irrespective of ΔT and the strength of the magnetic filed. However, the deformation-induced martensites in the Fe-30.2 Ni-0.4C and Fe-26.0 Ni-0.4C alloys are lenticular and butterfly, whereas the magnetic field-induced martensites formed at the same temperature are thin plate-like and lenticular, respectively. This fact indicates that the martensite morphology is not decided by the formation temperature alone.
