Effect of magnetic field and hydrostatic pressure on martensitic transformation and its kinetics

We performed recent studies on the effects of magnetic field and hydrostatic pressure on martensitic transformations in some ferrous and nonferrous alloys. The studies clarified the effects of magnetic field and hydrostatic pressure on martensitic transformation start temperature, the nature of magnetoelastic martensitic transformation and the morphology of martensites and transformation kinetics of athermal and isothermal transformations. Transformation start temperatures of all ferrous alloys examined increase with increasing magnetic field, but those of nonferrous alloys, such as Ti-Ni and Cu-Al-Ni shape memory alloys, are not affected. On the other hand, the transformation start temperature decreases with increasing hydrostatic pressure in some ferrous alloys, but increases in Cu-Al-Ni alloys. The magnetic field and hydrostatic pressure dependences of the martensitic start temperature are in good agreement with those calculated by our proposed equations. While investigations in the work on the ferrous Fe-Ni-Co-Ti shape memory alloy, we found that magnetoelastic martensitic transformation appears and, in addition, several martensite plates grow nearly parallel to the direction of the applied magnetic field in the specimen of Fe-Ni alloy single crystal. We further found that the isothermal process in Fe-Ni-Mn alloy changes to the athermal one under magnetic field and the athermal process changes to the isothermal one under hydrostatic pressure. Based on these facts, a phenomenological theory was constructed, which unifies the two transformation processes.