Stress-induced martensite to austenite phase transformation in Ni ₂MnGa magnetic shape memory alloys

The effects of temperature on the stress-induced variant reorientation process and the possibility to trigger stress-induced martensite to austenite phase transformation in Ni ₂MnGa single crystals were investigated under compression. It is revealed that, as the temperature decreases, both the critical stress (under 16kG magnetic field) and the critical magnetic field levels (under 1MPa) for martensite variant reorientation increase. Magnetic-field-induced strain is also found to increase with temperature. Near the austenite start temperature, it is possible to attain stress-induced martensite to austenite transformation under constant magnetic field. A thermodynamical guideline is introduced to explain the conditions for stress-induced martensite to austenite transformation in Ni ₂MnGa alloys. It is concluded that, when magnetocrystalline anisotropy energy is large enough, stress-induced martensite to austenite transformation can be achieved within a narrow temperature range below austenite start temperature.