The effect of magnetic field on shape memory behavior in Heusler-type nickel(,2)manganese-gallium-based compounds

Shape memory alloys (SMAs) have excellent mechanical properties showing large stroke and high power density when used as actuators. In terms of response speed, however, conventional SMAs have a drawback due to the isothermal nature of the associated phase transformation.; A new type of SMA, called ferromagnetic SMA, is considered to replace conventional SMAs and is hoped to overcome such a slow response drawback by changing driving mode of shape memory behaviors from thermal to magnetic. The new type of ferromagnetic SMAs is expected to exhibit not only a large displacement but also rapid response when magnetic field is applied and removed. There are three kinds of ferromagnetic SMAs and among them, Ni₂MnGa-based compounds exhibit prominent shape memory effects and superelasticity. In this study, Ni₂MnGa-based alloys were chosen and studied to characterize shape memory behavior upon the application and removal of magnetic field. The relevance of the magnetic field-induced shape memory behavior to the magnetization process was investigated by using transformation and/or the movement of martensite variant interfaces.; Two mechanisms have been proposed for controlling magnetic field-induced shape memory behaviors. One mechanism is related to shape memory behavior associated with magnetic field-induced martensitic transformation. The other is related to the rearrangement of martensite variants by magnetic field application.; Magnetic field-induced martensitic transformation and shape memory effects for single- and poly-crystalline Ni₂MnGa alloys were investigated under various conditions. In single crystalline specimens, it was observed that considerable strain changes are a function of magnetic field at temperatures below M_f (martensite finish temperature). Such strain changes, by application and subsequent removal of magnetic field, may be attributed to the martensite variant motion at lower temperatures than M_f. Magnetic field application made a significant contribution to the martensite transformation and related strain changes (0.3%–0.82%) at temperatures above A_f (austenite finish temperature) in some polycrystalline Ni₂MnGa alloys, where austenite and martensite phases possess paramagnetic and ferromagnetic properties, respectively.