In recent years, magnetic shape memory alloys (MSMAs) have attracted interest because of the ability to obtain one order of magnitude higher magnetic field induced strain (MFIS) than magnetostrictive materials and few orders of magnitude faster dynamic response than conventional shape memory alloys. They can also be used for sensing and passive power generation due to significant changes in magnetization upon the application of fluctuating mechanical forces or displacements making them truly multifunctional. Previous studies have mainly focused on field-induced martensite variant reorientation being the governing mechanism for the magnetic field-induced shape change, especially in NiMnGa alloys, which has resulted in low actuation stress levels. We have recently shown that field-induced phase transition is possible in NiMnGa alloys and it can be reversible or irreversible depending on the magnitudes of stress hysteresis, magnetocrystalline anisotropy and saturation magnetization. Utilizing field-induced phase transformation in NiMnGa alloys instead of variant reorientation, more than one order of magnitude increase in actuation stress is achieved. An extensive experimental program is undertaken on NiMnGa and NiMnCoIn single crystals in quest for identifying physical and microstructural parameters critical in field-induced phase transformation phenomena. A thermodynamical framework is constructed and will be discussed to identify the effects of magnetic field on martensitic phase transformations. Using the thermodynamical model, guidelines to increase the actuation stress levels and possible future directions for research on magnetic shape memory alloys will be presented.

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1. Cyclic Stability in Ultrafine Grained High Temperature Shape Memory Alloys