Ferromagnetic shape memory alloys (FSMA's) have attacted considerable attention due to the large field-induced strains (6-9%) exhibited at room temperature in magnetic fields on the order of 0.5 T. The focus of the research at MIT has been on alloys based on the Ni-Mn-Ga system. Recent progress in understanding and applications of these materials will be reviewed. The crystal structure of the room temperature martensites has been found to be extremely sensitive to alloy composition. Material that exhibits easy twin boundary motion fall in a narrow composition range that includes both tetragonal and orthorhombic martensites. Actuation of FSMA crystals measuring several mm on edge has been achieved using DC, AC, and pulsed magnetic fields. Application of pulsed fields greater than the anisotropy field (~0.6 T) does not induce greater strain, but only increase the speed at which full extension is observed. The large hysteresis associated with twin boundary motion has been used to demonstrate the use of FSMA loaded polymer composites for energy absorption.
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