Geometric Factors on Magnetically Driven Actuation Behaviour for Polycrystalline Ni-Mn-Ga and its Composites

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The thermo-mechanical behaviour of polycrystalline Ni-Mn-Ga depends on microstructure and several geometric sample properties like grain diameter and sample size. The interdependence of microstructure and shape is more pronounced in plate-shaped specimens. A significant increase of the magnetic field induced strain (MFIS) may be expected for coarse grained specimen because of fewer grain boundaries which are hindering twin boundary movement. It was found that larger grains do not result in higher reorientation strain in every case because the elastically stored energy is distributed on less grain boundaries. Thus, in this case the instability of grain boundaries against cracks is larger than in the case of fine grained samples. The findings were used to design new composites of Ni-Mn-Ga and polyurethane which show up to 1% MFIS and are stable over one million cycles of strain. The particles in this composite are still oriented in a {110} fiber texture and the volume fraction of polyurethane is up to 10%.