Over 2% magnetic-field-induced strain in a polycrystalline Ni50Mn28.5Ga21.5 alloy prepared by directional solidification

Ni–Mn–Ga single crystal alloys can exhibit giant magnetic shape memory effect through variant reorientation induced by the magnetic field. However, such effect is greatly weakened in polycrystalline alloys, due to the orientation differences of martensite variants and the constraints of grain boundaries. Here, to improve the magnetic shape memory effect, a polycrystalline Ni50Mn28.5Ga21.5 alloy with coarse columnar shaped grains and strong A texture is prepared by directional solidification. With the aids of mechanical training, the twinning stress of five-layered modulated (5 M) martensite in the directionally solidified Ni50Mn28.5Ga21.5 alloy is successfully lowered to ~0.9 MPa. A giant magnetic field induced strain up to ~2.1% is achieved under the magnetic field of 1 T, being much higher than those reported previously in polycrystalline alloys. In addition, a reversible magnetostrain of ~0.4% is also attained without the assistance of an external stress or a magnetic field. It is demonstrated that the microstructure control through directional solidification as well as mechanical training could be an effective solution to enhance the magnetic field induced output strain in polycrystalline Ni–Mn-Ga alloys.