| Ferromagnetic shape memory alloys possess some characteristics, such as large macroscopic strain, high frequency response and so on, which make them as a new generation of materials as actuators and sensors. A simple and low-cost method, unidirectional solidification, has been widely used to prepare polycrystalline ferromagnetic shape memory alloy, which has a lot of superior properties. In unidirectionally solidified ferromagnetic shape memory alloys the solidified direction is the preferred direction of grains, and their mechanical properties exhibit significant anisotropy.Polycrystalline Ni-Mn-Ga samples with different cutting angles to the solidification direction are studied in this paper. In order to improve the macroscopic strain of the material under external fields, mechanical training, namely alternately application of compressive stress in two mutually perpendicular directions of the specimens is carried out at first. In this way, the grain size can be increased, the number of martensite variant can be reduced, and the interference of twin boundaries moving with each other can be weaken.After the mechanical training, experimental programs are made according to the characteristics of the material. Then magneto-strain curves of specimens under constant compressive stresses, stress-strain curves of specimens under constant magnetic fields and temperature-strain curves of specimens under constant magnetic fields are measured, respectively. The magnetic fields are applied along the length and width of the specimen, respectively. The experimental results show nonlinear, hysteresis and anisotropic mechanical behaviors of the material. According to the results, the influence of coupling factors including stress, magnetic field, temperature and cutting angle on the mechanical behaviors of specimens are studied. Some useful conclusions are obtained, which may provide guidance for the materials used in engineering. |
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