Preparation And External Field Training Of Ni-Mn-Ga Unidirectional Solidification Alloy

With the increasing demand for miniaturized electronic devices and mechanical instruments with high efficiency, the sensor materials with properties of greater response strain, larger energy density and faster response are in great need. Ni-Mn-Ga alloys, with the combination of large output, high response frequence and easy control, become a promising magnetic field driven actuator material. Although Ni-Mn-Ga single crystals possess superior magnetic shape memory performance, their fabrication process is very complicated and the cost is high for practical application. It is of great significance to prepare strong textured polycrystalline alloys to realize the property improvement for the further application.In this thesis, the unidirectionally solidified Ni50Mn28.5Ga21.5 and Ni50Mn30Ga20 polycrystalline alloys were prepared. The crystal structure, micro structure, martensitic transformation and magnetic properties have been investigated. Moreover, external field trainings were performed on Ni5oMn3oGa2o alloy and the corresponding magnetic field-induced strains were measured. The main results are summarized as follows:(1) The unidirectionally solidified Ni50Mn28.5Ga21.5 alloy has FCC austenite and monoclinic 5M martensite with twins and micro-twins at room temperature. The unidirectional solidification Ni50Mn30Ga20 alloy possesses monoclinic 7M martensite at room temperature. TEM observation shows (1010) as the twinning plane for the compound twin in the 7M martensite. Moreover, two modulational directions have been also observed in the Fast Fourier transform diagram of the HREM at the interface, and six satellite spots were observed between two main spots. Four kinds of martensitic variants with different orientations were found in the bright and dark field images.(2) The Curie temperature of unidirectionally solidified Ni50Mn28.5Ga21.5 alloy is 371.88K under the field of 50G and the martensitic transformation temperature is around room temperature. The Curie temperature of Ni5oMn3oGa2o alloy is 367.79K and the martensitic transformation temperature is higher than room temperature. Strong magnetic field has less influence on the Curie temperature of the two alloys. With the increase of the temperature, the alloys transform from ferromagnetic martensite into ferromagnetic austenite, and finally into paramagnetic austenite. At the same magnetic field, saturation magnetization increases with the decrease of the temperature.(3) For the unidirectionally solidified Ni50Mn30Ga20 alloy, during the repeated mechanical training with multiple directions, the sample went through the twin boundary excitation stage, mobile stage and the twin boundary movement completion stage with the change of stress. The twin strain and residual strain increased with the increase of the training times. In the thermal-mechanical trainings, when the compression stress was applied perpendicular to the solidification direction, texture components did not change significantly before and after training. When the compression stress was applied parallel to the direction of the solidification, variant number reduced and the strong texture component with <010>// load direction appeared. The texture intensity increased with the increase of the training times. After six-cycles training, the sample mainly consisted of component with the <010> 7M// load direction. After three-field coupling trainings, the sample tended to be a single variant of the (100) [010] texture component, and [010]//the load direction and the magnetic field direction.(4) With the increase of the mechanical training times along the solidification direction in Ni5oMn3oGa2o alloy, the magnetic field induced strain gradually increased when the unidirectional solidification direction was perpendicular to the direction of the applied magnetic field. With the increase of the thermal-mechanical training cycles, the magnetic field-induced strain increased when the unidirectional solidification direction was parallel to the direction of the applied magnetic field. However, the magnetic field induced strain decreased when the unidirectional solidification direction was perpendicular to the direction of the applied magnetic field. In the three-field coupling trainings, when increasing the intensity of applied training magnetic field or the times of training cycles, the measured magnetic field-induced strain increased under the same stress conditions.