Effect of Magnetostatic Interactions on Twin Boundary Motion in NiMnGa Magnetic Shape Memory Alloy
We investigated the effect of magnetostatic interactions on the field-induced reorientation of martensite variants in Ni50.0Mn27.5Ga22.5. The reorientation, achieved by sweeping a single Type-II twin boundary along the sample, was triggered by a twinning stress of about 0.1 MPa. However, depending on the initial position of the twin boundary, the magnetic field providing the critical stress varied in the range 832 kA/m. By taking into account the variants sizes and their mutual interactions, we explained the observed dependence of the switching field on the location of the boundary. The resulting match between model predictions and measurements illustrates the fundamental role played by demagnetization effects and magnetostatic interactions in magnetic shape memory effect.
Sensing Strain with Ni-Mn-Ga
Abstract Deformation rearranges the crystal lattice in magnetic shape memory alloys, which changes all anisotropic properties of the material. This study investigates leveraging the deformation-induced change of magnetic permeability for a strain measurement technique. A Ni-Mn-Ga single crystal placed inside a doubly wound coil with a primary and a secondary winding was used as a strain sensor. An AC voltage excited the primary coil and the secondary voltage was measured as the sample was strained from 0 to 5.2%. This method varies from other methods that utilize complex magnetic circuits, require high magnetic fields, or other sensing methods such as Hall probes. When the sensor element was tested statically by compressing the element manually against a bias magnetic field perpendicular to the load axis, the voltage output varied from 129.7 mV to 164.2 mV. The dynamic performance of the sensor was tested by cycling the element between 25 and 100 Hz in compression against a bias magnetic field in a displacement controlled magneto-mechanical test system. The bias magnetic field was varied from 0.2 to 0.8 T (0.16 to 0.64 MA/m) while the cyclic displacement was varied from 0.5 to 4.5% strain. The voltage amplitude of the signal in the secondary coil increased with decreasing tensile strain. The full scale RMS voltage at a 200 mm stroke increased from 53.0 mV to 78.4 mV as the bias magnetic field decreased from 0.8T to 0.2 T. As the element was compressed, there was no difference in the sensor output voltage between the static and dynamic tests. When the element expanded during unloading, the voltage output of the sensor from the static test matched the voltage output during compression. For the dynamic testing, the voltage output of the sensor exhibited a hysteresis from the loading voltage output, the hysteresis increased when the strain rate increased.
DISLOCATION MECHANISM OF TWINNING IN Ni–Mn–Ga
Tensile tests were performed in situ in a transmission electron microscope to investigate the twinning mechanism in non-modulated Ni–Mn–Ga martensite. The reorientation of the twin variants occurs via twinning dislocations. Their generation and movement were followed; the glide plane and Burgers vector were verified. Individual twinning dislocations were visualized.