Ni–Mn–Ga is a ferromagnetic alloy that can exhibit the shape memory effect or superelasticity in the presence of a magnetic field. The behavior of the material is largely due to its microstructure, which is thought to be made of tetragonal martensite variants, each exhibiting an innate magnetization aligned approximately with the short side of the unit cell. Because the reorientation strain can be induced and recovered by either magnetic field or mechanical stress, it can be induced at frequencies larger than 1 kHz, which makes the material suitable for high-frequency actuation, sensing, or power harvesting applications. This paper investigates the power harvesting capability of Ni–Mn–Ga wrapped with a pick-up coil under a bi-axial magnetic field. In this work, both experimental tests and numerical simulations are used to identify the optimal direction of the externally applied magnetic field in order to achieve maximum open-circuit voltage output from a particular Ni–Mn–Ga based power harvester. Results suggest that significantly more power can be achieved with the bias field applied at an angle of 10°–20° off the perpendicular to the coil axis and the compressive stress. We believe that this increased power output is due to the saturation of domain walls due to a small component of the magnetic field along the direction of the coil.

Researchers have attempted to model the magneto-mechanical behavior of magnetic shape memory alloys (MSMAs) for over a decade, but all of the models developed to date have only been validated against experimental data generated under two-dimensional loading conditions. As efforts have been underway to develop models able to predict the most general (i.e. 3D) loading conditions for the material, there is a need for experimental data to support the calibration and validation of these models. This paper presents magneto-mechanical data from experiments where a MSMA specimen whose microstructure accommodates three martensite variants is subjected to three-dimensional magneto-mechanical loading. To the best of our knowledge, all prior experimental investigations on MSMA have been performed on samples accommodating two martensite variants and exposed to two-dimensional magneto-mechanical loads. The experimental results from the 3D loading of the three variant MSMA specimen are used to calibrate and validate a 3D model developed by this group [LaMaster et al. (2014)]. This model assumes that three martensite variants coexist in the material. The LaMaster et al. model captures the general trends seen in the experimental data, but does not predict the data with a high degree of accuracy. Possible reasons for the mismatch between experimental data and model predictions are discussed.

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Magnetic shape memory alloys are a relatively new class of materials that are suitable for actuation, sensing, and power harvesting. The power harvesting capability comes from the change in magnetization that the material exhibits when internal martensitic variants change orientation. In typical power harvesting tests, the material is loaded with axial compression in the presence of a bias magnetic field applied normal to the compressive loading direction. However, previous results suggest that having a component of the bias magnetic field applied axially, parallel to the compressive stress, can increase the power output of MSMAs. Furthermore, most of the MSMAs power harvesting results reported to date focused on the open circuit voltage that the material can generate during cyclic loading. However, this information is not indicative of the true power harvesting capability of the material and one has to focus on the power output of the material instead. This paper presents voltage trends and power output data for a MSMA sample exposed simultaneously to a cyclic compressive stress and bi-axial magnetic field.