Austenite-martensite phase transformation and magneto-mechanical studies on nickel manganese gallium single crystal alloy

The results of experiments on austenite-martensite phase transformation and magneto-mechanical behavior of a NiMnGa ferromagnetic shape-memory alloy are reported. This alloy undergoes a cubic to tetragonal martensitic phase transformation at about 5 °C and both phases are ferromagnetic. These experiments were conducted using a Magneto-Mechanical Testing Machine (MMTM) that is capable of simultaneously applying a uniaxial load and a magnetic field that can be varied in a plane containing the mechanical loading axis. One of the main accomplishments of this work was the extension of these measurements to tensile loads. Two types of experiments were conducted. First a set of experiments were performed to determine the effects of applied loads and magnetic fields on the transformation temperature of the alloy. The effect of compression on the phase transformation is about 0.2 to 0.4 K/MPa, while the effect of tension is only 0.1 to 0.2 K/MPa. These results agree with predictions made using the Clausius-Clapeyron equation. The effects of applied magnetic field are more complicated and do not follow the simple tends predicted by this equation. Observations of the microstructure that forms during transformation agree reasonably well with the predictions of the crystallographic theory of martensite.;The second set of experiments was conducted to measure the behavior of the alloy under a constant load and applied magnetic fields. These experimental results are directly applicable to using this material in an actuator. Several different magnetic fields paths were used to determine if this has an effect on the strains observed. Measurements of the specimen average magnetization where also made using Hall probes to measure the stray field produced by the specimen during these experiments. These results have some portions of the strain-magnetization curves that are linear, which is what is predicted by models that have both of these quantities directly related to variant volume fractions. Two of the key parameters for actuators that were measured are blocking stress and work output. It was found that the maximum work output occurs for a small tensile bias load due to the reduced effect of specimen demagnetization in this case. The blocking stress in tension appears to be just above the largest tensile stress of 3 MPa that could be applied, which is similar to the value in compression. The values of these two parameters compare well to values in the literature for compression, while the tensile results are the first to be reported.