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

Abstract

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 oC and both phases are ferromagnetic. These experiments were conducted using a Magneto- Mechanical Testing Machine (MMTM) that is capable of simultaneously applying a uni- axial load and a magnetic ¯eld 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 e®ects of applied loads and magnetic ¯elds on the transformation temperature of the alloy. The e®ect of compression on the phase transformation is about 0.2 to 0.4 K/MPa, while the e®ect of tension is only 0.1 to 0.2 K/MPa. These results agree with predictions made using the Clausius-Clapeyron equation. The e®ects of applied magnetic ¯eld 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 crystal- lographic theory of martensite. The second set of experiments was conducted to measure the behavior of the alloy un- der a constant load and applied magnetic ¯elds. These experimental results are directly applicable to using this material in an actuator. Several di®erent magnetic ¯elds paths were used to determine if this has an e®ect on the strains observed. Measurements of the specimen average magnetization where also made using Hall probes to measure the stray ¯eld produced by the specimen during these experiments. These results have some portions of the strain-magnetization curves that are linear, which is what is pre- dicted 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 e®ect 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 results to be reported.