Martensitic and magnetic transformations in polycrystalline Ni Mn Ga alloys

Abstract

There is an increasing demand for Ni-Mn-Ga Ferromagnetic Shape Memory Alloys (FSMAs) and the expectations from this material are very high. The ternary Ni-Mn-Ga ferromagnetic alloy system has been intensively studied by the researchers for more than a decade for its high Magnetic-Field- Induced Strain (MFIS) over a wide range of actuation frequency. A maximum strain of 6% has been reported so far in the tetragonal martensite phase and ~ 10% in the orthorhombic phase. The strain observed in this alloy system is much higher than that observed in piezoelectric and magnetostrictive materials. The present work is focused to study the thermal treatment on non-stoichiometric polycrystalline Ni-Mn-Ga alloy and to compare the as cast sample with annealed sample. The influence of temperature on the microstructure of the samples has been studied by using Scanning Electron Microscope (SEM) for all the five different Ni-Mn-Ga alloys. SEM images reveal that no martensite bands were observed in as-cast samples, but annealed samples showed the presence of martensite bands. From the SEM result, it is found that the formation of martensite bands is temperature-dependent and the thickness of the martensite band increases with annealing time. X-ray diffraction (XRD) study was performed to identify the phase constitution of the FSMAs. The measurements show that the short time annealing stabilizes the crystal structure, shifts the structural phase transformation to a high temperature. The observed result infers that just like the single crystalline Ni-Mn-Ga, the transformation temperature of polycrystalline Ni-Mn-Ga alloy is also influenced by the composition as well as the thermal treatment. Further, it is observed that annealing decreases the difference between martensite start and the finish temperature range and reduces the area of the magnetic hysteresis of a Ni-Mn-Ga alloy to nearly zero. As the magnetic hysteresis of Ni-Mn-Ga was very narrow, the potential utilization of this composition may be expected in micro-actuators.