The Effect of Manufacturing On The Crystalline And Magnetic Properties Of Ni-Mn-Ga Based Heusler Magnetocaloric Alloys
Date of Award
Doctor of Philosophy
Mechanical and Aerospace Engineering
Dr. Pnina Ari-Gur
Dr. Matthew Cavalli
Dr. Jinseok Kim
Dr. Yang Ren
Smart materials, magnetocaloric alloys, x-ray diffraction, synchrotron, melt spinning, severe plastic deformation
A modern refrigeration unit needs to be compact, operate noise and pollution-free and be energy- and cost-efficient. For these reasons, magnetic refrigeration is an attractive alternative to vapor-compression refrigeration. Magnetic refrigeration utilizes a solid-state refrigerant, which is quiet, energy-efficient and environmentally friendly, unlike the harmful hydrocarbon-based liquid refrigerants. The search for solid state refrigerant exhibiting optimum properties near room temperature is the motivation behind this research work. Heusler alloys are considered for such an application, as they exhibit important functional properties like magnetic shape memory effect, magnetocaloric effect, and are inexpensive. They undergo martensitic transformation from a high symmetry austenite phase to lower symmetry martensite phase. Under the influence of magnetic field, these alloys may show giant magnetocaloric effect (GMCE) which is associated with high entropy changes. When the crystalline and magnetic phase transformations coincide, the GMCE is largest. Manufacturing route and alloy composition both play important roles in defining the crystalline structure and magnetic properties of Ni-Mn-Ga based alloys. For this research, Ni-Mn-Ga based Heusler alloys of different compositions (at.%) were manufactured using melt-spinning (sample ID BMS219: Ni53.4Mn19.6Ga25.4Si1.56, BMS220: Ni55.4Mn18.9Ga23.9Si1.72, BMS222: Ni55.5Mn18.8Ga24Si1.7), high pressure torsion (Ga16-6: Ni54.1Mn19.6Ga24.6Si1.7) and multiple isothermal forging (Ga18-8: Ni56.2Mn18.8Ga23.2Si1.8) techniques. The study of crystalline properties like crystal structures, phase fractions, atomic site occupancies, transformation temperatures, and texture analysis were done using x-ray diffraction (XRD), non-ambient XRD, and synchrotron diffraction experiments. Rietveld refinements of diffraction patterns revealed a variety of crystal structures based on manufacturing route used. The melt-spun ribbons had austenite with cubic L21 crystal structure (Fm̅3m space group) and 7M modulated martensite (I2/m space group). The phase fractions of these austenite and martensite phases were dependent on the process parameters (wheel speed and cast temperature) used for melt-spinning. The texture analysis on melt-spun ribbons revealed the presence of <100>A fiber texture. The high-pressure torsion samples consisted of B2 austenite (Pm̅3m space group) and A1 austenite (Fm̅3m space group). Varying the load and number of revolutions changed the phase fractions in high pressure torsion samples. Pole figures of the high pressure torsion samples indicated the presence of <110>A1 fiber texture. In a multiple isothermal forged sample, a tetragonal (I4/mmm space group) and an orthorhombic (Pmmm space group) crystal structured martensite phases were found. This sample was highly textured and displayed a presence of (001) T and (100)[00̅1]O rolling type textures. Non-ambient XRD scans for BMS222 melt-spun ribbons and Ga18-8 alloys were used to obtain the martensitic transformation temperatures and to confirm the phase fractions in these alloys.
Thermomagnetic measurements and physical properties like electrical resistivity and heat capacity, were performed on the BMS222 melt-spun ribbon. The martensitic transformation temperatures were obtained by using electrical resistivity-temperature curves at varying applied magnetic fields. To confirm the magnetic ordering in the crystalline phases of BMS222 melt-spun ribbons, specific heat was measured with respect to temperature change under zero magnetic field. It was found that austenite phase in BMS222 melt-spun ribbons had ferromagnetic order, whereas the martensite phase exhibited lower magnetic order compared to austenite.
Restricted to Campus until
Bhale, Pranav, "The Effect of Manufacturing On The Crystalline And Magnetic Properties Of Ni-Mn-Ga Based Heusler Magnetocaloric Alloys" (2020). Dissertations. 3674.