The tensile behavior, fracture, and power harvesting potential of nickel-manganese-gallium magnetic shape memory alloys

D'Silva, Glen Jude (2023) The tensile behavior, fracture, and power harvesting potential of nickel-manganese-gallium magnetic shape memory alloys. Doctoral thesis, Northern Arizona University.

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Abstract

The shape memory effect in Ni2MnGa MSMAs is driven by the magnetic field-induced or stress-induced motion of twin boundaries. The Ni2MnGa microstructure consists of tetragonal martensite variants with magnetic easy axis aligned with the short axis (of the unit cell), which aligns in either the direction of the applied magnetic field or mechanical stress causing a reorientation of the microstructure. The reorientation strain and the change in the material’s magnetization during variant reorientation, drive the development of MSMA-based applications. The overarching objective of this dissertation study is to inform the development of new MSMA-based applications and to improve the efficiency and reliability of current MSMA-based applications by studying the tensile behavior, fracture mechanics, and power harvesting potential of Ni2MnGa MSMAs. Historically, MSMAs have been studied extensively under combined compressive and/or magnetic loads, and applications only use MSMAs in compression. The tensile study investigates the strain fields developed in Ni2MnGa samples, with fine and coarse twin structures when loaded in tension and/or with a magnetic field. The strain fields are recorded using digital image correlation, which allowed for the observation of the evolution of the strain field over the entire sample, concurrent with the evolution of the sample’s twin microstructure. The results show that the twin density, the uniformity of the magneto-mechanical loading along the sample, and the presence of pinning sites are all contributing to the profile of the tensile strain field; the presence of pinning sites along the sample inhibits variant reorientation and recovery. Both samples showed no visible signs of damage during tensile testing, and the magneto-mechanical response in tension was found to be comparable to that in compression for both sample types. The fracture mechanics study involves the experimental research of the fracture mechanisms in MSMAs and the development of an MSMA fracture mechanics modeling framework; the brittle nature of Ni2MnGa MSMAs causes cracks to develop in them hampering their function in MSMA-based applications. The phase-field method is proposed for the modeling framework since this method is able to capture complex crack patterns, and Vickers microindentation is used for the experimental study to determine the fracture energy of the material and study crack evolution characteristics under magneto-mechanical loading. The Vickers microindentation results suggest that transverse magnetic fields facilitate crack growth and decrease the fracture energy of the MSMA, while the axial compressive stress impedes crack growth and increases the fracture energy. Lastly, the power harvesting study reports new power harvesting data generated with a biaxial magnetic field and a surrounding coil, and full strain field data for an MSMA subject to load similar to what is seen during power harvesting, then compares theperformance of MSMA-based power harvesters with different designs to determine which give the best output. For this comparison, a framework for evaluating the performance of (side coil and surrounding coil type) MSMA-based power harvesters reported in the literature is developed. This framework involves normalizing the results to the design characteristics of the respective harvesters. The strain maps reveal the potential for perpendicular twin boundaries that limit variant reorientation and correspondingly the harvester’s power. The power harvesting study concludes that the largest change in magnetic flux density, which is the driver for power harvesting, occurs in the side coil setup with an optimized magnetic circuit and it recommends using this configuration for future MSMA-based power harvesters for maximum power.

Item Type:	Thesis (Doctoral)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	fracture mechanics; magnetic shape memory alloys; magneto-mechanical loading; Ni2MnGa; power harvesting; tensile testing
Subjects:	T Technology > TJ Mechanical engineering and machinery
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of Engineering, Informatics, and Applied Sciences > Mechanical Engineering
Date Deposited:	30 Aug 2023 17:12
Last Modified:	30 Aug 2023 17:12
URI:	https://openknowledge.nau.edu/id/eprint/6108

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