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dc.contributor.authorHeil, Todd M.en_US
dc.description.abstractThe martensite and magnetic transformations in Ni-Fe-Ga ferromagnetic shape memory alloys are very sensitive to both alloy chemistry and thermal history. A series of Ni-Fe-Ga alloys near the prototype Heusler composition (X2YZ) were fabricated and homogenized at 1423 K, and a Ni53Fe19Ga28 alloy was subsequently annealed at various temperatures below and above the B2/L21 ordering temperature. Calorimetry and magnetometry were employed to measure the martensite transformation temperatures and Curie temperatures. Compositional variations of only a few atomic percent result in martensite start temperatures and Curie temperatures that differ by about 230 K degrees and 35 K degrees, respectively. Various one-hour anneals of the Ni53Fe19Ga28 alloy shift the martensite start temperature and the Curie temperature by almost 70 K degrees. Transmission electron microscopy investigations were conducted on the annealed Ni53Fe19Ga28 alloy. The considerable variations in the martensite and magnetic transformations in these alloys are discussed in terms of microstructural differences resulting from alloy chemistry and heat treatments.

The phase-field method has been successfully employed during the past ten years to simulate a wide variety of microstructural evolution in materials. Phase-field computational models describe the microstructure of a material by using a set of field variables whose evolution is governed by thermodynamic functionals and kinetic continuum equations. A two dimensional phase-field model that demonstrates the ferromagnetic shape memory effect in Ni2MnGa is presented. Free energy functionals are based on the phase-field microelasticity and micromagnetic theories; they account for energy contributions from martensite variant boundaries, elastic strain, applied stress, magnetocrystalline anisotropy, magnetic domain walls, magnetostatic potential, and applied magnetic fields. The time-dependent Ginzburg-Landau and Landau-Lifshitz kinetic continuum equations are employed to track the microstructural and magnetic responses in ferromagnetic shape memory alloys to applied stress and magnetic fields. The model results show expected microstructural responses to these applied fields and could be potentially utilized to generate quantitative predictions of the ferromagnetic shape memory effect in these alloys.

dc.publisherVirginia Techen_US
dc.rightsI hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to Virginia Tech or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report.en_US
dc.subjectMartensite Transformationen_US
dc.subjectFerromagnetic Shape Memory Alloysen_US
dc.subjectFerromagnetic Shape Memory Effecten_US
dc.subjectPhase-Field Computational Modelen_US
dc.contributor.departmentMaterials Science and Engineeringen_US
dc.description.degreePh. D.en_US D.en_US Polytechnic Institute and State Universityen_US Science and Engineeringen_US
dc.contributor.committeechairReynolds, William T. Jr.en_US
dc.contributor.committeememberWang, Yuen_US
dc.contributor.committeememberKampe, Stephen L.en_US
dc.contributor.committeememberAning, Alexander O.en_US
dc.contributor.committeememberCorcoran, Sean Geralden_US

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