Development and modeling of iron-gallium alloys

Abstract

<p>Alloys of iron substituted with non-magnetic gallium (Galfenol) appear promising as mechanically robust actuator/sensing materials. They offer desirable properties including tensile strengths and magnetostrictive strains, respectively, on the order of 100 MPa and 100 x 10-6. To advance the understanding of these materials, this dissertation examines the alloys' magnetic and mechanical response as a function of applied magnetic field, mechanical stress, temperature, stoichiometry and crystallographic morphology. Characterizations of the alloys' single-crystal magneto strictive, elastic and plastic properties are used to facilitate the development and modeling of polycrystalline forms of the material having preferred crystallographic orientations (i.e. texture). The polycrystalline forms have potentially higher production yields and superior mechanical properties over those of single crystals. Irongallium alloys textured by different manufacturing processes reveal which production methods result in the most desirable magnetostrictive and mechanical performance envelopes. Growth and deformation processing techniques such as directional solidification, extrusion, forging and rolling were used to impart a variety of different texture distributions. Single-crystal tensile test were used to determine the material's elastic constants, yield stress, slip systems and their critical resolved shear stress as well as ultimate strength and percent elongation. Observations of the single-crystal alloys' remarkable in-plane auxeticity (or negative Poisson's ratio) and quadratic correlations of the Poisson's ratio magnitude to the level of gallium substitution are reported. Potential applications based on in-plane auxeticity are explored. Elastic properties, as determined from tensile testing on single-crystal specimens, provided the basis for the development and use of a M&barbelow;agnetostrictive M&barbelow;icromechanics F&barbelow;inite-E&barbelow;lement M&barbelow;odel (MMFEM) that captured the coupled magnetic and mechanical behavior of polycrystalline specimens. The MMFEM correctly reflects the bulk magnetostrictive capability of polycrystalline specimens having well-characterized texture distributions and provides a tool for predicting the magnetostrictive performance of textures yet to be produced.</p>