Browsing by Author "Noell, Philip James"

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    The influence of high-temperature tensile deformation on microstructure evolution in select BCC metals
    (2015-12) Noell, Philip James; Taleff, Eric M.; Bourell, David L; Engelhardt, Michael; Ferreira, Paulo; Kovar, Desiderio
    High-temperature plastic deformation can enhance the rate of grain boundary migration in some metals. In the body-centered-cubic (BCC) refractory metals molybdenum and tantalum, this increased rate of grain boundary migration produces abnormal grain growth at temperatures significantly lower than is possible by static annealing. This phenomenon is termed dynamic abnormal grain growth (DAGG). The influence of microstructure on DAGG is studied by examining the morphology of DAGG grains produced in two Mo sheet materials. DAGG grain propagation in these materials is not uniform throughout the sheet thickness. Variations through the sheet thickness in texture and grain size are explored as causes of these behaviors. DAGG grains in both materials preferentially grow into the finest-grained polycrystalline regions of the sheet. Direct effects of local crystallographic texture variation are not evident in microstructures containing DAGG grains. The initiation of abnormal grains in Mo materials by plastic straining at elevated temperatures is investigated. The minimum strain necessary to initiate DAGG, termed the critical strain, decreases approximately linearly with increasing temperature. The variation in critical strain values observed at a single temperature and strain rate is well described by a normal distribution. An increased fraction of grains aligned with the <110> along the tensile axis, a preferred orientation for DAGG grains, appears to decrease the critical strain for DAGG initiation. Uniaxial tension tests at temperatures between 1650 and 1950 °C were conducted with commercial-purity Mo rods to determine if DAGG can be used to produce large single crystals of Mo. Necking in these tensile rod specimens during the production of DAGG grains presents a potential issue with translation to commercial application. High-temperature tensile tests were also performed on another BCC metal, an interstitial-free steel. Grain growth in this interstitial-free steel during high-temperature plastic deformation is significantly accelerated above that of the static case. Grains generally oriented between {112}<110> and {111}<113> grow more readily than other grains during dynamic grain growth in this material.
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    The morphology and microstructure of dynamic abnormal grain growth in commercial-purity molybdenum
    (2014-05) Noell, Philip James; Taleff, Eric M.
    Dynamic abnormal grain growth (DAGG) is a phenomenon that produces abnormal grain growth at elevated temperatures during plastic deformation. It is distinct from classically studied static abnormal grain growth phenomena in that it only occurs during plastic deformation. Previous investigations of DAGG in a Mo sheet material produced using powder metallurgy techniques observed DAGG grains to grow more rapidly near the sheet surface than near the sheet center. This phenomenon is explored in the present study. A Mo sheet material produced using arc melting techniques is also studied to determine the morphology of DAGG grains. A preference for growth near the sheet center is observed in this material. The through-thickness variations in texture and grain size for both the arc-melted and powder-metallurgy Mo sheet materials are investigated. The preference for growth near the surface in the powder-metallurgy material is due to a through-thickness variation in grain size, with smaller grains near the surface and larger grains near the center. The preference for DAGG grain growth at the center of the arc-melted sheet material is because of very large grains that grow near the sheet surface. These large grains may be the product of multiple abnormal grains occurring near the sheet surface because of texture variation through the sheet thickness. Regardless, the DAGG grain cannot consume these large grains and leaves them as island grains decorating the region near the sheet surface. These results suggest that DAGG is driven primarily by grain boundary curvature. Microstructures that include DAGG grains are investigated with electron backscatter diffraction (EBSD). A new method to evaluate geometrically necessary dislocation densities using EBSD data is derived. DAGG grains are relatively undeformed compared to the polycrystalline microstructure. DAGG grains are not oriented either favorably or unfavorably for slip. Results of the analysis of the grain boundaries between DAGG grains and normal grains do not indicate any special character preference for these grain boundaries.