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    Plastic flow and microstructure evolution in niobium at elevated temperatures

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    BRADY-DISSERTATION-2021.pdf (2.971Mb)
    Date
    2021-12-09
    Author
    Brady, Emily Ann Dukes
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    Abstract
    Plastic flow and microstructure evolution are investigated at elevated temperatures in two unalloyed niobium sheet materials, Type 1 and Type 2 as designated in ASTM B393-18. Tensile tests are conducted at temperatures from 1473 to 1823 K (1200 to 1550°C) at constant true strain rates of 10⁻³ and 10⁻⁴ s⁻¹. Deformation microstructures are characterized using backscatter electron (BSE) imaging, electron backscatter diffraction (EBSD), and high-resolution EBSD (HR-EBSD). The mechanical behaviors of the Type 1 and Type 2 niobium materials are compared to relevant data from the literature. Elevated temperature deformation in both niobium materials is dominated by the five-power creep mechanism and the associated development of subgrains. The higher impurity content of the Type 2 niobium led to: 1. greater strength, 2. delayed recrystallization, 3. slower grain growth, 4. inhomogeneous microstructures, and 5. slower recovery which resulted in finer and less distinct subgrains compared to the Type 1 niobium. The smaller subgrain size of the Type 2 niobium produces, through the five-power creep mechanism, a higher strength at elevated temperature compared to the Type 1 niobium. This is the first mechanistic explanation supported by direct microstructural data for how impurity content affects strength in refractory metals. HR-EBSD analysis is performed on the deformed Type 2 niobium material by developing new techniques to: 1. utilize data from a new EBSD instrument, 2. expand the capabilities of the OpenXY open-source cross-correlation software, and 3. enable cross correlation calculations spanning the breadth of heavily deformed grains. This is the first successful implementation of these techniques.
    Department
    Materials Science and Engineering
    Subject
    Niobium
    High-temperature deformation
    Grain growth
    Creep
    Subgrains
    Electron backscatter diffraction (EBSD)
    Elastic modulus
    High resolution electron backscatter diffraction
    URI
    https://hdl.handle.net/2152/114155
    http://dx.doi.org/10.26153/tsw/41058
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    • facebook
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    • CONTACT US
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    • Emergency Information
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    © The University of Texas at Austin