Browsing by Author "Vinson, Katherine"

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    Plasticity mechanisms in HfN at elevated and room temperature
    (Nature Portfolio, 2016-10-06) Vinson, Katherine; Yu, Xiao-Xiang; De Leon, Nicholas; Weinberger, Christopher R.; Thompson, Gregory B.; University of Alabama Tuscaloosa; Drexel University
    HfN specimens deformed via four-point bend tests at room temperature and at 2300 degrees C (similar to 0.7 T-m) showed increased plasticity response with temperature. Dynamic diffraction via transmission electron microscopy (TEM) revealed < 110 > {111} as the primary slip system in both temperature regimes and < 110 > {110} to be a secondary slip system activated at elevated temperature. Dislocation line lengths changed from a primarily linear to a curved morphology with increasing temperature suggestive of increased dislocation mobility being responsible for the brittle to ductile temperature transition. First principle generalized stacking fault energy calculations revealed an intrinsic stacking fault (ISF) along < 112 > {111}, which is the partial dislocation direction for slip on these close packed planes. Though B1 structures, such as NaCl and HfC predominately slip on < 110 > {110}, the ISF here is believed to facilitate slip on the {111} planes for this B1 HfN phase.
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    Processing and deformation studies in high temperature ceramics
    (University of Alabama Libraries, 2018) Vinson, Katherine; Thompson, Gregory B.; University of Alabama Tuscaloosa
    Ceramics are high hardness materials with high melting temperatures, above 2000 C. As a consequence of those unique properties, they have found a niche in several extreme-environment applications. Their properties also present challenges in fabricating fully dense parts and understanding the underlying mechanisms that governing plasticity in the intended operating temperature range. Here, each of these topics is addressed using a range of case study materials. Specifically, this research will describe the fabrication of small-diameter C and SiC fibers for ceramic matrix composites. These fibers are derived using a hyperbaric pressure-laser chemical vapor deposition (HP-LCVD) method in which a laser is used to deposit a fiber directly from the vapor phase. Then, the topic of HP-LCVD will be explored further by a study in the phase and microstructure stability of tetramethylsilane derived fibers under an array of process conditions. Similarly, this dissertation will address the complications of incorporating SiC into HfB2 coatings by vacuum plasma spraying (VPS) and its effect on phase stability. Finally, the dissertation explores the role of intrinsic stacking faults in governing deformation mechanisms in HfN. The collective work reveals the link of structure in controlling either phase stability or mechanical deformation in these specific ceramics and their processing routes.