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Browsing by Author "Thompson, Gregory B"

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    Deformation Mechanisms in Nanocrystalline Copper Alloys at Ambient and Cryogenic Temperatures
    (University of Alabama Libraries, 2024) Robinson, Jarod Allen; Thompson, Gregory B
    When materials are subjected to deformation, the microstructure evolves. In this research, the microstructure response to deformation is considered in two extreme conditions: cryogenic temperatures, and nanocrystalline grain sizes. Prior reports have shown nanocrystalline materials with heightened grain growth at cryogenic temperatures. Simulations have shown faster twin boundary migration at lower temperatures in some cases as a result of coordinated dislocation motion, contributing to grain growth. This research explored the influence of solutes in this migration. Nanotwins were fabricated via sputter deposited copper with 2 at% and 8 at% aluminum. The aluminum content increases the hardness of the alloy contributing to shearing behavior at 8%. Both samples revealed grain bending when indented. This bending promoted detwinning by imposing a shear stress on the twin planes. An increase columnar bending with corresponding detwinning was observed as temperature decreased in the Cu-2Al sample whereas the Cu-8Al sample varied minimally with temperature. Simulated mobilities of an incoherent twin boundary matched well with these results. Grain growth was not observed highlighting the importance of grain aspect ratio. Follow-on work investigated mechanical properties of these alloys with temperature and twin configuration. These nano-twinned alloys were sputter deposited with twins parallel to the substrate and at an angle. Micro-pillars were deformed in-situ at ambient and sub-ambient temperatures. All samples got stronger as temperature decreased and with twins parallel to the substrate. Post-mortem characterization suggests that detwinning played a significant role as dislocations avoid cross-slipping through twin boundaries.Using nanocrystalline copper, deformation mechanisms were imaged with in-situ TEM testing. Digital Image Correlation (DIC) was applied to track the strain evolution. DIC has been minimally used in TEM imaging because of complex contrast mechanisms. This work demonstrates that DIC can produce reasonable strain values for TEM samples. This research captures strain evolution under deformation in nanocrystalline copper as a result of various dislocation mechanisms. By mapping the DIC strain onto the grain size, experimental verification of preferred plasticity within large grains is shown, supporting prior modeling predictions. Overall, this dissertation provides an understanding of the deformation of nanocrystalline copper and its alloys at ambient and sub-ambient temperatures.
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    Hollow Cathode Materials in an Iodine Plasma Enviroment
    (University of Alabama Libraries, 2023) Rogers, James Daniel; Branam, Richard D
    Current electric propulsion devices have proven their worth for large, conventional satellites. Trade studies using iodine as a propellant show superior system-level performance when compared to existing xenon-based systems. As a halogen, iodine introduces chemical reactivity issues not present in xenon plasmas, which are especially relevant for the extreme environment of hollow cathodes. An RF inductively coupled plasma source was designed, built, and characterized to simulate the environment found in a hollow cathode. Langmuir probe measurements were taken for a range of plasma conditions in iodine and argon plasmas. The iodine plasma required measurement with a double probe due to the occurrence of significant negative ionization. A combination of low-resolution survey spectra and high-resolution spectra were measured to measure plasma composition.Material samples of tungsten, tantalum, and molybdenum were tested for a range of temperatures in argon plasma, iodine plasma, and iodine vapor; their associated erosion ranges were measured. Molybdenum's erosion rate showed a roughly exponential relationship with respect to temperature, and a maximum erosion rate at 2000 K in iodine plasma of 70.1 µm/hr. Tungsten's erosion rate showed two distinct linear relationships with respect to temperature, and a maximum erosion rate at 2000 K in iodine plasma of 36.8 µm/hr. Tantalum experienced significant oxidation and expanded in size but had heavy cracking present which weakened the samples. Molybdenum was found to be the best material in iodine vapor, but tungsten was superior in iodine plasma. Tantalum demonstrated very unfavorable changes in surface structure and is not recommended for a system that contains any iodine or oxygen/water contamination. A significant challenge of this experiment was eliminating oxygen due to the oxygen and water presence in the iodine source. The primary improvements suggested for future studies are careful design of the iodine source to minimize moisture and oxygen contamination, a larger plasma source and sample loading area, and use of a differential vacuum system.
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    Hyperbaric Laser Chemical Vapor Deposition of Titanium Carbide Fibers
    (University of Alabama Libraries, 2023) Fronk, Kenan Timothy; Thompson, Gregory B
    Laser chemical vapor deposition (LCVD) is a technique in which material in the vapor phase can be deposited into a solid phase structure, such as a fiber, under the focus of a laser. This study reports on the deposition of titanium carbide (TiC) through LCVD. A system for the growth of TiC fibers at hyperbaric conditions was developed with subsequent fibers grown at 2 bar. The growth of the TiC fibers was done under three different laser intensities and under three different gas compositions of titanium tetrachloride (TiCl4), ethylene (C2H4), and hydrogen (H2). The combination of gas composition and laser intensity facilitated hollow fibers (or tubes) to hierarchical fibers of a distinct carbon-rich core with a titanium-rich outer shell. These fibers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The control of the tube-to-carbon rich core is explained by the Soret effect, which is where lighter weight molecules partition to the warmer regions of a reaction region.
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    The Influence of Hydrocarbon Precursors on the Deposition Characteristics of Carbon Fibers
    (University of Alabama Libraries, 2023) Cook, Chalrles; Thompson, Gregory B
    Laser Chemical Vapor Deposition (LCVD) is an additive processing technique in which freestanding fibers are deposited by the thermal dissociation of a precursor gas under a translating laser focal point. For carbon fiber deposition, the precursor is a hydrocarbon gas. In this study the relationships between deposited carbon fiber microstructure, mechanical properties, and processing conditions were compared using four different hydrocarbon gases that span two families, i.e., the alkanes and alkenes. The alkane gases were methane (CH4), ethane (C2H6) and propane (C3H8) and the alkene gas was ethylene (C2H4). These hydrocarbons were chosen because of their differences in carbon bond structure, molecular geometry and/or the number of available carbon atoms. At lower temperatures, all the fibers deposited in a surface reaction kinetically (SRK) limited growth regime whereas only two of the hydrocarbons, C2H4, and C2H6, were able to be induce a second growth regime that was mass transport (MT) limited. These growth regimes had significant impact to the changes in the structure and mechanical strength of the fibers, and, as noted, were not always accessible based on the hydrocarbon molecule. The reason for this inaccessibility is related to the Soret effect and steric resistance connected to the geometric structure of the molecule during deposition. Collectively, the outcomes reveal the impact of precursor chemistry selection in the deposition of LCVD carbon fibers.
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    Investigations into Structural, Magnetic, and Chemical Properties of Magnetic Metal-Amorphous Nanocomposites
    (University of Alabama Libraries, 2024) Cole-Piepke, Kayla; Thompson, Gregory B
    Metal-amorphous nanocomposites (MANCs) are currently seeing rigorous researchleading to the development of cleaner and more efficient materials for power applications.This dissertation reports on three topics related to MANCs, namely the stability ofnanocrystalline Co, magnetic properties derived from nano/microstructures usingmicromagnetic simulations, and the chemical partitioning characteristics in MANCs usingatom probe tomography (APT). Density functional theory was used to determine the influence of Mn doping on thestructural and magnetic behavior of face-centered cubic (FCC) Co. Prior reports show thatMn doping can have a tremendous impact on the permeability of Co-based MANCs. Here,it was found that the quantity and location of Mn atoms within the FCC Co supercellinfluence the local magnetic moments and can result in structural faulting in Co. Since a MANC is composed of nanocrystallites in an amorphous matrix, a Voronoitessellation construction method was implemented to create realistic representations ofthese nano/microstructures for micromagnetic simulations. The results reveal adependency of the coercivity response on the crystallite size, ?, and the crystallite volumefraction, with lower fractions deviating from established ?^(3−6) dependencies. Finally, APT studies examined how early transition metals in Fe-based MANCspartition to mitigate crystallite growth enabling MANCs to operate in warmerenvironments, such as engines. Using Ta, rather than Nb, is shown to increase the thermalstability of such materials. The APT studies conducted indicate that there is a reducedpartitioning of Fe out of the matrix of the MANC when Ta is used, showing that thisthermal stability is affected by partitioning.