Browsing by Author "Robertson, Jennifer E."

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    Multiscale Microstructural Investigation of the Ductile Phase Toughening Effect in a Bi-phase Tungsten Heavy Alloy
    Haag IV, James Vincent (Virginia Tech, 2022-06-03)
    A specialty class of alloys known as tungsten heavy alloys (WHAs) possess extremely desirable qualities for adoption in nuclear fusion reactors. Their high temperature stability, improvement in fracture toughness over other brittle candidates, and promising performance in initial experimental trials have demonstrated their utility, and recent advancements have been made in understanding and applying these multiphase materials systems. To that end, Pacific Northwest National Laboratory in collaboration with Virginia Tech have sought to understand and tailor the structure and properties of these materials to optimize them for service in fusion reactor interiors; thereby improving the robustness, efficiency, and longevity of structural materials selected for service in an extremely hostile environment. In this analysis of material viability, a multiscale investigation of the connections between structure-property relationships in these multiphase composite microstructures has been undertaken, employing advanced characterization techniques to bridge the macro, micro, and nanoscales for the purpose of generating a framework for the understanding of the ductile phase toughening effect in these systems. This analysis has yielded evidence suggesting the effectiveness of WHA microstructures in the simultaneous expression of high strength and toughness owes to the intimately bonded nature of the boundary which exists between the dissimilar phases in these bi-phase microstructures. Analytical techniques have been employed to provide added dimensionality to traditional materials characterization techniques, providing the first three-dimensional microstructure reconstructions exhibiting the effects of thermomechanical processing on these dual-phase microstructures, and the first time-resolved approach to the observation of WHA deformation through in-situ uniaxial tension testing. The contributions of purposefully introduced microstructural anisotropy and its contribution to texturing and boundary conformations is discussed, and an emphasis has been placed on the study of the interface between the dissimilar phases and its role in the overall expression of ductile phase toughening. In short, this collective work utilizes multiscale and multidimensional characterization techniques in the in-depth analysis and discussion of WHA systems to connect their structure to the properties which make them excellent candidates for fusion reactor systems.
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    Thermal Degradation Studies of Polycarbonate
    Robertson, Jennifer E. (Virginia Tech, 2001-05-04)
    Polymeric materials are increasingly being used in diverse, very demanding applications. Either pre- or post- application environments may require exposures to conditions hostile to the polymer's integrity. Frequently, these demanding conditions result in degradation of the polymer and subsequent decreases in desirable properties. Clearly then, a methodology to predict important properties, such as Tg, molecular weight, and tensile strength, from knowledge of the environmental history of a polymeric-based specimen is beneficial. The current study focuses on bisphenol A polycarbonate and tracks changes in the properties of this material as a function of the degree of degradation, t. For the purposes of the present research, the environmental effects have been limited to those associated with elevated temperature, although the methodology is general. This t parameter is a product of the kinetic rate constant, k, found from isothermal kinetics, and the time of degradation, t. Elucidation of t has been linked to measurement of the molecular weight distribution which in turn can be related to various properties to yield predictive relationships for these properties. Only the thermal history of the polymer and its initial properties are required for the model. This technique is not limited to a specific polymer or even to thermal degradation. As long as the kinetics of the process can be mathematically modeled, this approach should apply to a host of other situations, providing property prediction simply from knowledge of the material history. The research seeks to better understand the thermal degradation of polycarbonate. Kinetics of the process was explored, and the chemical mechanisms were examined. A key part of the project was the determination of the molecular weights and molecular weight distributions at each level of degradation. Furthermore, mechanical stress-strain properties, glass transition temperatures, and melt viscosities were also measured. This information, together with the kinetic expressions, facilitated prediction of these types of material properties for a known thermal history.