Temperature and Rate Effects on the Mechanical Behavior of Tungsten and Its Nanocomposites
2022-05
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Temperature and Rate Effects on the Mechanical Behavior of Tungsten and Its Nanocomposites
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2022-05
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Methods to predict material failure frequently rely on large experimental datasets tuned to the properties of one material or are based on computationally expensive modeling. Both approaches are slow and do not adapt well to changes in material chemistry or processing. Development of analytical models with easily measured, physically meaningful parameters are key to alleviating bottlenecks in new materials development. One such model is presented in this work, which relies on easily measured physical parameters, effective stress and activation volume, referred to as activation parameters. This approach requires knowledge of both the temperature and strain rate effects on mechanical properties, which is the primary focus of this dissertation.Nanoindentation is an ideal way to measure these physical parameters due to its ability to measure properties at a length scale appropriate for grain- or phase-dependent deformation mechanisms and the small quantities of material required for testing (~mm3). Newer nanoindentation tests, such as strain rate jump tests, can quickly determine activation parameters, but lack standardized protocols for experiments and analysis. For this work, tungsten (W) was chosen as a model system in which to understand temperature and strain rate effects. This is in part due to its elastic isotropy, which simplifies data analysis, and its relatively experimentally accessible brittle-ductile transition temperature.
In the first half of this dissertation, I will describe the use of nanoindentation strain rate jump tests to predict the fracture behavior of macroscale tungsten single crystals. I begin by presenting necessary protocol development for strain rate jump testing and analysis using a new, freely available software package. Included in the software package are a Python-based load function generator and a series of Matlab functions for data analysis. These tools were validated on single crystal tungsten, yielding a strain rate sensitivity nearly identical to that reported in the literature. The techniques were then applied at low temperature (-100 °C) and high temperature (50-300 °C) to measure activation parameters, effective stress and activation volume, as a function of temperature. The activation parameters, in combination with an analytical model for the strain energy release rate, accurately predict the brittle-ductile transition temperature along particular fracture systems in single crystal tungsten. Activation parameters measured from indentation of the (100) surface of single crystal tungsten accurately predict the brittle-ductile transition and fracture toughness along the {100}<011> fracture system of macroscale tungsten single crystals. Use of data from bulk tension of single crystal tungsten from the literature accurately predicts the fracture toughness in the {110}<110> fracture system of macroscale tungsten single crystals.
In the second half of this dissertation, I will describe the synthesis and mechanical properties of 3-dimensionally ordered macroporous (3DOM) tungsten and a nanocomposite based on the porous framework. 3DOM tungsten was made with 35-40 nm wide ligaments, which exploit material size effect to have a ligament yield strength of 6.1 GPa at room temperature, approaching the ideal strength of tungsten. Considerable plasticity was observed above 125 °C, implying a brittle-ductile transition around this temperature, consistent with the predictions based on the earlier-presented model. Filling of the 3DOM tungsten framework with a silicon oxycarbide (SiOC) reinforcing phase resulted in a heterogeneous structure containing tungsten, silicon oxycarbide glass, and small domains of free carbon. The failure strength of 3DOM W-SiOC was 1.1 GPa, a factor of 22 greater than the 3DOM W framework. Although an increase in deformability was observed at 225 °C, pillars still failed by abrupt crack propagation. Fracture was only prevented at the next testing temperature of 425 °C. This places the brittle-ductile transition temperature of the W-SiOC composite significantly higher than that of pure tungsten. Additionally, the composite retains high strength even to 425 °C, achieving a yield strength of 400 MPa.
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University of Minnesota Ph.D. dissertation. May 2022. Major: Chemical Engineering. Advisor: Nathan Mara. 1 computer file (PDF); xi, 234 pages.
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Schmalbach, Kevin. (2022). Temperature and Rate Effects on the Mechanical Behavior of Tungsten and Its Nanocomposites. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/241314.
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