Micromechanical investigation of geological materials

点击翻译按钮获取中文摘要

Creep of Earth materials is one of the processes that influences long term deformation on Earth, such as flow of the solid mantle and flow within ice sheets. Significant effort has been focused on the study of deformation mechanisms in key Earth materials (olivine, pyroxene, ice). Currently, most of our understanding is derived from either polycrystalline samples, or single crystals. Thus, we lack a robust understanding of the regions between crystals, the grain boundaries. Numerical studies indicate that grain boundaries are a key factor in creep of Earth materials, but we lack direct observations of grain-boundary sliding and measurements of the mechanical properties of grain boundaries. In this study, I propose to develop new techniques for directly investigating grain-boundary sliding and grain-boundary properties in geological materials and to integrate the results into a full-field micromechanical model. Therefore, I will directly measure grain-boundary viscosities in relevant geological material and assesses their role in large-scale geodynamic processes (mantle deformation, ice low). Firstly, I want to test the hypothesis that grain boundaries are sources of dislocations in olivine. The first set of methods I propose to use is small scale micromechanical testing via nanoindentation and micropillar compression. Room and high-temperature nanoindentation across a grain boundary in a synthetic olivine bicrystal will test pristine grain boundaries and their role as defect sources. Additionally, micro-pillars can be manufactured via focused ion beam (FIB) milling across the grain boundary. Novel compression testing can be done with a flat nanoindenter pressing on the top of the pillar, with the compressive axis at 45C to the grain boundary, in order to promote grain boundary sliding (Gong and Wilkinson, 2016). After testing, the samples will be investigated using High-Angular Resolution Electron Backscatter Diffraction (HR-EBSD) in order to map residual elastic fields and dislocation density. Secondly, I want to investigate the role of grain boundaries in the evolution of microstructure of Earth materials. The second technique I propose is a novel annular shear stage that would allow high strain torsion of materials within the Scanning Electron Microscope (SEM) chamber. The applications of this new apparatus include: ice weakening and improving predictions of ice sheet flow, a-Uranium deformation microstructures and twinning, grain-boundary sliding and cavitation in olivine (though its low-melting temperature analogous--borneol [C10H18O]), and creep deformation of Al-Mg alloys (for engineering applications). The new torsion stage would apply a torque up to 4.5 Nm on samples with varying geometry. Further improvements include the installation of cooling and heating stages and expanding the operation temperature range at -5- +100C. This set-up will allow simultaneous shear and EBSD mapping of materials for the first time. Further HR-EBSD analysis will allow observation of elastic field evolution and dislocation density with incremental torsion. Lastly, I want to compare the microstructures resulting from the torsion stage experiments with textures generated by microstructural modelling, and assess the implications of my experimental results on long term geodynamic process. I will use the full-field viscoplastic Fast Fourier Transform model developed by Llorens et al., 2016 and Lebensohn et al., 1994 and integrated within the ELLE software platform which simulates deformation of geological materials. This full-field approach allows for full control of the contribution of each deformation mechanism to the microstructural evolution. Moreover, the technique provides results in the same format as those from experimental EBSD, thus allowing direct comparison between model and experiment. This approach has already been implemented for modelling polycrystalline ice, halite, and a-Uranium deformation and recrystallization.