Micromechanical investigation of geological materials

地球物质的蠕变是影响地球长期变形的过程之一，如固体地幔的流动和冰盖内的流动。大量的工作集中在研究地球主要物质（橄榄石、辉石、冰）的变形机制上。目前，我们的大多数理解来自多晶样品或单晶。因此，我们对晶体之间的区域，即晶界缺乏深入的了解。数值研究表明，晶界是地球材料蠕变的一个关键因素，但我们缺乏直接的观察晶界滑动和晶界的力学性能的测量。在这项研究中，我建议开发新的技术，直接调查地质材料中的晶界滑动和晶界性质，并将结果整合到一个全场微观力学模型。因此，我将直接测量相关地质材料中的晶界粘度，并评估它们在大尺度地球动力学过程（地幔变形，冰低）中的作用。首先，我想验证晶界是橄榄石位错源的假设。我建议使用的第一组方法是通过纳米压痕和微柱压缩进行小规模微机械测试。室温和高温纳米压痕横跨合成橄榄石双晶体的晶界将测试原始晶界及其作为缺陷源的作用。此外，微柱可以通过聚焦离子束（FIB）铣削穿过晶界来制造。新型压缩测试可以通过在支柱顶部按压扁平纳米压头来完成，压缩轴在45 C处至晶界，以促进晶界滑动（Gong和威尔金森，2016）。测试后，将使用高角分辨率电子背散射衍射（HR-EBSD）对样品进行研究，以绘制残余弹性场和位错密度。其次，我想研究晶界在地球材料微观结构演化中的作用。我提出的第二种技术是一种新的环形剪切阶段，将允许高应变扭转的扫描电子显微镜（SEM）室内的材料。这种新装置的应用包括：冰的弱化和改进冰盖流动的预测，α-铀变形的微观结构和孪生，橄榄石中的晶界滑动和空化（尽管其低熔点类似于--[C10 H18 O]），以及Al-Mg合金的蠕变变形（用于工程应用）。新的扭转阶段将对不同几何形状的样品施加高达4.5 Nm的扭矩。进一步的改进包括安装冷却和加热阶段，并将操作温度范围扩大到-5-+100 ℃。这种设置将首次允许同时对材料进行剪切和EBSD绘图。进一步的HR-EBSD分析将允许观察弹性场的演化和位错密度与增量扭转。最后，我想比较从扭转阶段实验产生的微观结构与微观结构模拟产生的纹理，并评估我的实验结果的长期地球动力学过程的影响。我将使用Llorens等人开发的全场粘塑性快速傅立叶变换模型，2016和Lebensohn等人，1994年，并集成在ELLE软件平台，模拟地质材料的变形。这种全场方法允许完全控制的贡献，每个变形机制的微观结构的演变。此外，该技术提供的结果在相同的格式从实验EBSD，从而允许模型和实验之间的直接比较。这种方法已经实现了多晶冰，岩盐，和a-铀变形和重结晶的建模。

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.