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，例如固体地幔的流动和冰盖内的流动。在研究地球主要物质(橄榄石、辉石、冰)的变形机制方面做了大量工作。目前，我们的大部分认识要么来自多晶样品，要么来自单晶。因此，我们对晶体之间的区域，即晶界缺乏强有力的理解。数值研究表明，晶界是地球材料蠕变的关键因素，但我们缺乏对晶界滑动的直接观测和对晶界力学性质的测量。在这项研究中，我建议开发新的技术来直接研究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细观力学模型中。因此，我将直接测量相关地质材料中的晶界粘度，并评估它们在大范围地球动力学过程(地幔变形、冰低)中的作用。首先，我想测试一下假设，即晶界是橄榄石中位错的来源。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。在人工合成的橄榄石双晶中，跨越晶界的室温和高温纳米压痕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)球磨穿过晶界来制造。为了促进晶界滑动，可以用扁平的纳米压头压在柱子的顶部进行新颖的压缩测试，压力轴在45℃到晶界(Gong和Wilkinson，2016)。测试后，将使用高角分辨电子背散射衍射(HR-EBSD)对样品进行研究，以绘制剩余弹性场和位错密度图。其次，我想研究晶界在地球材料微结构演化中的作用。我建议的第二种技术是一种新颖的环形剪切阶段，它将允许在扫描电子显微镜(SEM)腔内对材料进行高应变扭转。这一新仪器的应用包括：冰弱化和改进冰盖流动的预测、α-铀变形微结构和孪生、橄榄石中的晶界滑动和空化(尽管其低熔点类似于冰片[C10H18O])，以及铝镁合金的蠕变变形(用于工程应用)。新的扭转台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，扩大运行温度范围在-5-+100℃。这一设置将首次实现材料的同时剪切和EBSD映射。进一步的HR-EBSD分析将允许随着增量扭转观察弹性场的演化和位错密度。最后，我想将扭转阶段实验产生的微结构与微结构模拟产生的纹理进行比较，并评估我的实验结果对长期地球动力学过程的影响。我将使用Llorens等人(2016)和Lebensohn等人(1994)开发的全场粘塑性快速傅立叶变换模型，并集成到模拟地质材料变形的ELLE软件平台中。这种全场方法允许完全控制每种变形机制对组织演变的贡献。此外，该技术以与实验EBSD相同的格式提供结果，从而允许在模型和实验之间进行直接比较。这种方法已经被用于模拟多晶冰、岩盐和a-铀的变形和再结晶。

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.