ANISOTROPY CHANGES DURING DEFORMATION AND PHASE TRANSFORMATIONS

地球上的大片区域对于地震波的传播是各向异性的。这对于上地幔来说是最好的理解，在那里各向异性是由于在对流过程中获得的橄榄石晶体的优先取向所致。各向异性在地壳中富含片状硅酸盐的岩石中是极端的，如页岩和板岩，尤其重要，因为它对油气勘探具有重要意义。它也存在于下地幔，特别是核幔交界处最低的D“带。大部分各向异性是由晶体的变形和相关的排列引起的，但地球深处的详细机制仍然知之甚少。这个项目将探索地球的宏观变形和相关的各向异性发展，方法是使用国家设施中已有的新方法，如同步辐射X射线和中子衍射，研究潜在的微观机制，然后应用物理模型来预测大的图景。该项目将提供对解释宏观地震观测至关重要的数据。但先进的研究方法不仅适用于工程学，也适用于广泛的物理科学。原位观察相变过程中的变化在材料科学(如过渡金属、高温超导体)和土木工程(如水泥矿物)中有直接的应用。实验方法、数据分析以及建模能力将通过跨学科合作向其他研究人员提供，并通过讲习班和培训传播给科学界。学生的参与将继续是这个项目的重要组成部分。在过去的几年里，PI和他们的合作者在高级光源(LBL)和高级光子源(ANL)上进行了钻石砧座(DAC)和多砧座设备的原位变形实验，这些实验针对钙钛矿、后钙钛矿和镁辉石等系统进行，并能够通过将实验的择优取向模式与多晶塑性理论的预测进行比较来推断变形机制。在应用微观机制时，他们可以对D“的宏观地震各向异性提出解释。然而，实验条件离真实的地球还很远。他们已经启动了一项计划，在更高的温度和更慢的应变速率下工作。一个主要的焦点是开发可靠的变形DAC加热技术，并将其应用于研究高压相的塑性变形、再结晶和相变。第二个焦点是多相材料，其变形行为在很大程度上仍然是谜，例如钙钛矿(强)-镁辉石(弱)混合物。本文还将实验结果与塑性模型进行了比较。洛斯阿拉莫斯大学开发的新的全场快速傅立叶变换公式考虑了颗粒之间的局部相互作用，具有很大的可能性。虽然重点放在地球深处，PI和他的团队也计划继续最近在石英上的工作，那里的机械孪生和残余应力是潜在的古地层压力。初步形变、X射线显微衍射和EBSD实验不仅有望估计应力场的大小，更重要的是可以估计应力场的方向性。

Large regions of the Earth are anisotropic for propagation of seismic waves. This is best understood for the upper mantle where anisotropy is attributed to preferred orientation of olivine crystals, attained during convection. Anisotropy is extreme in sheet-silicate-rich rocks in the crust such as shales and slates, especially important because of the significance for hydrocarbon exploration. It also is present in the lower mantle, particularly the lowermost D" zone at the core-mantle boundary. Much of the anisotropy is caused by deformation and associated alignment of crystals, but the detailed mechanisms in the deep Earth are still poorly understood. This project will explore macroscopic deformation and associated anisotropy development in the Earth by investigating the underlying microscopic mechanisms with novel methods that have become available at National facilities, such as synchrotron X-ray and neutron diffraction, and then applying physical models to project the large picture. The project will provide data that are critical for interpreting macroscopic seismic observations. But research methods that will be advanced are relevant for a broad range of physical sciences as well as engineering. In situ observation of changes during phase transformations has direct application in materials science (e.g. transition metals, high temperature superconductors) and civil engineering (e.g. cement minerals). The experimental methods, data analysis, as well as modeling capabilities will become available to other researchers through interdisciplinary collaborations and disseminated to the scientific community with workshops and training. Participation of students will continue to be an important part of this project.During the last years the PI and collaborators have performed in situ deformation experiments with diamond anvil cells (DAC) and multi anvil apparatus at the Advanced Light Source (LBL) and the Advanced Photon Source (ANL) on systems such as perovskite, postperovskite and magnesiowuestite, and were able to infer deformation mechanisms by comparing experimental preferred orientation patterns with predictions from polycrystal plasticity theory. When applying the microscopic mechanisms they could propose an explanation for macroscopic seismic anisotropy in D". However, conditions of experiments are still far from the real Earth. They have initiated a program to work at higher temperature and slower strain rates. A major focus is to develop reliable heating techniques for the deformation DAC and apply them to study plastic deformation, recrystallization and phase transitions of high pressure phases. A second focus are polyphase materials whose deformation behavior is still largely enigmatic, for example perovskite (strong)-magnesiowuestite (weak) mixtures. Also here experimental results will be compared with plasticity models. The new full-field fast Fourier transform formulation developed at Los Alamos which takes local interactions between grains into account has great possibilities. While emphasis is on the deep Earth, the PI and his team also plan to pursue recent work on quartz where mechanical twinning and residual stresses are potential paleopiezometers. Preliminary deformation, X-ray microdiffraction and EBSD experiments are promising for not only estimating magnitudes but more importantly the directionality of the stress field.