Anisotropy Changes during Phase Transformation

当岩石变形或受到地球温度和压力变化的影响时，组成岩石的晶体的方向就会发生变化。这些方向的变化，被称为纹理，被地震学家以地震波速度的方向性（各向异性）的形式记录下来，并在地球的大部分地区被记录下来。该提案解决了宏观各向异性性质的变化，并试图通过高温高压实验和建模，基于微观机制解释它们。特别重要的是矿物转变成不同结构时的相变。随着新仪器的出现，如新的中子衍射仪在洛斯阿拉莫斯和金刚石砧细胞在同步加速器源，它已成为可能的调查在变形，再结晶和相变过程中，在真实的时间，在广泛的温度和压力条件下的各向异性的变化的散装样品。初步的研究表明，往往是一个密切的取向关系之间存在的父和子阶段，这取决于结构的相似性和应力状态所施加的相邻晶粒。在这个提议中主要感兴趣的材料是石英（以及对变质岩解释古代应力场的影响），面心立方-体心立方金属（作为一个复杂的模型系统，以研究详细的机制，与核心相关）和橄榄石-林伍德石-钙钛矿+方镁石转换（解释地幔各向异性必不可少）。前两个将在LANSCE（稍后可能在SNS）高温下用中子衍射进行研究。最后一个系统将在APS和ALS用金刚石对顶砧进行研究。这些实验的结果，连同其他可用的数据，将被用来模拟下地幔的结构和各向异性的发展。各向异性，织构和相变是远远超出物理学的问题，这项研究的结果将对材料科学产生直接影响。用中子和X射线进行定量宏观织构分析以及用同步加速器束和电子显微镜进行显微分析的方法正在开发中，这些方法可以应用于金属、高温超导体和骨骼等各种材料，其中晶体取向对功能至关重要。

When rocks are deformed or subjected to temperature and pressure changes in the earth, the orientation of crystals that compose the rock changes. These orientation changes, known as texture, are recorded by seismologists in the form of directionality (anisotropy) of seismic wave speeds and have been documented for large parts of the earth. This proposal addresses the changes in macroscopic anisotropic properties and tries to explain them based on microscopic mechanisms, both through experiments at high temperature and pressure and by modeling. Of particular importance are phase changes when minerals transform into different structures. With the advent of new instrumentation such as the new neutron diffractometers at Los Alamos and diamond anvil cells at synchrotron sources, it has become possible to investigate changes in anisotropy of bulk samples during deformation, recrystallization and phase transformations in real time, at a wide range of temperature and pressure conditions. Preliminary studies revealed that often a close orientation relationship exists between parent and daughter phases that depends on structural similarities and the stress state imposed by neighboring grains. Materials of primary interest in this proposal are quartz (and implications for metamorphic rocks to interpret the ancient stress field), fcc-bcc metals (as a complex model system to investigate detailed mechanisms, with relevance for the core) and the olivine-ringwoodite-perovskite+periclase transformations (essential to explain anisotropy in the mantle). The first two will be investigated with neutron diffraction at high temperature at LANSCE (and later perhaps at SNS). The last system will be studied with diamond anvil cells at APS and ALS. Results from these experiments, together with other available data, will be used to model the development of texture and anisotropy in the lower mantle. Anisotropy, texture and phase transformations are issues that go far beyond geophysics and results from this research will have direct impact on materials science. Methods of quantitative macroscopic texture analysis with neutrons and X-rays as well as microscopic analysis with synchrotron beams and electron microscopy that are being developed can be applied to such diverse materials as metals, high temperature superconductors and bones where crystal orientation is critical for functionality.