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。地震学家以地震波速度的方向性（各向异性）的形式记录了这些方向上的变化，称为结构，并在地球的大部分地区都有记录。本研究通过高温高压实验和建模，探讨了宏观各向异性特性的变化，并试图从微观机制上解释这些变化。特别重要的是矿物转变成不同结构时的相变化。随着新仪器的出现，如洛斯阿拉莫斯的新型中子衍射仪和同步加速器源的金刚石砧细胞，在各种温度和压力条件下实时研究大块样品在变形、再结晶和相变过程中的各向异性变化已经成为可能。初步研究表明，父相和子相之间往往存在密切的取向关系，这取决于结构相似性和邻近晶粒施加的应力状态。本提案主要关注的材料是石英（以及对解释古代应力场的变质岩的影响），fcc-bcc金属（作为研究详细机制的复杂模型系统，与核心相关）和橄榄石-环橄榄石-钙钛矿+方长石转变（对解释地幔的各向异性至关重要）。前两个将在LANSCE（之后可能在SNS）用高温中子衍射进行研究。最后一个系统将在APS和ALS用金刚石砧细胞进行研究。这些实验的结果，连同其他可用的数据，将用于模拟下地幔结构和各向异性的发展。各向异性、结构和相变是远远超出地球物理学的问题，本研究的结果将对材料科学产生直接影响。正在开发的中子和x射线定量宏观结构分析方法，以及同步加速器束和电子显微镜的微观分析方法，可以应用于金属、高温超导体和骨骼等多种材料，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.