CAREER: Grain Growth and Topological Evolution of Polycrystals

技术摘要：最近的研究已经证明，对于非零边界迁移率，晶粒生长是材料微观结构稳定性的持久特征。这完全是由于多晶体中晶粒的几何形状和拓扑结构是可测量的。平均而言，具有各向同性表面能的晶粒在晶界平均曲率为零的同时不可能满足Plateau规则。这不可避免地导致多晶材料的晶粒生长和随后的环境不稳定。现有的晶粒生长理论可以用来计算稳态、自相似、晶粒尺寸分布的存在性。这已通过连续切片和计算机模拟对十粒至一百粒的团聚体进行了实验和理论上的证实。然而，对已发表数据的进一步检查表明，与构建颗粒的可能方法的数量相比，每个颗粒的面数和每个表面的边数的分布相对有限。这意味着多晶样品中的晶粒在所有允许的形状中具有相对较少的形状，导致PI提出以下问题：1)晶粒尺寸的自相似分布。是否也存在自相似的颗粒拓扑分布？2)当人们观察实验微观结构并将其分类为“可能”或“不可能”时，狭窄的分布是否会使人们的直觉产生偏差？3)形成条件或材料加工条件是否建立了可识别的初始拓扑分布？4)是否有一些分布比其他分布更能抵抗晶粒生长？5)拓扑分布是否具有普适性，或者晶体各向异性等材料特性是否会影响结果？这些问题的答案将提供一个更完整的晶粒生长理论，反过来，将提高我们设计和预测材料在使用条件下性能的能力。一般来说，金属材料是多晶的，宏观材料的性能很大程度上取决于晶粒尺寸。环境因素，如高温暴露，可以改变材料的晶粒尺寸，在某些情况下会导致有害的性能和工程部件的过早失效。因此，了解导致或抑制晶粒生长的因素并控制这些因素以改善对工程性能至关重要的材料性能非常重要。PI将在模拟和实验中系统地研究晶粒的拓扑结构，并将这些数据集综合为理解金属材料中晶粒生长的动力学和热力学奠定基础。本研究计划通过解决很少受到关注的晶粒拓扑方面来推进对晶粒生长的理解，并将有助于使已发表的理论预测、实验和模拟数据之间的差异合理化。在这个项目中，高中生、本科生和研究生将参与与材料界面、多晶形态和晶粒生长动力学相关的活动。与本研究项目相关的实验室技术和学生主导的研究项目的实际应用将补充动力学和电子显微镜的研究生和本科生课程。笔者将继续为高中和初中的学生和教师成功实施首都区教材营。作为该研究计划的一部分，所产生的结果和大型数据集将发表在期刊文章、会议报告中，并可供其他研究人员分析。

TECHNICAL SUMMARY: Recent studies have proven that for nonzero boundary mobility, grain growth is a persistent feature of the microstructural stability of materials. This is due entirely to the measurable geometry and topology of the grains in a polycrystal. On average, no grain with an isotropic surface energy can have zero mean curvature along a grain boundary and simultaneously satisfy Plateau's rules. This inevitably leads to grain growth and subsequent environmental instability in polycrystalline materials. Available grain growth theories can be used to compute the existence of a steady state, self similar, grain size distribution. This has been confirmed experimentally and theoretically through serial sectioning and computer simulation for aggregates between ten and one-hundred grains. However, further inspection of the published data reveals that the distribution of the number of faces per grain and the number of edges per face are relatively limited as compared to the number of possible ways of constructing grains. This implies that grains in a polycrystal sample a relatively small number of shapes out of all permissible shapes, leading the PI to ask the following questions: 1) There is a self similar distribution of grain sizes. Is there also a self similar distribution of grain topologies? 2) Is the narrow distribution that which biases someone's intuition when they observe experimental microstructures and classify them as "likely" or "unlikely"? 3) Do the formation conditions or the materials processing conditions establish an initial topological distribution that can be identified? 4) Are there some distributions that are more resistant to grain growth than others? 5) Is the topological distribution universal, or, do material properties such as crystalline anisotropy affect the outcome? Answers to these questions will provide a more complete theory of grain growth that, in turn, will improve our ability to engineer and predict materials properties in service conditions.NON-TECHNICAL SUMMARY: In general, metallic materials are polycrystalline and macroscopic materials properties are strongly dependent on the grain size. Environmental factors, such as high temperature exposure, can vary the grain size of materials and in some cases lead to deleterious properties and premature failure of engineered components. Therefore, it is important to understand the factors that lead to or inhibit grain growth and to control those factors to improve the materials properties critical to engineering performance. The PI will systematically investigate the topology of grains in both simulation and experiment and will synthesize these data sets into a foundation for understanding the kinetics and thermodynamics of grain growth in metallic materials. This research program advances the understanding of grain growth by addressing aspects of grain topology that have received little attention and will help to rationalize discrepancies between published theoretical predictions, experiment, and simulation data. In this program high-school students, undergraduates, and graduate students will participate in activities related to material interfaces, morphology of polycrystals, and kinetics of grain growth. Hands-on application of laboratory techniques and student-led research projects related to this research program will complement graduate and undergraduate courses in kinetics and electron microscopy. The author will continue the successful implementation of the Capital District Materials Camp for high-school students and teachers in both high- and middle-school. The results and large data sets generated as part of this research program will be published in journal articles, conference presentations, and made available for analysis by other researchers.