Microstructure Evolution in Solids with External Constraints and Defects

该奖项支持理论和计算研究和教育，以研究固体中微观结构的演变。该提案的主要科学目标是了解外部约束对相变和微观结构演变的影响，以及相和缺陷微观结构之间的相互作用。PI将使用相场方法结合细观弹性理论研究两个具体问题。第一个问题是铁电薄膜在衬底约束下的相变和畴结构演化。 建立了一个描述单晶薄膜铁电畴演化的相场模型。该模型将包括长程弹性和电偶极-偶极相互作用，以及适当的机械和电气边界条件。最初的重点将是一些重要的氧化物，PbTiO 3，BaTiO 3，PbZrxTi 1-xO 3，有广泛的实验测量和理论热力学分析。 PI将系统地研究基板约束和薄膜厚度对相变温度，体积分数和每个取向域的大小的影响。重点将放在铁电畴结构的时间演变过程中的成核，生长和粗化，以及在畴壁运动和极化开关下的电场。将研究内部缺陷，不动和扩散，畴壁迁移率和铁电/介电响应的影响。第二个问题涉及先进合金中相和位错微观结构之间的相互作用。基于位错的相场模拟的最新进展，将开发一个综合模型的相位和位错微观结构的同时时间演化，将弹性各向异性和弹性不均匀性。PI将通过改变溶质-溶剂尺寸失配、弹性不均匀性以及相对溶质扩散率和位错迁移率来研究静态和移动位错周围的局部相平衡、溶质偏析动力学以及成核和生长过程。一个主要的努力将致力于模拟溶质偏析和第二相沉淀物的动力学的孤立和合奏的位错施加应力下的影响。特别是，对于一个给定的应变速率，溶质浓度，溶质扩散系数，沉淀物的大小和形状，沉淀物-沉淀物间距，晶格失配，和弹性不均匀性，对合金的临界屈服应力的影响将被系统地研究。申请资助两名研究生。PI将与实验学家密切互动，以验证理论预测。他还计划与其他理论家合作，将电子结构计算和介观相场模拟联系起来，以模拟相变和微观结构演变。拟议的研究将影响材料研究生教育，因为相变和微观结构演变的相场模拟正在作为热力学和动力学教育计划的一部分纳入研究生课程。将开发方便用户的图形界面软件，并分发给其他机构用于教育目的。拟议的项目还将产生新的计算工具，这些工具有可能应用于工业上重要的材料问题，PI和工业界之间现有的合作证明了这一点。该奖项支持理论和计算研究和教育，以研究原子和宏观之间的长度尺度上的材料结构，微观结构，其在相变中的作用，以及在外部约束和内部缺陷存在下的演变。 这是一个直接影响材料加工的基本难题。PI将使用相场方法，重点研究铁电材料中畴的演化以及先进合金中相和位错微结构之间的相互作用。位错在固体的扩散过程和相变中起着重要的作用。铁电材料可用于传感器和光学元件 *

This award supports theoretical and computational research and education to study the evolution of microsctructure in solids. The main scientific objective of this proposal is to understand the effect of external constraints on phase transformations and microstructure evolution, and the mutual interactions between phase and defect microstructures. The PI will investigate two specific problems using the phase-field approach in combination with mesoscale elasticity theory. The first problem is concerned with phase transformations and domain structure evolution in ferroelectric thin films constrained by a substrate. A phase-field model will be developed for ferroelectric domain evolution in single-crystal films. The model will include long-range elastic and electric dipole-dipole interactions, and the appropriate mechanical and electrical boundary conditions. The initial focus will be on a number of important oxides, PbTiO3, BaTiO3, PbZrxTi1-xO3, for which there have been extensive experimental measurements and theoretical thermodynamic analyses. The PI will systematically investigate the effect of substrate constraints and film thickness on transformation temperatures, volume fractions, and the size of each orientation domain. The focus will be on the temporal evolution of ferroelectric domain structures during nucleation, growth and coarsening, as well as during the domain-wall motion and polarization switching under an electric field. The effect of internal defects, both immobile and diffusive, on domain-wall mobility and ferroelectric/dielectric responses will be studied. The second problem involves the mutual interactions between phase and dislocation microstructures in advanced alloys. Based on recent advances in phase-field modeling of dislocations, a comprehensive model for the simultaneous temporal evolution of phase and dislocation microstructures will be developed, incorporating both elastic anisotropy and elastic inhomogeneity. The PI will study the local phase equilibria, solute segregation kinetics, and nucleation and growth processes around both static and moving dislocations, by varying the solute-solvent size mismatch, elastic inhomogeneity, and the relative solute diffusivity and dislocation mobility. A major effort will be devoted to modeling the influence of solute segregation and second-phase precipitates on the dynamics of both isolated and an ensemble of dislocations under applied stresses. In particular, for a given strain rate, the effect of solute concentration, solute diffusivity, precipitate size and shape, precipitate-precipitate spacing, lattice mismatch, and elastic inhomogeneity, on the critical yield stress of an alloy will be systematically studied. Financial support for two graduate students is requested. The PI will interact closely with experimentalists for validation of theoretical predictions. He also plans collaborations with other theorists to link electronic structure calculations and mesoscale phase-field simulations for modeling phase transformations and microstructure evolution.The proposed research will impact graduate education in materials, as phase-field simulations of phase transformations and microstructure evolution are being incorporated into a graduate course as part of an educational program on thermodynamics and kinetics. User-friendly software with graphical interfaces will be developed and distributed to other institutions for educational purposes. The proposed project will also result in new computational tools that can potentially be applied to industrially important materials problems as evidenced by the existing collaborations between the PI and industry.%%%This award supports theoretical and computational research and education to study the structure of materials on length scales between the atomic and the macroscopic, the microstructure, its role in phase transformations, and its evolution in the presence of external constraints and internal defects. This is a difficult fundamental problem which directly impacts materials processing. The PI will use phase field methods and focus on evolution of domains in ferroelectric materials and the mutual interactions between phase and dislocation microstructures in advanced alloys. Dislocations play an important role in diffusion processes and phase transformations of solids. Ferroelectric materials have applications in sensors and optical components***