Microstructure Evolution in Solids with External Constraints and Defects

该奖项支持理论和计算研究和教育，以研究固体微观结构的演变。本课题的主要科学目标是了解外部约束对相变和微观组织演化的影响，以及相与缺陷微观组织之间的相互作用。PI将使用相场方法结合中尺度弹性理论研究两个具体问题。第一个问题是关于受衬底约束的铁电薄膜的相变和畴结构演变。本文将建立单晶薄膜中铁电畴演化的相场模型。该模型将包括远程弹性和电偶极子-偶极子相互作用，以及适当的机械和电边界条件。最初的重点将放在一些重要的氧化物上，PbTiO3, BaTiO3, PbZrxTi1-xO3，为此已经进行了广泛的实验测量和理论热力学分析。PI将系统地研究衬底约束和薄膜厚度对转变温度、体积分数和每个取向域大小的影响。重点研究了在电场作用下，铁电畴结构在成核、生长和粗化过程以及畴壁运动和极化开关过程中的时间演化。内部缺陷（包括不动缺陷和扩散缺陷）对畴壁迁移率和铁电/介电响应的影响将被研究。第二个问题涉及先进合金中相和位错组织之间的相互作用。基于位错相场模拟的最新进展，将建立一个结合弹性各向异性和弹性非均质性的相和位错微观结构同时时间演化的综合模型。PI将通过改变溶质-溶剂尺寸错配、弹性不均匀性、相对溶质扩散率和位错迁移率，研究静态和移动位错周围的局部相平衡、溶质偏析动力学、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对施加应力下的孤立和整体位错动力学的影响。特别是，在给定应变速率下，将系统地研究溶质浓度、溶质扩散率、析出相尺寸和形状、析出相间距、晶格错配和弹性不均匀性对合金临界屈服应力的影响。申请资助两名研究生。PI将与实验人员密切互动，以验证理论预测。他还计划与其他理论家合作，将电子结构计算和中尺度相场模拟联系起来，以模拟相变和微观结构演变。拟议的研究将影响材料的研究生教育，因为相变和微观结构演化的相场模拟作为热力学和动力学教育计划的一部分被纳入研究生课程。本署亦会开发附有图形界面的易用软件，分发予其他院校作教育用途。拟议的项目还将产生新的计算工具，可以潜在地应用于工业上重要的材料问题，PI和工业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，以研究材料在原子和宏观之间的长度尺度上的结构，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难题。PI将使用相场方法，重点研究铁电材料中畴的演变以及先进合金中相和位错微观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。铁电材料在传感器和光学元件中有应用

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***