Imaging Local Stress Anisotropy and Determining Its Role in Driving Defect Mobility in Crystals

非技术摘要：金属等材料弯曲、拉伸和变形的难易程度取决于晶体内部的缺陷。了解这些缺陷如何相互作用是困难的，因为目前还没有任何实验工具来成像这些缺陷在原子尺度上彼此之间的三维相互作用。这个项目需要研究悬浮在液体中并有序形成晶体的称为胶体的微小颗粒。由这些粒子制成的晶体显示出许多与原子晶体相同的缺陷和缺陷相互作用，但这些粒子足够大，足够慢，可以用显微镜检查。这个项目将使用由主要研究者和共同原理研究者开发的一种新的计算技术来确定晶体缺陷通过这些粒子对彼此施加的力。此外，这些缺陷相互作用在大范围内产生的力将使用原理调查者开发的设备进行测量。正在研究的缺陷包括空位，即晶体中缺少单个粒子；位错，缺少整个原子平面；晶界，不同取向的晶体相遇；以及裂纹，粒子之间失去联系。通过比较驱动这些缺陷在不同长度尺度上移动和相互作用的力，可以使晶体的设计和操作确定其大规模的机械性能。涉及凝聚态物理基本问题的基础研究与实验开发中的实际问题相结合，提供了一个很好的培训基础；从事这个项目的学生将接受最先进的实验和理论技术培训，这将为他们进入学术界、政府实验室和高科技行业的职位做好准备，因为他们的基础工作和应用工作都有背景。技术摘要：在晶体中产生大的不可逆应变的主要机制是缺陷的形核和动态演化，包括空位、位错和晶界。这些缺陷是我们理解原子晶体的屈服、加工硬化、断裂、疲劳和随时间变化的弹性的核心。材料科学界面临的一个主要障碍是了解由这些缺陷造成的空间不均匀和与历史相关的局部应力环境如何进一步影响它们的演化，并决定晶体的整体力学性质。使用一种名为“局部结构各向异性应力评估”(SALSA)的新技术，该项目正在绘制胶体晶体中此类缺陷及其相互作用引起的应力不均匀。至关重要的是，SASA允许在单个粒子尺度上绘制出这些压力。三轴共焦流变仪被用来测量块体性质，并确定局部应力不均匀如何导致晶体的宏观响应。该项目专注于在晶体中发现的几种主要的缺陷中介现象。空位与其他缺陷相互作用的非线性动力学理论正在发展之中。此外，这些技术还被用来研究位错与空位、其他位错、层错和晶界的相互作用。当薄膜厚度接近单个颗粒的长度尺度时，整体压缩和剪切的组合将决定多晶区在剪切下的力学响应如何变化。最后，耗尽相互作用被用来形成吸引人的晶体，以便测量这种晶体中裂纹扩展引起的应力。这项研究使人们能够从根本上理解驱动缺陷迁移率的应力场与由此产生的晶体整体机械响应之间的联系。所选择的项目反映了空位、位错、晶界和裂纹在调节这些特性中所起的关键作用。

Non Technical Abstract:The ease with which materials like metals bend, stretch and deform depends on the defects inside the crystals. Learning how such defects interact with one another is difficult since there aren't any experimental tools for imaging the three dimensional interactions of these defects with one another at the atomic scale. This project entails looking at tiny particles called colloids that are suspended in a fluid and ordered into crystals. Crystals made from these particles exhibit many of the same defects and defect interactions found in their atomic counterparts, but the particles are big enough and slow enough to examine using microscopes. This project will use a new computational technique developed by the principle investigator and the co-principle investigator to determine the forces that crystal defects exert on one another through these particles. In addition, the forces these defect interactions produce on a large scale will be measured using equipment developed by the principle investigator. The defects being investigated include vacancies, where single particles are missing from the crystal, dislocations, where an entire plane of atoms is missing, grain boundaries, where crystals with different orientations meet, and cracks, where particles lose contact with one another. Comparison of the forces driving these defects to move and interact with one another on different length scales is enabling the design and manipulation of crystals to determine their large scale mechanical properties. The combination of basic research involving fundamental issues in condensed matter physics with practical issues in experiment development provides an excellent training ground; students working on this project will recieve training in state-of-the-art experimental and theoertical techniques which will provide them with an excellent preparation to enter positions in academia, government laboratories, and high-tech industry, given their background in both basic and applied work. Technical Abstract:The dominant mechanisms for creating large irreversible strain in crystals is the nucleation and dynamic evolution of defects including vacancies, dislocations, and grain boundaries. Such defects are central to our understanding of yield, work hardening, fracture, fatigue, and time-dependent elasticity in atomic crystals. A major hurdle faced by the materials science community is to understand how the spatially heterogeneous and history dependent local stress environment created by such defects further affects their evolution and determines the bulk mechanical properties of the crystal. Using a newly developed technique called Stress Assessment from Local Structural Anisotropy (SALSA) this project is mapping out stress inhomogeneities that arise from such defects and their interactions in colloidal crystals. Crucially, SALSA allows for mapping out these stresses at the single particle scale. A triaxial confocal rheometer is being used to measure bulk properties and determine how the local stress inhomogeneities lead to the macroscopic response of the crystal. The project focuses on several major defect mediated phenomena found in crystals. A nonlinear dynamics theory of vacancy interactions with other defects is being developed. In addition these techniques are being used to study the interaction of dislocations with vacancies, other dislocations, stacking faults and grain boundaries. A combination of bulk compression and shear will determine how the mechanical response of polycrystalline domains under shear varies as the film thickness approaches the length scale of a single grain. Finally, depletion interactions is being used to form attractive crystals in order to measure the stresses arising from crack propagation in such crystals. This research is enabling a fundamental understanding of the connection between the stress fields driving defect mobility and the resulting bulk mechanical response of the crystal. The projects chosen reflect the crucial roles played by vacancies, dislocations, grain boundaries, and cracks in mediating these properties.