CAREER: Spatiotemporal Avalanche Kinetics in Size-Dependent Crystal Plasticity

非技术摘要当金属部件受到一定程度的应力而塑性变形时，许多缺陷会导致永久的形状变化。在晶体金属中，这意味着几乎所有的工业合金，这些缺陷被称为位错。协同作用，许多位错可以开始同时移动。这一过程可能导致突然的塑性不稳定性，使部件的结构稳定性恶化，并最终引发故障。这种集体的、类似雪崩的过程的一个主要问题是它们是自发发生的，这意味着它们很难预测。此外，这些位错雪崩仅限于纳米尺度，并且进行得非常快。因此，人们对它们在空间和时间中的运动方式知之甚少。在这项研究工作中，PI和他的学生将解开位错雪崩的精确动力学。我们不仅将跟踪它们的时空动态，还将定义它们如何对温度变化做出反应。这将通过独特的微尺度和温度依赖性变形实验来完成，具有极快的响应动力学。预测描述雪崩行为的一般统计和物理模型将用实验数据进行测试，并提出新的变形模型。我们的研究的成功完成将导致更好地控制结构稳定性，并推动数学模型的发展，可以预测雪崩，从而失败。由于雪崩发生在许多其他系统，如地震，无序材料，或磁性，这里获得的结果的意义将远远超出金属的塑性。为了增加国家的多样性和保留在STEM教育中代表性不足的群体，PI将在中学年龄段的坚实材料领域制定一项教育计划，他将在学校的外展活动中展示，并为女孩开创一个新的中学营地。这些干预措施将与积极的学习技术，PI目前正在实施本科education.Technical AbstractThis建议将解决一个臭名昭著的难题，控制金属材料的结构完整性：如何进行局部结构不稳定性的空间，时间，温度域？这些不稳定性是由集体缺陷动力学引起的，称为晶体中的位错雪崩。挑战在于这些进程的空间局限性和短期性。在温度依赖的微尺度应变实验中，使用纳秒时间分辨率与亚纳米位移分辨率相结合，目标是在真实的时间内追踪位错雪崩。这将通过扩展具有MHz数据采样能力的市售纳米压头来实现，并将该系统集成到低温恒温器中。四个主要的推力组成了这个研究计划的核心：1）非线性建模的设备-样品动力学，2）实验验证理论预测的标度律，3）解开从间歇到平滑的塑性流动的过渡，和4）确定热激活参数的位错雪崩动力学。如果成功的话，由此产生的大型实验数据集将是预测材料建模开发的独特基础，并可能导致更好地控制脱钉过渡，从而控制结构材料的强度。该项目的关键将是一个统一的实验方法，具有高度时间分辨率和温度依赖性的小规模变形实验，可以评估位错雪崩的速度分布，从而仔细检查最近提出的理论，雪崩附近的脱钉过渡。这些努力的影响是对晶体塑性动态阶段的首次实时评估，这将提高我们对最终决定金属机械稳定性或小型金属部件成型过程的物理理解。结果将是相关的散装金属一般，并提供了许多重要的参数，材料建模和系统，经历类似的动态相变，从晶体到粒状材料。解开雪崩特性将进一步提供位错塑性的粗粒度视图，可以在位错动力学和本构晶体塑性建模之间架起桥梁，这可能直接导致更有效的多尺度建模框架。

Non-Technical Abstract When a metallic component is stressed to the extent that it plastically deforms, many defects operate to allow the permanent shape change. In crystalline metals, which means practically all technical alloys, these defects are called dislocations. Acting cooperatively, many dislocations can begin to move at the same time. This process can lead to abrupt plastic instabilities that deteriorate the structural stability of components and eventually trigger failure. One main problem with such collective, avalanche-like, processes is that they occur spontaneously, which means that they are hard to predict. In addition, these dislocation avalanches are confined to the nanometer scale and proceed extremely fast. As a result, very little is known about how they proceed in space and time. In this research effort, the PI and his students will unravel the precise dynamics of dislocation avalanches. We will not only track their spatiotemporal dynamics, but we will also define how they respond to changes in temperature. This will be done by unique micro-scale and temperature-dependent deformation experiments with extremely fast response dynamics. General statistical and physical models that are predicted to describe the avalanche behavior will be tested with the experimental data, and novel deformation models will be proposed. A successful completion of our research will lead to a better control of structural stability, and drive the development of mathematical models that can predict avalanches and therefore failure. Since avalanches occur in many other systems, such as earthquakes, disordered materials, or magnetism, the significance of the here-obtained results will extend well beyond plasticity of metals. In order to increase the nation's diversity and retention of underrepresented groups in STEM education, the PI will develop an educational program in the area of solid materials for the middle-school age-bracket, which he will present in outreach activities at schools, and also pioneer a new middle-school camp for girls. These interventions will be integrated with active learning techniques that the PI is currently implementing in undergraduate education.Technical AbstractThis proposal will tackle a notoriously difficult problem that controls the structural integrity of metallic materials: How do local structural instabilities proceed in the space-time-temperature domain? These instabilities are caused by collective defect dynamics, called dislocation avalanches in crystals. The challenge lies in the spatial confinement and the short time scales of such processes. Using nanoseconds time resolution in combination with sub-nanometer displacement resolution during a temperature-dependent micro-scale straining experiment, the objective will be to trace dislocation avalanches in real time. This will be achieved by extending a commercially available nanoindenter with MHz data sampling capabilities, and to integrate the system into a cryostat. Four main thrusts compose the core of this research program: 1) non-linear modeling of the device-sample dynamics, 2) experimental validation of theoretically predicted scaling laws, 3) unraveling the transition from intermittent to smooth plastic flow, and 4) determining thermal activation parameters for dislocation avalanche dynamics. If successful, the hereby generated large experimental data set will be a unique basis for the development of predictive materials modeling, and may lead to a better control of the depinning transition and thus the strength of structural materials. Key of this project will be a unified experimental approach with highly time-resolved and temperature-dependent small-scale deformation experiments that can assess the velocity-profiles of dislocation avalanches, thereby scrutinizing recently proposed theories for avalanches near the depinning transition. The impact of these efforts is a first real-time assessment of a dynamic phase in crystal plasticity, which will improve our physical understanding of a process that ultimately dictates the mechanical stability of metals, or forming of small metallic components. The results will be relevant for bulk metals in general, and provide numerous important parameters for materials modeling and systems that undergo similar dynamic phase transitions, ranging from crystals to granular materials. Unravelling avalanche characteristics will furthermore provide a coarse-grained view on dislocation plasticity that can bridge between dislocation dynamics and constitutive crystal plasticity modeling, which may directly lead to more efficient multi-scale modeling frameworks.