CAREER: Spatiotemporal Avalanche Kinetics in Size-Dependent Crystal Plasticity

非技术摘要当金属部件的应力达到塑性变形的程度时，许多缺陷的作用是允许永久形状变化。在晶体金属中，这几乎意味着所有的工业合金，这些缺陷被称为位错。在协同作用下，许多位错可以同时开始运动。这一过程可能会导致突然的塑性不稳定，从而恶化部件的结构稳定性，并最终引发故障。这种类似雪崩的集体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，这些位错雪崩被限制在纳米级，并以极快的速度进行。因此，人们对它们在空间和时间上是如何进行的知之甚少。在这项研究中，PI和他的学生将揭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依赖温度的变形实验来完成，并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检验，并将提出新的变形模型。我们的研究的成功完成将导致对结构稳定性的更好控制，并推动能够预测雪崩从而失败的数学模型的发展。由于雪崩发生在许多其他系统中，如地震、无序物质或磁性，这里所获得的结果的意义将远远超出金属的可塑性。为了增加国家在STEM教育中的多样性和留住代表不足的群体，PI将制定一项针对中学年龄段的固体材料领域的教育方案，他将在学校的外联活动中介绍这一方案，并开创一个新的针对女孩的中学营地。这些干预措施将与PI目前在本科教育中实施的主动学习技术相结合。技术摘要这项建议将解决一个众所周知的控制金属材料结构完整性的难题：局部结构不稳定性如何在时空温度域中进行？这些不稳定性是由晶体中称为位错雪崩的集体缺陷动力学引起的。挑战在于这种过程的空间限制和短时间尺度。在依赖温度的微尺度应变实验中，使用纳秒时间分辨率和亚纳米位移分辨率相结合，目标将是实时跟踪位错雪崩。这将通过扩展商业上可用的具有MHz数据采样能力的纳米压头来实现，并将该系统集成到低温恒温器中。四个主要推力构成了本研究计划的核心：1)装置-样品动力学的非线性建模，2)理论预测标度律的实验验证，3)解开从间歇塑性流动到光滑塑性流动的转变，以及4)确定位错雪崩动力学的热激活参数。如果成功，由此产生的大型实验数据集将为预测材料建模的发展提供独特的基础，并可能导致对脱钉转变的更好控制，从而导致结构材料的强度。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相关的小规模变形实验，可以评估位错雪崩的速度分布，从而仔细审查最近提出的脱钉转变附近的雪崩理论。这些努力的影响是第一次实时评估晶体塑性的动态阶段，这将提高我们对最终决定金属机械稳定性或小金属部件形成的过程的物理理解。研究结果对大块金属具有普遍意义，并为材料建模和经历从晶体到颗粒材料的类似动态相变的系统提供了许多重要参数。雪崩特征的揭示将进一步提供一个关于位错塑性的粗粒度视图，它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.