CAREER: Spatiotemporal Avalanche Kinetics in Size-Dependent Crystal Plasticity

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

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 Abstract This 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.