Dislocation escape-related size effects in single-crystal micropillars under uniaxial compression

The size-dependence of the plastic response of single-crystal micropillars at submicron/micron length scales under compression was investigated using three-dimensional discrete dislocation dynamics (DDD) simulations. In the simulations, the initial dislocation configuration consists of randomly distributed Frank–Read-type dislocation sources. The simulation results are compared with a dislocation evolution model for geometrically confined systems with free surfaces, intended to approximate the evolution behavior of the dislocation density at sufficiently high velocities or stress levels. The dependence of the effective stress on both the sample dimension and source density was shown to take the form τeff∝1/a〈N〉 at a fixed strain rate, where a is the sample dimension and 〈N〉 is the number density of activated sources. This relationship is found to be in good accord with the DDD simulation results. The new finding in this study is that the size dependence of the plastic response can be independent of source strength in the high-velocity or high-stress regime. The length-scale effects we observe are due to dislocation escape through free surfaces. Mobile dislocations can typically escape faster in a smaller sample, leading to a lower mobile dislocation density and an increased resistance to plastic flow. Thus, the dislocation-escape mechanism provides a possible explanation of the experimentally observed size effects in the testing of micropillars.