The size-dependence of the plastic response of single-crystal micropillars at sub-micron/micron length-scales under compression was investigated by using 3-dimensional discrete dislocation dynamics simulations. In the simulations, the initial dislocation configuration consisted of randomly distributed Frank–Read-type dislocation sources. The simulation results were 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 was the sample dimension and <N> was the number density of activated sources. This relationship was found to be in good accord with the discrete dislocation dynamics simulation results. The new finding in this study was that the size dependence of the plastic response could be independent of source strength in the high-velocity or high-stress regime. The length-scale effects observed were due to dislocation escape through free surfaces. Mobile dislocations could 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 provided a possible explanation of the experimentally observed size effects in the testing of micropillars.

Dislocation Escape-Related Size Effects in Single-Crystal Micropillars under Uniaxial Compression. H.Tang, K.W.Schwarz, H.D.Espinosa: Acta Materialia, 2007, 55[5], 1607-16