Effect of High Angle Grain Boundary on Deformation of Aluminum Bicrystalline Nanopillars

Abstract

Nanoscale single crystal metals are known to exhibit size dependent strength and plasticity, where the strength of the nanopillar increases with a decrease in the sample size. Due to the small dimension of the nanopillar, the dislocations leave the pillar before multiplication occurs to result in a dislocation starved state. The plasticity would then be governed by dislocation nucleation at the surface of the nanopillar. In this study, the effect of inclusion of a high angle grain boundary is studied using microcompression method. Single and bicrystal aluminum nanopillars with the diameters ranging from 200nm to 2000nm were tested in compression that revealed a typical size effects exponent found in face-center-cubic single crystals. However, the plastic flow behavior and the magnitude of the flow stresses at 5% were distinctly different for the single and the bicrystal Al nanopillars. The bicrystal Al nanopillars were shown to display small, frequent strain bursts that resulted in a homogeneous deformation, while the single crystal Al nanopillars displayed stochastic flow behavior with large strain bursts that resulted in an inhomogeneous deformation. The observed difference in the deformation behavior of the bicrystal Al nanopillars can be explained by the fact that the grain boundary may act as a dislocation source that continuously emits dislocations into the nanopillar during deformation. The flow stresses at 5% plastic strain were plotted as a function of diameter for both the single crystals and bicrystals that revealed similar size exponents. However, the flow stresses of the bicrystal pillars for the relevant dimension of half of the diameter was lower than those for single crystals of the full diameter. The difference in flow stresses can be attributed to the emission of dislocations from the grain boundary or easier dislocation nucleation as a result of inclusion of a grain boundary in the middle of the nanopillar.