On the experimental characterization of crystal plasticity in polycrystals

Abstract

An experimental procedure has been developed to systematically investigate the local deformation behavior of polycrystalline materials at the micrometer scale. The procedure consists of a combination of the measurement of the local in-plane strains and the local crystal orientation during an in-situ deformation test in the scanning electron microscope. The local in-plane strains are determined from micrographs that are taken before and after a considered deformation step. By means of a digital image analysis system, homologue points are found on the two micrographs. The homologue points form a deformation field that can be derived to determine the local strain field. The local crystal orientation is measured by orientation image microscopy analyses, again taken before and after the deformation step. From these measurements, the local rotation of the crystal lattice during the deformation step and the Miller indices of the rotational axis are determined. The combination of the data of the local crystal orientation and the local in-plane strains is used to estimate, with a simple kinematic model, the slip systems that are locally active at arbitrary positions within a grain. A polycrystalline copper specimen was loaded in two deformation steps of 7.4 and 14% global tensile strain. A region of ≈250×200 μm was analyzed. The experiment shows that very strong heterogeneities in the in-plane strain and in the local lattice rotation exist within the single grains. At some grain boundaries, very high lattice rotations are observed. Even far from the boundary, the activation of slip systems can be quite different in different regions of a grain. On the specimen surface, more than three independent slip systems must be activated in some grain boundary regions to accommodate the influence of the neighboring grain. In most regions of the grain interior, three slip systems seem to be enough. With the developed procedure, we have generated a tool for a very comprehensive experimental characterization of crystal plasticity in polycrystals.