Grain-scale measurement of slip resistances in aluminum polycrystals using spherical nanoindentation

In this work, we develop and demonstrate novel protocols based on spherical nanoindentation and orientation image mapping (OIM) for quantifying the local increases in slip resistances in the individual grains of a deformed (or strain hardened) polycrystalline sample. These new protocols utilize the recently developed data analyses methods for extracting indentation stress-strain (ISS) curves in conjunction with the measurements of the local crystal orientations at the indentation sites using the OIM technique. The proposed protocols involve two main steps. In the first step, spherical nanoindentation measurements are conducted on fully annealed samples of the material of interest to map out the functional dependence of the indentation yield strength (Yind) on the crystal lattice orientation in the annealed condition. In the second step, spherical nanoindentation and OIM measurements are conducted on the deformed samples of the same material and are analyzed rigorously to reliably estimate the increase in the local slip resistance at the indentation sites. The function established in the first step is utilized in the second step to properly account for the influence of the local crystal orientation on the measured Yind in the deformed sample. This novel measurement and data analysis protocol is demonstrated in this paper on polycrystalline samples of high purity aluminum. From this study, it was noted that the influence of the crystal lattice orientation on the measured Yind in Al crystals can be as high as 40%, with the lowest values corresponding to the [1 0 0] (cube) orientation and highest values corresponding to the [1 1 1] orientation. The measurements on the deformed samples showed a significant variation in the strain hardening rates in the individual grains of the polycrystalline sample. A positive correlation was observed between the percentage increase in the local slip resistance and the value of the Taylor factor computed for the local crystal orientation at the indentation site subjected to the macroscale imposed deformation.