Effects of Nanoindentation Loading Rates on Pop-in Events and Deformation Mechanisms in Single-Crystal Zinc

Abstract

Single crystals of pure zinc were examined using instrumented spherical indentation on the basal-oriented surface at three different loading rates. The residual impressions and the surrounding surface were investigated using atomic force microscopy and electron backscatter diffraction in order to reveal information about the topography and orientation changes. Noticeable pop-ins were reproducible in the majority of the tests. Contrary to reports in the literature, the pop-in initiation load and loading rates were found to have an inverse relationship, and this behaviour was attributed to twinning deformation and dislocation movement. In tests with all loading rates, evidence of twinning could be found both within and outside of the imprints. Residual impressions composed of slip and twin ‘subgrains’ and external twins manifested as sink-in patterns were observed in most of the tests, and the difference lay in the dissimilar twin formations at different loading rates. A hypothesis was developed to address the change in deformation behaviour within the subsequent loading process after a pop-in event.