Interplay between sample size and grain size: Single crystalline vs. ultrafine-grained chromium micropillars

To gain insights into the influence of the microstructure on the strengthening behaviour in confined volumes, single crystalline (sxx) and ultrafine-grained (ufg) Cr micropillars were investigated using in-situ scanning electron microscope microcompression tests. Post-compression images of the sxx pillars indicate crystallographic slip, while the ufg pillars reveal a bulk-like deformation behaviour and an emergence of grains from the sample surface. Stress-strain curves of sxx samples show intermittent flow and a scaling behaviour agreeing well with other bcc metals investigated previously. Also for ufg samples a size-dependent strength with a reduced but non-negligible scaling exponent is determined. This latter ufg size effect contributes to an increasing influence of near-surface grains controlling plastic flow with decreasing pillar diameter. While for micron-sized pillars the strength differs between the two microstructures, the two scaling trends converge for sub-micron pillars with diameters close to the grain size, indicative of a transition from boundary-mediated to single crystal plasticity.