Effect of specimen size and crystallographic orientation on the nano/microscale mechanical properties and deformation behavior of CrCoNi medium-entropy alloy

CrCoNi medium-entropy alloy (MEA) shows an exceptional combination of macroscale mechanical properties, however, knowledge of its mechanical properties at nano/microscale is lacking. In this paper, nano/microscale single crystal MEA pillars with orientation of <116>, <014>, <102> were fabricated using focused ion beam milling, and the compressive strength was determined with micropillar compression testing using a flat-end tip in an indentation machine. Results showed that the yield and flow strength of these MEA pillars were orientation-dependent, i.e., the lower the Schmid factor, the higher the strength. At the same time the strength of the pillars was dependent on the pillar’s size, the strength increasing as the pillar diameter reduced. From the results of the micropillar compression testing, the bulk critical resolved shear stress (CRSS) was determined to be 92 MPa, which is the highest value of the group of CrCoNi-based medium and high-entropy alloys. Analysis of transmission electron microscope images and large-scale molecular dynamics simulations revealed a group of complimentary deformation mechanisms, including dislocation slides, dislocation annihilation, dislocation-slip band interaction, formation of nanoscale stacking-faulting networks, and deformation nanotwins.