Deformation mechanisms in FCC Co dominated by high-density stacking faults

It has been known that twin boundaries dominate the plasticity of nanotwinned metals. However, the role of stacking faults on the deformation mechanisms in face-centered-cubic (FCC) metals is less well understood. Here we investigate the deformation mechanisms of FCC Co with high-density stacking faults using in situ micropillar compression and atomistic simulations. In situ compression tests show a prominent strain rate dependence, with either strain softening or strain hardening observed at different strain rates. Molecular dynamics simulations reveal that the stacking faults and partial dislocations dominate the plastic deformation of FCC Co. Abundant dislocation junctions form at high strain rate, leading to strengthening.