Effect of grain size on critical twinning stress and work hardening behavior in the equiatomic CrMnFeCoNi high-entropy alloy

While the impact of grain boundary strengthening on dislocation slip is particularly effective in the equiatomic CrMnFeCoNi high-entropy alloy (HEA), its effect on deformation twinning remains unclear. To better understand how a grain size reduction affects the onset of deformation twinning and the work hardening behavior of the CrMnFeCoNi HEA, chemically homogeneous, nearly untextured, and single-phase face-centered cubic alloys with different grain sizes were investigated. Tensile tests were performed at 293 and 77 K and interrupted at different strains followed by systematic transmission electron microscopy observations. In all cases, deformation twinning occurs above a critical stress that is independent of temperature. This uniaxial twinning stress decreases from similar to 785 to similar to 615 MPa when the grain size increases from 6 to 242 mu m, respectively, following the Hall-Petch equation. The resistance of the grain boundaries against slip and twinning is found to be nearly identical (Hall-Petch slope: similar to 500 MPa.mu m(1/2)) but the twinning stress extrapolated to infinite grain size (592 +/- 30 MPa) is larger than the uniaxial friction stress against dislocation glide at 293 and 77 K (130 and 320 MPa, respectively). Deformation twinning at 77 K is found to sustain a high work hardening rate when it is triggered in a plastic regime dominated by planar glide of dislocations. In contrast, it does not significantly contribute to the work hardening rate at 293 K when dislocation cells have already formed and the dislocation mean free path is smaller than the mean twin spacing.