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

• Published in: International journal of plasticity Vol. 166; p. 103651
• Main Authors: Wagner, Christian, Laplanche, Guillaume
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.07.2023
• Subjects: CoCrFeMnNi Cantor HEA; Hall-Petch effect; mechanical twinning; Strain hardening; transmission electron microscopy (TEM); transmission electron microscopy (TEM); CoCrFeMnNi Cantor HEA; Hall-Petch effect; mechanical twinning; Strain hardening
• ISSN: 0749-6419; 1879-2154
• DOI: 10.1016/j.ijplas.2023.103651

•The uniaxial twinning stress decreases from ∼785 MPa for a grain size d = 6 µm to ∼615 MPa for d = 242 µm, following the hall-petch equation.•The resistance of grain boundaries against slip and twinning is nearly identical. This contrasts with what was reported in the case of pure cu and binary Cu-based alloys.•The estimated CRSS for twinning is 193 ± 10 MPa, assuming a taylor factor of 3.06.•When the sample thickness-to-grain size ratio is only 4, free surfaces affect the work hardening behavior but their influence on the yield and twinning stresses is not significant.•Deformation twinning enhances the work hardening rate only when it is activated well before the generalization of cross slip with the formation of dislocation cells. 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 ∼785 to ∼615 MPa when the grain size increases from 6 to 242 µm, 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: ∼500 MPa·µm1/2) 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. [Display omitted]