Effects of stacking fault energy and temperature on grain boundary strengthening, intrinsic lattice strength and deformation mechanisms in CrMnFeCoNi high-entropy alloys with different Cr/Ni ratios

Tensile properties and deformation mechanisms at 293 and 77 K of single-phase face-centered-cubic (FCC) CrxMn20Fe20Co20Ni40-x (0 ≤ x ≤ 26 at.%) high-entropy alloys (HEAs) with grain sizes (d) in the range [6–283 µm] were studied. A specificity of these alloys is that their stacking fault energy (SFE) strongly decreases with increasing the Cr/Ni ratio while all the other characteristics affecting strength (elastic moduli and solute misfit volumes) remain approximately constant. Therefore, these alloys allow to investigate how the SFE affects the tensile behavior and deformation mechanisms of FCC HEAs. Grain boundary strengthening is found to be nearly independent of SFE, composition, and temperature. While the intrinsic lattice strength (σ0 ≈ 136 MPa) remains approximately constant at 293 K, it strongly increases with increasing Cr/Ni ratio and thus decreasing SFE at 77 K, presumably due to a reduction of dislocation line tension. Deformation twinning occurs at 293 K when x ≥ 20 at.% (SFE ≤ 35 mJ/m2) and in all alloys at 77 K. The strength-ductility combination is found to increase with decreasing SFE only when 14 ≤ x ≤ 24 at.% at 77 K because twinning must be triggered at relatively low plastic strains to yield a significant dynamic Hall-Petch effect, allowing to sustain a high work hardening rate and postpone necking instability. In addition to twinning, an FCC-to-HCP (hexagonal close-packed) martensitic transformation was observed at 77 K in the Cr26Mn20Fe20Co20Ni14 HEA, which induced early rupture when d = 212 µm.