Effect of Strain Rate on the Tensile Behavior of CoCrFeNi and CoCrFeMnNi High-Entropy Alloys

High-entropy alloys (HEAs), a novel class of metal alloys, have been receiving increasing attention from the scientific community. HEAs have the potential to be used in critical load-bearing applications in replacement of conventional alloys such as stainless steel and nickel-base superalloys. Tensile experiments at quasi-static to dynamic strain rates (10⁻⁴-10³ s⁻¹) were performed on two single-phase face-centered cubic HEAs, CoCrFeNi and CoCrFeMnNi. Electron backscatter diffraction was used to study the microstructure of the samples before the experiments, and transmission electron microscopy was performed postmortem. The dominant deformation mechanisms were dislocation slip at quasi-static strain rates with the addition of deformation nano-twins at dynamic strain rates. Ultimate dynamic tensile strength and ductility improved with the increase in strain rate, which can be attributed to the activation of deformation nano-twins in HEAs. CoCrFeNi and CoCrFeMnNi both have low stacking fault energies, which could promote twinning at high strain rates to accommodate plastic deformation. The strain rate sensitivity of the flow stress increased with increasing strain rate, beginning with negligible strain rate sensitivity in the quasi-static range to high strain rate sensitivity in the dynamic range. CoCrFeMnNi showed greater strain rate sensitivity of flow stress. CoCrFeNi, with less configurational entropy, had higher mechanical properties and strain-hardening rates compared to CoCrFeMnNi, which was attributed to the weakening effect of the addition of Mn on the solid solution hardening.