Strain-rate-dependent plasticity in NiCoFe medium-entropy alloys: Insights from single-crystal and polycrystalline nanoindentation simulations

Abstract Medium-entropy alloys (MEAs) are emerging materials known for their remarkable mechanical properties. This study employs molecular dynamics simulations to investigate the deformation mechanisms of face centered cubic structure NiCoFe MEAs with a typical 〈111〉 orientation. The mechanical response is evaluated under various indentation velocities to understand the influence of the deformation rate on the mechanical behavior. The results show that the strain rate significantly influences deformation behavior. In single crystals, lower strain rates promote extensive Shockley partial dislocation and prismatic dislocation loop formation, while higher strain rates limit dislocation nucleation due to reduced relaxation time. In polycrystalline samples, grain boundaries (GBs) impede dislocation glide, leading to heterogeneous plastic deformation and increased residual stress at higher velocities. Smaller GBs enhance strain localization, resembling a Hall-Petch-like effect. These behaviors are governed by thermally activated dislocation interactions that are sensitive to indentation velocity, highlighting the critical role of strain rate in controlling the plastic response of NiCoFe MEAs. These findings advance our understanding of plastic deformation in MEAs and provide insights for designing alloys with improved strength and rate-sensitive performance.