Influence of severe straining and strain rate on the evolution of dislocation structures during micro-/nanoindentation in high entropy lamellar eutectics

Eutectic high entropy composites (EHECs) can exhibit an excellent combination of high strength and high ductility; however, the mechanisms responsible for the strength-ductility trade-off remain unpredicted. The influence of strain rate (ε˙) on the severe deformation imposed by high-pressure torsion (HPT) was used to evaluate the deformation mechanisms for a series of CoCrFeNiNbx (x molar ratio, 0 ≤ x ≤ 0.80) EHECs. Systematic and detailed micro-/nanoindentation investigations were performed and the results suggest that strain hardening (Taylor hardening) and grain-boundary strengthening (H-P strengthening) are the predominant strengthening mechanisms. Nanoindentation at different loading conditions (varying ε˙) revealed that the measured hardness in the eutectic regime increases gradually because of dislocation-lamellae-interface interactions. Based on the deformation mechanisms operating at different strain rates (ε˙), the density of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs), determined by the Nix-Gao approach, are used to explain the strain hardening phenomena. The results reveal that a large volume fraction of lamellae-interfaces accommodate more dislocations upon straining these EHECs. Lamellae-interface GNDs (ρGG) are activated at higher strain rates and can be effectively stored, thereby improving the global strain and strain hardening.