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 (epsilon) over dot 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 (epsilon) over dot) 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 (epsilon) over dot, 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 (rho(GG)) are activated at higher strain rates and can be effectively stored, thereby improving the global strain and strain hardening.