Utilization of brittle σ phase for strengthening and strain hardening in ductile VCrFeNi high-entropy alloy

General design concept used in high-entropy alloys (HEAs) have deviated from forming an fcc single phase to utilizing hard intermetallic phases in ductile fcc matrix. Here, we effectively exploited strengthening effects of a brittle intermetallic sigma (σ) phase to improve cryogenic tensile properties of a non-equi-atomic ductile VCrFeNi four-component HEA. We preferentially selected vanadium as a candidate alloying element to efficiently produce the σ phase through computational thermodynamic approach. This σ phase has beneficial effects on grain refinement through retardation of grain growth due to grain-boundary pinning, thereby leading to yield strength of 0.79–0.93 GPa. The extensive strain hardening results in tensile strength of 1.33–1.49 GPa and ductility of 23–47% at cryogenic temperature, which are enabled by nano-sized dislocation substructures rather than deformation twinning. Our results demonstrate how the intermetallic σ phase, which has been avoided in typical HEAs because of ductility deterioration, could be used in high strength HEA design.