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

Abstract

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 (sigma) 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 a phase through computational thermodynamic approach. This( sigma) 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 sigma phase, which has been avoided in typical HEAs because of ductility deterioration, could be used in high strength HEA design.