Fe–Mn–Co–Al–C duplex high-entropy alloys with high hardness, high strength and good ductility processed via high-pressure torsion

High-entropy alloys exhibit good mechanical properties, but there is still a demand to develop high-entropy alloys with ultrahigh strength and good ductility. In this study, Fe–Mn–Co–Al–C high-entropy alloys with a duplex FCC + BCC structure were processed by high-pressure torsion (HPT) to induce microstructural defects. They exhibited hardness levels up to 570 HV, exceptional yield and ultimate tensile strength values exceeding 2 GPa with elongation to failure of up to 30% after HPT processing. The best combination of strength and ductility was achieved in the presence of elongated grains, while the generation of equiaxed nanograins led to a reduction of ductility. Short-term aging induced additional hardening to 700–800 HV due to polygonization and partial phase ordering, but no evidence for carbide precipitation was found. Detailed analysis through the Hall-Petch relationship suggests that grain boundary strengthening is the dominant hardening mechanism after HPT processing, despite the presence of solute atoms and formation of dislocations. The combination of ultrahigh strength, good ductility realized by microstructural control through severe plastic deformation by HPT highlights the potential of these alloys for advanced structural applications.