Enhanced mechanical properties of a carbon and nitrogen co-doped interstitial high-entropy alloy via tuning ultrafine-grained microstructures

C-N co-doped interstitial high entropy alloy (iHEA) was reported to have high strength and ductility. However, iHEA with fully recrystallized ultrafine grains (UFGs) and underlying thermally activated pro-cesses associated with dislocation slip, twinning, and solute drag have not been reported yet. In this work, a C-N co-doped iHEA with nominal composition Fe48.5Mn30Co10Cr10C0.5N1.0 (at.%) was prepared, and the microstructures were tuned by cold-rolling and annealing treatments to improve mechanical properties. Upon cold-rolling with a strain of 1.74, the main microstructures in the iHEA are composed of nano-grains, nano-twins, HCP laminates, and high density of dislocations, leading to ultrahigh hardness of 466.7 HV and tensile strength of 1730 MPa at the expense of ductility (2.4 4%). Both the nanostructures and the high hardness of the iHEA can be maintained up to an annealing temperature of 600 degrees C (462.5 HV). After annealing at 650 degrees C for 1 h, the UFG microstructures are obtained in the iHEA, containing re-crystallized grains with an average grain size of 0.91 mu m and nanoprecipitates with an average diameter of 90.8 nm. The combined strengthening and hardening effects of UFGs, nanoprecipitates, twinning, and solutes contribute to high strain hardening ( n = 0.81), gigapascal yield strength (984 MPa), and good duc-tility (20%). The C-N co-doping leads to a strong drag effect on dislocation slip, resulting in a nano-scale mean free path of dislocation slip lambda over line (1.44 nm) and much small apparent activation volume V * (15.8 b 3 ) of the UFG iHEA.(c) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science &Technology.