Abstract: 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 processes 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 Fe_48.5Mn₃₀Co₁₀Cr₁₀C_0.5N_1.0 (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 (2.44%). Both the nanostructures and the high hardness of the iHEA can be maintained up to an annealing temperature of 600 °C (462.5 HV). After annealing at 650 °C for 1 h, the UFG microstructures are obtained in the iHEA, containing recrystallized grains with an average grain size of 0.91 µm 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 ductility (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 λ¯ (1.44 nm) and much small apparent activation volume V* (15.8b³) of the UFG iHEA.

Key words: Ultrafine-grained high-entropy alloys, Interstitial atoms, Thermomechanical processing, Microstructures, Mechanical properties