In-situ alloyed nitrogen doping enhances mechanical properties of medium-entropy alloys through lattice distortion and increased dislocation density

CoCrNi medium-entropy alloys (MEAs) containing 0−0.84 at.% nitrogen were prepared utilizing selective laser melting via in-situ alloying, and the influence of interstitial nitrogen on microstructure, mechanical characteristics, and deformation mechanisms was thoroughly examined. Nitrogen was uniformly integrated into the FCC matrix without the formation of nitrides; increasing nitrogen concentrations caused significant lattice expansion, indicating pronounced local lattice distortion, and raised the as-fabricated dislocation density from 6.65 ± 0.5 × 10¹⁴ m⁻² to 9.01 ± 0.6 × 10¹⁴ m⁻². The 0.84 at.% N-doped MEA attained a yield strength of 724 ± 10 MPa and an ultimate tensile strength of 966 ± 15 MPa, surpassing the undoped alloy by 106 MPa and 108 MPa, respectively, with a consistent elongation of 28 %. Nitrogen doping elevated the stacking fault energy, inhibited deformation twinning, and altered the primary deformation mechanism to dislocation slip. Quantitative research indicated that nitrogen-induced strengthening was mostly governed by increased dislocation storage, rather than traditional interstitial solid-solution strengthening. This study clarifies the function of interstitial nitrogen in additively made MEAs, informing the design of high-performance MEAs through in-situ alloying.