Enhanced strength of N-doped NiCoCr medium-entropy alloy produced by plasma arc melting in nitrogen atmosphere

Nitrogen doping is an effective method to enhance the mechanical properties of medium-entropy alloys (MEAs). However, some nitride inclusions may be formed during nitrogen process of alloys, which is detrimental to their properties. Therefore, a method that maximizes nitrogen content in alloys without forming nitride inclusions is urgently needed. In this study, a novel nitriding technology, nitrogen plasma arc melting, was introduced for the first time to prepare N-doped NiCoCr alloys. This study focused on the impact of nitrogen atom interstitial doping on the microstructure and plastic deformation of NiCoCr alloys. Results demonstrated that a remarkably high concentration of nitrogen (up to 0.27 wt%) was dissolved in the matrix without forming any nitride inclusions. The tensile strength of the nitrogen-doped NiCoCrN_0.27 alloy reached 1325 MPa, and the average grain size was 14 μm. Calculation results of density functional theory (DFT) revealed that N atoms exclusively occupied the octahedral interstitial sites, particularly those with higher concentrations of Cr atoms. N atoms formed the localized short-range ordered (SRO) structures with surrounding Cr atoms, creating strong ionic bonds among adjacent metal atoms. This SRO structure, with N atoms at the center, effectively facilitated the dislocation storage and increased the lattice friction, which were crucial for improving the alloy's strength. However, the presence of N atoms in octahedral interstitial sites increased the stacking fault energy of the {111}<101> slip system, leading to a significant decrease in ductility. Interstitial doping of N atoms in NiCoCr alloys could significantly enhance the alloy's strength through multiple strengthening mechanisms.