Effect of Nitrogen doping on the tribological behavior and mechanism of Fe₅₀Mn₃₀Co₁₀Cr₁₀ high entropy alloys at ambient and cryogenic temperatures

The FeMnCoCr high entropy alloys (HEAs) have been extensively studied in recent years. The low yield strength and wear resistance, however, limit practical application. In this work, 1 at% nitrogen addition and proper thermo-mechanical treatment have been adopted in Fe₅₀Mn₃₀Co₁₀Cr₁₀ alloy to overcome this problem. The wear resistance of the N-doped alloy has improved 1/3∼1/2 compared with the base alloy. At room temperature, the wear resistance improvement is closely related to the increase in hardness, which originates from interstitial effect, precipitation strengthening and grain refinement strengthening. At cryogenic temperatures, the increase in intrinsic hardness and the activation of rich deformation mechanisms for the decreasing stacking fault energy (SFE), such as transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP), enhance the strength, toughness and the wear resistance of the two alloys. While, the addition of N improves the stability of the FCC phase, and the nano-HCP phase in-situ and Cr₂N precipitates on the wear surface improve the surface hardness and reduce the thickness of the severe deformed zone. At the same time, more HCP grains and FCC twins appear in the deformation-affected zone, providing a more stable plastic deformed layer support. Thus, the interstitial strengthening induced by N-doped at cryogenic temperatures and a multi-layered structure of soft and hard combination in-situ, improve the wear resistance of N₁ alloy. The current work clarifies the cryogenic wear mechanisms of the Fe₅₀Mn₃₀Co₁₀Cr₁₀ and (Fe₅₀Mn₃₀Co₁₀Cr₁₀)₉₉N₁ HEAs and provides a novel strategy for designing wear-resistant alloys by doping the interstitial atoms.