Plasma Nitriding Properties of Sintered CoCrFeMnNi High-Entropy Alloy with Pure Ni Screen

Abstract

Recent reports highlight the CoCrFeMnNi high-entropy alloy's (HEA) impressive tensile strength and ductility at low temperatures, but Cantor alloys, including CoCrFeMnNi, tend to exhibit lower hardness compared to typical steel. To address this, surface modification treatment is investigated as an effective method of enhancing Cantor alloys' hardness. This study focuses on varying temperatures with a Ni metal screen to optimize thickening the plasma-nitrided layer in sintered Cantor alloys. Additionally, it aims to assess nitrided layer properties through direct-current plasma nitriding (S-DCPN) treatment, varying the sintered HEA's temperature using a pure Ni screen. Gas-atomized CoCrFeMnNi HEA powder, ball-milled for 10 h, forms the sintered body. S-DCPN treatment is applied at 673-873 K for 15 h, gas pressure at 200 Pa (75% N₂-25% H₂). A pure Ni screen aids uniform heating and increased nitrogen supply. The nitrided samples undergo analyses including X-ray diffraction (XRD), microstructure observation, surface morphology inspection, hardness testing, glow discharge optical emission spectroscopy (GD-OES), corrosion testing, and wear testing. Observations reveal a black modified layer on the nitrided sample's surface, with an edge effect. Surface roughness and SEM images show increased roughness with higher nitriding temperatures. XRD identifies specific compounds, and expanded fcc is noted in the 673 and 723 K samples. After nitriding, HEA sample hardness significantly increases, with GD-OES revealing greater depth of dissolved nitrogen at higher nitriding temperatures.