Thermochemical nitriding treatments, well-known for improving mechanical properties and corrosion resistance of conventional alloys, could find a potential interest for some families of high entropy alloys (HEA) as well. In this perspective, this study focuses on the microstructural evolutions induced by plasma nitriding at moderate temperature on AlxCoCrFeNi alloys, produced by hot isostatic pressing (HIP).

The main objective is to study the effects of nitriding, in relationship with the Al content, on the different phases - FCC A1, BCC A2 and B2 - present in the AlxCoCrFeNi alloy produced by HIP at T=1300°C and P=150MPa. This study mainly focuses on the equimolar AlCoCrFeNi alloy because it contains simultaneously the three phases mentioned above. Nitriding treatments with plasma assistance (N2/H2 mixture, P = 8 Pa) were carried out at 400°C for 1h or 6h on the AlCoCrFeNi alloy.

The nitrided samples were characterized by X-Ray Diffraction, Scanning Electron Microscopy, and Transmission Electron Microscopy coupled with local analyses by energy dispersive X-ray spectroscopy and electron diffraction. Cross-sectional observations reveal that the nitriding front progresses more rapidly within the FCC phase than in the BCC phases. As a result, higher Al content in the alloy induces lower nitidriding depth. The FCC A1 phase incorporates about 20-30 %at. of nitrogen to form the supersaturated solid solution known as the expanded austenite, similar to what is observed for austenitic stainless steels or Ni-Cr or Co-Cr alloys. Al content affects the induced lattice parameter. The two BCC phases, A2 and B2, both undergo a decomposition according to their main elements as shown in Figure 1. The B2 phase, initially rich in Al and Ni, decomposes into a Ni-rich fiber-shaped FCC phase and alternative nanolamellas (2-3 nm thick) of Ni-rich and Al-N-rich phases, the latter phase being probably HCP AlN nitride. The A2 phase, initially rich in Fe and Cr, shows BCC spherical precipitates of about ten nanometers rich in Fe in a matrix rich in Cr and N similar to FCC CrN nitride. Surface observations show nanometric precipitates (≤ 100 nm) in variable quantities depending on the phases involved.

Finally, the AlxCoCrFeNi alloys were oxidized at 1000, 1100 and 1200 °C in dry air during 50h, before and after nitriding, to evaluate the oxidation resistance of such HEAs and the effect of the former plasma nitriding treatment.