Supersaturation and dissolvable α-Cr phase enable superior oxidation resistance in FeCrNi medium-entropy alloys

Increasing Cr content in Fe-based alloys is expected to improve oxidation and corrosion resistances, but its effectiveness always degrades because of the over-doping induced precipitation of harmful σ-FeCr and Cr₂₃C₆ phases. In this study, the precipitation and oxidation behaviors of Fe_xCrNi alloys were systematically investigated. It is found that the precipitation behavior could be controlled by adjusting the Cr content. The α-Cr phase precipitates at 800 °C in supersaturated FeCrNi medium-entropy alloy (MEA), while it is absent in Fe₃CrNi alloy due to the lower Cr content. FeCrNi MEA shows much better oxidation resistance than Fe₃CrNi, the oxidation rate constant of FeCrNi is two to three orders lower than that in the Fe₃CrNi alloy. The oxidation activation energy of FeCrNi (545 kJ/mol) is much higher than that of the Fe₃CrNi alloy (80 kJ/mol), which can be attributed to the synergistic effects of α-Cr dissolution, high configuration entropy and Cr supersaturation. Unlike σ-FeCr and Cr₂₃C₆ in stainless steels, the metastable α-Cr phase can be controllably introduced in FeCrNi as a “Cr resource”, and will dissolve during long-term oxidation to provide the required Cr element for the stabilization of the Cr₂O₃ scale. These discoveries shed new insights into the innovative design of stainless materials for heat-resistance structural applications.