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

Abstract

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 FexCrNi 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.