Oxidation Performance of Fe-Ni-Co-Cr-Mn High Entropy Alloy and its Al-Containing Variants in Supercritical CO2

Abstract In this study, a new material type, known as high entropy alloy (HEA), is being evaluated for use in supercritical CO2 (sCO2) Brayton power cycle. This cycle is a promising power generation technology that offers an increased efficiency and smaller footprint compared to the conventional Rankine steam cycle. However, the construction of components that operate in the high temperature and pressure regions of the cycle requires the use of expensive high-performance alloys such as Inconel 740H and Incoloy 800HT. As an alternative to these alloys, three compositions of HEAs are evaluated, namely, FeNiCoCrMn (HEA-1), FeNi1.5CoCrMnAl0.5 (HEA-2), FeNi1.5CoCrAl0.5 (HEA-3). The alloy samples were exposed to sCO2 at 700 °C, 20 MPa for 600 hours. They were then evaluated through weight measurements and characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The results showed that HEA-3 outperformed the two other compositions. It experienced an impressively low mass gain of 0.04 mg/cm2, compared to 0.65mg/cm2 and 0.25 mg/cm2 for HEA-1 and HEA-2, respectively. The lack of Mn enabled HEA-3 to form protective Cr2O3 oxides, whereas the other two compositions formed porous oxides containing MnO and Mn3O4. Furthermore, the results showed that HEA-3 has the potential to outperform many conventional superalloys being evaluated for sCO2 applications. Additional test campaign has been planned to study the effects of prolonged exposure of HEA-3 to high temperature and pressure sCO2 environment to further assess its performance and to do initial benchmarking with respect to other Ni-based alloys.