Investigation of Oxidation Performance of Co-Cr-Fe-Ni based High Entropy Alloys in Supercritical-CO2 Environment

Abstract

This work presents the first oxidation studies of Co-Cr-Fe-Ni-based high entropy alloys (HEAs) in supercritical-CO2 (sCO2) environment. These materials are being evaluated as an alternative to commercial Ni-based superalloys for construction of select components of sCO2 power cycles. The alloys investigated in this research include CoCrFeMnNi, Al0.5CoCrFeMnNi1.5, Al0.5CoCrFeNi1.5, (CoCrFeMnNi)Si0.01, and CoCrFe0.5MnNi1.5 specifically designed to study the impact of Mn, Al, Si and Fe on the oxidation behaviour. The HEAs were characterized through scanning electron microscopy (SEM), energy-dispersive Xray spectroscopy (EDS), X-ray diffraction before and after exposure to sCO2. The samples were tested inside a high pressure sCO2 corrosion test rig located at Natural Resources Canada's, capable of mimicking the operating conditions of indirectly-fired sCO2 power cycles. The oxidation behaviour was evaluated by comparing experimentally obtained mass gain data and the characterized oxide formations. The pre-exposure results show that Al0.5CoCrFeMnNi1.5 and Al0.5CoCrFeNi1.5 compositions form γ/γ' microstructure, desirable for high strength applications, while the exposure to sCO2 at elevated temperatures (500-700°C) and pressure (20 MPa) causes formation of ordered B2 NiAl phases dispersed within the matrix. Al0.5CoCrFeNi1.5 is the only composition that forms detectable protective Cr2O3 after 300 hours at 500°C/20MPa, as well as after 600 hours at 700°C/MPa. The other four compositions were shown to form a porous unprotective MnO oxide layer. Additionally, the results indicate that similar to conventional alloy systems, the oxidation of HEAs increases with temperature, however, Al0.5CoCrFeNi1.5 exhibited nearly the same mass gain at both temperatures after 300 and 600 hours. Lastly, the results suggest that Al0.5CoCrFeNi1.5 composition has the potential to outperform commercial superalloys in sCO2 applications.