Oxidation behavior of CoCr2−xFeNi2.1Nbx high entropy alloys

High entropy alloys (HEAs) are one of the prominent alloy systems being studied for high-temperature applications. One of the key aspects to consider for high-temperature applications is to understand their oxidation behaviour. When Nb is added as the alloying element to base FCC alloy like CoCrFeNi it promotes the formation of dual phase HEA. In the present study, the high-temperature oxidation behaviour of annealed CoCr2-xFeNi2.1Nbx (x=0.25, 0.5 and 1) isothermally at 800˚C in the air is studied. As-homogenized microstructure of these alloys shows a dual phase containing FCC and Laves phase. The amount of Laves phase increases with increasing Nb content. The oxidation kinetics of the three compositions follows parabolic kinetics at 800˚C. The characterization (XRD, Raman, SEM) of the oxide layer reveals the formation of Cr2O3 as the main oxide layer. The cross-sectional SEM images of the oxide samples after 96 h of oxidation confirms absence of internal oxidation and a thin Laves-rich region below the Cr2O3 layer. The preferential formation of Cr2O3 over Nb oxides is explained using the assessment of their thermodynamic activity in the FCC phase.