The high temperature oxidation behavior of a refractory high-entropy alloy (HEA) 20Nb-20Mo-20Cr-20Ti-20Al at 900°C, 1000°C and 1100°C was investigated. The oxidation kinetics of the alloy was found to be linear at all temperatures. Oxide scales formed are largely inhomogeneous showing regions with thick and porous layers as well areas with quite thin oxide scales due to formation of discontinuous chromium-rich oxide scales. However, the oxidation resistance can be moderately improved by the addition of 1 at.% Si. The thermogravimetric data obtained during oxidation of the Si-containing alloy at 1000°C and 1100°C reveal pronounced periods of parabolic oxidation that, however, change towards linear oxidation after prolonged exposure times. Microstructural investigations using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) document that the Si addition gives rise to a nearly continuous alumina-rich layer which seems to be responsible for the good protection against further oxidation. Pronounced zones of internal corrosion attacks consisting of different oxides and nitrides were observed in both alloys. In order to determine the chemical composition of the corrosion products and their mass fraction, quantitative Xray diffraction (XRD) analysis was performed on powdered oxide scales that formed on the alloys after different oxidation times. Rutile was identified as the major phase in the oxide scales rationalizing the relatively high mass gain during oxidation.

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