On the oxidation mechanism of refractory high entropy alloys

The high-temperature oxidation mechanism of a series of refractory high entropy alloys: TaMoCrTiAl, NbMoCrTiAl, NbMoCrAl and TaMoCrAl at 1000 degrees C in air was studied. A complex protective oxide layer consisting of Al2O3, Cr2O3 and CrTaO4 oxides was observed for the quinary Ta-containing alloy. The formation of CrTaO4 in this alloy after a short incubation period decreased the oxidation kinetics from a parabolic to a quartic rate law. Ti was found to support the formation of CrTaO4. In the Nb-containing alloys, the formation of different Nb2O5 polytypes near the metal/oxide interface caused a highly porous oxide scale and severe oxide spallation.