On the oxidation mechanism of refractory high entropy alloys.

• The formation of CrTaO 4 after a short incubation period decreased the oxidation kinetics of Ta-containing alloys. • Activation energies for scale growth comparable to those of Cr 2 O 3 and CrTaO 4 forming Ni-based superalloys were found. • The oxygen inward diffusion was found to be rate determining in CrTaO 4 forming alloys. • In Nb-containing alloys, the formation of Nb 2 O 5 caused a porous oxide scale as well as severe oxide spallation. • Ti had a positive effect on the high-temperature oxidation behaviour because it supports the formation of protective CrTaO 4. The high-temperature oxidation mechanism of a series of refractory high entropy alloys: TaMoCrTiAl, NbMoCrTiAl, NbMoCrAl and TaMoCrAl at 1000 °C in air was studied. A complex protective oxide layer consisting of Al 2 O 3 , Cr 2 O 3 and CrTaO 4 oxides was observed for the quinary Ta-containing alloy. The formation of CrTaO 4 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 CrTaO 4. In the Nb-containing alloys, the formation of different Nb 2 O 5 polytypes near the metal/oxide interface caused a highly porous oxide scale and severe oxide spallation. [ABSTRACT FROM AUTHOR]