On the oxidation mechanism of refractory high entropy alloys

• Published in: Corrosion science Vol. 159; p. 108161
• Main Authors: Müller, Franz, Gorr, Bronislava, Christ, Hans-Jürgen, Müller, Julian, Butz, Benjamin, Chen, Hans, Kauffmann, Alexander, Heilmaier, Martin
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Ltd 01.10.2019; Elsevier BV
• Subjects: Alloys; Aluminum oxide; Compositionally complex alloy; CrTaO4 formation; Heat resistant alloys; High entropy alloys; High temperature; High temperature corrosion; Niobium oxides; Oxidation; Polytypes; Quartic rate law; Reaction kinetics; Refractory alloys; Refractory high entropy alloy; Refractory materials; Scale (corrosion); Spallation; Tantalum; Titanium; Compositionally complex alloy; Quartic rate law; Refractory high entropy alloy; High temperature corrosion; CrTaO4 formation
• ISSN: 0010-938X; 1879-0496
• DOI: 10.1016/j.corsci.2019.108161

•The formation of CrTaO4 after a short incubation period decreased the oxidation kinetics of Ta-containing alloys.•Activation energies for scale growth comparable to those of Cr2O3 and CrTaO4 forming Ni-based superalloys were found.•The oxygen inward diffusion was found to be rate determining in CrTaO4 forming alloys.•In Nb-containing alloys, the formation of Nb2O5 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 CrTaO4. 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 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.