Surface oxidation mechanism of a refractory high-entropy alloy

High-entropy alloys (HEAs) synthesized using refractory elements are being strongly considered as candidates for high temperature structural applications. The role of compositional changes of HEA surfaces due to oxidation is crucial to sustain the material properties, but a detailed description of the thermodynamic mechanism driving the adsorption of oxygen on such complex surfaces is absent. We examine and explain the reaction process of oxygen on a representative refractory HEA surface using first principles and atomistic thermodynamic models. The HEA surface is highly reactive to oxygen yielding a full monolayer coverage at temperatures between 300 and 1500 K. The preferential adsorption of oxygen to specific sites of the HEA surface is attributed to the electronic configuration of the bonding shells of the constituent surface atoms. On further oxygen addition, the oxygen atoms diffuse into the bulk regions of the alloy. Manipulation of temperature and oxygen pressure reveals that it is difficult to rid the alloy surface of oxygen even at extremely low pressures of 10⁻⁹ bar at 2000 K.