Formation of rutile (Cr,Ta,Ti)O2 oxides during oxidation of refractory high entropy alloys in Ta-Mo-Cr-Ti-Al system

The chemical identity of rutile oxides formed during oxidation of alloys with different Ti content within the Ta-Mo-Cr-Ti-Al system at 1200 °C for 24 h was determined. Rutile-type oxide (Cr,Ta,Ti)O2 was observed on 20Ta20Mo20Cr20Ti20Al and 67.5(TaMoCr)10Ti22.5Al (at%). Based on the experimental results, the valence states of Cr, Ta and Ti were unequivocally determined. Higher concentrations of Ti in (Cr,Ta,Ti)O2 lead to the formation of thinner oxide scales and zones of internal corrosion. It is concluded that the occupation of higher-valent Ti4+ cations on the Cr3+ sub-lattice reduces the oxygen vacancy concentration and thus improves the oxidation behavior of n-type conducting (Cr,Ta,Ti)O2-forming alloys.