| Summary: | Herein, the oxidation behaviour of refractory (HfCo)<sub>100−x</sub>(NbMo)<sub>x</sub> (x = 0; 10; 25; 40; 75; 100 (at.%)) high-entropy alloys with a bcc+B2 structure subjected to cyclic oxidation at 1000 °C was studied. The single-phase B2-ordered HfCo alloy demonstrated the best spallation resistance and retained a pristine form after 100 h. The oxidation kinetics of the HfCo alloy was near-parabolic, accompanied by the formation of external HfO<sub>2</sub> or CoO layers after 1 or 100 h, respectively. Additions of (NbMo)<sub>x</sub> deteriorated the spallation resistance (x ≤ 25 at.%) or led to complete disintegration (x > 25 at.%). Among the (NbMo)-containing alloys, the (HfCo)<sub>90</sub>(NbMo)<sub>10</sub> alloy with the dual-phase bcc+B2 structure showed the most promising oxidation resistance. This alloy withstood cyclic oxidation up to 15 h with a mass gain close to the HfCo alloy and survived 100 h without changes in geometry of the specimen. Unlike the HfCo alloy, in the (HfCo)<sub>90</sub>(NbMo)<sub>10</sub> alloy, the external CoO layer was found already after 1 h. The effect of chemical and phase compositions on the formation of certain oxides was discussed. Comparison with the other refractory high-entropy alloys was also presented.
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