## 5 Conclusions  
In this study, the oxidation behaviour of the bcc+B2 (HfCo)\({}_{100\sim\)(NbMo)\({}_{\rm K}}\) (x = 0; 10; 25; 40; 75; 100 (at.%)) RHEAs during cyclic oxidation at 1000 \({}^{\circ}\)C was investigated. The following conclusions were drawn:  
1. The single-phase B2-ordered HfCo alloy demonstrated the best spallation resistance and retained a pristine form after 100 h. The near-parabolic oxidation kinetics of the HfCo alloy with n = 0.58 and k = 5.23 mg/cm\({}^{2}\) h\({}^{0.58}\) was found. The protection was provided by the formation of the external HfO\({}_{2}\) or CoO layers after 1 or 100 h, respectively.
2. In the (NbMo)-containing alloys, the oxidation and spallation/disintegration resistances degraded with an increase in the fraction of the (Nb, Mo)-rich bcc phase that acted as a reservoir for the harmful MoO\({}_{3}\). Only the (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloy with \(\sim\)13% of the bcc phase could withstand the spallation up to 15 h and preserved the acceptable integrity after 100 h, while the alloys with \(>\)50% of the bcc phase oxidized and disintegrated completely after 1-5 h.
3. Before spallation, the (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloy showed near-parabolic oxidation kinetics with n = 0.46 and k = 7.75 mg/cm\({}^{2}\) h\({}^{0.46}\). Unlike the HfCo alloy, the protection of the (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloy initially relied on the exclusive CoO layer. Due to this, the (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloy was more oxidation-resistant compared to many other RHEAs covered by scales composed of complex and/or simple oxides.  
Figure 6: Comparison of the mass gain after 1 h at 1000 \({}^{\circ}\)C of the (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloy and other RHEAs [9; 18; 19; 21; 23; 24; 31; 56; 57; 58; 59; 60].  
**Author Contributions:** Conceptualization, N.Y., E.P. and N.S.; methodology, N.Y. and E.P.; validation, N.Y., E.P. and N.S.; formal analysis, N.Y. and E.P.; investigation, E.P.; data curation, N.Y., S.Z. and N.S.; writing--original draft preparation, N.Y.; writing--review and editing, N.Y., E.P., S.Z. and N.S.; visualization, N.Y. and E.P.; supervision, N.Y., S.Z. and N.S. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Russian Science Foundation, Grant no. 19-79-30066 ([[https://rscf.ru/en/project/19-79-30066/](https://rscf.ru/en/project/19-79-30066/)]([https://rscf.ru/en/project/19-79-30066/](https://rscf.ru/en/project/19-79-30066/)) (accessed on 26 July 2023)). A part of this research (XRD characterisation) was partially funded by the Ministry of Science and Higher Education of the Russian Federation as part of the World-Class Research Center program: Advanced Digital Technologies (Contract no. 075-15-2022-312 dated 20 April 2022).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The work was carried out using the equipment of the Joint Research Center of Belgorod State National Research University \"Technology and Materials\". The authors also thank [PERSON], Belgorod National Research University, for aid in preparing the ingots of the alloys.  
The authors declare no conflict of interest.