## 4 Discussion  
### Oxidation Behaviour  
The results of this study show that the chemical and phase composition has a profound effect on the oxidation behaviour of the (HfCo)\({}_{100-\mathrm{x}}\)(NbMo)\({}_{\mathrm{x}}\) alloys during cyclic testing at 1000 \({}^{\circ}\)C. The single-phase B2-ordered HfCo alloy demonstrated the best oxidation and spallation/disintegration resistances. Dual-phase bcc+B2 (HfCo)\({}_{00}\)(NbMo)\({}_{10}\) and (HfCo)\({}_{75}\)(NbMo)\({}_{25}\) alloys with \(f_{hcc}\leq 30\%\) could withstand spallation at short exposures and preserved an acceptable level of the integrity after 100 h, but, when \(f_{hxx}>50\%\) in the (HfCo)\({}_{40}\)(NbMo)\({}_{60}\) and (HfCo)\({}_{25}\)(NbMo)\({}_{75}\) alloys, the alloys disintegrated rapidly. The single-phase bcc NbMo alloy appeared to be the most vulnerable to oxygen attack and disintegrated completely after 1 h.  
Analysis of oxidation kinetics (Equation (1)), which could only be performed for the HfCo and (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloys, revealed a near-parabolic regime in both alloys that was an indication of a diffusion-controlled oxidation mechanism, involving the formation of the protective scales. In the HfCo alloy, the protection was provided by the external HfO\({}_{2}\) or CoO scales formed at early (1 h) or late (100 h) stages, respectively (Figures 3a, 4a, and 5; Tables 1 and 2). In turn, the addition of (NbMo)\({}_{\mathrm{x}}\) could alleviate the outward diffusion of Co, leading to the creation of the CoO layer on the top surface of the oxidized (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloy from the initial stages of oxidation (Figures 3b, 4b, and 5; Tables 1 and 2). In the latter alloy, CoO could also be considered as the protective oxide, which seemed to partially counter the negative effect of the presence of the (Nb, Mo)-rich bcc phase. Specifically, CoO could (i) block the outward diffusion of Mo and (ii) prevent  
Figure 5: XRD patterns collected from the surfaces of the HfCo, (HfCo)\({}_{90}\)(NbMo)\({}_{10}\), and (HfCo)\({}_{75}\)(NbMo)\({}_{25}\) alloys oxidized at 1000 \({}^{\circ}\)C for 1 (**a**) and 100 (**b**) h.  
the possible emergence of binary Nb-rich oxides [8] due to scavenging of the Nb cations to a complex Nb\({}_{2}\)Co\({}_{4}\)O\({}_{9}\) oxide(Figure 5). In the meantime, the external CoO layer was permeable for the inward diffusion of O that resulted in the development of deep internally reacted zones in the HfCo and (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloys as the testing time progressed. These IRZs consisted of a dominant HfO\({}_{2}\) and, in the case of the (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloy, traces of Nb\({}_{2}\)Co\({}_{4}\)O\({}_{9}\) and MoO\({}_{3}\) (Figures 3a,b and 4a,b; Tables 1 and 2). Although no spallation occurred in the HfCo alloy, we observed occasional cracks originated, most likely, from the repeated heating-cooling cycles [52; 53; 54]. In the (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloy, the cracks could additionally appear due to the nucleation of the fast-growing and highly volatile MoO\({}_{3}\)[14]. However, although the single-phase B2-ordered HfCo alloy was extremely ductile [36] and capable of preventing the propagation of the cracks and, thus, to avoid the spallation, the less ductile dual-phase bcc+B2 (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloy remained pristine for a much shorter period of time, accumulating the stresses which were finally relieved by partial spallation [31]. The poor oxidation and spallation/disintegration resistances of other alloys could be rationalized by the example of the (HfCo)\({}_{75}\)(NbMo)\({}_{25}\) alloy (Figures 3c and 4c). Despite the CoO layer that also covered the 1 h-oxidized (HfCo)\({}_{75}\)(NbMo)\({}_{25}\) alloy, it was unable to protect this alloy due to its loose constitution. Simultaneously, the high fraction of the (Nb, Mo)-rich bcc phase that served as reservoirs for the harmful MoO\({}_{3}\) intensified the spallation (Figure 4c).