## 3 Results
### Structure after Oxidation
#### 3.3.1 Cross-Section Analysis after 1 h  
Figure 3 illustrates the cross-sections of the HfCo, (HfCo)\({}_{90}\)(NbMo)\({}_{10}\), and (HfCo)\({}_{75}\)(NbMo)\({}_{25}\) alloys after 1 h of oxidation. In the HfCo alloy (Figure 3a), the top of the oxidized specimen was covered by a \(\sim\)10 \(\upmu\)m-thick oxide layer (OL). The OL was enriched with Hf and appeared to be rather dense (Figure 3a, Table 1). Beneath the OL, a \(\sim\)20 \(\upmu\)m-thick internally reacted zone (IRZ) was located. Inside the IRZ, we found alternate (Hf, O)-rich and Co-rich (O-lean) areas. The bottom part of the IRZ was decorated with a \(\sim\)4 \(\upmu\)m-thick dense Co-rich layer (Figure 3a, Table 1).  
Figure 2: Mass gains for the (HfCo)\({}_{100-x}\)(NbMo)\({}_{x}\) alloys after cyclic oxidation tests at 1000 \({}^{\circ}\)C (**a**); an overall view of the oxidized alloys after certain periods (**b**).  
Unlike the HfCo alloy, a \(\sim\)10 \(\upmu\)m-thick OL of the (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloy was enriched with Co (Figure 3b, Table 1). The OL was dense, though it contained occasional cracks and pores. The bottom part of the OL was framed with a thin and discontinuous Mo-rich layer. Below the OL, a \(\sim\)45 \(\upmu\)m-thick IRZ composed of two characteristic regions was located (Figure 3b, Table 1). The top of the first \(\sim\)30 \(\upmu\)m-thick region in the IRZ (IRZ_1) was covered by thin (Nb, Co)-rich islands. The remaining part of IRZ_1 was presented by a mixture of internally reacted B2 grains and bcc interlayers. Notably, the former B2 grains were mostly composed of Hf, while the bcc interlayers were enriched with Nb. The Co content was reduced in this region; Mo appeared to be evenly distributed. As the O penetration  
Figure 3: Characterisation of typical cross-section areas of the HfCo (**a**), (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) (**b**), and (HfCo)\({}_{75}\)(NbMo)\({}_{25}\) (**c**) alloys oxidized at 1000 \({}^{\circ}\)C for 1 h: SEM-BSE images combined with SEM-EDS maps. On the maps of the O distribution, the oxide layer (OL) and the internal reaction zone (IRZ) are denoted. In the (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) (**b**) and (HfCo)\({}_{75}\)(NbMo)\({}_{25}\) (**c**) alloys, the IRZ consisted of IRZ_1 and IRZ_2 with higher and lower concentration of O, respectively. Yellow arrows in the Co distribution maps of the HfCo (**a**) and (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) (**b**) alloys highlight the formation of a Co-rich layer on the boundary between the IRZ and an unoxidized core.  
deepened, IRZ_1 transitioned to IRZ_2, the overall constitution of which partly resembled that of the IRZ found in the HfCo alloy. Specifically, there were Co-rich (and O-lean) zones that filled the gaps between Hf-rich areas. The distribution of Mo in this zone was similar to Co. Analogous to the HfCo alloy, the boundary between the IRZ and unoxidized core was delineated by a \(\sim\)3 \(\upmu\)m-thick Co-rich layer (Figure 3b, Table 1).  
Compared to the HfCo and (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloys, the \(\sim\)10 \(\upmu\)m-thick OL of the (HfCo)\({}_{75}\)(NbMo)\({}_{25}\) alloy was rather loose (Figure 3c). Its upper part was covered by a Co-rich layer, while the bottom part was enriched with Mo (Figure 3c, Table 1). A \(\sim\)35 \(\upmu\)m-thick IRZ, the outmost part of which was wetted with a (Nb, Co)-rich layer, was found beneath. Similar to the (HfCo)\({}_{90}\)(NbMo)\({}_{10}\) alloy, the IRZ was composed of two specific regions, namely IRZ_1 and IRZ_2. The microstructure of both regions replicated that of unoxidized core, yet differed in the distribution of constitutive elements. Specifically, inside IRZ_1, we found former B2 and bcc lamellae, which were composed of Hf and Nb, respectively. Meanwhile, this region was lean in Co and Mo. Within IRZ_2, profuse Co-rich (and O-lean) islands surrounded by Hf-rich lamellae were found. A tiny discontinuous Co-rich layer that separated the bottom part of IRZ_2 and unoxidized core should also be noted (Figure 3c, Table 1).