## 3 Results and Discussion  
Figure 1 shows the XRD patterns of the as-cast ingot samples with different chemical composition, as listed in Table 1. The HT-1 and HT-2 samples containing relatively higher Hf (lower Ta) content were revealed as mixtures of the hexagonal close-packed (HCP) \(\alpha\)-Hf and body-centered cubic (BCC) Hf-Ta phases. Except for a peculiar peak observed near 65\({}^{\circ}\), the HT-1 sample showed the same tendency as the HT-2 sample. On the other hand, in the case of the HT-3 sample, no peak of the HCP (\(\alpha\)-Hf) phase was observed and only the BCC (Hf-Ta) phase was identified. Despite incomplete identification due to the unknown peaks, it is clearly shown that a phase transition occurred between the HT-2 and HT-3 samples. The same tendency was also observed in a previous paper: constituent phases changed from a mixture of both \(\alpha\)-Hf and Hf-Ta into Hf-Ta phase alone with increasing Ta content from 19.7 to 26.7 [12]. Meanwhile, the critical chemical composition leading to phase transition appears to lie between 27.2 and 35.2 (% Ta) in this experiment, considering the chemical compositions of the HT-2 and HT-3 samples. This discrepancy can be attributed to a locally different chemical composition that arises during the arc melting process, even when the cross section appears to be uniform without an unmelted phase, as shown in the inset photograph in Figure 1. Nevertheless, the well-known Hf\({}_{6}\)Ta\({}_{2}\)O\({}_{17}\) phase as a superstructure was expected from the HT-2 and HT-3 samples because its formation range is known to be sufficiently broad in terms of chemical composition [12]. To form the superstructure, the same method of oxidation was applied. Figure 2 shows the SEM and EDS results on a cross section of the HT-1 sample after oxidation heat treatment. The cross-sectional images reveal the outermost layer (surface layer), which is distinguished by its contrast from the substrate region. In a low-magnification micrograph, the thickness of the surface layer appears to be quite uniform along the surface. In both SEM images in Figure 2, the position of the interface between the substrate and surface layer is denoted by dashed lines. Taking into account the applicability of this oxidized alloy as a crucible material in pyro  
Figure 1: XRD spectra of as-cast ingots with different composition. provides practical benefits. This is attributed to the even diffusion depth of oxygen by the solid/vapor reaction from the surface. As long as solid/vapor contact is secured between the alloy and gaseous oxygen, the oxide scale can be formed uniformly regardless of the shape of the sample. This gives practical significance where other well-known passivation coating methods are not effective due to the irregular shape of the materials to be passivated. Across the well-defined interface between the substrate and surface layer, an EDS scan was carried out, as shown in Figure 2. It provides the compositional distribution of Hf, Ta, and O across the interface. To minimize any local fluctuation that may result from local heterogeneity, the EDS signal was collected from a specific area. As a result, every element shows typical diffusional profiles: the oxygen content drastically increased and then became saturated near the surface, while both Hf and Ta gradually decreased and then became saturated towards the surface layer. The newly formed oxide scale was examined more precisely from the HT-3 sample by SEM and EDS, as shown in Figure 3. The HT-3 sample, containing a relatively high amount of Ta, revealed a thicker layer than did the other samples with a lower amount of Ta. Despite the edge effect of SEM (abnormally bright contrast), the micrographs with higher magnifications clarify two different layers distinguished by their morphology. The inner layer in the vicinity of the substrate reveals a heterogeneous phase, while the outer layer appears as a uniform phase.