## 1 Introduction
These phases significantly reduce the ductility of the material, and this phenomenon is described in detail in a number of works [23; 24; 25; 26; 27; 28]. The C-family of Ni-Cr-Mo alloys used in this study includes three materials, i.e., the newly developed corrosion-resistant alloy KhN62M-VI, created in collaboration with Russian researchers from UrFU, NIIKhIMMASH, and TsNIIChERMET [29], as well as two corrosion-resistant alloys from VDM Metals--VDM(r) Alloy C-4 [30; 31; 32; 33], an analog of Hastelloy(r) C-4, and VDM(r) Alloy 59, which offers greater phase stability [34; 35; 36; 37], even after prolonged exposure (up to 20,000 h) at 200-427 \({}^{\circ}\)C [38]. The selected alloys are characterized by low carbon and silicon content, the absence of tungsten, and reduced iron content. During long-term high-temperature aging, Ni-Cr-Mo alloys can form both ordered phases (Pt\({}_{2}\)Mo-type) and topologically close-packed (TCP) phases, such as \(\sigma\), P, and \(\mu\)[11]. These phenomena depend on the chromium and molybdenum content in the materials. The indicated phases influence mechanical and corrosion properties of the alloys, often reducing ductility and increasing brittleness, thus limiting their application as structural materials for molten salt reactor applications. For example, both a \(\sigma\)-phase and a P-phase were identified in VDM(r) Alloy C-4 [39]. The presence of these phases in the face-centered cubic nickel-based matrix has a significant impact on the material's properties, resulting in strengthening, while simultaneously increasing the risk of embrittlement. Hastelloy(r) G-35(r) alloy represents the fourth generation of the G-family of nickel alloys. It differs from its predecessors by reduced iron content, which significantly affects the phase composition. This alloy is primarily used in wet-process phosphoric acid production [40, 41] and fertilizer manufacturing, and it is also found applications in ore processing [20]. Two types of precipitates can be formed in a wide temperature range (550-1000 \({}^{\circ}\)C), i.e., a \(\sigma\)-phase with a characteristic lamellar morphology appears along the grain boundaries at higher temperatures; and chromium-rich (\(\alpha\)-Cr) precipitates are formed independently at relatively low temperatures [42]. All G- and C-family alloys selected for investigation exhibit a tendency toward ordering phase formation. An ordered Ni\({}_{2}\)(Cr,Mo) phase (Pt\({}_{2}\)Mo-type) significantly influences the mechanical properties of Ni-Cr-Mo alloys. This phase is observed in both the binary Ni-Cr system (as Ni\({}_{2}\)Cr, stable up to 475 \({}^{\circ}\)C) and the ternary Ni-Cr-Mo system (as Ni\({}_{2}\)(Cr,Mo)). The addition of molybdenum extends the temperature stability range of this phase and affects ordering kinetics, depending on the Mo/ratio [43]. Ni\({}_{2}\)(Cr,Mo) phase has also been identified in VDM\({}^{\otimes}\) Alloy C-4 after long-term aging at temperatures of up to 600 \({}^{\circ}\)C [9, 14, 28, 44, 45]. Except for KhN62M-VI, which was specifically developed for operation in molten halides and exhibits high resistance to such environments, all other studied commercial alloys were originally designed for use in the chemical industry at moderate temperatures. However, previous studies have demonstrated that these materials can be used as structural components under conditions that combine high temperatures and aggressive chemical environments [12, 13, 28]. Phase transformations in Ni-Cr-Mo alloys play a key role in the change of their performance characteristics. The formation of secondary and ordered phases has a complex effect on mechanical and, in some cases, corrosion properties, and therefore, it must be considered when nickel alloys are intended for application in high-temperature and aggressive environments. However, the relationship between the mechanical properties of prospective construction materials and their microstructure after long-term aging was considered only for the C-4 alloy [9, 14].