## 1 Introduction  
Molten salt reactors (MSRs) are fourth-generation nuclear power systems where halide-based salts can be used as both coolants and nuclear fuel. The practical realization of MSRs requires core and structural materials that exhibit high corrosion and radiation resistance, mechanical stability, and manufacturability for long-term operation in a high-temperature molten salt environment [1, 2, 3, 4]. Nickel-based alloys are promising construction materials due to their corrosion resistance in molten chloride- and fluoride-based media [5, 6, 7, 8]. However, the investigation of properties of nickel-based alloys solely in their as-received state is insufficient to predict their service life in MSRs, because prolonged exposure under real operating conditions at elevated temperatures and in aggressive environments leads to structural and phase transformations in these alloys, inevitably altering their corrosion and mechanical properties [9, 10, 11]. The precipitation of secondary phases along the grain boundaries can significantly influence the corrosion resistance and increase susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) [12, 13, 14, 15]. High capacity for work hardening is reflected by the tensile curves, where an extended uniform deformation stage and low ratio of the yield strength to the tensile strength are demonstrated [9; 14]. The processes occurring during the aging of nickel-based alloys (depletion of the FCC solid solution, precipitation of secondary-phase particles, their growth, and ordering) result in dispersion hardening. The extent of dispersion hardening is determined by the density of the particles, their size, and the degree of coherence with the matrix. As a result, dispersion hardening leads to the degradation of tensile properties, increasing yield strength and the ratio of the yield strength to the tensile strength. Thus, these mechanical properties can be used for assessing the alloy's embrittlement during aging [16; 17; 18; 19]. The investigation of precipitation of secondary phases and the determination of their impact on the mechanical properties of nickel-based alloys during long-term thermal exposure is therefore an important task. This study is focused on the effect of prolonged high-temperature aging (up to 5000 h) of Ni-Cr, Ni-Mo, and Ni-Cr-Mo alloys on their short-term mechanical properties (yield strength, YS; tensile strength, TS; relative elongation, El; the ratio of the yield strength to the tensile strength, YS/TS) and their dependence on the type and morphology of secondary-phase precipitates. All alloys considered in the present study are nickel-based materials that vary in molybdenum and chromium content, as well as in their ratio, which determines different structural and operational properties. The ternary Ni-Cr-Mo system has been widely recognized as a base for the development of a variety of corrosion-resistant alloys [20; 21]. Historically, these materials were classified into three main families, i.e., B, C, and G.  
The B-family of nickel alloys with 26-30 wt.% molybdenum as a primary alloying element is represented in this study by Hastelloy(r) B-3(r). This alloy is an improved version of Hastelloy(r) B and B-2. The B-series alloys were specifically developed for environments containing reducing agents such as hydrochloric acid. Hastelloy(r) B-3(r) was also designed for the use in hydrochloric acid environments at various concentrations and temperatures, offering corrosion resistance comparable to that of more expensive materials like tantalum and zirconium [22]. Unlike its predecessors, it also exhibits improved thermal stability, which allows eliminating post-veld annealing. Despite their excellent corrosion resistance, Ni-Mo-based alloys are thermally unstable at elevated temperatures and are subjected to the formation of ordered phases such as Ni\({}_{4}\)Mo (D1a) and Ni\({}_{3}\)Mo (DO2\({}_{22}\)).