## 3 Results
### Microstructure and Stress-Strain Curves of Nickel Alloys After Long-Term Aging
Figure 9: Excessive phases formed in Hastelloy® G-35® after 5000 h of aging at 650 \({}^{\circ}\)C.  
3.3 Microstructure and Stress-Strain Curves of KhN62M-VI Alloy After Long-Term Aging at 600 \({}^{\circ}\)C  
One can see that the microstructure of the KhN62M-VI alloy after 1000 h and 5000 h at 600 \({}^{\circ}\)C (Regime 1) exposure is similar to the initial state (Figure 11). However, prolonged aging at 600 \({}^{\circ}\)C resulted in significant changes not only in the shape but also in the behavior of the stress-strain curves (Figure 12), and it led to substantial material strengthening and gradual embrittlement as the aging duration increased. The rise in strength properties and strain hardening, reflected in the higher yield stress and a steeper slope of the uniform deformation stage, indicates an increasing volume fraction of coherent precipitates. Additionally, the increase in barriers for dislocation movement caused a sharp decline in ductile properties after just 100 h of aging, including a drastic reduction in uniform plasticity, the disappearance of the localized deformation stage, and an increase in the YS/TS ratio. Prolonged aging for up to 2000 h resulted in complete embrittlement of the alloy. However, further aging for up to 5000 h led to a partial recovery of plasticity, likely due to the process of coherent precipitate coarsening. \begin{table}
\begin{tabular}{c c c c c} \hline
**Spectrum** & **Cr** & **Fe** & **Ni** & **Mo** \\ \hline Spectrum 1 & 49.76 & 0.66 & 37.62 & 11.96 \\ Spectrum 2 & 21.67 & 0.47 & 72.59 & 5.27 \\ Spectrum 3 & 16.73 & 0.18 & 77.42 & 5.68 \\ Spectrum 4 & 45.66 & 0.48 & 41.51 & 12.35 \\ Spectrum 5 & 50.18 & 0.55 & 34.02 & 15.24 \\ Spectrum 6 & 12.64 & 0.24 & 80.81 & 6.32 \\ \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of phases in Hastelloy® G-35® aged at 650 \({}^{\circ}\)C for 5000 h (at.%). Figure 10: Typical engineering stress–strain curves of Hastelloy® G-35® specimens after long-term aging at 650 \({}^{\circ}\)C.  
Figure 11: Microstructure of KhN62M-VI alloy after long-term aging at 600 \({}^{\circ}\)C for 1000 h (**a**) and 5000 h (**b**). 3.4 Microstructure and Stress-Strain Curves of KhN62M-VI Alloy After Long-Term Aging at 700 \({}^{\circ}\)C  
The behavior of the KhN62M-VI alloy during aging at a higher temperature (Regime 2) differed significantly from Regime 1, as evident from both microstructural changes and stress-strain curve evolution (Figure 13). TCP-phase particles enriched in chromium and molybdenum were formed in the material during prolonged aging (Figure 14, Table 3). At relatively short exposure times, these phases formed thin plates along the grain boundaries and chains along the intergranular boundaries. The density and size of these precipitates grew with increasing aging time, and the particles became larger and had a tendency to coagulate. Grain boundary TCP-phase precipitates did not significantly influence mechanical properties similar to Regime 2 behavior in Hastelloy\({}^{\text{\textregistered}}\) G-35\({}^{\text{\textregistered}}\). [PERSON] et al. [48] noted a similar effect--the sigma phase precipitated along the grain boundaries caused a slight reduction in tensile strength due to the formation of cracks at the \(\gamma/\sigma\) interface but had no obvious effect on ductility. However, potential contact of these particles with molten electrolytes may initiate intergranular corrosion [14]. Figure 12: Typical engineering stress–strain curves of KhN62M-VI specimens after long-term aging at 600 \({}^{\circ}\)C.  
Figure 13: _Cont._  
Figure 14: Excessive phases formed in KhN62M-VI after 5000 h of aging at 700 \({}^{\circ}\)C for 5000 h.  
Figure 13: Microstructure of KhN62M-VI alloy after long-term aging at 700 \({}^{\circ}\)C for 1000 h (**a**) and 5000 h (**b**). Analysis of the stress-strain curves indicates that both uniform and localized deformation stages remained for up to 2000 h at 700 \({}^{\circ}\)C. Further aging for up to 5000 h led to a gradual reduction in ductility to 44%.