## 3 Results
### Microstructure and Stress-Strain Curves of Nickel Alloys After Long-Term Aging
#### 3.3.5 Effect of Long-Term Aging on VDM(r) Alloy 59 and VDM(r) Alloy C-4 Properties  
Analysis of the stress-strain curves of VDM(r) Alloy 59 and VDM(r) Alloy C-4 indicates that their deformation behavior during aging was similar to that of KhN62M-VI. A summary plot of the short-term mechanical properties of VDM(r) Alloy 59 and VDM(r) Alloy C-4 for selected aging temperatures is presented in Figures 16 and 17. It is important to note that prolonged exposure of VDM(r) Alloy 59 at 700 \({}^{\circ}\)C led to the formation of TCP-phase precipitates at grain boundaries but did not affect strength properties. However, the ductility of the alloys was significantly reduced, particularly after 5000 h.  
Figure 15: Typical engineering stress–strain curves of KhN62M-VI specimens after long-term aging at 700 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c} \hline
**Spectrum** & **Cr** & **Ni** & **Mo** \\ \hline Spectrum 1 & 21.78 & 37.55 & 40.67 \\ Spectrum 2 & 21.95 & 37.83 & 40.22 \\ Spectrum 3 & 70.40 & 18.75 & 10.85 \\ Spectrum 4 & 26.31 & 64.90 & 8.79 \\ \hline \end{tabular}
\end{table} 
Table 3: Chemical composition of phases in KhN62M-VI aged at 700 \({}^{\circ}\)C for 5000 h (at.%). The correctness of our experimental data is confirmed by the results of [PERSON] [28], who observed similar effects in VDM(r) Alloy C-4 after long-term aging (up to 16,000 h). 3.6 Microstructure and Stress-Strain Curves of Hastelloy(r) B-3(r) After Long-Term Aging at 500 \({}^{\circ}\)C  
The microstructure of Hastelloy(r) B-3(r) after high-temperature exposure (Figure 18) did not show much difference from the initial state but contained coherent intermetallic phases based on nickel and molybdenum. A comparative analysis of the stress-strain curves of Hastelloy(r) B-3(r) in the as-received state and after aging reveals that exposure at 500 \({}^{\circ}\)C for up to 5000 h led to a gradual increase in strength and a more pronounced reduction in ductility (Figure 19). This was primarily due to the shortened (to 21%) uniform deformation stage and an increase in the YS/TS ratio from 0.45 to 0.77. Figure 16: Tensile properties of VDM® Alloy 59 after long-term aging. Figure 17: Tensile properties of VDM® Alloy C-4 after long-term aging. Figure 18: Microstructure of Hastelloy\({}^{\text{\textregistered}}\) B-3\({}^{\text{\textregistered}}\) alloy after long-term aging at 500 \({}^{\circ}\)C for 500 h (**a**) and 5000 h (**b**). 3.3.7. Microstructure and Stress-Strain Curves of Hastelloy\({}^{\circledR}\) B-3\({}^{\circledR}\) After Long-Term Aging at 650 \({}^{\circ}\)C  
At the aging temperature of 650 \({}^{\circ}\)C, precipitates of various types and morphologies were formed in Hastelloy\({}^{\circledR}\) B-3\({}^{\circledR}\) alloy, both at grain boundaries and within the grain body. Intragranular precipitates primarily appeared as thin plates (below 1 \(\upmu\)m thick) oriented at different angles (Figure 20). This morphology is characteristic of the ordered intermetallic Ni\({}_{4}\)Mo phase, as confirmed by EDS analysis (Figure 21, Table 4). Some precipitates with a lack of distinct interfaces between their surface and the matrix are coherent or semi-coherent. These results agree well with previous studies of Ni\({}_{4}\)Mo precipitates, which form through solid solution ordering and may retain partial coherence with the matrix [25; 49]. In certain regions, darker zones are observed that can correspond to a Ni\({}_{3}\)Mo phase. The plate-like morphology of Ni\({}_{4}\)Mo particles is highly unfavorable for maintaining adequate ductility and viscosity during aging. In this case, interphase boundaries act as stress concentrators and crack nucleation sites. As a result of these processes, aging at 650 \({}^{\circ}\)C led to complete embrittlement of Hastelloy\({}^{\circledR}\) B-3\({}^{\circledR}\) (\(\delta\to 0\), YS/TS = 1), and these phenomena did not depend on aging duration (Figure 22). Similar results were obtained for the Hastelloy B-2 alloy by [PERSON] [27].