## 3 Results
### Fractographic Studies and Microstructure Analysis after Tensile Testing  
Fractures of the specimens were examined after the mechanical tensile tests at various test temperatures (Figures 6-8). An increase in the temperature of the tests led to a change in the nature of fracture from quasi-brittle-brittle (with shallow dimples) at room temperature to viscous (with clear dimples) at elevated temperatures. This behavior is characteristic of TiNi alloys both in coarse-grained and ultrafine-grained states; however, the diameter of the dimples in the ultrafine-grained state was much smaller (the diameter of the dimples ranged from 2 to 3 microns) than in the coarse-grained (the diameter of the dimples was about 5-7 microns). In the case of a quasi-brittle and brittle fracture, the depth of the dimples was small (about 1 \(\upmu\)m, the area on the upper right in Figure 6), and in the case of a viscous fracture (the area on the lower right in Figure 6), dimples with a depth of several \(\upmu\)m were observed. In the GG and UFG states at elevated test temperatures, a noticeable narrowing of the sample was observed in comparison with the tests at room temperature. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**State** & **Temperature**, \({}^{\circ}\)**C** & \(m\) & \(\mathbf{V\times 10^{23}}\) & **Mechanism of Deformation** \\ \hline \multirow{3}{*}{CG} & 25 & 0.016 & 69 & annihilation of dislocations along the grain boundaries \\  & 250 & 0.019 & 121 & deformation is a slip of dislocations and the formation of \\  & 400 & 0.024 & 185 & a forest of dislocations \\ \hline \multirow{3}{*}{UFG} & 25 & 0.025 & 134 & deformation is a slip of dislocations and the formation of \\  & 250 & 0.06 & 147 & a forest of dislocations \\ \cline{1-1}  & 400 & 0.07 & 315 & a forest of dislocations \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of determining the parameter of speed sensitivity and activation deformation volume. Figure 5: Graphs of the dependence “stress–time” of the Ti\({}_{49.1}\)Ni\({}_{50.9}\) alloy: (**a**) CG, (**b**) UFG. Figure 6: Fractography of fractures of the Ti\({}_{49.1}\)Ni\({}_{50.9}\) alloy in a coarse-grained state at different test temperatures. Figure 7: Fractography of fractures of the Ti\({}_{49,1}\)Ni\({}_{50,9}\) alloy in an ultrafine-grained state at different test temperatures. In the nanocrystalline state, an increase in the test temperature, as a rule, does not lead to a change in the nature of fracture--both at room temperature and at 400 \({}^{\circ}\)C--the fracture pattern remains brittle with characteristic cleavage facets. Only in some areas of the fracture, dimples characteristic of viscous fracture could be observed. At the same time, it was noted that the ductility of the NC alloy at a temperature of 400 \({}^{\circ}\)C was noticeably higher than that of CG and UFG. Additionally, the general view of the fracture indicates that there was no noticeable localization of deformation in the NC alloy at a temperature of 400 \({}^{\circ}\)C (Figure 8b), and the curve indicates a longer homogeneous flow (without hardening) (Figure 4c, curve at 400 \({}^{\circ}\)C). By contrast, both the fractures and the shape of the curves at a temperature of 400 \({}^{\circ}\)C of the CG and UFG alloys indicated a strong localization of deformation and necking (Figure 4a,b, curves at 400 \({}^{\circ}\)C). In the case of the CG and UFG states, the curves at 400 \({}^{\circ}\)C after the ultimate strength showed a noticeable decrease in stress, which indicates active necking. In the case of the NC state, a noticeably less intense decrease in stress was observed, which may indicate that there is a certain mechanism, perhaps a change in the strain rate sensitivity parameter \(m\), which actively prevents neck formation. However, in this study, it was not possible to determine this mechanism; this requires additional research.