## 2 Materials and Methods  
Test specimens were prepared from a commercially available TiNi alloy (MATEK-SMA Ltd., Russia), manufactured using the vacuum-induction melting method. The chemical composition of the alloy was: Ti (balance); 55.08 Ni; 0.051 C; 0.03 O; 0.002 N (wt.%)--resulting in the Ti\({}_{49.1}\)Ni\({}_{50.9}\) alloy (at.%) of stoichiometric composition. The temperatures of the onset and completion of the direct and reserve martensite transformation (MT) of the TiNi alloy were determined using the differential scanning calorimetry method (DSC): \(M_{\rm S}=-7\pm 2\)\({}^{\circ}\)C; \(M_{\rm f}=-35\pm 2\)\({}^{\circ}\)C; \(A_{\rm s}=5\pm 2\)\({}^{\circ}\)C; \(A_{\rm f}=28\pm 2\)\({}^{\circ}\)C, and this alloy has a B2 austenite structure at room temperature. The Ti\({}_{49.1}\)Ni\({}_{50.9}\) alloy was preliminary quenched from 800 \({}^{\circ}\)C in water. To obtain the UFG state, the Ti\({}_{49.1}\)Ni\({}_{50.9}\) alloy was subjected to equal-channel angular pressing at a temperature of 450 \({}^{\circ}\)C, the angle of intersection of the equipment channels was 120\({}^{\circ}\), and the number of ECAP cycles was n = 8 [9; 10]. It was previously shown that an amorphous-nanocrystalline state is formed in TiNi during the high pressure torsion (HPT), and an NC state is formed during subsequent annealing [7]. Accordingly, the nanocrystalline structure was formed using the HPT method with a number of rotations of the anvils of n = 5 with a speed of 1 rpm at room temperature and a pressure of 6 GPa, and then the samples after HPT were subjected to subsequent annealing at 350 \({}^{\circ}\)C for 1 h.  
Mechanical tensile tests under quasistatic uniaxial tension at various temperatures were carried out on a tensile testing machine designed using the Instron 5982 tensile testing machine on small flat samples with a gauge section of 1 \(\times\) 0.25 \(\times\) 4 mm. Tensile specimens were deformed to failure with constant crosshead speed corresponding to the initial strain rate \(\dot{\epsilon}=1\ \times 10^{-3}\) at test temperatures of 25 \({}^{\circ}\)C, 250 \({}^{\circ}\)C and 400 \({}^{\circ}\)C. At least three samples of each type were tested. Based on the analysis of the recorded loading diagrams (stress-strain curves), in addition to the conventionally used strength and ductility properties, some functional properties of the TiNi alloy under study were determined; that is, the martensite shear stress (the critical stress to induce the B19\({}^{\prime}\) transformation), namely \(\sigma_{\text{M}}\). Samples for testing were cut from billets using electric-spark cutting, grinded and polished with a diamond paste. To determine the parameter of the strain rate sensitivity of the plastic flow stress \(m\), the method of varying the tensile strain rate from \(\dot{\epsilon}=1\ \times 10^{-3}\ \text{s}^{-1}\) to \(\dot{\epsilon}=1\times 10^{-4}\ \text{s}^{-1}\) and back was applied. The parameter \(m\) was determined from Relation (1) [24]:  
\[m=\frac{\lg(\sigma_{2}/\sigma_{1})}{\lg(\dot{\epsilon}_{2}/\dot{\epsilon}_{1} )}, \tag{1}\]  
where \(m\) is the parameter of strain rate sensitivity; \(\sigma_{2}\), \(\sigma_{1}\)--flow stress, MPa; \(\dot{\epsilon}_{2}\), \(\dot{\epsilon}_{1}\)--strain rate, \(\text{s}^{-1}\). The activation volume of deformation was calculated using Formula (2) [25]:  
\[\Delta V=\sqrt{3}kT/m\sigma \tag{2}\]  
where \(m\) is the parameter of strain rate sensitivity; \(\sigma\)--flow stress, MPa; \(k\)--Boltzmann's constant, 1.38 \(\times\) 10\({}^{-23}\) J/K; \(T\)--test temperature, K.  
The microstructure in the initial state was examined with an OLYMPUS GX51 optical microscope. The fine structure of the samples was investigated using a JEOL JEM-2100 transmission electron microscope at an accelerating voltage of 200 kV. Foils for transmission electron microscopy were obtained on a TenuPol-5 double-sided electropolishing unit using a standard electrolyte composition: 10% HClO\({}_{4}\) + 90% CH\({}_{3}\)(CH\({}_{2}\))\({}_{3}\)OH.