**2. Materials and Methods**

As a research material, a two-component alloy of the TiNi system was chosen—the stoichiometric alloy Ti-50.8 at.% Ni enriched with nickel, manufactured by MATEK-SMA Ltd. (Moscow, Russia). It has a bcc lattice ordered by type B2 (CsCl) and a phase enriched with nickel Ti2Ni3 [1,4], and the chemical composition of the alloy is presented in Table 1. To obtain a solid solution and eliminate processing history, quenching was carried out from the homogeneity region (heating at a temperature of 800 ◦C in a Nabertherm furnace for 1 h) into the water. The average grain size of the hardened alloy was about 20 ± 2 μm.


**Table 1.** Chemical composition of the alloy of titanium nickelide, % (by atomic).

To carry out the deformation by the ECAP method, the equipment of the Ufa State Aviation Technical University (USATU) design in isothermal mode was used. To form the UFG structure, quenching samples of cylindrical TiNi alloys (Ø20 mm, length 100 mm) were subjected to 8 passes along the Bc route at 450 ◦C, and the channel intersection angle (ϕ) was 120◦ [9]. Thermal cycling of the samples in different initial states was carried out as follows: the samples were successively immersed in liquid nitrogen (−196 ◦C), then they were heated to a temperature of 150 ◦C, which is actually lower and higher than the temperatures Mf of direct and Af reverse martensitic transformation. The number of heating–cooling thermal cycles ranged from 0 to 250. The thickness of the samples subjected to TC in the section was less than 1 mm to ensure their rapid heating and cooling. The exposure time at the heating and cooling temperatures was 5 min [25]. Quantitative and qualitative analyses of the initial structure were carried out using an OLYMPUS GX51 metallographic microscope. To detect the

microstructure, an etchant with a composition of 60% H2O + 35% HNO3 + 5% HF was used. Using the random secant method, the sizes of structural elements were calculated.

X-ray diffraction studies of the samples were carried out on a Rigaku Ultima IV diffractometer (*U* = 40 kV and *I* = 35 mA) at room temperature in the angle range 2θ = 30◦–120◦. The main structure parameters were determined by the Rietveld method using the Materials Analysis Using Diffraction (MAUD) program. The dislocation density was calculated by processing X-ray diffraction data using MATLAB software. The formula was used to calculate the density of dislocations [8]: ρ = 2 √ 3 < ε<sup>2</sup> >1/2/*Db*, where <ε2>1/<sup>2</sup> represents the microdistortions, *D* is the average grain size, and *b* is the Burgers vector.

The fine structure of the material was studied at room temperature using a JEOL JEM-2100 transmission microscope ("JEOL Ltd.", Tokyo, Japan) with an accelerating voltage of 200 kV. Samples for thin foils cut by the electro-erosion method were made by double-sided jet electrolytic polishing using a Tenupol-5 device ("Struers", Copenhagen, Denmark) in a solution of 10% perchloric acid (HClO4) and 90% butanol (CH3(CH2)3OH).

The average size of structural elements (grains, subgrains, martensitic twins) was estimated using the "GrainSize" software package by measuring chord lengths, the relative measurement error of which did not exceed 5%. In this work, the calorimetric testing of the material was carried out on a Mettler Toledo high-sensitivity differential scanning calorimeter ("Mettler Toledo", Columbus, OH, USA) on samples weighing up to 50 mg (diameter 3.5 mm, thickness 0.5–0.7 mm), and the change in heat flux was studied during cooling and heating in the temperature range from −196 ◦C to 150 ◦C at a rate of 10 ◦C/min. The temperatures of the beginning (Ms and As) and the end (Mf and Af) of the direct and reverse transformations were determined by standard tangent methods (ASTM 2004-05).

The microhardness *Hv* in this work was determined by the Vickers method on a Micromet 5101 instrument with a diamond indenter ("Buehler", Lake Bluff, OH, USA). Mechanical tensile tests of small flat specimens in compliance with all dimensional ratios with a working part of 1 × 0.25 × 4 mm were carried out with a strain rate of 1 <sup>×</sup> 10−<sup>3</sup> s−<sup>1</sup> on a special installation of the USATU design at room temperature. According to the test results, the strength characteristics (phase yield stress σm, dislocation yield strength σYS, ultimate tensile strength σUTS) and ductility (elongation, δ) were determined. The difference between the dislocation and phase yield limits allows one to estimate the stress σreac = σYS − σ<sup>m</sup> (estimated reactive stress) [4,25]. The length of the plateau at the phase yield stress stage was adopted as an estimate of the recovery strain εrec (Figure 1). The values of the conditional stresses of phase yield σm, dislocation yield stress σYS, and ultimate tensile strength σUTS are calculated as average statistical values for three samples.

**Figure 1.** Diagram of a tensile stress indicating mechanical characteristics.
