*3.4. Mechanical and Functional Characteristics of Ti-50.8 at.% Ni Alloy during Thermal Cycling*

In both CG and UFG states of the Ti-50.8 at.%Ni alloy, the values of microhardness as a result of multiple martensitic transformations increased slightly compared to the state before thermal cycling (Figure 13). However, in the first 50–100 cycles, a more intense increase in microhardness is characteristic, then the values stabilize and saturation occurs after 100 cycles of transformations.

**Figure 13.** The dependence of microhardness on the number of thermal cycles in various states.

The results of mechanical tensile tests are presented in Figures 14 and 15 for CG and UFG states. The functional characteristics determined from the analysis of mechanical tensile tests are presented in the form of graphs of the dependence of the estimated reactive stress (σreac) and plateau length at the phase yield stage, used as an estimate of the recovery deformation (εrec) on the number of thermal cycles (Figure 16).

The most sensitive characteristic to thermal cycling is the yield strength. In both states of the alloy, it increases with an increase in the number of cycles. UFG states up to TC are characterized by higher values of strength and yield strength due to the contribution of grain boundary hardening. The tensile strength in the CG state increased to 150 cycles, then the values decreased and did not change. This is probably due to the fact that after a certain number of cycles (in CG state, *n* = 150), the material is saturated and no further increase in characteristics is observed. In the UFG state, with an increase in the number of cycles of multiple transformations, the values of the tensile strength increased until reaching 100 thermal cycles; then, a decrease in the parameter was observed, which can be explained by the heterogeneity of the formed structure.

**Figure 14.** The dependence of the ultimate tensile stress of the alloy on the number of thermal cycles.

**Figure 15.** The dependence of the yield stress of the alloy on the number of thermal cycles.

**Figure 16.** The dependence of the functional characteristics of the alloy on the number of thermal cycles: (**a**) coarse-grained state, (**b**) ultrafine-grained state.

According to the constructed graphs (Figure 16), the functional characteristics (the magnitude of the reversible deformation and the estimated reactive stress) increased slightly and then remained stable both in the CG and in the UFG states with an increase in thermal cycles.
