## 1 Introduction  
TiNi (Nitinol) alloys are an important class of shape memory alloys (SMAs). They are well known for their shape memory effect and superelasticity, and have found several important applications. These alloys are used as functional materials in mechanical actuation systems, couplings [1], actuators, biomedical equipment and biomedical implants [1, 2]. In some cases, the level of properties of alloys in a coarse-grained state is insufficient. There are various methods for increasing the mechanical and functional properties of these alloys--rolling, rotational forging, and others. Methods for enhancing performance include severe plastic deformation (SPD) methods [3, 4, 5, 6, 7, 8, 9, 10]. As a result of equal-channel angular pressing (ECAP), an ultrafine-grained (UFG) structure with a grain size of about 500 nm is formed in TiNi alloys, which leads to a significant increase in the mechanical properties and functional characteristics of shape memory effects [9, 10]. Recently, many works have been devoted to the study of alloys with an ultrafinegrained structure [11, 12, 13]. However, the mechanical behavior and deformation mechanisms of UFG TiNi are poorly studied. Note that martensitic transformations during deformation of TiNi alloys play a strategic role in their behavior during deformation [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]. The mechanical behavior of UFG and nanocrystalline (NC) metals is markedly different from the behavior of microcrystallines (with grain sizes greater than 1 \(\mu\)m) [25, 26, 27, 28, 29, 30, 31, 32]. In UFG materials, an increase in the rate of diffusion creep and an increase in the contribution of grain-boundary slip to deformation have been observed [25; 26; 27; 28; 29; 30; 31; 32]. To assess the mechanisms of deformation, the following parameters are used--the parameter of strain rate sensitivity \(m\) and the activation volume of deformation \(V\)[24; 25; 26; 27; 28; 29; 30; 31; 32]. The value of \(m\) characterizes the ability of the material to deform uniformly, or in other words, the tendency of the material to form a neck. The physical meaning of the indicator \(m\) is that when the neck is formed, the strain rate in it will be greater than in the rest of the sample. At high values of \(m\), this will lead to hardening of the material in the neck; therefore, the flow is concentrated in another part of the sample. Analysis of the activation volume allows us to estimate the contribution of various deformation mechanisms to a particular structural state. It is recognized that in materials with bcc and hcp lattices, the activation volume does not change with decreasing grain size, and in fcc materials, \(V_{\sigma}\) decreases with decreasing grain size. As a rule, different mechanisms of plastic deformation correspond to different ranges of \(m\), and when the mechanisms of superplastic deformation (slip due to the elongation and rotation of the grain along grain boundaries) prevail, \(m\) reaches its highest level of 0.4 - 0.7 [26; 27; 28; 29; 30]. The effect superplastic deformation in alloys with a refined structure can be observed not only at elevated test temperatures. In some ultrafine-grained materials, this effect can be observed at relatively low temperatures and/or relatively high strain rate. It was shown that sensitivity to the strain rate increases with decreasing grain size for fcc metals such as Cu, Al, and Ni [30; 31]. For pure Ni, for example, a decrease in grain size leads to an increase in the indicator of strain rate sensitivity \(m\)[31]. In the case of hcp metals, the dependence of \(m\) on the grain size is similar [32; 33; 34; 35]. Experimental results on the effect of grain size and temperature on the sensitivity to the strain rate of TiNi showed significantly small effects. The mechanical behavior of TiNi alloys is studied at elevated temperatures and low strain rates that create favorable conditions for the creep of coarse-grained (CG) TiNi alloys.