Effect of submicron Ti–Ni–Ta-based surface alloy on the mechanical properties of the TiNi alloy before and after thermal cycling of the system [Ti–Ni–Ta-based surface alloy|TiNi-substrate]

The dependencies of changes in the mechanical properties of the TiNi alloy with synthesized surface alloy based on the Ti–Ni–Ta system with a thickness of $<$ 1 $\mu$m were investigated in torsion tests. The synthesis was carried out by alternating the operations of deposition of a Ti$_{60}$Ta$_{40}$ (at.%) alloying film and liquid-phase mixing of the film/substrate system using a low-energy high-current electron-beam. It has been shown that the presence of the surface alloy results in the preservation of martensite shear stresses $\tau_{\mathrm{М}}\approx$ 460 MPa, an increase in the width of the stress hysteresis loop $\Delta\tau$ by $\sim$ 25 MPa, and an improvement in the ability of the material to accumulate and recover superelastic strain $\gamma_{\mathrm{SE}}$ by $\sim$ 0.2% more than the values in TiNi samples without irradiation and alloying. It was found that reducing of the thickness of the surface alloy does not lead to an increase in the crack resistance of the synthesized surface layer. It was found that thermal cycling of the modified samples in the temperature range of B2-B19' martensitic transformation leads to a change in the elastic-stress state in the [surface alloy|TiNi-substrate] system, resulting in a reduction of stresses $\tau_{\mathrm{М}}$ by $\sim$ 100 MPa, and an increase in the crack resistance of the synthesized surface layer.