Low-cycle fatigue life and fracture of superelastic NiTi with surface layers modified by tantalum ions

The effect of surface treatment in the form of high-dose implantation of Ta⁺ ions (D_i = 5 × 10¹⁶ cm⁻²) on the regularities and mechanisms of deformation and fracture under cyclic loading of specimens from a superelastic NiTi alloy has been studied. It has been shown that under the loading amplitude \({\sigma }_\text{max}=250 MPa\), closed to the conditions of practical use of this alloy, ion-beam treatment leads to ~ 1.3–1.7 times increase in its fatigue life. A comparative analysis of the evolution of the strain components fields, as well as the sample average and local values of these components as a function of time (number of cycles) in unirradiated and irradiated samples, was performed. The parameters \({\nu }_\text{ave}^{*}=-\frac{{\varepsilon }_{xx \text{ave}}}{{\varepsilon }_{yy \text{ave}}}\) and \({\nu }_\text{local}^{*}=-\frac{{\varepsilon }_{xx \text{min}}}{{\varepsilon }_{yy \text{max}}}\), referred to in the paper as the average and local "Poisson's-like ratio", are proposed as fatigue criteria for NiTi-based alloys with shape memory effects and superelasticity. The inflection points in the dependence of these parameters on the number of cycles can be interpreted as points of transition from the stage of relative stability of the functional properties revealed by NiTi specimens during their cyclic deformation under uniaxial stretching to the stage of accelerated degradation of these properties. Scenarios of deformation and fracture development in unirradiated and irradiated NiTi specimens subjected to cyclic uniaxial tensile loading are proposed. The fatigue loading conditions under which the ion-beam treatment resulted in an almost twofold increase in the fatigue life of this alloy were determined.

Graphical abstract