Effect of electron-beam treatment on the nature of fatigue crack propagation and formation of plastic zones on fracture surfaces in TiNi shape memory alloy

Influence of surface electron-beam processing on character of fatigue cracks propagation and crack growth rates under cyclic tension in a mode of low-cycle loading of TiNi samples before and after irradiation by low-energy high-current electron beam (LEHCEB) is studied in the work. Correlation between stages of fatigue cracks propagation and formation of plastic zones on surfaces of failure in TiNi samples before and after LEHCEB treatments is established. The LEHCEB treatment of specimen surfaces was carried out at the electron-beam installation “RITM-SP” with the electron beam parameters: energy density ES = 3.7 J/cm2, pulse duration τ = 2.5 μs, number of pulses n = 5. Differences in the stages of propagation of main fatigue cracks during cyclic stretching of TiNi samples before and after LEHCEB have been revealed. Preferential mechanisms of quasistatic and fatigue fracture at different stages of crack propagation are determined. It is shown that LEHCEB treatment leads to a shift of fatigue fracture initiation and the beginning of all stages by ∆N ≥ 3000, increasing the cyclic durability of the specimens by ~ 1.5 times. The greatest influence of surface modification is shown on Stage I of fatigue crack propagation. The lower rate of fatigue crack propagation at this stage in irradiated specimens leads to an increase in its duration as compared to unirradiated specimens. It is concluded that to effectively increase the fatigue life of TiNi specimens by means of LEHCEB treatments it is necessary to create conditions for increasing the number of cycles before the Stage I initiation and maximizing the duration of this stage.