Multiscale Mechanism of Fatigue Fracture of Ti—6A1-4V Titanium Alloy within the Mesomechanical Space-Time-Energy Approach

Ultrasonic impact treatment (UIT) of alloy Ti-6Al-4V (VT6) causes a high lattice curvature, nanostructuring of thin surface layers, and the formation of complex band structures of T Al pre-precipitates in the a phase of the underlying sublayer, as well as the formation of the martensitic a ' phase. In so doing, the fatigue life of the alloy increases only by a factor of 1.3 due to the negative influence of complex band structures. Positron annihilation spectroscopy revealed a nonequilibrium vacancy concentration in the treated surface layer equal to 10^-5, which is by five orders of magnitude greater than the equilibrium vacancy concentration. This makes possible reversible structural transformations through plastic distortion under cyclic loading of VT6 and underlies the increase in fatigue life. There is a convergence of the electron energy distribution curves for VT6 + UIT and Al obtained from the Doppler broadening spectra of annihilation radiation. This result suggests the formation of Ti-Ti-Al clusters and Ti₃Al pre-precipitates in etch-resistant banded structures. Hydrogen charging of the ultrasonically treated VT6 surface layers leads to a 4-fold decrease in the fatigue life of the material. This effect is due to the formation of α"-phase martensite laths in the a phase which rearranges the hcp lattice into an orthorhombic structure under the functional influence of hydrogen, with the segregation of vanadium atoms in the α"-phase bands. The segregation causes a convergence of the electron energy distribution curves of VT6 + UIT + HN and V, as evidenced by the Doppler broadening spectra of annihilation radiation. Bundles of α"-phase bands reinforce the nanostructured surface layer, which drastically reduces the fatigue life of the alloy. Its microhardness in the zone of fatigue fracture greatly increases. The multiscale structural analysis of fatigue fracture is carried out on the basis of the mesomechanical space-time-energy approach.