The stress effect on very high cycle fatigue and fracture of near β titanium alloy

The structural-phase state of an ultrafine-grained Ti-55511 alloy produced by radial shear rolling and subsequent aging was investigated after very high cycle fatigue (VHCF) tests with stress amplitudes close to the fatigue limit. The study revealed that a discontinuity region is formed in the central part of the specimen during VHCF testing at a stress amplitude below the fatigue limit (sigma(a) = 580 MPa) and a number of cycles without failure of 8.79 & sdot; 10(9). The size of the region (30-40 mu m) is in good agreement with the size of the fatigue crack initiation site. The selected VHCF conditions cause structural-phase transformations in the alloy, leading to a 52-75 % increase in the volume fraction of the alpha phase. This occurs with the accumulation of deformation defects and diffusioncontrolled redistribution of alloying elements in local regions at the particle/matrix interface, due to which the interfacial energy increases while the fatigue microcracking resistance of the interface decreases. A higher stress amplitude above the fatigue limit (sigma(a) = 660 MPa) intensifies the above processes, leading to internal microcracking and fatigue failure of the alloy at a much smaller number of cycles (6.(7) & sdot; 10(8)).