Fatigue Behavior and Fracture Features of Ti-15Mo Alloy in β-, (α plus β)-, and Ultrafine-Grained Two-Phase States

The influence of the ultrafine-grained structure formed by equal-channel angular pressing via the "Conform" scheme on the fatigue behavior of metastable beta-alloy Ti-15Mo has been studied. It is shown that the alloy with a two-phase ultrafine-grained structure achieved the best mechanical properties and enhanced fatigue endurance limit (up to 710 MPa on the basis of 10(7) cycles) due to the total contribution of grain boundary, dislocation, and phase strengthening mechanisms. A fractographic analysis of the fracture surface of samples after fatigue tests showed the features of fatigue crack propagation depending on the type of alloy microstructure. The general and distinctive features of fatigue failure of alloy samples in the initial coarse-grained (alpha + beta)-, single-phase coarse-grained beta-, and ultrafine-grained (alpha + beta)-states are revealed. In all of the samples, a fatigue crack nucleated on the surface and propagated downward, i.e., perpendicular to the direction of the applied pressures. It is shown that fracture surfaces of the ultrafine-grained samples had a high roughness and were characterized by the presence of a large number of secondary cracks, as compared to the coarse-grained analogues.