Titanium alloy (Ti-6Al-4V) having bimodal "harmonic structure", which consists of a coarse-grained structure surrounded by a network structure of fine-grains, was produced by mechanical milling (MM) and spark plasma sintering (SPS) to achieve high strength and good plasticity. The microstructure of the MM-processed powder and the sintered compacts were characterized using a micro-Vickers hardness tester, an optical microscope, scanning electron microscope (SEM) and an electron backscatter diffraction technique (EBSD). Harmonic structure was created in the sintered Ti-6Al-4V compacts prepared from the MM-processed powders having fine grains at its surface. The effects of the harmonic structure on the 4-points bending fatigue properties of Ti-6Al-4V alloy was investigated under the stress ratio R = 0.1 in ambient air without any controls of the temperature. The compacts with harmonic structure exhibited higher fatigue strength compared to the conventional coarse-grained material prepared from as-received powders. This was because the harmonic structure increased the tensile strength of Ti-6Al-4V alloy. Moreover, fatigue fracture mechanism of Ti-6Al-4V alloy with harmonic structure was discussed from viewpoints of fractography and microstructural orientations. As results of observing the fracture surfaces, the Ti-6Al-4V alloy with harmonic structure failed from the coarse grained structure with almost the same crystal orientation in the surface fracture mode.
