Impact of Morphology on the High Cycle Fatigue Behavior of Ti-6Al-4V for Aerospace

The mechanical properties of Ti-6Al-4V alloy are affected by its microstructures. However, the effects of these microstructures on the high cycle fatigue behavior of Ti-6Al-4V alloy with a mixed structure (alpha + beta phases) remain unknown. In this study, three alloy specimens were prepared using different hot-deformation methods, and their microstructures were investigated by optical microscopy and scanning electron microscopy. Fatigue tests were then performed to determine their high cycle fatigue and fatigue crack propagation behavior. All specimens showed a bimodal structure, but the morphology of each phase (e.g., diameter, shape, and volume fraction) showed notable differences. Among the samples prepared, the forged sample (FS) showed the lowest fatigue strength in all cycles. The fatigue strength of the homogeneously rolled sample (HS) was slightly higher than that of the rolled sample (RS) below 10(6) cycles but lower above 10(6) cycles. Compared with those of RS and HS, the secondary alpha (alpha(s)) grain width of FS was twofold larger. The interconnected primary alpha (alpha(p)) phase clusters in HS appeared to promote microcrack propagation.