Characteristic and mechanism of crack initiation and early growth of an additively manufactured Ti-6Al-4V in very high cycle fatigue regime

In this paper, very high cycle fatigue (VHCF) behavior of an additively manufactured (AM) Ti-6Al-4V by selective laser melting process and post-heat treated by hot-isostatic pressing is investigated by ultrasonic frequency fatigue test and rotating bending fatigue test. It is shown that the fatigue crack initiation is related to loading types in VHCF regime. Under rotating bending fatigue test, the fatigue crack initiates from specimen surface. While for ultrasonic frequency fatigue test, both the surface and the interior crack initiations are observed. For interior crack initiation, the fracture surface presents fish-eye like pattern and fine granular area (FGA) morphology. Electron backscatter diffraction and transmission electron microscopy observations indicate that there are discontinuous refined grain regions beneath the fracture surface in crack initiation and early growth region (i.e. FGA). It is proposed that the mechanism of crack initiation and early growth of titanium alloys in VHCF regime is attributed to the grain refinement caused by dislocation interaction over a number of cyclic loadings followed by cracks combined with the cracks formed at defects, alpha-phase, interfaces, etc. during cyclic loadings. The paper also indicates that the fatigue performance of the present AM Ti-6Al-4V is comparable to that of the conventionally processed Ti-6Al-4V.