Crack initiation mechanism in a high-strength Ti-5Al-7.5V alloy subjected to high cycle fatigue loading

Fatigue crack initiation mechanism at high cycle regime in a bimodal Ti-5Al-7.5V alloy are investigated by fracture observations, focused-ion-beam cross-section and electron back-scattered diffraction characterizations. The aged Ti-5Al-7.5V exhibits superior tensile and fatigue properties, alongside a wide range of fatigue life when tested at the stress levels of - 700-800 MPa. Fatigue failure was dominated by surface and subsurface crack initiation at facets matching basal planes. These faceted primary alpha (alpha p) grains generally have a medium or high Schmid factor (SF) for basal < a > slip. Moreover, a basal faceting process also occurred in the "hard" grains, in which the c-axis of the HCP-alpha grains was approximately parallel to the loading axis so that the < a > type slips were thought to be suppressed. The observed internal microcracks were formed along the grain boundaries located in clusters of alpha p grains, especially along (0001) twist boundaries. Furthermore, transmission electron microscope observations evidenced the absence of intense slip bands within faceted grain and fatigued alpha p grains, which indicates that the facet growth should not along the pre-existing slip bands resulting from high cycle fatigue loadings. The pre-existing structural defects including dislocation networks and subgrain boundaries within alpha p grains are relatively stable configurations under fatigue loadings. The observed fine (-500 nm in diameter) silicides were widely dispersed, whether the silicide can act as the crack nucleation site still needs further more detailed studies and this remains an open question.