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

Abstract

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 α (αp) 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-α 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 αp grains, especially along (0001) twist boundaries. Furthermore, transmission electron microscope observations evidenced the absence of intense slip bands within faceted grain and fatigued αp 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 αp 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.