The effects of grain morphology and crystallographic orientation on fatigue crack initiation in a metastable β titanium alloy Ti-7333

Introducing fine α particles into the soft β matrix contributes significantly to the strengthening of the metastable β titanium alloy microstructures. However, this microstructure design often leads to microcracks initiated at the α/β interface. In this work, fatigue crack-initiation (FCI) modes and FCI mechanisms at high-cycle fatigue regime in a metastable β titanium alloy, Ti–7Mo–3Nb–3Cr–3Al, are investigated by fracture surface observation and focused-ion-beam cross-section characterizations on a 2-D section through the faceted grains. Based on fracture observation, four categories of FCI modes were summarized from the given microstructures. Fatigue damage mechanism is non-unique and strongly dependent on microstructural constituent combinations although only subtle differences among them. FCI site revealed the compound primary-α (α_p)/β facets or isolated α_p facets. Elongated α_p particle or multiple equiaxed α_p particles favorably oriented for basal <a> slip are preferred crack-initiation sites, resulting in the formation of near basal facets. The β facet was in close correspondence to the {110} slip plane with high Schmid factor. The occurrence of the elongated α_p facet is usually accompanied by the rugged β facet. Further, three classes of fatigue-critical microstructural configurations are deduced. This work provides an insight into the fatigue damage process of the α precipitate strengthened metastable β titanium alloys.