Microstructural influences on the high cycle fatigue life dispersion and damage mechanism in a metastable β titanium alloy

In this work, the effect of microstructure features on the high-cycle fatigue behavior of Ti-7Mo-3Nb-3Cr-3Al (Ti-7333) alloy is investigated. Fatigue tests were carried out at room temperature in lab air atmosphere using a sinusoidal wave at a frequency of 120 Hz and a stress ratio of 0.1. Results show that the fatigue strength is closely related to the microstructure features, especially the α_p percentage. The Ti-7333 alloy with a lower α_p percentage exhibits a higher scatter in fatigue data. The bimodal fatigue behavior and the duality of the S-N curve are reported in the Ti-7333 alloy with relatively lower α_p percentage. Crack initiation region shows the compound α_p/β facets. Faceted α_p particles show crystallographic orientation and morphology dependence characteristics. Crack-initiation was accompanied by faceting process across elongated α_p particles or multiple adjacent α_p particles. These particles generally oriented for basal slip result in near basal facets. Fatigue crack can also initiate at elongated α_p particle well oriented for prismatic <a> slip. The β facet is in close correspondence to {110} or {112} plane with high Schmid factor. Based on the fracture observation and FIB-CS analysis, three classes of fatigue-critical microstructural configurations are deduced. A phenomenological model for the formation of α_p facet in the bimodal microstructure is proposed. This work provides an insight into the fatigue damage process of the α precipitate strengthened metastable β titanium alloys.