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

Abstract

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 αp percentage. The Ti-7333 alloy with a lower αp 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 αp percentage. Crack initiation region shows the compound αp/β facets. Faceted αp particles show crystallographic orientation and morphology dependence characteristics. Crack-initiation was accompanied by faceting process across elongated αp particles or multiple adjacent αp particles. These particles generally oriented for basal slip result in near basal facets. Fatigue crack can also initiate at elongated αp 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 αp 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.