The influence of microstructure on dwell sensitivity of fatigue in Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy

The fatigue behaviour of titanium alloy Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) was examined at 520 °C to study the effects of microstructural variation (equiaxed, tri-modal and basketweave microstructure) on the dwell sensitivity. It was found that when a three-minute dwell time was imposed at the peak of each cycle a significant fatigue life reduction was observed for all microstructures tested. Among the three microstructures, equiaxed microstructure shows the largest fatigue life reduction. Basketweave microstructure has a little higher dwell-time fatigue life than tri-modal microstructure and has the best fatigue-creep life. Extensive SEM and TEM observations show that the fractures for equiaxed and tri-modal microstructures are characterized by extensive quasi-cleavage facets and planar slips with track-like dislocations. The amount of quasi-cleavage facets and planar slips decrease with the decrease of the alpha phase percentage. While the fracture for basketweave microstructure was characterized by cyclic cleavage and dislocation networks. The planarity of dislocation slip was interpreted by a model based on the mechanism of dislocations shearing α₂ particles.