The influence of microstructure on dwell sensitive 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 on the dwell sensitivity. Three microstructures (equiaxed, tri-modal and basketweave) were used in this study. When a 3-min 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 showed the strongest fatigue life reduction. The basketweave microstructure had a little higher dwell-time fatigue life than tri-modal microstructure at low maximum stress levels. In all cases, extensive quasi-cleavage facets and planar slips with track-like dislocations have been intimately linked with the dwell sensitive fatigue response. The amount of quasi-cleavage facets and planar slips decreased with a decrease of the α phase content. A rationalization for planar slip was proposed based on the mechanism of dislocations shearing α2 particles. It is believed that α2 particle formation and oxidization effects played an important role in dislocation planar slip.