DWELL-SENSITIVE FATIGUE UNDER BIAXIAL LOADS IN THE NEAR-ALPHA TITANIUM-ALLOY IMI685

Low-temperature dwell sensitivity is an industrially relevant problem in the near-alpha titanium alloys. It is well documented that components in these materials can display life reductions of over an order of magnitude when the fatigue cycle contains a dwell period at peak stress. Despite much work, the mechanisms responsible for the phenomenon are not understood in full, although a poorly aligned alpha microstructure in thick sections, plastic strain accumulation, hydrogen level and stress biaxiality have all been invoked. The research programme has addressed the problem by focusing on IMI685 as a typical representative of this class of material. To probe the mechanisms involved, tension and torsion loading modes were employed and hydrogen concentration was varied in the range 20-60 ppm. Quasi-cleavage facet formation is an essential element of the failure process and a model for its occurrence on basal planes approximately perpendicular to the principal tensile stress direction in both tension and torsion specimens is introduced. On the basis of this model, the life decrement in large components can be related to variations in prior beta grain size. Hydrogen content is shown to have an effect on deformation response but not a significant influence on the failure process.