Effects of Deformation-Induced Constraint on High-Cycle Fatigue in Ti Alloys with a Duplex Microstructure

The effects of the deformation behaviors of individual phases in Ti alloys with a duplex microstructure containing a mixture of primary α grains and lamellar α + β colonies have been analyzed by applying the finite-element method (FEM) to the relevant microstructure. The softer primary α grains and the harder lamellar α + β colonies are treated as distinct microstructural units with different constitutive properties. The FEM results indicate that the softer (primary α) phase, which reaches yielding first, tends to concentrate both the plastic strain and the hydrostatic stress, whose magnitudes depend on the volume fraction of the primary α grains and the load levels. The insight from the FEM analyses has been used to modify the linear relation between alternating stress and mean stress in a Haigh or modified Goodman diagram by incorporating a microstructure-induced hydrostatic stress term, leading to a set of bilinear relations for high-cycle fatigue (HCF) failures of Ti alloys with a duplex microstructure. The applicability of the micromechanical approach to treating the mean stress effects in HCF failure of Ti-6Al-4V with various duplex microstructures is elucidated by comparison against experimental data in the literature.