Effects of microstructure on high cycle fatigue properties of dual-phase Ti alloy: Combined nonlocal CPFE simulations and extreme value statistics

The nonlocal crystal plasticity finite element (CPFE) simulations and extreme value statistics were combined to study the effects of microstructure on the high cycle fatigue (HCF) behavior of dual-phase Ti alloy. A modified Armstrong-Frederick nonlinear kinematic hardening equation accounting for cyclic softening effect was employed in the crystal plasticity constitutive model. Three-dimensional equiaxed microstructure models and two-dimensional duplex microstructure models with real lamellar structure were generated based on Voronoi method, serving as statistical volume elements (SVEs). The effects of morphological and crystallographic features, including grain size, grain orientation, phase volume fraction and lamellae width, on the fatigue performance were investigated. By simulating multiple SVEs, extreme value distributions of the driving force for fatigue crack formation were predicted using Fatemi-Socie (FS) parameter as a fatigue indicator parameter (FIP). Meanwhile, whether the extreme values of geometrically necessary dislocation (GND) density can be taken as a FIP was discussed with compared to FS FIP. The GND density, related to local stress and strain gradient, has considerable potential as a FIP to estimate the fatigue performance of titanium alloys. Based on simulated results, it is suggested that microstructure with small grain size, low volume fraction of primary grains, and thinner lamellae width have the lowest probability of crack formation during HCF.