Lengthscale-dependent, elastically anisotropic, physically-based hcp crystal plasticity: Application to cold-dwell fatigue in Ti alloys

A crystal plasticity model for hcp materials is presented which is based on dislocation glide and pinning. Slip is assumed to occur on basal and prismatic systems, and dislocation pinning through the generation of geometrically necessary dislocations (GNDs). Elastic anisotropy and, through the coupling of GNDs with slip rate, physically-based lengthscale effects are included. A model polycrystal representative of the alloy Ti–6Al, which shows creep and strain rate effects at 20 °C, is developed and it is shown that the primary effect of elastic anisotropy during subsequent plastic flow is to increase local, grain-level, accumulated slip. Lengthscale effects, however, are shown to lead to quite considerable increases in grain-boundary stresses, and to the re-distribution of accumulated slip local to grain boundaries. In particular, an initially highly non-uniform slip distribution tends to be made more uniform through the hardening effect of sessile GNDs at grain boundaries.