The formation of precipitate-free zones in β-quenched TC18 titanium alloy during two phase field heat treatment

The formation of a precipitate-free zones (PFZs) has great influence on fatigue strength, tensile ductility and fracture toughness of titanium alloys. In this paper, the formation process of PFZ and the relationship between PFZ, beta grain boundary and alpha phase in TC18 titanium alloy were investigated through beta phase field deformation and alpha + beta phase field heat treatment. It was found that the formation of PFZ was affected by the misorientation of beta grain boundaries and the morphology of grain boundary alpha phase (alpha(GB)). The thick and continuous aGB phase would precipitate at low angle beta grain boundaries (LAGBs), and led to the depletion of alpha phase stable elements near the beta grain boundaries, which further resulted in the formation of PFZ. At high angle beta grain boundaries (HAGBs), both discontinuous alpha(GB) phase and grain boundary Widmanstatten alpha phase (alpha(WGB)) can precipitate to occupy beta matrix, no obvious PFZ. The pre-deformation in beta phase field can increase the content of HAGBs and promote the formation of discontinuous alpha(GB) phase. In this case, the area of PFZ can be effectively reduced.