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

Abstract

The formation of α 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, β grain boundary and α phase in TC18 titanium alloy were investigated through β phase field deformation and α + β phase field heat treatment. It was found that the formation of PFZ was affected by the misorientation of β grain boundaries and the morphology of grain boundary α phase (αGB). The thick and continuous αGB phase would precipitate at low angle β grain boundaries (LAGBs), and led to the depletion of α phase stable elements near the β grain boundaries, which further resulted in the formation of PFZ. At high angle β grain boundaries (HAGBs), both discontinuous αGB phase and grain boundary Widmanstätten α phase (αWGB) can precipitate to occupy β matrix, no obvious PFZ. The pre-deformation in β phase field can increase the content of HAGBs and promote the formation of discontinuous αGB phase. In this case, the area of PFZ can be effectively reduced.