Grain-Refined Metastable Β Titanium Alloy with Suppressed Ω-Assisted Nucleation and Enhanced Mechanical Properties

The mechanical characteristics of metastable β titanium alloys are related to the α precipitates that form during aging. Although precipitation behavior can be modulated by ω-assisted nucleation, the influence of grain size (GS) on this process has not been fully considered. In this study, the effects of grain refinement on mechanical properties and ω-assisted nucleation were investigated by aging the Ti-5.3Cr-4.9Mo-4.9 V-4.3Al-0.9 Nb-0.3Fe (TB18) alloy with coarse grains (87 μm) or fine grains (25 μm) at different heating rates (RH). It was found that ω-assisted nucleation depends on the RH, with a lower RH promoting the processes of ω-assisted nucleation and the precipitation of finer secondary α (αs) plates. The vacancy concentration after annealing reduced with decreasing GS, indicating that ω-assisted nucleation is suppressed by grain refinement. Additionally, grain refinement significantly improved the ductility of the TB18 alloy. This effect was attributed to enhanced homogeneous plastic deformation and an increase in slip systems, such as pyramidal slip on αs plates. At ultimate tensile strengths of 1300–1550 MPa, the elongation of the fine-grained specimens was typically 2–3 times greater than that of the coarse-grained specimens. Furthermore, the corresponding mixed fracture mode involved a greater extent of ductile fracture. Therefore, the mechanical properties of metastable β titanium alloys are primarily limited by coarse β grains.