Tuning α_s precipitation behavior via introducing sub-grain boundaries to significantly enhance the ductility of a high-strength metastable β titanium alloy

To overcome the brittleness in high-strength metastable β titanium alloys, a novel thermomechanical processing route combining hot rolling and dual-step aging was applied to a Ti-4Al-5Mo-5V-5.5Cr-1Nb alloy. By combining hot rolling with a dual-step aging treatment, a high density of sub-grain boundaries and low-angle grain boundaries were introduced, effectively partitioning the prior β grains. Coupled with the synergistic effects of embryonic ω phase and spinodal decomposition, this approach enabled the uniform precipitation of nanoscale secondary α phase (α_s) both near grain boundaries and within grain interiors. The resulting homogeneous microstructure successfully suppressed strain localization and intergranular fracture. Consequently, the processed alloy achieved an exceptional combination of an ultra-high tensile strength of 1648 MPa and a significantly enhanced ductility of 7.1 % elongation. This work provides a new pathway for designing metastable β titanium alloys with superior strength-ductility synergy.