Four-scale hierarchical α microstructure via ω and α″ synergistic refinement: Overcoming strength–ductility Trade-off in an α/β Ti-alloy

The strength–ductility trade-off remains a central challenge in structural titanium alloys. While heterogeneous microstructure design is a promising solution, existing strategies rely on single metastable phase refinement. Here, we demonstrate a novel paradigm in a Ti-3Al-5Mo-4.5V (wt.%) alloy by synergistically activating dual metastable phase refinement pathways —ω-assisted α nucleation and α″ decomposition— for the first time . This approach successfully fabricates a four-scale heterogeneous α (FSH-α) microstructure, comprising micron-scale primary α_p alongside three distinct secondary α morphologies: micron-scale α_s-fine, nanoscale α_s-ultra, and ladder-like α_s-ladder. Advanced characterization reveals that α_s-ultra forms via ω-assisted nucleation, while α_s-fine and α_s-ladder evolve from the decomposition of α″, with the latter originating from α″ with lattice distortion regions. Compared to the conventional annealed microstructure and two-scale heterogeneous α (TSH-α) microstructure refined solely through ω-assisted α_s-ultra nucleation, the FSH-α structure exhibits a superior yield strength (990–1050 MPa vs. 820–850 MPa and 880–970 MPa) without sacrificing ductility (11–16% elongation vs. 12–15% and 14–18%). This enhancement stems from hetero-deformation induced (HDI) strengthening due to a multi-tiered network of hetero-interface, where plastically deformable α_s-fine domains act as mechanical buffers, generating additional HDI stress while coordinating strain to maintain ductility. This work establishes a transformative strategy for designing hierarchical heterostructures by harnessing the synergy of multiple phase transformation pathways to overcome property trade-offs in α/β titanium alloys.