Breaking the strength-ductility trade-off in metastable β21s alloy via high silicon content and heterogeneous lamellar architecture

The metastable β21S titanium alloy faces significant challenges in practical applications due to its insufficient yield strength and restricted uniform elongation. While silicon addition has proven effective in enhancing mechanical properties of titanium alloys, conventional wisdom restricts Si content to ≤0.5 wt.% to avoid embrittlement from coarse silicide formation. This study challenges this paradigm through innovative alloy design, incorporating 0.9 wt.% Si combined with isothermal treatment and hot extrusion to create a heterogeneous lamellar structured (HLS) β21S-Si alloy. Our approach achieves dual microstructural control: isothermal pretreatment induces ∼10 nm nanowire silicide precursors that refine final precipitates to 230 nm (from 700 nm in conventional processing), while subsequent extrusion disrupts continuous grain boundary silicides and constructs a well-defined heterogeneous lamellar architecture comprising recrystallized and substructured lamellae. The optimized HLS β21S-Si exhibits remarkable mechanical performance, demonstrating a 1035 MPa yield strength (10% enhancement) and 12% uniform elongation (8 × improvement) compared to baseline β21S. Multiscale characterization combining SEM-DIC and first-principles calculations reveals a unique sequential work-hardening mechanism: heterogeneous deformation-induced (HDI) hardening dominates early stages, followed by silicon-promoted cross-slip activity, culminating in stress-induced ω phase transformation during advanced deformation. This synergistic interplay of microstructure-engineered deformation mechanisms establishes a new pathway for overcoming the persistent strength-ductility trade-off in metastable β-Ti alloys, with significant implications for aerospace applications demanding high-performance structural materials.