A novel dual-phase high strength titanium alloy fabricated by laser directed energy deposition and duplex annealing

The strength-ductility trade-off is the primary challenge currently faced by additively manufactured high-strength titanium alloys, which stems from the inherent drawbacks caused by the massive addition of β-stabilizing elements. Targeting laser direct energy deposition technology, this work proposes a novel high-strength dual-phase titanium alloy Ti-6.7Al-2Zr-4Mo-1Fe-1Cr with a low addition of β-stabilizing elements. A multiscale precipitates microstructure consisting of micron-sized primary α phases (α_p) with fork-like morphology, nano-sized secondary α phases (α_s) with bimodal distribution, and retained β phases forms via duplex annealing, achieving an excellent combination of ultimate tensile strength up to 1313 MPa and elongation of 10.8%. The strength-ductility synergy is closely related to its unique microstructure characteristics. The α_p phase with high Al content significantly improves strain compatibility through sequentially activated multiple slip systems and FCC phase transformation, thereby promoting uniform plastic deformation. The bimodal distribution of α_s phase further optimizes the strength-ductility balance: on the one hand, fine α_s mainly contributes to high strength through interface strengthening; on the other hand, relatively coarse α_s can provide channels for plastic deformation, synergistically enhancing ductility. This work demonstrates the advantages of low molybdenum equivalent dual-phase titanium alloys in breaking through the strength-ductility trade-off.