Achieving ultrahigh strength and toughness in as-cast Ti-7.5Mo-4Al-4Zr-3Nb-2Cr-1Fe alloy through the formation of nanotwins

Formation of nanotwins (NTs) is an effective strategy to simultaneously improve the strength and toughness of metallic materials. However, the formation of NTs within the secondary α (α_s) phase of as-cast and heat-treated titanium (Ti) alloys is difficult due to their intrinsically high stacking fault energy (SFE). In this study, NTs are successfully introduced into the α_s phase of Ti alloys for the first time by precisely tuning the Mo content to tailor the SFE and phase transformation stress to regulate the thermodynamics and kinetics conditions. First-principles calculations reveal that Mo doping reduces SFE and promotes perfect dislocation dissociation into extended dislocations, which creates thermodynamically favorable conditions for twin nucleation. Meanwhile, the localized high internal stress from β → α_s phase transformation provides the kinetic driving force for twinning. As a result, typical lens-shaped NTs with an average width of approximately 21 nm and a volume fraction of 7.4% are successfully activated within the α_s phase in the 7.5Mo alloy. Benefiting from the higher proportion and the nanoscale, the alloy exhibits an ultrahigh tensile strength of 1358 MPa and a fracture toughness of 84 MPa m^1/2, realizing an excellent strength-toughness synergy. This study provides a new strategy for the development of high-performance as-cast Ti alloys through NTs-induced simultaneous strengthening and toughening.