Hierarchical Twin Networks Enable Exceptional Strength and Fracture Toughness in Titanium

Hexagonal close-packed (HCP) metals such as titanium are widely used in aerospace applications, where simultaneously achieving high specific strength and fracture toughness is essential for operational safety. However, their intrinsically low symmetry crystal structures and the adverse effects of conventional toughening methods have long imposed a trade-off between strength and fracture toughness. Here, we demonstrate that introducing pre-engineered hierarchical twin networks endows commercial-purity HCP titanium with an exceptional fracture toughness (K_JIc) of ∼187 MPa·m^1/2 and a specific yield strength of 140 MPa·cm³·g⁻¹, its overall performance is superior to most titanium alloys and even some benchmark high-toughness metals at room temperature. The activation and multiplication of <c+a> pyramidal dislocations, typically difficult in titanium, are remarkably promoted by the pre-engineered high-density twin network. The resultant profuse <c+a> and <a> dislocation activities substantially enhance crack-tip plastic flow and suppress damage nucleation. This strategy is further validated in commercial-purity HCP zirconium, achieving nearly a twofold improvement in both fracture toughness and yield strength. Our findings establish twin-network engineering as a powerful microstructural design strategy for developing high-performance HCP metals for safety-critical applications.