Abstract: Ultra-high strength titanium alloys have substantial application potential, but the match of strength-ductility-toughness has consistently been a bottleneck restricting their application. In this work, the fork + multi-scale structure was developed using a three-step heat treatment method for TB17 alloy. The fork + multi-scale structure encompasses the forked structure, mesoscale α lamellae and secondary α phase, such a structure exhibits unparalleled match of strength-ductility-toughness with tensile strength of 1378 MPa, elongation of 8.6 % and fracture toughness of 81 MPa m^1/2. The formation of the forked structure is attributed to the combined action of epitaxial growth, connection elongation and merge coarsening. The forked structure can activate a large number of <a>/<c + a> dislocations and has a remarkable capacity for strain hardening. In comparison with the conventional microstructure, the forked structure offers space for the precipitation of mesoscale α lamellae and secondary α phase without compromising the crack propagation resistance. Put simply, it manages to elevate the strength while maintaining the toughness. Meanwhile, the forked structure, mesoscale α lamellae, and secondary α phase jointly form a multi-scale structure, which has a similar function to gradient-structured materials. Non-uniform deformation occurs in different scales of microstructures under constraints, additional geometrically necessary dislocations (GNDs) are generated, achieving simultaneous enhancement of the strength and ductility. Additionally, the coordinated deformation capability of the fork + multi-scale structure effectively augments the size of the plastic zone of the crack tip. The forked structure can effectively deflect crack propagation, exhibiting a robust GND storage capacity. The toughness is dramatically enhanced by the fork + multi-scale structure. This novel microstructure provides an effective approach to address the application of ultra-high strength titanium alloys.

Key words: Ultra-high strength titanium alloy, Forked structure, Multi-scale microstructure, Gradient effect, Strength-ductility-toughness