Microstructure Evolution and Strengthening–Toughening Mechanism of TC4 Titanium Alloy With Minor Nb and Ni Additions During Posthot‐Rolling Heat Treatment

This study systematically compared the microstructural evolution and mechanical properties of TC4 titanium alloy and a new TC4‐0.5 wt.% Nb‐0.5 wt.% Ni (TC4‐NbNi) titanium alloy in three conditions (hot‐rolled, single‐annealed, and double‐annealed). The results show that the addition of trace amounts of niobium and nickel enhances the stability of the β phase, raising the β‐phase transformation temperature to 986 °C. In the hot‐rolled and single‐annealed conditions, these trace elements significantly refined the spacing of the α lamellar phase. After double annealing, the TC4‐NbNi alloy formed a bimodal microstructure consisting of an interlaced primary equiaxed α phase and a fine, strip‐like secondary α phase. Ni and Nb are enriched in the β phase and exist primarily in a solid‐solution state, resulting in a high density of intragranular dislocations and substructures. Mechanical testing indicates that, thanks to solid‐solution strengthening, microstructural refinement, and the bimodal structure formed by dual annealing, the TC4‐NbNi alloy achieves an excellent balance between strength and toughness. The tensile strength of the hot‐rolled alloy is 1021 MPa, and its impact toughness is 44.9 J; after dual annealing, the tensile strength and yield strength are further increased to 1052 and 993 MPa.