Microstructure evolution, interfacial bonding and surface energy mechanism of Ti₂AlNb/Ti60 brazed joint with Cu/Ni-free Ti-Co-Fe-Sn amorphous filler

Reliable brazing of Ti₂AlNb to Ti60 alloy is critical for advanced hypersonic vehicles but remains challenging due to the formation of brittle intermetallics containing Cu and Ni. A novel Cu/Ni-free Ti-25Co-8Fe-5Sn (at. %) amorphous filler, developed independently, was compared with conventional Ti-Zr-Cu-Ni-based fillers. Multiscale characterization combined with first-principles calculations revealed the microscopic origins of performance differences between the joints. The Ti–Co–Fe–Sn amorphous filler exhibits an exceptionally low surface energy (0.0570 eV/Ų), which reduces the thermodynamic barrier for wetting and spreading, enabling defect-free joints with only 20–30 °C of superheat. Microstructural analysis reveals a “tough α-Ti matrix + discretely distributed bulk Ti₃Sn and strip-like Ti₂(Co, Fe) hard phases” architecture. The key α-Ti/Ti₃Sn interface was semi-coherent (lattice mismatch 21.1%) and achieved strong bonding through intense Ti-d and Sn-d orbital hybridization, with an interfacial separation work of 0.3248 eV/Ų, 5.8% higher than that of the reference interface. Consequently, the Ti-Co-Fe-Sn brazed joint achieved a room-temperature shear strength of 247.7 MPa, ∼46% higher than conventional joints, along with significantly improved energy absorption capability. This study provides a design strategy and theoretical foundation for developing novel filler systems for high-performance titanium alloy brazing.