Enhancement of mechanical properties in Ti₂AlNb/Ti60 brazed joints via Nb foam-induced in-situ formation of tough Ti₄Nb phase

Brazing titanium alloys with TiZrCuNi filler typically leads to numerous eutectic structures and brittle intermetallic compounds, compromising the mechanical properties of joints, particularly their toughness. To mitigate this issue, this study employs Ni and Nb foams as interlayers to join Ti₂AlNb and Ti60 alloy. The results show that using Ni foam as the interlayer only forms 17.8 % β-Ti phase in the brazing seam. In contrast, Nb foam as the interlayer promotes the in-situ formation of 38.5 % Ti₄Nb phase within the brazing seam, effectively balancing the coefficient of thermal expansion difference and reducing residual thermal stresses. Comparative analysis reveals that the Ti₄Nb phase has superior plasticity and toughness compared to the β-Ti phase, allowing for substantial strain energy storage. The interface between β-Ti phase and Zr₂Cu phase exhibits significant lattice mismatch, resulting in an incoherent interface. Conversely, the Nb foam interlayer produces a semi-coherent interface between Ti₄Nb and Zr₂Cu phases, characterized by reduced lattice mismatch, which enhances the interfacial bonding strength of the brazed joint. The Ti₂AlNb/Ti60 joints, when brazed with Nb foam under consistent conditions, achieved a shear strength of 414.6 MPa, which is approximately 18 % superior to that of joints brazed using the Ti-36.5Zr-10Ni-15Cu-0.5Co-0.5Nb amorphous filler solely. Additionally, the shear stress-strain curves of the joints with Nb foam exhibit a more pronounced yield stage compared to those with only filler. This study introduces a novel approach for improving the toughness of brazed joints in practical applications using titanium-based fillers.