Microstructure and mechanical properties of Ti₂AlNb diffusion bonding using multi-phase refractory high-entropy alloy interlayer

This paper conducts diffusion bonding of Ti₂AlNb based alloy, in which a novel refractory high-entropy alloy (Ti₄₀Nb₃₀Hf₁₅Al₁₅; RHEA) was used as the interlayer. The RHEA interlayer is designed to eliminate bond line and restrain the aggregation of orthorhombic (O) phase at bonding interface. The microstructures of joints were investigated by scanning and transmission electron microscopy (SEM and TEM, respectively). Defect-free joints were obtained when bonding was performed in the temperature range of 950–1000 °C at 30 MPa for 2 h. The joint microstructure was mainly composed of a disordered bcc-type solid solution and nanosized basketweave ordered O phase. According to the TEM results, the matrix was rich in Hf, which strengthened the matrix by solid solution strengthening. The tiny O phase retained a specific coherent relationship with the bcc matrix, (001)_O// (011‾)_bcc and [01‾1]_O// [1‾11]_bcc, which strengthened the bonding interface by precipitation strengthening. This multiphase coupling interface indicated reliable metallurgical bonding and guaranteed excellent joint performance. The mechanical properties of the joints were evaluated using nanoindentation and shear tests. The microhardness and Young's modulus were distributed evenly without any noticeable fluctuation and ranged from 5.01 to 5.52 GPa and 103.97–117.22 GPa, respectively, illustrating the suitable property matching between the high-entropy interlayer and Ti₂AlNb. The maximum shear strength of the joint was 463 MPa with bonding at 970 °C and 30 MPa for 2 h. The main crack was significantly deflected into the parent materials, rather than propagating along the interface, which further demonstrated that the bonding face had higher strength than the base metals. The precipitation mechanism of the nanoscale O phase was revealed through transmission Kikuchi diffraction–electron backscatter diffraction (TKD-EBSD). The O phase variants formed with equal probability could lead to a basketweave morphology. The successful bonding of Ti₂AlNb using RHEA as the interlayer provides a new interlayer system to bond Ti-based intermetallic compounds.