Vacuum diffusion bonding of TC4 titanium alloy to T2 copper with VCrAl_1.21Ni_0.93Co_1.85 high entropy alloy interlayer

VCrAl_1.21Ni_0.93Co_1.85 eutectic high entropy alloy interlayer was designed based on the pseudo-binary strategy and design concept of eutectic high entropy alloy, which was used to join TC4 titanium alloy and T2 copper via vacuum diffusion bonding. The influence of bonding temperature on the microstructure and mechanical property of TC4/T2 diffusion bonded joint was investigated. The results showed that all the joint were well combined without defects. TC4 titanium alloy adjacent to the diffusion layer underwent a phase transition from α-Ti to β-Ti, forming part of the Widmanstatten structure. Ti₂(Co, Ni), Ti₂(V, Cr, Al), Ti(V, Cr, Co) and (V, Cr)(Al, Ni, Co) phases appeared in the TC4/VCrAl_1.21Ni_0.93Co_1.85 interface while (V, Cr)(Al, Ni, Co) and (V, Cr)(Cu, Ni, Co)₃ phases formed in the VCrAl_1.21Ni_0.93Co_1.85/T2 interface. The high entropy microstructure formed in the interface, which played the role of high entropy solution strengthening. The growth activation energy of diffusion layer I and II was 229 kJ/mol and 191 kJ/mol, respectively. The maximum shear strength of joint at 880 ℃ reached 194 MPa. The joint fracture was mainly along the BCC matrix phase and B2 phase alternately, and the fracture surface had the feature of river pattern, which belonged to a typical cleavage fracture mode.