Microstructure and mechanical response of the DD5 single crystal and FGH98 superalloy joint diffusion bonded using Al_0.3FeCoNiCr high entropy alloy interlayer

In this study, the DD5 single crystal and FGH98 superalloy were diffusion bonded using Al_0.3FeCoNiCr high entropy alloy interlayer (HEA interlayer). The microstructure evolution and mechanical property of the joint were investigated in detail. During the diffusion bonding process, the C, Ti, and Ta released by the FGH98 and HEA interlayer reacted to form carbide at the FGH98/HEA interlayer interface, achieving reliable interfacial bonding. The concentration gradient at the HEA interlayer/DD5 interface promoted the Ni and Al to diffuse from the DD5 to the HEA interlayer, inducing the decomposition of the γ′ phase in the DD5 substrate adjacent to the joining interface. The diffusion zone comprised of tiny γ′ and residual γ’ bulk formed at the HEA interlayer/DD5 interface. With the increase of the diffusion bonding temperature, the diffusion of elements at the interface was sufficient. The carbide at the FGH98/HEA interlayer interface grew into a continuous line, and the diffusion zone at HEA interlayer/DD5 interface thickened significantly. The maximum shear strength was 541 MPa when the DD5 and FGH 98 were diffusion bonded at 1130 °C for 1 h. The fracture initiated from the carbide and propagated in the FGH98/HEA interlayer interface.