Microstructure and mechanical properties of the brazed region in the AlCoCrFeNi high-entropy alloy and FGH98 superalloy joint

In this study, brazing of Al_0.3CoCrFeNi high-entropy alloy to FGH98 superalloy was achieved using a BNi-2 filler metal. The microstructure of the Al_0.3CoCrFeNi/FGH98 joint was examined, and mechanical properties of the joints brazed at different brazing temperatures were evaluated. During the brazing process, boron diffused from the filler alloy into the Al_0.3CoCrFeNi substrate, and mainly reacted with Cr to form the Cr-rich boride exhibiting Cr₅B₃ structure. Eventually, the reaction zone was composed of a Cr-rich boride and Ni-rich HEA matrix with a coherent relationship of [0 1 1]_{Ni-rich HEA}//[2 4 0]_{Cr-rich boride} and (2 0 0)_{Ni-rich HEA}//(0 0 2)_{Cr-rich boride}. The dissolution of the FGH98 substrate induced the island-like (γ + γʹ) phase in the brazing seam. With the brazing temperature increasing, the voids and borides disappeared, and finally the brazing seam transformed into homogeneous Ni(s.s). Moreover, when the brazing temperature reached 1090 and 1110 °C, the volume of intergranular Cr-rich borides increased significantly, and the precipitation of carbides (or borides) in the FGH98 substrate adjacent to the brazing seam was dispersive. The optimum shear strength of 454 MPa was obtained when the brazing was carried out at 1070 °C for 10 min, and ductile fractures occurred in the joint during the shear test.