Diffusion Brazing Metallurgy of IN718/Ni-Cr-Si-B-Fe/IN718

This paper investigates the effect of microstructure development on mechanical properties of diffusion brazed IN718 nickel-based superalloy using a NiCr-Si-B-Fe filler metal. The phase transformations during diffusion brazing of IN718/Ni-Cr-Si-B-Fe/IN718, which dictate the microstructure of the bonds are governed by diffusion-induced isothermal solidification, cooling-induced athermal solidification, and diffusion-induced solid-state precipitation. It was found that when partial isothermal solidification occurs at the bonding temperature, the residual liquid is transformed into eutectic-type microconstituents. Considering solute redistribution and segregation behavior of the melting point depressant elements (Si and B), the solidification behavior of the liquid phase is discussed. Moreover, extensive Cr-Mo-Nb-rich boride precipitates were formed in the substrate region due to solid-state boron diffusion into the base metal during the bonding process. Increasing bonding time resulted in progress of isothermal solidification and an intermetallic-free joint centerline was obtained after 40 min holding at 1050 degrees C. Mechanical properties of the joints are described using hardness profile and room-temperature shear testing. It was found that the presence of a continuous network of intermetallic-containing eutectic-type microconstituents in the joint centerline acts as the preferential failure source and leads to a semi-cleavage brittle fracture with low load-bearing capacity. Completion of isothermal solidification, which guarantees the formation of a ductile single-phase solid-solution joint centerline, improves the shear strength of the bond up to 75% of the base metal.