Behzad Binesh

In this study, diffusion brazing was employed to join IN718 superalloy and AISI 316L ‎stainless steel using MBF-20 amorphous interlayer. The effect of interlayer thickness (50-150 ‎‎µm) was studied on the microstructure and mechanical properties of the joints. The joints ‎were characterized by using optical and scanning electron microscopes, energy dispersive ‎spectroscopy and X-ray diffraction analysis. Microhardness and shear tests were used to ‎assess the mechanical properties of the joints. The microstructural investigations revealed that ‎eutectic phases including γ solid solution, Ni3B, CrB, Cr2B3 and Ni6Si2B were formed at the ‎joint centerline of the sample with incomplete isothermal solidification during holding at the ‎brazing temperature. Fine Ni3Si particles also precipitated via a solid state transformation ‎during cooling in the vicinity of eutectic phases. Mechanism of microstructure evolution and ‎the solidification behavior of the centerline intermetallic compounds were discussed. In ‎addition, Cr-, Mo-, Nb-rich borides precipitated in the DAZ of IN718 side and the ‎precipitates formed in the DAZ of AISI 316L side were Cr-, Mo-rich and Fe-, Cr-rich ‎borides. The maximum shear strength of 440 MPa was obtained for the sample with complete ‎isothermal solidification. Fractography of the samples after the shearing test indicated that the ‎sample with complete isothermal solidification exhibits a ductile failure mode, however a ‎brittle fracture happens in the samples containing hard eutectic constituents at the joint ‎centerline.‎