Morphology evolution and precipitation behavior of in-situ borides in dissimilar brazed Superalloy joints and their influence on strength-ductility

To reduce boride content and improve control over in-situ boride morphology for enhanced strength–ductility balance of brazed joints, a NiPd-based amorphous brazing filler metal (NP-ABFM) was employed to braze dissimilar superalloys Inconel 718 and Inconel 625. Through experimental characterization and modeling analysis, the roles of granular, needle-like, short rod-like, and continuous grain-boundary borides in joint performance were examined. Dispersed intragranular borides strengthened the matrix with limited ductility loss, whereas continuous grain-boundary borides could reduce tensile shear strength, with a maximum decrease of 80 MPa. By optimizing the brazing time at 1060 °C, continuous grain-boundary borides were effectively suppressed while dispersed borides within the isothermal solidification zone (ISZ) were promoted at brazing durations of 15 and 30 min, yielding an ultimate shear strength of 552 MPa and an elongation of 37.5%, with fracture occurring in the Inconel 718 base metal (BM), comparable to that of the original BM. The effects of brazing time on elemental diffusion, boride precipitation, and joint performance were systematically analyzed, and the formation tendencies of intergranular versus intragranular borides were quantified, revealing a critical B concentration for boride formation: negligible below 0.23%, preferential at grain boundaries at 0.23%, dispersed within grains above 0.59%, and coexistence within 0.23–0.59%.