Transient liquid phase bonding of Inconel 625 with Mar-M247 superalloy using Ni–Cr–B interlayer: Microstructure and mechanical properties

The transient liquid phase (TLP) bonding of Inconel 625 with Mar-M247 superalloy was performed using Ni–Cr–B interlayer material. The effects of bonding time and temperature on the microstructure evolution and mechanical properties of the joints of the dissimilar superalloys were investigated. Results showed that the precipitated phases in the diffusion affected zone (DAZ) on both sides of the joint were M5B3- and M3B2-type borides on the grain boundary and needle-like M5B3-, granular M5B3-, and M3B2-type borides in the grain. In addition, granular HfTaC2 and fine γ′ particles precipitated in the isothermally solidified zone near the Mar-M247 matrix. The non-isothermally solidified zone (NSZ) was composed of Mo–Cr–Nb-enriched borides and Nb-rich nickel-based γ solid solution. The formation mechanism and the asymmetry caused by the differences in grain size and element composition of the TLP-bonded joint were discussed accordingly. The mechanical properties of the joints were found to be closely related to the number, size, and location of the precipitated borides. Given the high hardness and poor plastic deformation resistance, the considerable amount of large borides in the NSZ and DAZ became the preferential source of cracks and led to the decrease of the mechanical properties of the joints. The maximum tensile strength and elongation of 865 ± 11.4 MPa/42.2% were achieved when the joint was bonded at 1150 °C/60 min, at which point isothermal solidification completely occurred and only the discontinuous block or granular borides with an area fraction of 17.2% remained in the DAZ on the Inconel 625 side. Specimen fracture occurred within the Inconel 625 base metal.