Effect of bonding temperature on the microstructure and mechanical properties of Hastelloy X superalloy joints bonded with a Ni–Cr–B–Si–Fe interlayer

The effect of the bonding temperature during transient liquid phase (TLP) bonding of Hastelloy X using temperatures below (1070 °C), within (1110 °C), and above (1160 °C) the range of eutectic temperatures of the binary Ni–B was studied. Isothermal solidification was completed for all samples after 40 min of thermal treatment; this was in good agreement with the times predicted from an analytical model. Electron probe microanalysis displayed that the isothermally solidified zone (ISZ) was composed of a solid-solution of a Ni-rich γ phase. However, (Cr, Mo)-rich borides with various morphologies were observed in the diffusion-affected zone (DAZ). A Ni-rich boride and binary Ni–Si eutectic were also detected in a centerline of the sample bonded at 1070 °C for 5 min. Increasing the bonding temperature to 1160 °C resulted in a decrease in the volume fraction of borides and a change to rounder particles, accompanied by an increase in the uniformity of the hardness profile. The best shear test results were observed for the specimen bonded at 1160 °C; this was attributed to a low density of DAZ borides and effective solid-solution strengthening. The joint efficiencies of samples bonded at 1070, 1110, and 1160 °C were about 88%, 82%, and 89%, respectively. Chromium, silicon, and tungsten alloying elements had the maximum contribution to solid-solution strengthening of the ISZ. Cellular fracture with both the brittle and ductile characteristics was observed when analyzing the fracture surfaces of shear test specimens. The fractures for all joined samples originated close to hard and brittle DAZ precipitates. Therefore, improved mechanical properties are expected to be achieved by reducing the volume of DAZ precipitates.