The effect of heat treatment sequence on microstructure and mechanical properties of diffusion brazed IN-939 superalloy

Abstract

Conducting the diffusion brazing cycle at 1120 °C for 45 min led to the completion of isothermal solidification and formation of a single-phase Ni-rich γ-solid solution at the joint centerline. Chromium-rich boride precipitates with blocky and acicular morphologies were detected in the diffusion affected zone (DAZ) of all the bonds. The post bond heat treatment of the brazed joint (HT-3 sample) resulted in complete elimination of DAZ boride precipitates and obtaining high levels of homogeneity across the joint region. Performing the brazing cycle between the solution treatment and the double aging cycle (HT-1 sample), as well as after the full heat treatment of the base metal (HT-2 sample), had a minor effect on the morphology of the joint region. While all the heat treatment sequences raised the microhardness values across the bonding regions, the HT-2 sequence reduced the microhardness levels in the isothermally solidified zone (ISZ). The HT-3 sequence improved the joint efficiency up to the remarkable value of 91%. In the as-brazed sample, the fracture path was observed in the ISZ and a dimple-like morphology was detected in the fracture surface. In HT-1 sample, the fracture occurred within the hard DAZ precipitates and a semi-cleavage morphology was detected in the fracture surface.