The Role of Base Metal Chromium in Determining the TLPB Behavior of Ni-Based Alloys Using a Boron-Containing Braze

The TLPB behavior of Ni/Ni-Si-B and Ni-Cr-Fe/Ni-Si-B base metal/braze joints was examined to determine the role chromium in the base metal has during joining of Ni-based alloys using a boron-bearing braze metal. The isothermal solidification (IS) rate in these two braze joints at 1075 °C was measured using DSC. Microstructural evolution within the diffusionally affected zone (DAZ) was examined both in terms of the type of borides formed and their volume fraction, using image analysis and phase equilibria predictions. Overall, the results show that when the base metal (BM) contains an appreciable concentration of Cr the IS rate decreases significantly. This was due to the formation of a low volume fraction of CrB within the DAZ, dictated by phase equilibria. Conversely, in the pure Ni base metal (BM) the thermodynamically favored boride was Ni₃B, present at high volume fractions, resulting in rapid IS. A direct link between the boride microstructure of the DAZ and the rate of IS was established.