Application of Diffusion Path Analysis to Understand the Mechanisms of Transient Liquid-Phase Bonding in the Ni-Si-B System

Diffusion path analysis was applied to the Ni-Si-B system to explain the deviation from classical transient liquid-phase bonding observed when using multicomponent systems containing Boron as a melting point depressant. This was achieved using a combination of differential scanning calorimetry, SEM-EDS microstructural and chemical analysis, and Thermo-Calc modeling software of Ni/Ni-Si-B couples. Compositional analysis identified differing distributions of B and Si across the braze joint which were mapped onto isothermal sections of Thermo-calc-generated phase diagrams. Below 1093 degrees C, the inevitable formation of diffusionally affected zone (DAZ) and isothermally solidified zone is a direct consequence of the need for phase equilibria in the Ni-Si-B ternary system and is predicted by diffusion path analysis. Above 1100 degrees C, diffusion path analysis also correctly predicts a persistent liquid phase observed to occur through the melting of the DAZ (i.e., a gamma-Ni(Si)-Ni3B eutectic reaction).