Microstructure development during isothermal brazing of Ni/BNi-2 couples

Heat treatments of the Ni–BNi-2 assemblies were performed at 1050 °C, for different brazing times, in a Differential thermal analysis apparatus. The nature, chemical composition and crystallography of the involved phases were identified. Limited predictive capability of Thermo-Calc software with the TTNI7 data base (developed for Ni-based alloys) was pointed out. The microstructure evolution with brazing time was quantitatively analyzed, and used to discuss the mechanism of isothermal brazing and further solidification during cooling. The fraction of dissolved base metal was found proportional to the initial thickness of brazing material. This suggests that the composition of liquid stays homogeneous with a precise initial equilibrium composition during the brazing process. Finally a specific analysis of DTA curves is proposed to estimate the brazing kinetics.