First melting stages during isothermal brazing, of Ni/BNi-2 couples

Ni/BNi-2 brazing couples were continuously heated at 25 °C/min and their melting temperature range was determined by differential thermal analysis (DTA). Samples were cooled down from different temperatures in the melting domain. Their microstructures were observed and quantitatively characterized through image analysis. Melting proceeds in two steps: The first one (lower temperatures) involves the filler metal being only partially melted, and boron diffusion in pure Ni leading to the precipitation of fine Ni3B borides at the initial interface between the filler alloy and the base metal. In the second step (higher temperatures), melting completes and thermodynamic equilibration requires significant dissolution of nickel, which also involves the dissolution of the borides already formed. Afterwards, transient liquid bonding can proceed as described in the usual models, involving re-precipitation of Ni3B borides on the new fresh interface between liquid alloy and solid metal. High heating rates allow avoiding the first step (direct melting of brazing alloy then dissolution of base metal), whereas very low heating rates can result in brazing without significant dissolution of base metal (no second step).