Isothermal solid–liquid transitions in the (Ni,B)/ZrB₂ system as revealed by sessile drop experiments

In the framework of joining processes of ultrahigh temperature ceramics (UHTCs), sessile drop experiments were performed in the Ni–B/ZrB₂ system in the range 1110° ≤ T ≤ 1200 °C. They show that, at temperatures between 1110 and 1150 °C, isothermal solid–liquid transitions occur in a sequence; while in fact at T ≥ 1200 °C, the drop melts without any further phase transition, at lower temperatures, complete melting is followed by a solidification stage and final remelting. This complex behavior, which can be very relevant when utilizing Ni–B alloys for brazing processes (e.g., by the transient liquid phase bonding technique), is successfully interpreted on the basis of the complete B–Ni–Zr phase diagram newly computed by CALPHAD: Isothermal sections, calculated between 1110 and 1150 °C, show that the composition of the drop enters, crosses, and leaves the primary solidification region of the Zr₂Ni₂₁B₆ ternary compound. The use of thermodynamic modeling for the explanation of experimental data sets a link between wetting and joining experiments and phase diagrams assessment.