D. S. Grummon, Michigan State University, East Lansing, MI; K. B. Low, University of Illinois at Chicago, Chicago, IL; J. Foltz, The Ohio State University, Columbus, OH; J. A. Shaw, The University of Michigan, Ann Arbor, MI
We have recently demonstrated the ability to join Nitinol components successfully using a novel brazing technique. The technique employs niobium as the braze filler, which reacts with the parent NiTi above 1170°C in high-vacuum to form a braze liquid that eventually solidifies predominantly into eutectic micro-constituents. The braze liquid is self-fluxing as it readily dissolves any native oxide present on the NiTi surfaces, and wets the mating interfaces efficiently. Nitinol components produced by this brazing method possess remarkably high tensile strengths at the joints without severely compromising the functional properties of the parent NiTi alloys after post-braze annealing. This work aims at understanding the driving force governing the contact melting between NiTi and Nb by reviewing the Ni-Ti-Nb ternary phase equilibria. The kinetics associated with the brazing was investigated through a time-resolved series of contact melting at the NiTi-Nb interface, where their respective microstructures were observed ex-situ by scanning electron microscopy. The Ni-Ti-Nb ternary phase diagrams indicate that the driving force for contact melting stems from the need for the NiTi and Nb melting fronts to maintain their respective chemical equilibria dynamically with the braze liquid. A one-dimensional diffusion-based numerical model was successfully established to describe the melting process. Results from simulation suggest that the diffusion-controlled melting is rate-limited by Nb diffusion across the liquid.
Summary: The driving force and kinetics governing the contact melting between NiTi and Nb during the reactive eutectic brazing of Nitinol were understood through the Ni-Ti-Nb phase equilibria, experimental observations and numerical modeling of the melting sequence.
