Microstructure Evolution during Heat Treatment of Cold-Sprayed Ni-Ti Composite Coatings

Direct deposition of intermetallic compounds, such as NiTi, is a major challenge in cold spraying, because of the relatively high hardness and brittleness of these materials. On the other hand, intermetallic deposits can be obtained by heat treatment of cold-sprayed mixtures of elemental powders. Following this method, intermetallic phases were obtained by allowing diffusive reactions between particles of nickel and titanium. The fraction of different Ni-Ti intermetallic phases and the sequence of phase formation were found to be temporally variable and strongly depend on the selected annealing temperature. The results indicate that microstructure evolution is also influenced by the preceding severe deformation of particles. Concurrent recrystallization and interdiffusion as well as prominent diffusion through defects can promote formation of intermetallic phases at unexpected locations within the diffusion couple. The results show that cold spraying and annealing treatments may be tuned for optimal intermetallic and microstructure formation.