Reactive diffusion in Ni-Ni2Al3 laminated structures

The sequence and kinetics of intermediate layer growth have been explored in Ni-Ni2Al3 laminated structures with the characteristic dimension of several microns, annealed in the temperature range 550 to 750 degrees C. It has been found that all phases appear in the interdiffusion zone that are thermodynamically stable at the diffusion temperature; these phases are NiAl, Ni3Al and Ni5Al3 (below 690 degrees C). The formation of Ni5Al3 is outstanding since this phase has never been previously observed in diffusion couples, and its nucleation in the course of reactive diffusion was believed factually impossible. Sequential formation and growth of two separate Al- and Ni-rich NiAl layers has been observed in the studied laminated structure, in contrast to the earlier reported and theoretically predicted growth of a single NiAl layer with a sharp S-shaped concentration profile. The peculiarities of the reactive diffusion process in the laminated structures are discussed in terms of kinetic instability induced by interfacial reactions.