Simulation of magnetron-sputtered Ni/Ti thin films and effect of annealing

Nanometric metallic multilayers of nickel (Ni)/titanium (Ti) were obtained by alternate deposition of nickel and titanium by magnetron sputtering. Subsequently, the multilayered films were annealed at various temperatures with the goal of obtaining nickel/titanium shape memory thin films. The annealing was accompanied by interdiffusion, amorphization and precipitation of various intermetallic phases. Classical molecular dynamics (MD) simulations were performed for the atomic diffusion during annealing of the nickel/titanium multilayer by using embedded atomic method interaction potentials. The transition to the amorphous structure can be understood in terms of the distortion of the crystal structure by removal of several atoms to create vacancies. MD simulation showing angular diffusion at the interface was clearly observed in the cross-sectional transmission electron microscopy micrograph. It is observed that there is continuous diffusion of nickel into titanium due to atomic displacement; the amorphous phase will continue to form. Since the diffusivities of amorphous structures are many orders of magnitude higher than that of their crystalline counterparts, a path where diffusion has already taken place becomes a zone of high diffusivity.