The high density of interfaces, lattice defects and point defects in miscible metallic multilayer thin films synthesized by physical vapor deposition induce kinetics and chemical reactions that are far from equilibrium conditions. Specifically, very thin individual layer thicknesses (typically d<10 nm) having superlattice structures demonstrate diffusion coefficients that are several orders of magnitude higher than multilayers with larger layer thicknesses (typically 50<d<300 nm), leading to unique phase formations by thermal treatment [1,2]. The mechanisms for mass-transport and phase formation in nanocrystalline materials still remain unclear, where developments of superlattice structures are of great industrial interest .
We present here an extensive atom probe tomography (APT) analysis on epitaxial Ag/Al multilayers with 10 nm bi-layer periodicity where interdiffusion and phase transformations are investigated as a function of deposition conditions and film composition. Ag-Al exhibits a simple phase diagram and identical lattice structures of the parent phases, offering an ideal model system for studying interdiffusion and nucleation characteristics in such structures. APT technique allows substantial spatial resolution and chemical sensitivity to quantify concentration gradients and atomic exchange along reactants with such sizes of a few nanometers. Transmission electron microscopy, differential scanning calorimetry and X-ray diffraction techniques were also implemented. The correlative analysis displayed formations of intermediate metastable phases and supersaturated solid solutions during annealing which are highly dependent on the film composition .
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