Nucleation and growth during reactions in multilayer Al/Ni films: The early stage of Al₃Ni formation

Solid-state interdiffusion reactions at Al/Ni interfaces in multilayer films have been studied using differential scanning calorimetry, cross-sectional transmission electron microscopy/microanalysis, and thin-film x-ray diffraction. Multilayer films with various modulation periods and an overall atomic concentration ratio of three Al to one Ni were prepared by alternate electron-beam evaporation in high- and ultrahigh-vacuum systems. We show calorimetric, microstructural, and compositional evidence that interdiffusion of Al and Ni leading to solid solutions precedes the formation of intermetallic crystalline compounds. Isothermal calorimetry indicates that Al3Ni subsequently nucleates in the interdiffused region at preferred sites. Calorimetric analyses also suggest that nucleation sites quickly saturate in the early stage of Al3Ni formation and that the nucleation site density strongly depends on the grain sizes of the deposited films. After coalescence into a continuous layer at the interface, Al3Ni thickens through a diffusion-limited process, in agreement with previous reports. A kinetic model is developed which yields calculated calorimetric traces in good agreement with experimental data. Our results suggest the importance of prenucleation interdiffusion, in addition to nucleation, in the selection of the first phase during thin-film reactions.