Phase Transformations in Au-Fe Particles and Thin Films: Size Effects at the Micro- and Nano-scales

Abstract

Thin Au-Fe bilayers (3–30 nm in total thickness) were deposited on sapphire substrates. Annealing in a temperature range of 600–1100°C resulted in solid-state dewetting and the subsequent formation of micro- and nano-particles. Electron microscopy, atomic force microscopy and in situ x-ray diffraction were employed to systematically study two phase transformations in the Au-Fe system: (1) precipitation of α-Fe from supersaturated Au-Fe solid-solution particles; and (2) α↔γ transformation in Fe and Au-Fe thin films and particles. In both cases, the transformations proceeded differently than in the bulk already for sub-micron (100 nm to 1 μm) particles. These results were explained by the low defect concentration in the particles, nucleation difficulties, slow diffusivity on facets, and Au segregation. A “reverse size effect” was observed in thin Fe films, and discussed in terms of nucleation model taking into account the small size of the parent phase. The main conclusion is that phase transformations in the particles and in the bulk proceed differently, not only for nano-sized particles as was customarily believed but also for particles of sub-micrometer size. We suggest that this size effect is governed by two different length scales: the inter-defect spacing (upper limit) and the bulk critical nucleus size (lower limit).