Nucleation and thermal stability of an icosahedral nanophase during the early crystallization stage in Zr-Co-Cu-Al metallic glasses

The early stage of crystallization in amorphous phases is a key step in the overall microstructural evolution. In a Zr-based amorphous alloy the formation of metastable nanophase was observed at the earliest crystallization stage, with an ultra-high nucleation density of 6.5 × 10²⁴ m⁻³ and a size of ∼1–2 nm. The nanophase has an icosahedral structure and exhibits an extremely high thermal stability up to 150 K above the crystallization onset. The chemical and structural heterogeneity in the early stage of the glass crystallization were observed using high-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), which can be applied to study materials with atomic resolution. A nucleation kinetics analysis of the nanophase formation accounts for the high product number density and indicates a low interface energy value between the metastable clusters and the amorphous matrix, σ_a,i of 14 mJ/m². The thermal stability and resistance to size change by growth or coarsening are attributed to a stalled growth due to solute depletion and the low σ_a,i value. With the high nucleation rate and the low barrier, it is remarkable that the kinetics follow conventional nucleation theory.