A two-step surface crystallization reaction was observed on $\mathrm{A}{\mathrm{u}}_{60}\mathrm{C}{\mathrm{u}}_{15.5}\mathrm{A}{\mathrm{g}}_{7.5}\mathrm{S}{\mathrm{i}}_{17}$ metallic glass under the isothermal annealing below the glass transition temperature ($T_g$). First, at $T_{g-20\mathrm{K}}$, the surface enriched with Cu forms a few nanometers thick distribution of plate-shaped Cu crystals. Then, at $T_{g\text{−}10\mathrm{K}}$, catalyzed by the surface Cu crystal, Si with a small amount of Cu crystallizes into a 100-nm-thick nanostructure deeper into the liquidlike-near-surface region. The fast surface diffusion of glass provides the favorable environment for nanocrystal development where the growth mode and structure can be effectively tuned by the temperature.

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• Microstructure
• Nucleation on surfaces
• Phase transitions
• Surface diffusion
• Metallic glasses