Glasses have markedly different stability around their glass transition temperature ( T _g ), and metallic glasses (MGs) are conventionally regarded as metastable compared to other glasses such as silicate glass or amber. Here, we show an aging experiment on a Ce-based MG around its T _g (~0.85 T _g ) for more than 17 years. We find that the MG with strong fragility could transform into kinetic and thermodynamic hyperstable state after the long-term room temperature aging and exhibits strong resistance against crystallization. The achieved hyperstable state is closer to the ideal glass state compared with that of other MGs and similar to that of the million-year-aged amber, which is attributed to its strong fragility and strong resistance against nucleation. It is also observed through the asymmetrical approaching experiment that the hyperaged Ce-based MG can reach equilibrium liquid state below T _g without crystallization, which supports the idea that nucleation only occurs after the completion of enthalpy relaxation.

The evolution of flow units associated with the flow “defects” in metallic glass is characterized by monitoring the metallic glassy density change upon isothermal annealing far below their glass transition temperature. A meaningful function for the density variation with the concentration of flow units is obtained for the metallic glasses. We show that the correlation between the density variation and the flow unit have implications for understanding the fragility, structural heterogeneous, and structural relaxation behaviors in metallic glasses.