Natural aging of metallic glasses

Metallic glasses (MGs) are intrinsically metastable, and tend to relax and crystallize at elevated temperatures. However, it is generally believed that their amorphous nature at room temperature can be retained for a sufficiently long time. Herein, we report the long-term stability of a series of Cu-Zr MGs in natural environment after twelve years. Surprisingly, the Cu-Zr MGs with a low Zr content (i.e., below 66.7 at%) encountered severe instability, which was induced by room-temperature oxidation. Although those with a high Zr content preserve the shiny surfaces due to the formation of a dense ZrO2 layer, their thermal properties (e.g., glass transition temperature T-g and onset crystallization temperature T-x) are appreciably changed due to oxygen permeation. Our findings not only shed light on the understanding of the thermal stability of MGs in general, but also have important implications for engineering applications of these disordered solids.