METALLIC GLASSES

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For most of us, glass is the clear material that’s used to make windows and wine bottles. But glasses, which lack the conditions necessary to reach thermal equilibrium, come in many forms. They can be transparent or opaque, metallic or insulating, and their fundamental building blocks can be as small as atoms or as large as polymers. Finding similar behaviors in different types of glasses could lead to a better understanding of the materials, and potentially to an overarching theory for them. A team led by Weihua Wang of the Chinese Academy of Sciences in Beijing has made such a connection. They report the first evidence that a strained metallic glass relieves mechanical stress, or “relaxes,” via two steps, one fast and one slow—a two-step relaxation process that has so far only been observed in “soft” glasses like colloids. The fact that the process also occurs in a rigid, or “hard,” glass like a metal suggests that it may be a universal feature of glassy systems. In general, the results provide a fuller picture of the dynamics of the out-of-equilibrium nature of glasses, which is poorly understood.

Metallic glasses are alloys with amorphous atomic structures. They are obtained by rapidly cooling, or “quenching,” the alloy from its high-temperature, liquid phase to below its glass-transition temperature. The quenched metal’s structure is out of equilibrium from its preferred crystalline form. As such, the structure will spontaneously start to evolve very slowly over time, a process known as aging, which can last for a much longer time than any laboratory experiment. The strength and resistance to corrosion of metallic glasses often exceeds that of crystalline metals, making them of interest for certain applications. But they are also model glassy systems because of their simple atomic structure, which can be described as a dense random packing of hard spheres.

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