ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2024 25 The quest for extreme applications of materials inevitably leads to the development of extreme materials. Metallic glasses are non- crystalline solid metals that have high strength, excellent corrosion resistance, and impressive hardness compared to their crystalline counterparts. Metallic glasses are materials that have been developed for nearly 80 years and whose potential is just being realized. Applications are appearing in biomedical applications, the aerospace industry, nanoscale printing, and even in advanced ballistics as well as a host of other applications[1]. As the name implies, metallic glasses are metals that retain a unique ordering among their atoms incompatible with their highly ordered crystalline counterparts. While crystalline materials are built of a cell of ordered atoms, repeated to fill space, metallic glasses are far more like liquids in their structure but also can be heated from a solid state to flow; plastically deforming like a silicate might to form an intricate or ornate glass ornament. Part of what makes these metallic glasses so technologically important is that they can be formed at the large scale and small scale (nanometers) by molding in a way that conventional metallic alloys cannot because the glasses shrink very little upon resolidification. While metallic glasses are brittle, much like window glass, their ultimate yield strength is exceptionally high. METALLIC GLASSES: MATERIALS FOR TODAY AND TOMORROW A modern understanding of how glasses are formed along with an appreciation for the high ultimate yield strength of metallic glasses are leading to a boon in applications for these unique materials. Nicholas Mauro, St. Norbert College, De Pere, Wisconsin Fig. 1 — Groupings of zirconium (green) atoms around palladium (purple) atoms in a simulation of a Zr75.5Pd24.5 liquid. Image reproduced with permission from Ref. 6. state of matter; that is, there is no persistent ordering in the gas. Crystals, on the other hand, are solid states of matter that have extensive short-, medium-, and long-range ordering and their structural coherence is very high. Liquids and glasses fall in between these two extremes, but it is difficult to definitively say what the structure of a liquid or a glass is, for somewhat different reasons. When considering a metallic glass, it is thought that the atoms don’t flow. This is an excellent assumption as long as the temperature is low enough (well below the “glass Despite the promise of these materials, metallic glasses have not achieved their full potential. A key challenge that remains is fully under- standing why some alloys can be easily formed into glasses (for example, zirconium[2] and palladium[3] based metallic systems) and others must be rapidly cooled at rates exceeding 100,000 times cooling achieved by conventional casting techniques. At the heart of addressing this challenge is identifying the most compelling set of questions to answer. Those revolve around understanding how liquids cool to form solid phases and how to control that process. FORM AND FUNCTION/ FUNCTION AND FORM The various states of matter have extraordinarily different atomic and molecular ordering. At a very high temperature, all materials can be turned into a gas. While this may not be practical and requires a tremendous amount of energy, it is a useful starting point because there generally is no short range or nearest neighbor ordering. It is believed that there is no “structural coherence” in that
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