The regular crystal structure is a defining property for metals. It has been known since the 1960s that certain alloys can be transformed into a disordered state. This is done by rapidly cooling them down to room temperature from their liquid phase, whose atomic structure is reminiscent of glass. For a long time, the possibilities for processing were limited. In their article "Thermoplasticity of metallic glasses: Processing and applications", OeAW researchers Baran Sarac and Jürgen Eckert from the Erich Schmid Institute of Materials Science (ESI) show what progress has been made and how better processing methods open the door to new applications.
First comprehensive overview since the 60ies
"I have been researching metallic glasses since my master’s and doctoral theses at Yale University and, together with my boss Jürgen Eckert, we have summarized our own results and other work from this area to provide a comprehensive overview of the state of research and a wide range of current and potential areas of application for the first time since the 1960s. At first, alloys were restructured from melts to glasses by very rapid cooling. Today we use different glass formers from several elements, including transition and precious metals as well as possible semi-metal additions, to mass produce them at relatively low cooling rates and can manipulate the properties much better through the application of pressure and temperature," says Baran Sarac from the Erich Schmid Institute of Materials Science.
Glasses become malleable
With classic processing methods, high temperatures are usually applied, as a result of which metallic glasses crystallize and lose their special properties. Thermoplastic methods developed since 2005, on the other hand, make it possible to bring the materials into different shapes and provide them with almost any structure. "We start with disks, plates or pellets, which are formed by ultra-rapid cooling of metal alloys. We then treat these with heat and pressure while they have a honey-like viscosity. Relatively favorable process conditions at around one third of the melting temperature allow a sufficient time window to deform them and create micro- or even nanostructures on these materials. The glass structure is retained after cooling," Sarac says.
There are still limits to the size of the pieces that can be produced; expansions of more than a few centimeters are still a long way off. But thanks to the thermoplastic processes, the materials can now be processed. The extremely tough metallic glasses can be used where steel or titanium is not strong enough, for example, to make joint implants that last longer and do not struggle with abrasion. But that is just the tip of the iceberg.
From artificial organs to invisibility cloaks
"Different geometries and structures give metallic glasses a wide variety of interesting properties. They have the potential to enable breakthroughs in medicine, renewable energy and electronics," Sarac says. Metallic glasses with net-shaped, hair-like nanowires, for example, are biocompatible and are ideal as scaffolds for biological cells, for example, if organs are to be cultivated in the laboratory in the future. The optical properties of such nanowires can be very finely tuned by the structure, making "invisibility cloaks" conceivable, which can make objects invisible under light with certain wavelengths. High aspect ratio nanopatterning can also store energy as heat very efficiently. The materials can be formed into micro-scale textures to create extremely water-repellent surfaces. In electronics, thermoplastically formed metallic glasses can be used as information storage with very high density.
Porous structures made of metallic glasses store enormous amounts of kinetic energy, which could help protect satellites from micrometeorites. The magnetic properties are also unique and allow the production of transformers with unprecedented efficiency. These materials are also very well suited for hydrogen storage and production, micro fuel cells, and blood sugar sensors. The glasses examined so far are mainly based on iron, zirconium, titanium, magnesium, or precious metals, which are then alloyed with other metals or semi-metals. The number of potential metallic glass alloys is huge, and nobody knows which new glass properties are still waiting to be discovered.
"One must not forget that we are just at the beginning of exploration, and yet the number of possible applications is already enormous. The use of artificial intelligence and other new methods are accelerating the discovery of new alloys and structures. In the future, metallic glasses will replace other materials in many areas because they can do almost everything better. The next ten years will be very exciting because we will learn to combine the interesting properties and to master the glasses better," Sarac says.