Metallic glasses are rather curious materials. They may appear to look like stainless steel, but yet differ from ordinary metals in that they are amorphous, lacking an orderly, crystalline atomic arrangement. Actually this is how they got their name – random atomic structure is actually a defining characteristic of glass materials.
However, metallic glasses are very strong and elastic materials with unique mechanical properties but unpredictable internal structure. Now scientists at California Institute of Technology, better known as Caltech, have shown that metallic glasses do have an atomic-level structure.
This structure has not been yet identified before, because you have to zoom in quite a lot to notice it. Looking at the metallic glasses on a scale larger than a few atomic diameters only tightly packed, jumbled clusters of atoms can be observed. However, now scientists noticed that by zooming in further one can notice that atoms have a predictable arrangement called a fractal inside each of these clusters, but scale has to be about two to three atomic diameters. Furthermore, this atomic-level structure differs significantly from the periodic lattices that characterize crystalline metals.
Fractals are very interesting structures, yet hard to explain. They occur naturally and are self-similar on different scales. David Chen, lead author of the study, suggests imagining this fractal structure like a piece of paper crumpled into a ball.
He explained: “if you look at the folds of the paper when it is flattened back after crumpling, it will look qualitatively the same if you zoom in on a smaller portion of the same paper. The scale that you use to examine the paper more or less does not change the way it looks”. To get to know that the research team examined the atomic structure of metallic glass alloys of copper, zirconium, and aluminium.
The normal crystalline solids like diamond or gold have an orderly lattice pattern, in which atoms or molecules are arranged. This means that the local neighbourhood around an atom in a crystalline material is identical to everywhere else in the material.
This is completely different in amorphous metals, where every location within the material looks different. However, now scientists discovered that zooming in to a scale of about one nanometre a structure can be noticed. At this level of zoom the same fractal pattern is present, regardless of location within the material. In fact, it is a particular kind of fractal pattern called percolation.
This is somewhat of a paradox. Even though there were scientists who hypothesized that the atoms in metallic glasses are distributed fractally, it did not gain much support, since it would mean that there should be empty space between atoms. But there is no such emptiness between these fractals – metallic glasses are as dense as regular metals. This means they lack significant pockets of empty space.
However, researchers solved this paradox by demonstrating that fractal pattern is only present to a certain level. Looking at larger than that scale, atoms appear to be packed randomly and tightly, making a fully dense material, just like a regular metal. This means metallic glasses are both fractal and fully dense at the same time.
Even though discovery was made in metallic glasses, fractal arrangements of atoms are equally true for pretty much any rigid amorphous material, like the glass in a windowpane or a frozen piece of chewing gum. These amorphous metals sometimes have pretty unique properties, such as unusual strength and elasticity. Knowing these structural parameters of metallic glasses will allow scientists to study how this atomic-level arrangement affects large-scale properties of amorphous metals.
This discovery will have implication not only in materials science, but also in mathematics, physics, and computer science, all of which are particularly interested naturally occurring fractal distributions. In fact, fractals interested mathematicians and physicists for centuries. And now this new knowledge about how these fractals emerge in a metallic alloy provides a physical foundation for something that has only been studied theoretically. We will have to wait and see what kind of practical implications this discovery may have in the future.