Metallic glasses are very strong and elastic materials that appear with the naked eye to be identical to stainless steel. But metallic glasses differ from ordinary metals in that they are amorphous, lacking an orderly, crystalline atomic arrangement.
This random distribution of atoms, which is the primary characteristic of all glass materials (such as windowpanes and tableware), gives metallic glasses unique mechanical properties but unpredictable internal structure.
Researchers in the Caltech lab of Julia Greer, professor of materials science and mechanics, have shown that metallic glasses do have an atomic-level structure—if you zoom in closely enough—although it differs from the periodic lattices that characterize crystalline metals.
If you looked at a metallic glass on a scale larger than a few atomic diameters, you would see tightly packed, jumbled clusters of atoms. A new study from the Greer group—published in Science—shows that inside each of these clusters, on a scale of about two to three atomic diameters, atoms have a predictable arrangement called a fractal.
Repeating Pattern
Fractals are patterns that are self-similar on different scales, and they can occur quite naturally.
Mathematicians and physicists have studied them for centuries. Showing how they emerge in a metallic alloy provides a physical foundation for something that has only been studied theoretically.
“Take for example a piece of paper crumpled into a ball. 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,” says David Chen, a fourth-year graduate student in the Greer lab and first author on this new paper.
