Identifying icosahedron-like clusters in metallic glasses

Since the discovery of the first metallic glass (MG) with the composition of Au75Si25 in 1960, vast efforts have been devoted to understanding the mechanisms of glass formation in metals, because this class of glassy alloy usually possesses unique properties that may have the potential application as engineering material. As is well known, structure determines properties of material. Therefore, understanding the glass formation of MG from the structural perspective is helpful for guiding researchers in developing more MGs. So far, icosahedral clusters are regarded as the preferred clusters contributing to the formation of amorphous structure due to its five-fold symmetrical feature and high atomic packing. However, it has been found that an ideal icosahedron usually does not have a high concentration in many MG compositions. Thus, we wonder whether icosahedral clusters are popular in microstructures of amorphous alloys. In this work, a feasible scheme for identifying the icosahedron-like clusters in MGs is developed to address this issue. It is found that icosahedron-like clusters are popular structural units in amorphous structure indeed, contributing to the glass formation in alloy. A projection method of reflecting the styles of shell-atom connections in Voronoi-tessellation indexed clusters is developed in detail, so that all clusters can be further geometrically indexed as different projected types of polyhedra. It is revealed that there are three kinds of clusters (0, 2, 8, 1, 0, 2, 8, 2 I-type, and 0, 1, 10, 2) which have the most similar geometrical features to that of the so-called ideal icosahedron, 0, 0, 12, 0. Therefore, besides the ideal icosahedron, these three types of clusters can be regarded as the icosahedron-like clusters. The ideal icoshahedron (0, 0, 12, 0) has a coordination number (i.e., the number of shell atoms) of 12, while these three icosahedron-like clusters have coordination numbers ranging from 11 to 13, so that structural balance between the geometrical atomic stacking and the chemical interactions among various elements in MGs (especially multicomponent MGs) is more easy to achieve. Furthermore, structural models of three selected ZrCu compositions are studied, which are obtained by systematic experimental measurements combined with reverse Monte Carlo simulation. It is found that both the icosahedron-like cluster and the ideal icosahedron have the similar values of some structural parameters, in terms of high atomic packing efficiency, high cluster regularity, fruitful five-fold symmetrical feature, etc. In addition, it is revealed that these ideal icosahedra and icosahedron-like clusters can contain almost all the atoms in these structural models, enhancing the space filling efficiency. In conclusion, these identified icosahedron-like clusters should be the popular building blocks, contributing to the glass formation in alloy. This work provides an insight into the glass formation in alloy from the cluster-level structural angle and will shed light on developing more MGs.