Atomistic study on medium-range order structures in amorphous metals

Amorphous metals have excellent properties, such as large elastic elongation, high fracture toughness and high corrosion resistance. These properties may arise from their disordered atomic structures, which are totally different from the crystalline structures with long-range periodic order. However, detailed structure of amorphous metals is still a controversial issue, because the experimental diffraction methods have a difficulty to reveal detailed structural features of amorphous metals. Using molecular dynamics methods with binary Lennard-Jones Cu-Zr interatomic potentials, we in this study investigated topological feature of amorphous metals focusing on a medium-range order (MRO). We conducted melt-quenching simulations and constructed amorphous alloy models with three different Cu concentrations. MRO clustering of icosahedra structures which consist of interpenetrating icosahedra links were found in a constructed amorphous structure. In the amorphous model, there are many small and a few large MRO clusters, and the number of MRO clusters monotonically decreases with increasing its size. During the melt-quenching process, the size of MRO cluster rapidly increases with increasing the number of icosahedra at around the glass transition temperature Tg. It is implied that the rapid increase of the size of MRO cluster is caused by a clustering between MRO clusters as well as a growth of each MRO cluster. An amorphous model with more icosahedral clusters tends to have larger, denser and more MRO clusters, and a maximum size of the icosahedral MRO cluster is ∼4nm. Based on both the radius of gyration and the density correlation function analyses, we found that the size of icosahedral MRO cluster has fractal feature, of which fractal dimension are 1.46-1.80. ©2015 The Society of Materials Science.