Short to medium range order and atomic dynamic property within the surface of nanoscale metallic glasses revealed by molecular dynamics simulations

The surface microstructure and its atomic dynamic property of a metallic glass nano pillar were investigated by using molecular dynamics simulations. The short-range order motifs in the surface contain a large number of unstable bonded pairs and clusters, leading surface to approach the structural property of liquid phase. A proportion of <0, 0, 12, 0> clusters in the surface constitute chain-like super clusters, but its overall degree of medium-range order is lower than that in the core region. The atomic mobility in the surface at different scales are enhanced significantly. Generally, the lower the degree of order for the cluster, the stronger its migration ability. The surface region could be divided into three consecutive sublayers, which are solid-like layer, semi-solid layer and liquid-like layer. The content of liquid-like clusters increases sequentially in these three sublayers. A schematic illustration of the relationship between the hierarchical structure and the possible relaxation modes was proposed.