Icosahedral medium-range order formed in Mg₇₀Zn₃₀ metallic glass: a larger-scale molecular dynamics simulation

A larger-scale Mg₇₀Zn₃₀ alloy system including 100000 atoms has been simulated by using the molecular dynamics method to investigate the icosahedral medium-range order (IMRO) formed in the Mg₇₀Zn₃₀ metallic glass. It is found that the simulated pair distribution function of Mg₇₀Zn₃₀ metallic glass is in good agreement with the experimental results. The glass transition temperature T_g is near 450 K under the cooling rate of 1×10¹² K/s. The icosahedral local structures play a critical role in the formation of metallic glass, and they are the dominant local configurations in the Mg₇₀Zn₃₀ metallic glass. The IMRO in the Mg₇₀Zn₃₀ metallic glass is characterized by certain types of extended icosahedral clusters combined by intercross-sharing atoms in the form of chains or dendrites. The size distributions of these IMRO clusters present a magic number sequence of 19, 23, 25, 27, 29, 31, 33, 35, 37, 39, ..., and the magic clusters can be classified into three types according to their compactness. The IMRO clusters grow rapidly in a low-dimensional way with cooling, but this growth is limited near T_g.