Role of nanosize icosahedral quasicrystal of Mg-Al and Mg-Ca alloys in avoiding crystallization of liquid Mg: Ab initio molecular dynamics study

We have used highly accurate first-principles molecular dynamics simulations to explore the structural (both topological and chemical ordering) evolution during solidification of Mg90Ca10 and Mg90Al10 liquids upon cooling. Using various structural analysis methods, we find that local atomic structure of both amorphous alloys are characterized by major distorted and perfect icosahedral symmetry. The fraction of fivefold icosahedra around Mg atoms in Mg-Ca alloy is higher than in Mg-Al alloy. Eutectic Mg90Ca10 alloy shows strong chemical short range order, which further increases upon cooling. In contrast, Mg90Al10 shows weak chemical short range ordering which seems to show weak temperature dependency behavior. The fraction of fivefold icosahedral symmetry, which is responsible for geometrical frustration, and the atomic size are responsible for the dynamic slowdown and causing glassy formation.