Sluggish mobility and strong icosahedral ordering in Mg-Zn-Ca liquid and glassy alloys

The dynamic properties and atomic configuration of Mg, Mg-Zn, Mg-Zn-Ca and Mg-Zn-Ca-Cu liquid and solid systems were comprehensively studied using ab initio molecular dynamics calculation. The viscosities of an Mg-Zn-Ca alloy were measured from 700 K to 960 K. It was found that Zn and Ca in Mg liquid induce remarkable slowing down of atoms, but Cu acted as a stimulus to the diffusion. Icosahedral short-range orders are most dominant in Mg-Zn-Ca alloys in view of the existence of a majority of 1551 type of bond pairs, 〈0, 0, 12, 0〉 type of Voronoi polyhedra. Icosahedral medium-range orders can be formed in Mg-Zn-Ca alloys by the perfect icosahedra via the linkage of vertex-, edge-, face- and intercross-shared atoms. It is suggested that the high glass forming ability of Mg-Zn-Ca alloys is essentially related to the sluggish mobility and strong icosahedral short- and medium-range ordering in the undercooled liquid.