Atomic size and chemical effects on the local order of Zr2M (M=Co, Ni, Cu, and Ag) binary liquids

First-principles molecular dynamics simulations are performed to investigate the atomic size and chemical effects on the short-range order (SRO) in superheated and undercooled Zr-based metallic liquids, Zr2M (M=Co, Ni, Cu, and Ag). We demonstrate that the local atomic structures in liquids are quite sensitive to the atomic size ratio and the electronic interactions between component elements. The large negative heats of mixing for Zr-M do not favor icosahedral SRO in these binary liquids, contrary to the common belief. Full icosahedral structure units are few in the superheated liquids, although the number of icosahedral clusters increases upon undercooling. Comparing Zr2Co, Zr2Ni, and Zr2Cu, all of which have very similar atomic size ratios, we find that the degree of local icosahedral order increases with decreasing interaction strength between the d electrons in Zr-Co, Zr-Ni, and Zr-Cu. A comparison of Zr2Cu and Zr2Ag alloys shows that the degree of icosahedral order increases much more in Zr2Ag than in Zr2Cu with decreasing temperature. The difference in atomic sizes of Cu and Ag may account for the subtle discrepancy in the evolution of short-range ordering in undercooled Zr2Cu and Zr2Ag liquids.