Effects of oxygen on local atomic order and diffusion properties in Al-Ni glass-forming liquids

Abstract

Oxygen has long been a concern in metallic glass-forming liquids since a minor addition can have a significant influence on liquid properties and glass formation, especially by inducing crystallization. However, how exactly oxygen affects the atomic structure and liquid properties on atomic level remains unclear to date. Here, by using extensive reactive molecular dynamics simulations, we conduct a comparative study of the atomic structures and diffusion behavior in both Al-Ni and its oxygen-containing liquids. We find that the oxidation occurs with the formation of extremely stable oxide clusters with the bonding preference between Ni and O atoms. The presence of the oxide clusters results in an increase in free volumes around them. It is the covalent bonding as well as the local excess volumes around the oxides that lead to changes in diffusion coefficients. The diffusion coefficients follow the Arrhenius relation at high temperatures, but a dynamical crossover happens above the glass transition temperature. We show that the diffusivity still correlates well with the free volume as predicted by Cohen and Turnbull although the effect of oxide clusters is not considered. This fundamental discrepancy may challenge our current understanding of liquid properties.