Interplay between structural and atomic transport properties of undercooled Al-Cu binary alloys

We perform ab initio molecular dynamics simulations in stable and undercooled Al1-xCux liquids, with copper composition x = 0.3 and 0.4, to examine the relationship between structure and dynamics as a function of temperature. The temperature evolution of the diffusivity, relaxation times and viscosity display a change from an Arrhenius behavior at high temperature to a non-Arrhenius one at low temperature, which occurs at a crossover temperature TX yet in the vicinity of the liquidus temperature. We find that TX corresponds also to an onset of dynamic heterogeneities (DHs) that develop in the undercooled states. The structure factors and pair-correlation functions display characteristic features compatible with the occurrence of the icosahedral short-range order (ISRO) as well as the development of a medium range order (MRO) upon cooling. A common neighbor analysis further identifies that ISRO undergoes a strong increase below TX. Finally, we demonstrate that the medium-range order is formed by interconnected fivefold bipyramids and is correlated with the emergence of DHs.