Coupling between dynamic slowing down and chemical heterogeneity in a metallic undercooled liquid
Résumé
We investigate the connection between local structure and dynamics in the liquid and undercooled Al93Cr7 alloy using ab initio molecular dynamics simulations. In the liquid phase, we show the striking effects of Cr alloying on the atomic-scale structure, characterized by a heterogeneous local ordering around each component, due to a strong interplay between chemical short-range order and icosahedral short-range order. In the undercooled phase, this interplay leads to the formation of an icosahedral-based medium-range order (IMRO) referring to Cr atoms. In examining the dynamic properties, we observe that this chemically induced structural heterogeneity gives rise to a substantial decoupling of component diffusion in the liquid phase, far above the liquidus temperature. In the undercooled regime, we find the breakdown of the Stokes-Einstein relation and a pronounced increase of the non-Gaussian parameter, indicating the onset of dynamic heterogeneities. Using the isoconfigurational ensemble method, we evidence that the structural origin of dynamics heterogeneities is clearly related to the formation of IMRO. Finally, we discuss the role played by IMRO in nucleation on the preexisting icosahedral cluster in the liquid phase.