Influence of Cr on local order and dynamic properties of liquid and undercooled Al-Zn alloys
Résumé
Using ab initio molecular dynamics, we present a systematic study of structural and transport properties of liquid Al90Zn10 and Al83Zn10Cr7 alloys. In the liquid phase, we find that Cr additions promote the formation of a heterogeneous local ordering characterized by a strong five-fold symmetry (icosahedral short-range order (ISRO)) around Cr atoms. In the undercooled phase, we observe the extension of ISRO to icosahedral medium-range order (IMRO) length scale referring to Cr atoms. In examining dynamic properties, we show that this Cr induced structural heterogeneity leads to a substantial decoupling of Cr diffusion from the diffusion of Al and Zn components by a factor of 3 at 1000 K, the liquidus temperature. Below this temperature, the formation of IMRO gives rise to a non-Arrhenian temperature dependence of diffusivity and viscosity, a breakdown of the Stokes-Einstein relation, as well as the onset of dynamic heterogeneities. Using the isoconfigurational ensemble method, we evidence that the structural origin of dynamics heterogeneities is clearly related to IMRO. Finally we discuss the role of IMRO in a quasicrystal-enhanced nucleation mechanism discovered recently in Al-Zn-Cr alloys. Published by AIP Publishing.