Revealing mechanism of non-Arrhenius viscosity change in metal melts based on Wulff cluster model

Abstract

In some metal melts, like Pb and Al, viscosity as a function of temperature does not obey Arrhenius law in some temperature range, so called non-Arrhenius viscosity change. In this paper, the Wulff cluster model combined with phonon calculations is used to investigate the relation between the structural characteristic and the non-Arrhenius viscosity change in metallic melts. Although the surface energy of the Wulff clusters in melts does not change significantly with temperature, the Wulff shape changed evidently. For both Pb and Al, one surface of the Wulff shape disappears (Pb(321) and Al(100)) when the non-Arrhenius viscosity change occurs upon heating. The trend of the non-Arrhenius viscosity change is strongly related to the properties of the disappeared surface. The non-Arrhenius viscosity drop corresponds to the disappearance of the surface with the highest interaction energy between surface and free atoms (Pb(321) surface), while the non-Arrhenius viscosity rise corresponds to the disappearance of surface with the lowest interaction energy (Al(100) surface). All evidence indicates that the viscosity anomaly and the change of Wulff shape is closely related. The non-Arrhenius viscosity change in metallic melts is caused by the appearance/disappearance of certain surfaces of clusters, while the interaction strength of these surfaces determines the different features (non-Arrhenius drop/rise) of corresponding viscosity change.