Transport Properties And Abnormal Breakdown Of Stokes-einstein Relation For Liquid Al₇₂Ni₁₆Co₁₂ Alloy By Molecular Dynamics Simulations

The transport properties of liquids,such as self-diffusion coefficient and viscosity,play an important role in the nucleation and growth of crystals,liquid-to-glass transition and the mechanical properties of glasses.The coupling and decoupling of self-diffusion coefficient and viscosity elucidate the microscopic details of liquid dynamic behavior.Therefore,Stokes-Einstein relation（SER）is always the hotspot in the material science and condensed matter physics.Many recent studies found that the breakdown of SER appears abnormally at temperatures far above the melting temperature for some alloy system,which is highly different from public knowledge.So,the study is of great significance for the breakdown behavior and mechanism of SER.In this paper,the molecular dynamic simulations were performed for Al₇₂Ni₁₆Co₁₂ liquid alloy to study the transport properties and the Stokes-Einstein relation（SER）.Furthermore,the mechanism of abnormal high-temperature breakdown of SER is explored based on the studies for structure properties,decoupling of element dynamics and dynamic heterogeneity of the metallic melt.The self-diffusion coefficient of the elements in Al₇₂Ni₁₆Co₁₂ liquid alloy was calculated by mean square displacement.The viscosity was calculated by reversed non-equilibrium molecular dynamics method,and theα-relaxation time was obtained by self-intermediate scattering function.The SERs in terms of viscosity and relaxation time were studied and the breakdown temperatures were calculated for different components.It is found that the breakdown temperatures of SER for Al and Ni are much higher than theoretical prediction temperatures,and the failure of SER for Co happens at all temperatures in the simulations.It is observed that the effective diameters for Co changes abnormally with temperatures.The dynamic heterogeneity of Al₇₂Ni₁₆Co₁₂ liquid alloy was characterized by calculating the coupling parameter ofα-relaxation process and the non-Gaussian parameter.The ratio of the two diffusivities for different elements were used to describe the decoupling of element dynamics.It is indicated that the dynamic heterogeneity dramatically enhances at low temperature,and the decoupling between Co and other elements happens at low temperature,which lead to the abnormal breakdown of the liquid and abnormal behavior of Co atoms.The pair correlation function and Voronoi analysis were used to characterize the static structure properties and local five-folder symmetry of Al₇₂Ni₁₆Co₁₂ liquid alloy,respectively.By analyzing the structure properties of the liquid at different temperatures,it is found that the interaction between Co atoms is very strong and the structure properties of the liquid significant change at low temperatures,which result in the abnormal breakdown of the liquid and abnormal behavior of Co atoms.In summary,the dynamic heterogeneities and static structural properties were calculated and analyzed,and the potential reasons were obtained for the abnormal breakdown of SER and the abnormal behavior of Co atoms.