Transport properties and abnormal breakdown of the Stokes-Einstein relation in computer simulated Al72Ni16Co12 metallic melt

Abstract Molecular dynamic simulations were performed for liquid Al72Ni16Co12 to study the transport properties and the Stokes-Einstein relation (SER). It was observed that the self-diffusion coefficient for Co is much lower and α-relaxation time is much longer than the other two elements. The viscosity as a function of temperature was obtained with a method of reversed non-equilibrium molecular dynamics simulation. The SER in terms of viscosity and α-relaxation time for all elements was evaluated. It is found that the effective diameter for Co increases abnormally with the decrease of the temperature. The breakdown of SER for Co between self-diffusion coefficient and viscosity or α-relaxation time happens in the whole temperature range covered in this work, whereas the breakdowns of SER for both Al and Ni occur respectively at 1500 K and 1200 K in term of viscosity and α-relaxation time, which are far above the critical temperature of mode coupling theory, Tc, around 814 K according to transport properties, and 797 K based on the α-relaxation time. The decoupling of element dynamics and the dynamic heterogeneity are studied by calculating the ratio of the two diffusivities for different elements, D1/D2, and the non-Gaussian parameter. To check the structural-dynamics correlation, the total and partial pair correlation functions are calculated. It is proposed that for Co the decoupling of self-diffusion coefficient and viscosity or α-relaxation time persists at all temperatures due to the strong interactions between Co atoms. And the sudden increase of the dynamic heterogeneity, accompanied by a subtle change of liquid structure, is supposed to be the reason for the abnormal breakdown of SER for Al and Ni at a temperature far above Tc.