The self-diffusion coefficients D and the viscosities η of elemental Ni, Cu, and Ni-Si alloys were calculated over a wide temperature range. For elemental Ni and Cu, Arrhenius-law variations of D and η with temperature predominated. The temperature dependence of Dη could be approximated by a linear relationship, whereas the Stokes-Einstein relationship was violated. The calculations of D and η were extended to regions close to the crystallization of Ni95Si5, Ni90Si10 and the glass transitions of Ni80Si20 and Ni75Si25. The results showed that both D and η strongly deviated from the Arrhenius law in the vicinity of phase transitions, exhibiting a power-law divergence. A decoupling of diffusion and viscous flow was found just above the crystallization point of Ni95Si5 and Ni90Si10. For the two glass-forming alloys, Ni80Si20 and Ni75Si25, the relationship, Dη = constant, was obeyed as the glass transition was approached, indicating a dynamic coupling as predicted by the mode-coupling theory. This coupling was enhanced with increasing Si composition and, at 25, Si spanned a wide temperature range across the melting point. The decoupling was found to be related to the distribution of local ordered structure in the melts. The power-law governing the growth of solid-like clusters prior to crystallization created a dynamic heterogeneity that was responsible for decoupling.

A Molecular Dynamics Examination of the Relationship between Self-Diffusion and Viscosity in Liquid Metals. Lü, Y., Cheng, H., Chen, M.: Journal of Chemical Physics, 2012, 136[21], 214505