Structure and chemistry of liquid Al–Cu alloys: molecular dynamics study versus thermodynamics-based modelling

Classical molecular dynamics simulations, employing a modified embedded atom model (MEAM) parametrization recently developed by Trybula, have been performed and combined with thermodynamics-based modelling for weakly interacting compound-forming molten alloys, to investigate the structure and chemistry of liquid Al–Cu alloys over a broad Cu concentration range. The compound-forming model (CFM) based on experimental thermodynamic data revealed the importance of the Al₂Cu "associate" in the determination of transport properties such as diffusion and viscosity as well as confirmation of the compound formation ability with regard to the available experimental data. Adequately to this fact, molecular dynamics simulation results showed strong evidence of deviation from regular metallic solution resulting from a preponderance of chemical short-range ordering, expressed by Warren–Cowley parameter and increasing abundance of icosahedral motifs with increasing Cu content. In addition, their strong impact on mass transport properties as well as the excess entropy has been detected which exhibits nonlinear compositional behaviour. Thus, we find that the Stokes–Einstein relation is unsuitable for atom transport properties determination at investigated Cu concentration range, while the Green–Kubo formalism can fully account for the experimentally observed physical phenomena. We obtain a compact and compatible view onto the structure and chemical behaviour, including atom kinetics and thermodynamics, of Al–Cu liquid alloys, which allowed us to find another hard-sphere-like metallic system in which transport properties and thermodynamics are strongly affected by packing effects. The hybrid approach presented herein gave a broader and deeper look into the liquid state of the Al–Cu alloys being missing in the literature.