Ab initio molecular dynamics simulations of molten Al _1-xSi _x alloys

Recent increases in computational efficiency have enabled large-scale ab initio molecular dynamics simulations to be performed on molten eutectic Al _1-xSi _x alloys (x = 0.12). Atomic forces were calculated using real-space pseudopotential density-functional theory, and the pair correlation, structure factor, coordination number, velocity autocorrelation, and mean-square displacement were computed at temperatures 973 K, 1223 K, and 1473 K. The calculated structure factor agrees very well with data from neutron-diffraction experiments. In addition an analysis of partial coordination numbers suggests that Si and Al are well mixed. This finding does not support an earlier conjecture attributing anomalous density variations to Si-aggregation phenomena. For dynamical properties the velocity autocorrelation function calculated for Al atoms demonstrates a "cage effect" similar to that found in pure liquid Al. In addition, the diffusion constants of Al are consistently lower than that of Si over the temperature range we have studied.