Anomalous Diffusion of Fe in Liquid Al Measured by the Pulsed Laser Technique

Abstract

The diffusivity of Fe and Cu in liquid Al was measured by using a nanosecond-duration pulsed laser to melt thin Al films ion implanted with solute. The thin film geometry eliminates convection in the melt during the experiment. The time-dependent electrical conductance and optical reflectance of the Al film during melting were measured to determine the melt duration, allowing the diffusivity to be calculated based on the one-dimensional broadening of an ion-implanted solute depth profile. The measured diffusivity of Cu is about three times that of Fe, which is consistent with Turnbull's cluster model for liquid Al-Fe. The diffusion coefficients measured for both Fe and Cu changed very little as the peak concentration decreased with time, implying little or no concentration dependence. The temperature dependence of the diffusivity was examined by using heat-flow simulations to extract temperature information from the transient conductance data. Our results for Fe diffusion in liquid Al are consistent, within experimental uncertainties, with extrapolations of Ejima's data to lower temperatures, but we observe Cu diffusivities approximately twice as large as would be expected from extrapolations of Ejima's data.