Diffusion-coefficient measurements in liquid metallic alloys

The value of the diffusion coefficient in the liquid (D_l) is generally obtained from the measurement of composition profiles ahead of a quenched planar interface. The experimental results show significant scatter. The main reason for this scatter will be shown to be due to the presence of fluid flow in the liquid. Directional-solidification studies in the Al-Cu system have been carried out to first establish the experimental conditions required for diffusive growth. The composition profiles are then measured to obtain the values of D_l for alloy compositions ranging from 4.0 to 24.0 wt pet Cu. The value of D_l = 2.4 × 10^-3 mm²/s was obtained along the liquidus line, and this result is significantly smaller than the values reported in the literature, which vary from 3.0 to 5.5 × 10^-3 mm²/s. It is shown that the scatter in the reported values can be correlated with the diameter of the sample used and, thus, with the fluid flow present in their experiments. Detailed experimental procedures to obtain and verify diffusive-growth conditions are outlined, and appropriate analyses of the data are discussed.