Importance of reliable phase equilibria in studying microsegregation in alloys: Al-Cu-Mg

The solid and liquid phase equilibrium of Al-rich Al-Cu and Al-Cu-Mg alloys was investigated in order to examine the effect of small uncertainties in the solidus on model-calculated microsegregation during solidification. The phase boundaries were determined using EPMA on quenched two-phase alloys. The liquidus was obtained using a raster beam scan technique. The phase boundaries obtained for Al-Cu are in accord with experimental data available in the literature. The measured (Al) solidus in terms of Cu is found to be similar to 0.5 at% higher than model-calculated values from a thermodynamic description reported in the literature. On the other hand, the measured (Al) liquidus in terms of Cu are in good agreement with model-calculations. Moreover, the difference between the measured and calculated solidus in ternary Al-Cu-Mg was found to be the same as that in binary Al-Cu. This difference is believed to be responsible for the deviations in the calculated microsegregation from experimental data for Al-rich Al-Cu-Mg alloys. The (Al) phase in both the Al-Cu and Al-Cu-Mg systems was thus remodeled so that the calculated solidus using the new model description is in better agreement with experimental data. The microsegregation model-calculations show that even small differences in the phase boundaries of 0.5 at.% or less have a pronounced effect on the calculated concentration profiles of Cu in the dendrites. We thus conclude that in order to test the validity of a microsegregation model, accurate phase boundary data are needed! (C) 2000 Elsevier Science S.A. All rights reserved.