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 ∼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!