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 bou
ndaries obtained for Al-Cu are in accord with experimental data available i
n the literature. The measured (Al) solidus in terms of Cu is found to be s
imilar 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. Mo
reover, the difference between the measured and calculated solidus in terna
ry Al-Cu-Mg was found to be the same as that in binary Al-Cu. This differen
ce is believed to be responsible for the deviations in the calculated micro
segregation from experimental data for Al-rich Al-Cu-Mg alloys. The (Al) ph
ase in both the Al-Cu and Al-Cu-Mg systems was thus remodeled so that the c
alculated solidus using the new model description is in better agreement wi
th experimental data. The microsegregation model-calculations show that eve
n small differences in the phase boundaries of 0.5 at.% or less have a pron
ounced effect on the calculated concentration profiles of Cu in the dendrit
es. We thus conclude that in order to test the validity of a microsegregati
on model, accurate phase boundary data are needed! (C) 2000 Elsevier Scienc
e S.A. All rights reserved.