COST 507 - Repositório Aberto da Universidade do Porto

three different sections starting from binary liquid Cu-Mg alloys (50, 75, and 90 at%
Mg), and one section starting from a binary liquid Mg-Y alloy (8 at% Y).
The obtained experimental results for ternary liquid alloys were interpreted using an
association model [82Som2, 90Som]. It was found that a good description was possible
with the assumption of three binary associates (Cu 2 Mg, CuY, Mg 2 Y). With the model
parameters obtained from the three limiting binary systems it was possible to calculate
thermodynamic properties for ternary liquid alloys. Although there are some
indications for ternary interactions, the existing experimental data are still not
extensive enough to warrant a corresponding evaluation. All experimental details and
the final results are summarized in Appendix 2 and are published as [97Gan2].
4 The Ternary Cu-Mg-Si System
4.1 Thermodynamic Properties
Magnesium vapor pressures were determined along an isopleth with a constant concentration
ratio of Xcu/xsi = 7/3 using again the described isopiestic method [93Gna]. This
particular composition was selected because of a minimum melting temperature of
802°C at the eutectic point (30 at% Si) in the Cu-Si binary system [8601e]. It was
supposed to guarantee a maximum stability range of the liquid phase in the corresponding
ternary section which, however, was reduced considerably by the appearance
of a melting point maximum of about 950°C around 33 at% Mg and 13 at% Si [39Wit,
60Asc]. From the vapor pressure measurements, partial thermodynamic properties of
magnesium were derived; they are presented for a temperature of 1173 K. A Gibbs-
Duhem integration was performed based on the method of Peiton and Flengas [69Pel],
using the obtained magnesium activities to calculate integral Gibbs energies of mixing
for liquid alloys in the investigated isopleth.
The enthalpy of mixing in this isopleth with Xc u /x s ¡ = 7/3 was determined by solution
calorimetry [80Som]. Measurements were performed starting both from liquid binary
Cu 0 7 Si 0 3 -alloys (and adding solid magnesium) and from liquid magnesium (adding
solid Cu 07 Si 03 -samples). Partial enthalpies of mixing of magnesium were derived from
the obtained data and were found to be in very good agreement with those derived
from the temperature dependence of the magnesium activities.
The obtained thermodynamic results are again interpreted by the described association
model [82Som2, 90Som]. All experimental details as well as the final results can be
found in Appendix 3 and will be published soon [97Gan3].
4.2 Phase Diagram
The Cu-Mg-Si phase diagram along the isopleth with XcAsi = 7/3 was studied by
means of differential thermal analyses (DTA) and X-ray powder diffraction methods.
With these it was possible to derive the shape of the liquidus curve. The phase
relationships at lower temperatures are very complicated (see also [60Asc]), and it was
found that additional measurements will be necessary outside the investigated section
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