Thermochemistry of ternary liquid Cu-Mg-Si alloys

Abstract

The thermodynamic properties of ternary liquid Cu-Mg-Si alloys with a constant concentration ratio of x Cu /x Si ϭ 7/3 were determined using a combination of different experimental methods. Enthalpies of mixing were measured by isoperibolic calorimetry, and magnesium vapor pressures were obtained by an isopiestic method. Partial thermodynamic properties of magnesium were derived from the vapor pressure data, and the composition dependence of the magnesium activities is given for 1173 K. Integral Gibbs energies of mixing for liquid alloys were calculated by a Gibbs-Duhem integration. The agreement of the thermodynamic data obtained from different methods was found to be very good.

Thermodynamic assessment of the Cu‐Mg‐Si system in its copper‐rich region

“…They agree reasonably well with the surfaces assessed tentatively by Aschan [9]. The clearest disagreement is the much lower stability of τ by [9], but as stated in section "Experimental data", this disagrees also with the observations of Ganesan et al [10]. Using the data of [13], the overall agreement is much poorer.…”

“…All calculations were carried out with the ThermoCalc software [16]. 3 Calculated activity of Cu-Mg-Si alloys at 1173 K and x Cu :x Si =7:3, together with experimental data points [11]. Solid lines refer to the present calculations and dotted lines refer to those of [13].…”

“…As an example, see the position of the τ/Cu 2 Mg curve indicating a very high stability for τ or very low stability for Cu 2 Mg by [13]. On the other hand, [13] may not have relied on the data of Aschan [9] but that of Ganesan et al [9,10] only. The calculated and experimental [9] four-phase equilibria of the system Cu-Mg-Si are presented in table 3.…”

“…The phase equilibrium data of Witte [12], however, shows that this is not the case. Consequently, the high temperature data of Ganesan et al [10] is preferred to that of Aschan [9] for Cu 3 Mg 2 Si.…”

“…As an example, the liquidus surfaces of [9] look "ragged", which may be due to his choice to rely on each measurement, regardless of the iron content of the alloy. More recent data comes from Ganesan et al [10,11] but only along the x Cu :x Si =7:3 section, which is not so close to the copper-rich corner. The stability of the Cu 3 Mg 2 Si (τ) phase above 700 °C by [9] seems to be far too low in regard to the observations by [10,11].…”