THE AG-AU-SI SYSTEM - EXPERIMENTAL AND CALCULATED PHASE-DIAGRAM

In the course of a systematic thermodynamic investigation of ternary alloys constituted of silver, gold, and an IVb metal, experimental investigations and calculations have been performed on the Ag-Au-Si system. Two techniques were employed: calorimetry at a very high temperature (T max = 1750 K) and differential thermal analysis. The former allowed the enthalpies of formation of the ternary liquid alloy to be determined in a very wide compositional range. The analysis of the plots Δ mix H = f(X Ag or Au or Si ) X(Au)/X(Ag)(or X(Ag)/X(Si) or X(Au)/X(Si)) = Ct for several values of the ratio: X Au /X Ag = 7/3, 1/1, X Ag /X Si = 9/1, and X Au /X Si = 4/1, 3/2, 1/1 enabled us to obtain some points of the liquidus surface; indeed, the appearance of the equilibrium L 1 ↔L 2 +S results in a break in these Δ mix H curves. Differential thermal analysis of 35 ternary alloys belonging to the sections X Au /X Ag , X Au /X Si , and X Ag /X Si yielded liquidus, first and second crystallization surfaces. These results, in good agreement with those obtained by calorimetry, were then interpolated in order to localize the ternary eutectic valley. The second step of this work was to calculate the equilibrium phase diagram of Ag-Au-Si. For this purpose, all the thermodynamic data available for the three binary systems (Ag-Au, Au-Si, and Ag-Si) were taken into account, after analysis, together with the ternary information obtained experimentally. Thus, it was possible to propose a quasi-complete description of the equilibrium phase diagram of Ag- Au-Si. It should be stressed, however, that the lack of thermodynamic data relative to solid solutions does not allow the existence zone of these solid solutions to be determined. Crystallographic investigations of these solids will be necessary to fully complete the description of this phase diagram.