The Ag-Au-Si system: Experimental and calculated phase diagram

Abstract

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}/_{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, andX _Au/X _Si = 4/1, 3/2, 1/1 enabled us to obtain some points of the lithe three binary systems (Ag-Au, Au-Si, and Ag-Si) were taken into account, after analysis,quidus surface; indeed, the appearance of the equilibrium L₁ ↔ L₂ + S results in a break in these Δ_mixH 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 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.