Phase equilibria in the system Au–Cu–Si and structural characterization of the new compound Au5±xCu2±xSi

• Three isothermal sections of Au–Cu–Si established.
• Crystal structure of the new compound Au5±xCu2±xSi determined.
• Partial Scheil diagram and liquidus surface projection constructed.

The ternary Au–Cu–Si system was investigated by means of powder X-ray diffraction (XRD) for phase identification, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX) for microstructures and chemical compositions, light optical microscopy (LOM), and differential thermal analysis (DTA) for the determination of thermal effects. Three isothermal sections were constructed at 250 °C, 400 °C and 650 °C. A new ternary compound τ, Au5±xCu2±xSi, was identified and its crystal structure was determined by means of single crystal X-ray diffraction. It adopts a new crystal structure type in space group Pnma, Pearson symbol oP32 and shows a composition range between Au5.6Cu1.4Si and Au4.4Cu2.6Si at 250 °C. Lattice parameters were found to vary between a = 9.64–9.50 Å, b = 7.61–7.64 Å and c = 6.90–6.89 Å from the Au-rich to the Au-poor composition limit. Three vertical sections, at 10 and 30 at.% Si and at 10 at.% Cu, were constructed based on DTA data and four invariant ternary phase reactions were identified. A partial ternary reaction scheme (Scheil diagram) and a partial liquidus projection are given.

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