Al-Au-Cu (Aluminum-Gold-Copper)

Recently, [2002Lev] and [2003Lev] investigated the phase equilibria in this system and presented an isothermal section at 500 °C and a vertical section at 76 mass% Au.

Binary Systems

The Al-Au system [2005Oka] depicts the following intermediate phases: AuAl₂(C1, CaF₂-type cubic), AuAl (AuAl-type monoclinic), Au₂Al (α, β, and γ modifications with MoSi₂-type or related structures), Au₈ Al₃ (rhombohedral, space group R $\bar{3}$ c), Au₄Al (cubic, space group P2₁3), and β (80-81.2 at.% Au; bcc). The Al-Cu phase diagram [1998Liu] depicts a number of intermediate phases: CuAl₂ (θ, C16-type tetragonal), CuAl (η_1,orthorhombic), CuAl (η₂, monoclinic), Cu₅Al₄(LT) (ζ₂, orthorhombic), ɛ₁(bcc), ɛ₂ (B8₂, Ni₂In-type hexagonal), Cu₃Al₂ (δ, rhombohedral), Cu₉Al₄(HT) (γ₀, D8₂, Cu₅Zn₈-type cubic), Cu₉Al₄(LT) (γ₁, D8₃-type cubic), and Cu₃Al (β, bcc). In the above, HT = high-temperature and LT = low-temperature. Au and Cu form a continuous face-centered cubic (fcc) solid solution at high temperatures. At lower temperatures, at least three ordered structures Au₃Cu (L1₂, AuCu₃-type cubic), AuCu-I (L1₀, AuCu-type tetragonal), and AuCu₃-I (L1₂-type cubic) are known with formation temperatures of 240, 385 and 390 °C respectively [Massalski2].

Ternary Phase Equilibria

With starting metals of at least 99.9% purity, [2002Lev] arc-melted or air-melted about 50 alloys. The alloys were annealed at 500 °C for 2 h and quenched in ice-water or ice-brine mixture. [2002Lev] pointed out that the annealing time of 2 h corresponds to the cast-and-solution-anneal kind of treatment and may or may not have produced the equilibrium structures. The isothermal section at 500 °C constructed by [2002Lev] is shown in Fig. 1. A ternary phase labeled β with the nominal formula AlAu₂Cu with the B2-type of ordered bcc structure is stable below about 800 °C in an approximately-triangular region having the coordinates of Al_1.08Au_1.96Cu_0.96, Al_0.68Au_2.12Cu_0.80, and Al_1.0Au_1.0Cu₂ [2002Lev]. The ternary phase β, the binary phaseAu₄Al (labeled as β by [2002Lev]) and the binary phase Cu₃Al (also labeled β, stable only above 567 °C) all lie approximately along the 25 at.% Al line, suggesting the possibility of a continuous solid solution between them at higher temperatures. Cu₉Al₄(LT) (γ₁) dissolves a large amount of Au, which substitutes for Cu at constant Al content. [2002Lev] found some evidence for the ternary ordering of Au and Cu atoms in the γ₁-based solid solution. The line corresponding to 75 mass% Au (the 18-carat line) (Fig. 1) passes through the single-phase regions of the ternary β and the γ₁-based solid solution.

Fig. 1

Al-Au-Cu isothermal section at 500 °C [2002Lev]

[2003Lev] used about half of the samples prepared by [2002Lev], which had an approximate Au content of 76 mass%. Differential thermal analysis and differential scanning calorimetry were employed at a heating/cooling rate of 5-10 °C per min to identify the thermal arrests. Samples were also annealed at 700, 600, 500, and 400 °C for 2-4 h, followed by ice-brine quenching. The phase equilibria were studied with optical microscopy, x-ray powder diffraction, and energy dispersive spectral analysis. The vertical section constructed by [2003Lev] at ∼76 mass% Au is redrawn in Fig. 2.

Fig. 2

Al-Au-Cu vertical section at ∼76 mass% Au [2003Lev]