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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 (η1,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 Al1.08Au1.96Cu0.96, Al0.68Au2.12Cu0.80, and Al1.0Au1.0Cu2 [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.
[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.
References
X.J. Liu, I. Ohnuma, R. Kainuma, and K. Ishida, Phase Equilibria in the Cu-Rich Portion of the Cu-Al Binary System, J. Alloys Compd.,1998, 264, p 201-208
F.C. Levey, M.B. Cortie, and L.A. Cornish, A 500°C Isothermal Section for the Al-Au-Cu System, Metall.Mater. Trans. A,2002, 33A, p 987-993
F.C. Levey, M.B. Cortie, and L.A. Cornish, Determination of the 76 Mass Percent Au Section of the Al-Au-Cu Phase Diagram, J. Alloys Compd.,2003, 354, p 171-180
H. Okamoto, Al-Au (Aluminum-Gold), J. Phase Equilib. Diffus.,2005, 26(4), p 391-393
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Raghavan, V. Al-Au-Cu (Aluminum-Gold-Copper). J Phs Eqil and Diff 29, 260–261 (2008). https://doi.org/10.1007/s11669-008-9286-z
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DOI: https://doi.org/10.1007/s11669-008-9286-z