A 500 °C isothermal section for the Al-Au-Cu system

Abstract

The Al-Au-Cu system and its associated ternary alloys and intermetallic compounds is surprisingly poorly known, and the authors could find no phase diagram for it in the literature. This article addresses this omission by presenting an isothermal section at 500 ЊC, derived with the aid of X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), metallography, and hardness measurements. The samples studied had generally received an anneal of 2 hours at 500 ЊC, primarily in order to complete any transformations that occurred during solidification and cooling of the castings. The possibility of further changes on protracted annealing at 500 ЊC is not ruled out, and the diagram presented is, therefore, applicable only to material prepared by thermal processing of an industrial nature. The presence of a ternary ␤ phase with a nominal stoichiometry of AlAu 2Ϫx Cu 1+x (0 Յ x Յ 1) was confirmed, and its phase field at 500 ЊC was determined. A number of the binary intermetallic phases were found to exhibit some solid solubility of the ternary element. In particular, the ␥ -Al 4 Cu 9 phase extends deep into the ternary and, in the vicinity of the commercially interesting 18-carat line, appears to exist in a ternary ordered form, designated here as ␥ 2 .

Ternary β and γ phases in the Al–Au–Cu system at 750°C

“…This is similar to the situation reported previously for the 50 at.% Au region of the phase diagram [21]. Overall, the Sample 6 composition lies within the α-phase field of the previously published 500˚C isothermal section [20], which is continuous along the Au-Cu edge at that temperature. It should also be continuous at 750 °C.…”

“…From previous knowledge [20] and the characterisation results shown below, the thirteen samples can be classified according to how much of the phases α, β, γ or Al 2 Au they contain. The α-phase is the face-centred cubic solid solution of Au and Cu, also denoted as (Au,Cu).…”

“…Finally, Al 2 Au, also known as 'purple gold', is a purple-colored intermetallic compound. Consideration of the microstructures, compositions and electron-toatom (e/a) ratios of the experimental samples, taken together with the published binary edge phase diagrams [12,22], 500˚C isothermal section [20] and 18K pseudobinary [21], permitted the construction of a 750˚C isothermal section, Figure 1. We will now briefly discuss the main features of this ternary section, and justify its construction.…”

“…This raises the question of whether the stability and resistance to aging of Spangold might also be found in Al-Au-Cu β-phase alloys of lower Au content. Previous work in this system has been focussed on the 500˚C isothermal section and on 18 K compositions [20,21]. The focus of the present work was, however, to establish the boundaries of the β-phase at lower Au contents and at 750˚C, which is a more realistic betatising temperature for the lower karat compositions.…”

Phase diagram of Au-Al-Cu at 500 °C

“…The isothermal sections of Au-AlCu at 500°C were reported by Levey et al [9][10][11] and those at 750°C by Bhatia et al [12]. Levey et al [13] also constructed isopleths for the 76 wt.% Au section.…”