Determination of the 76 wt.% Au section of the Al–Au–Cu phase diagram

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

“…Therefore, there is a peritectic relationship in this system, with the solidification occurring first as L (liquid) →L+α and then, once the composition of the remaining liquid has become enriched in Al, L+α→β. 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.…”

“…In this case the solid material must contain an increasingly greater proportion of Cu (metallic radius 0.128 nm, compared to the 0.144 nm of Au) as the temperature rises, and the lattice of the phase in equilibrium with the melt must therefore contract, as observed. There is no α-phase in these patterns indicating that in Spangold itself the reaction is β→(L+β)→L as the temperature is increased, as indeed was shown on the previously published 18 karat pseudobinary [21]. The approximate proportion of phases present in the melting zone can be obtained by plotting the normalised {011} peak area against temperature and taking this parameter to be proportional to the fraction of solid phase present, Figure 5(c).…”

“…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.…”

“…However, EDS measurements indicated that the boundary of the β-phase field on the γ side was at an e/a ratio of about 1.46 at 750˚C. Finally, from the published 18K isopleth [21] it is known that, at 750˚C, the material would be partially molten, and the range of solid β-phase must therefore transition into (L+β), (L+β+γ), and (L+β+α) phase fields on the 750˚C isothermal section. The β-phase of the Au-Al binary phase diagram, which actually has the approximate stoichiometry Au 9 Al 2 (rather than Au 3 Al), has a eutectic with Au 8 Al 3 at 525˚C and a peritectic with L and α at 545˚C, so it is clear that the β-phase stoichiometries for 0<x<1 in Au 8-x Cu 4+x Al will be completely molten on the 750˚C isothermal section.…”

“…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.…”