In Memoriam

138 Gold: Science and Applications
Ag
Weight percent Ag
9kt
80
60
14kt
18kt
40
20
α 1 + α 2
α
α 1 AuCu II
α 1 AuCu 3
α 1 + α 2
α 1 + α 2
in 10ct alloys. However, during slow cooling and even during quenching from the solid solution at
elevated temperatures, they decompose into copper- and silver-rich phases (also referred to as phase
separation), leading to already high hardness and difficult workability, for example, of castings or
soft-annealed and subsequently air-cooled material. For lower Ag contents down to ~ 5 wt% in 14kt
and 10kt alloys as well as higher Ag contents up to roughly 37 wt% in 14kt and 50 wt% in 10kt
alloys, single-phased and hence more easy-to-work alloys can be more easily obtained by quenching,
with a good potential for subsequent age-hardening. Beyond those boundaries for the Ag content,
little to no age-hardening potential is available in the ternary 14kt and 10kt alloys.
The miscibility gap extends much less into the area of 18kt alloys, so that alloys with Ag contents
above ~20 wt% are soft and not age hardenable. With lowering of the Ag content the hardness in the
aged condition increases steadily, even down to single-phased alloys with low Ag content. This is
attributed to the extension of the stability field of the ordered AuCu solid solution into the ternary system
and the related hardening effects (see Disorder–Order Transformation Hardening earlier). Hence
precipitation hardening and disorder-to-order transformation hardening overlap for a wide range of
ternary 18kt Au alloys. In fact, the two mechanisms also overlap for alloys with gold contents down to
~35 wt%, since the Cu-rich precipitates can undergo disorder-to-order transformations during aging
or slow cooling as illustrated by the isothermal section shown in Figure 7.13 [33– 36].
From the many further alloying elements that are added to Au-Cu-Ag-based alloys, zinc is of
particular importance. It is added in large amounts of 4 wt% and significantly above especially to
14kt and 10kt jewelry alloys for color adjustment, but also because it narrows the volume of the
immiscibility gap present in the ternary system. This leads to softer and more workable alloys in
both the age-hardened and air-cooled states [32]. The addition of 6 wt% Zn to a 14kt alloy or 9 wt%
Zn to a 10kt alloy leads to single-phased alloys that are not age hardenable.
As obvious from Figure 7.12, ternary Au-Ag-Cu–based 18kt jewelry alloys with medium to high
Ag content are comparably soft and show little or no age-hardening potential. While the hardness
and age-hardening potential of these alloys can be increased by additions of Zn, In, Sn, or Ga for
casting applications, there is a demand for alloys with improved age hardenability for applications
involving mechanical working, especially in the watch-making industry [43,44]. The influence of
80
AuCu I
AuCu II
60
AuCu 3
20 40 60 80
Weight percent Cu
Weight percent Au
FIGURe 7.13 Isothermal section at 300°C in the Au-Ag-Cu system [2]. (From Phase Diagrams of Ternary
Gold Alloys, A. Prince, G. V. Raynor, and D. S. Evans, The Institute of Metals, London, UK, 1990. With
permission.)
40
20
Cu