Christopher W. Corti is now retired from World GoldCouncil, but he works for them now as a Consultant. He isalso Consultant to the Goldsmiths Company, in London(the ancient association who operates the London AssayOffice). The consultancy company of C. W. Corti is calledCoreGold Technology Consultancy. He has organizedseminars on metallurgy of precious alloys and on jewellerytechnology for the World Gold Council all over the world.
This paper focuses on microalloyed gold and examines themetallurgy - the theoretical basis for hardening - anddiscusses some candidate alloying elements, which couldform the basis of microalloyed 24 ct golds. Publishedinformation on the compositions and properties of actualmicroalloyed 24 ct golds is discussed. The scope foradapting the microalloying approach to 22 ct and othercarat golds as well as to platinum and silver is alsodiscussed in terms of current developments. Theadvantages and disadvantages of such materials forjewellery application are considered.
Christopher W. CortiCoreGold Technology Consultancy, London, UK
Pure gold, platinum and silver, like all pure metals, are relatively soft with low yieldpoints and this has several drawbacks in the fabrication of 24ct gold, pure platinumand pure silver jewellery, limiting design possibilities as well as making suchjewellery prone to scratching and wear. This problem has traditionally beenovercome by alloying to increase hardness and strength and has led to the use ofthe carat golds, sterling silver and 950 platinum and the lower finenesses of platinumand silver in modern jewellery. However, for gold especially, 24 carat gold of purity>99.0% is the alloy of choice in the Far East (1, 2) where it is known as Chuk Kam,meaning pure gold, and in the largest market, India, 22 carat gold dominates. Thesetwo markets account for around 40% ot total gold jewellery fabrication and therelative softness of both 22 and 24 ct gold is seen as a weakness: the developmentof stronger 22/24 ct golds has long been desired.
For gold, the development of 990 gold, a 99.0% gold - 1% titanium alloy in the late1980s, hallmarkable as 24 ct, overcame many of the deficiencies of 24 ct gold, withgood strength and hardness, but has not met with much commercial success forseveral reasons (1,2). In recent years, however, there have been a number ofhardened (or improved strength) 24 ct gold materials developed, with finenesses of99.5% or higher, some in commercial production, where improved hardness andstrength have been achieved by microalloying, i.e the addition of very small amounts(typically
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The metallurgy - the theoretical basis for hardening - and possible candidatealloying elements, which could form the basis of microalloyed 24 ct golds.
Published information on the compositions and practical aspects of jewellerymanufacture in actual microalloyed 24 ct golds.
Progress in adapting the microalloying approach to 22 ct and other high carat goldsas well as to platinum and silver is also discussed in terms of current developments.The advantages and disadvantages of such materials for jewellery application areconsidered.
Improved Strength 24 Carat Golds
In recent years, a number of improved strength 24 carat golds have been developed(4 - 13), some commercially available, and jewellery produced in these materials arein the market place, particularly in Japan, Figure 1. All have virtually the same meltingpoint, colour and density as normal pure gold. These are listed, with theirmechanical properties in Table 1.
Figure 1. Jewellery in High Strength Pure Gold (8)
It is evident from Table 1 that, whilst annealed hardness is usefully higher than that fornormal commercial pure gold, cold working results in significant hardness increases andthat some materials can be further hardened by low temperature ageing heat treatments.
Table 1. Improved Strength 24 ct Golds
Perhaps not surprisingly, the highest hardnesses are achieved in the lower puritygolds of 99.5 - 99.7% fineness. Most can be cast, but the best hardness values areachieved in the wrought condition, often coupled with ageing heat treatments. Froma practical standpoint, as far as published information tells us, these materials cannotbe simply remelted and recycled without loss of strength (8), as the hardeningmicroalloying additions lose their effect on remelting. As we shall see later, this is dueto the oxidation of the microalloying metals on melting.
The superior strength of these materials has a beneficial effect in manufacturingjewellery in that certain processes can be done that are difficult with normal 24 ctgold (8, 14,15). For example, findings such as lobster claws and some difficult chaindesigns become feasible.
When compared to standard yellow carat golds, Table 1, it can be seen that theimproved strength, microalloyed 24 ct golds approach the hardness of 22 ct gold in bothannealed and cold worked conditions, but are still some way behind those of 18 ct gold.
Material Manufacturer Purity AnnealedHardness,
99.9% 55 123 500 2 Castable
Honten, Japan99.9% 35 - 40 90 - 100 - - Castable
Mintek, S. Africa
Aged: 131 - 142
PureGoldThree O Co,
99.5+% - ca.130 - -
DiAurum 24 Titan, UK 99.7%60
(as cast)95 - - Castable
Pure Gold - 99.9% 30 50 190-380Anneal:40
C.W.: 122ct Yellow
(5.5 Ag - 2.8 Cu)
- 91.7% 52 100-138 220-440Anneal:27
18ct Yellow(12.5 Ag -12.5 Cu)
- 75.0% 150190 - 225
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It is surprising that such improvements in strength and hardness can be achieved ingold with alloying additions of only 0.5% wt. or lower. Such small additions can bedescribed as microalloying. It is instructive, therefore, to examine how such propertyimprovements are possible in microalloyed gold in terms hardening mechanisms ingold alloys. As will be apparent, this exercise will have relevance to silver andplatinum too.
Basic Mechanisms Of Hardening Gold
There are several mechanisms for hardening pure metals and more than one can beutilised in practice:
Grain size control (Hall-Petch effect) Solid solution hardening by alloying Cold working (Work hardening) Two phase microstructures (including ordering) Dispersion hardening by a second phase (age- or precipitation-hardening)
For those wanting a simple explanation of each, I suggest you read the originalpapers on this topic (1,2, 15). In carat golds, all mechanisms of hardening may beutilised. As we shall see, hardening by microalloying is accomplished primarily bydispersion hardening. There is some evidence in the literature of substantialhardening by oxide dispersions in gold (16,17). Poniatowski and Clasing (16)reported that a dispersion of 0.42% wt (1.85% vol) of TiO2 particles, 0.5mm diameter,gave an annealed hardness of HB 55 compared to HB 20 for pure gold, and that thisrose to HB 80 after cold working by 80% reduction. Tensile strength was about 190N/mm2 compared to about 75 N/mm2 for pure gold. Hill (17) studied mixtures of goldpowder and various oxides to produce dispersions of oxides up to 1.0% by volume(0.18 - 0.38% wt). Annealed hardnesses ranged from HV 51 - 65, which increased toHV 67 - 82 after 82% cold work. Tensile strengths ranged from 153 - 207 N/mm2
compared to 112 N/mm2 for pure gold. These studies demonstrate that dispersionhardening can enable substantial hardening of gold at low concentrations.
Microalloying of Gold
Compositions: Density effect To preface this section, mention must be made of the difference between atomweight and volume. The higher atomic numbered metals are heavier and denser.Gold is a heavy metal, with a density of 19.32, whereas silver has a density of 10.5.and copper a density of 8.93 Thus, in describing alloy compositions, we mustdifferentiate alloy compositions given in terms of weight percent - the relative weightsof alloying metals present - and compositions given in terms of atomic percent, i.e.how many atoms there are of each metal in the alloy. This difference is illustrated withgold-copper alloys. An alloy of 50% gold atoms and 50 % copper atoms, i.e. 1 gold
atom to each copper atom, has a weight % composition of about 75% gold and 25%copper, reflecting the difference in weight of the gold and copper atoms!
The theoretical basis for microalloyingIn the development of improved strength 24 carat golds, we are looking at totalalloying additions of 0.5 wt.% or less, even down to only 0.1 wt. % in some instances,to effect a dramatic strengthening of the gold crystal lattice. Such small additions areapproaching those values typically used to control grain size, such as cobalt oriridium in carat golds. As gold is a low stacking fault metal (stacking faults are a typeof crystal lattice defect), control of grain size alone or in combination with cold workwill not yield significant hardening in pure gold, so such small additions cannot workthrough grain size control only.
Significant solid solution hardening by such small weight additions is only possibleif the alloying metal is very light, i.e. it has a low density and has a small atomicweight compared to gold. If we examine the Periodic Table,