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 called‘CoreGold 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

133June 2005

Introduction

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 <0.3% wt.) of certain metals and these are attaining some limitedpenetration of the market. Very recently, microalloyed silver and platinum materials,with increased strength and hardness, have been developed and are beingcommercialised (3).

Until relatively recently, the little published on such microalloyed precious metalswas mainly in patents and there was little understanding of the metallurgical basis forsuch materials. The work on microalloyed gold was reviewed by the author (1) at the1999 Santa Fe Symposium and attempted to explain the theoretical basis for themicroalloying of gold. Since then, some further information has been published andinterest in microalloyed silver and platinum has emerged. It is, thus, an appropriatetime to review progress and our understanding and to widen the scope to includethe new platinum and silver materials.

This paper initially focuses on microalloyed gold, which serves as a model preciousmetal to examine microalloying, and covers the following aspects:• The microalloyed golds that have been developed in terms of their properties in

comparison to those of conventional carat golds

Microalloying of High Carat Gold,

Platinum and Silver

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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.

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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,

HV

ColdWorked

Hardness,HV

Strength,MPa

Ductility,% Comments

HighStrength

Pure Gold

Mitsubishi,Japan

99.9% 55 123 500 2 Castable

TH GoldTokuriki

Honten, Japan99.9% 35 - 40 90 - 100 - - Castable

Hard 24Carat

Mintek, S. Africa

99.5% 32100

Aged: 131 - 142

- -Age

Hardenable

PureGoldThree O Co,

Japan99.7% 63

106Aged:

145-176- -

Castable,Hardenable,

ChainUno-A-Erre24ct Gold

Uno-A-Erre,Italy

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

C.W.: 3Castable

18ct Yellow(12.5 Ag -12.5 Cu)

- 75.0% 150190 - 225

Aged:230

520-900

Anneal:40C.W.: 3Aged:

15

Castable,Age

Hardenable

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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

137June 2005

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, the light metals thatmight be possible microalloying additions are, in order of density: Lithium,Potassium, Sodium, Calcium, Magnesium and Beryllium, Table 2.

Table 2. Possible Light Metals for Alloying into Gold

Assuming a maximum alloying level of 0.5%, and taking the lightest metal in Table 2,lithium, then a gold - 0.5 wt % lithium alloy, for example, is 12.55 atomic % lithium,which is within the solid solubility range. This is 1 atom of lithium to every 7 atoms ofgold. In comparison, a gold - 12.55 at.% copper alloy is 4.4% copper in weight %terms which would increase hardness in the annealed condition to about HV40 andto about HV80 in the cold worked condition. So maybe a gold-lithium alloy couldprovide some of the necessary property improvement by solid solution hardening.

If we look at another light metal, calcium, a gold- 0.5 wt.% calcium alloy is only 2.41at.% calcium which is quite small - only 1 atom in 40 - and, therefore, would not beexpected to provide much solid solution strengthening. However, reference to thephase diagram, Figure 2, shows that there is virtually no solid solution of calcium ingold and that there is a eutectic comprising 2 phases, gold and an intermetalliccompound, Au5Ca, which has a high gold content. If this phase is finely dispersedin the microstructure, then we have the basis of a possible alloy system that couldprovide improved properties through dispersion hardening.

Metal Atomic number Atomic Weight Density, g/cm3

Lithium 3 6.9 0.53

Potassium 19 39.1 0.86

Sodium 11 23.0 0.97

Calcium 20 40.1 1.53

Magnesium 12 24.3 1.74

Beryllium 4 9.0 1.85

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Figure 2. The gold-calcium phase diagram

We find similar features to the gold-calcium system in the phase diagram for the gold- potassium system, but less strongly (i.e. lower gold content intermetallic phases) inthe gold - beryllium, gold - magnesium and gold - sodium phase diagrams,suggesting that they are less favourable for a microalloying approach.

Another alloying approach would be to add the rare earth metals, such as cerium,lanthanum and dysprosium, as these also tend to have limited solid solubility in goldand to form eutectics and intermetallic compounds with gold. Table 3 lists somerelevant features of their phase diagrams with gold. Some rare earth metals havebeen omitted for brevity.

Table 3 Features of Gold - Rare Earth Phase Diagrams

* Solubility at the eutectic temperature; this reduces as the temperature falls.

From this table, it can be seen that the light rare earths are potentially suitable. Figure3 shows the phase diagram for gold-cerium.

Rare Earth Solid Solubility in gold Intermet compd. Eutectic, at %

(temp, °C) Comment

Lanthanum v.low Au6La 91 (808) OK

Cerium v.low Au6Ce 90.5 (808) OK

Praeodymium v.low Au6Pr 88 (808) OK

Neodymium v.low Au6Nd 90.5 (796) OK

Samarium v.low Au6Sm 88.5 (770) OK

Gadolinium low (0.7 at %*) Au6Gd 90.5 (804) Age-hardenable?

Dysprosium 2.1 at %* Au6Dy 90.5 (808) Age-hardenable?

Erbium 5.7 at %* Au4Eb 88.6 (734) Age-hardenable?

Terbium 1.5 at %* Au6Tb 90.3 (798) Age-hardenable?

Lutetium 7.7 at %* Au4Lu 84.8 (890) Age-hardenable?

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The similiarity to the gold-calcium system at the gold-rich end is evident. Also, as the‘heavy’ rare earths have a solid state solubility in gold at the eutectic temperature inexcess of 0.5%, but a very low solubility as the temperature falls, it is possible thatthey may be amenable to age-hardening treatment with the precipitation of fineparticles of the intermetallic on annealing quenched material at low temperatures.

Figure 3. The gold-cerium phase diagram

Figure 4 shows the region of solid solubility for gold-erbium alloys.

Figure 4. The limit of solid solubility at the gold-rich end of the gold-erbium phase diagram

In the development of the 990 gold-titanium alloy, Gafner (18) describes work doneby the Degussa company in Germany on other candidate alloy systems whichincluded the heavy rare earths. The basis for selection was the possibility of secondphase precipitation as the alloy containing a 1% wt alloying addition in solution wascooled from 800°C to 400°C. From this, a table of probable hardening effectivenesswas constructed, Table 4.

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Table 4. Candidate Alloy Systems and Probable Hardening Effect,[from Gafner, reference (18)]

*Fraction of 1 wt % of alloying element precipitating at 400°C.

The fraction of hardening phase in the last column (calculated as the fraction of the1% alloying addition precipitating in atomic percent multiplied by the number ofatoms of the alloying addition in the precipitating intermetallic compound) was takenas an indication of hardening effectiveness. The reason for developing the 990 gold-titanium alloy is obvious from this table. The promise of the rare earths and zirconiumshould also be noted.

However, in this work, a 1 wt% alloying addition was being evaluated. If we consideronly a 0.5 wt% addition of rare earth, then from the solubility data at 400°C in Table4, we cannot expect much hardening phase to precipitate on annealing solutionisedmaterial at 400°C.

Fortunately, Degussa carried out some tests (18) on gold-rare earth alloys at alloyinglevels of 1 wt% and lower. Cast alloys were annealed at 800°C for 1 hour. They werealso cold rolled up to 95% deformation and subjected to age hardening treatmentsat a range of temperatures. Table 5 shows the hardness values attained for goldalloys containing 0.5% alloy or less.

Table 5. Hardnesses of Gold - Rare Earth Alloys (from reference 19)

* Approximate values taken from graphs. C.W. - cold worked

System Solubil’y 800°C

Solubil’y 400°C

Fraction*wt%

Ratio, atom. wt. Fraction* at % Fraction

harden. phaseAu - Ti 1.2 0.4 0.6 4.1 2.5 12.5 Au - Rh 0.6 0.2 0.4 1.9 0.8 0.8Au - Ru 1.0 0 1.0 2.0 2.0 2.0Au - Zr 2.0 0.3 0.7 2.2 1.5 7.5 Au - Tb 1.2 0.3 0.7 1.2 0.8 5.6 Au - Dy 1.9 0.3 0.7 1.2 0.8 5.6Au - Ho 3.2 0.4 0.6 1.2 0.7 4.9Au - Er 4.8 0.4 0.6 1.2 0.7 3.5

Alloy Composition,wt%

Hardness, As cast,

HV

Hardness,Annealed

HV

Hardness*,95% C.W.

HV

Hardness*,Aged 300°C

HVAu - 0.3 Gd 44 30 130 63Au - 0.5 Gd 34 48 115 85 Au - 0.5 Tb 44 30 110 67Au - 0.5 Dy 70 29 120 75 Au - 0.3 Y 35 24 110 45Au - 0.4 Y 32 34 120 -Au - 0.5 Y 61 38 145 174

141June 2005

From this work, it can be seen that the annealed hardness is little different from normalpure gold, although cold worked material is much harder and in the range of theimproved strength 24 carat materials (Table 1). Age hardening heat treatments are notvery effective at these low concentrations with the exception of the 0.5% gold-yttriumalloy (and yttrium is not strictly a rare earth metal), confirming the view expressedearlier in that consideration of the solubility data, Table 4, of the heavy rare earthssuggested little age hardening was possible at these low alloying levels. Whetheralloys of gold with the light rare earths show good properties is not known. It is difficultto comment on the results for gold-yttrium alloys as there is no published gold-yttriumphase diagram (20), but recent work by Ning (21) indicates that it is similar to theheavy rare earths with some solid solubility (about 2%) of yttrium in gold.

To summarise, the mechanism of hardening by microalloying would appear to bebased on some form of dispersion (precipitation) hardening by intermetallic phasesof high gold content in alloy systems that form eutectics at high gold contents andwhere the microalloying addition has little solid solubility in gold.

Compositions of Microalloyed Golds

The possible theoretical basis for microalloying of gold has been discussed and it isnow appropriate to compare this with what is known about the microalloyed goldsthat have been developed, Table1.

1. High Strength Pure Gold - Mitsubishi Materials Corporation Mitsubishi have several patents in this area. In their main patent (4), they claimgold alloys of 99% purity or higher containing 200 - 2000 ppm of one or more ofthe following elements: calcium, beryllium, germanium and boron. From othersources (22), it is clear that calcium is the principal hardening metal in HighStrength Pure Gold. Examination of the phase diagrams for gold-beryllium, gold-germanium and gold-boron shows similarities with the diagram for gold-calcium,so similar effects on microstructure and properties are anticipated. The patentalso includes further additions of 10 -1000 ppm of one or more of many metalsincluding magnesium, aluminium and cobalt and /or 10-1000 ppm of rare earthmetals and yttrium. The hardness values for over 50 alloys quoted in their patentlie typically in the range HV 100 - 140 which is consistent with the claimedproperties for High Strength Pure Gold.In a further patent (5), an alloy of 99% gold or higher is claimed containing 500-2000 ppm calcium and 1-50 ppm carbon. The role of carbon is not clear, but mayharden interstitially or preferentially segregate with some calcium to grainboundaries.

2. PureGold - Three O CompanyIn their patent (10), an age hardenable alloy of 99.7% gold with a hardnesscomparable to an 18 carat gold is claimed containing 50 ppm or more gadoliniumand optionally a third metal - calcium, aluminium or silicon - the total being in the

142 Jewelry Technology Forum

range 100-3000 ppm. For an alloy containing gadolinium and calcium, amaximum hardness after a combination of working and ageing of HV 176 isdescribed. The optimum ageing temperature is 250°C.

3. Hard 24 ct Gold - MintekAt the time of the original review, there was no published information on thisdevelopment. Since then, a patent (23) and paper (24) have provided anunderstanding of this alloy. As the authors explain (24), this alloy was developedto overcome the problems associated with the other microalloyed golds that usecalcium and/or the rare earths, i.e. the loss of strength on remelting, due to lossof the microalloying metals,and the expense and difficulties in making them. Hard24 ct Gold can be made and processed on conventional equipment and isamenable to remelting without significant loss of strength. The age-hardenablealloy is of 995 fineness and contains 0.2% cobalt and 0.3% antimony. It is basedon precipitation hardening by a gold-antimony intermetallic, AuSb2, with thecobalt retarding recrystallisation. The gold-antimony phase diagram is a eutecticsystem at the gold-rich end and shows the features described in the previoussection. As shown in Table 1, this alloy can achieve hardnesses of HV 100 in thecold-worked condition and up to HV 142 in the cold-worked and aged condition.

4. Other GoldsFrom private discussions, I am aware of the use of calcium in combination withother alloying metals in some of the other golds listed in Table 3. In a patent fromTanaka KK, Japan (6), an alloy for precision casting is claimed containing smallamounts of hafnium and rare earth metals.In some reports (8,9), the cold working of the surface during finishing plays animportant role in hardening the surface.

5. Other literatureDoped pure gold wires are used extensively in the electronics industry forbonding. In a recent paper (25), Lichtenberger and colleagues doped high puritygold (5-9’s purity) with 3-30 ppm of aluminium, calcium, copper, silver and/orplatinum. They showed that most dopants strengthened the wire during extrusion(beryllium had the largest effect) but only calcium and beryllium had significantstrengthening effects after annealing. This is explained on the basis of the atomsize difference in the gold lattice: Calcium atoms are about 30% larger andberyllium atoms are about 30% smaller than gold. There will be a tendency forcalcium atoms to sit on grain boundaries and pin them. Various patents for improved strength gold bonding wires cite additions ofbismuth, rare earths, calcium with beryllium, europium and niobium, germanium,barium, yttrium and rare earths, or calcium and lead. The use of calcium,beryllium and/or the rare earths seems to be a popular choice in this application. Recent work by Sarawati and co-workers (26) on calcium and palladium additionsto gold bonding wire materials showed calcium to be more effective in increasingstored energy, strength and ductility of cold-worked material than palladium andthis is attributed to grain boundary segregation and dislocation loop generation.

143June 2005

Practical aspects of making jewellery

The practical attributes of microalloyed 24 ct golds for jewellery making have beenfully discussed elsewhere, e.g references 8,14, 15 and 24. In summary, these are:

• Can be investment cast and fabricated by conventional techniques• Can make some products that are difficult with normal 24 ct gold, such as some

chain designs, springs, screws, catches and other findings• Can make lightweight strong chains with good wear properties• Melting generally needs to be done in an inert atmosphere to prevent oxidation

of the microalloying metals. Therefore, they cannot be made easily in smallworkshop situations by conventional melting in air

• As with normal 24 ct gold, soldering must be consistent with national hallmarkinglaws. In some countries, use of lower (22 ct) solders is allowed although theremay be limits on the amount of solder used. However, another factor is the temperature of soldering, which can lead to a lossof work hardening in the vicinity of the joint. Laser welding may overcome bothproblems. There are also very low melting point 22 ct gold solders. Onesolderpaste has a melting point of 361°C.

• Polishing should be easier than conventional 24 ct gold as they are harder.Bernadin (14) notes that castings require procedures similar to those for platinum.

• Recycling of scrap is generally not viable due to loss of strength on remelting, asnoted earlier. The Mintek alloy is an exception here. However, remelting shouldallow the gold content to be fully recovered for further use.

Application to high carat golds

The microalloying approach described in this paper should be applicable to 22 ct andother high carat golds. However, significant improvements to 22 ct golds have beendescribed (24, 27) using conventional alloying approaches. Van der Lingen and co-workers at Mintek and Fischer-Bühner of FEM have both demonstrated use of circa 2.0 -2.5% cobalt additions to effect significant strengthening. Taylor (28) has also patented acobalt-containing alloy, but with up to 1% boron included. However, as will be seen inreference 29, the microalloying approach can be effective in 22 ct and lower carat goldstoo, using gadolinium and calcium additions.

Microalloying of Silver

In principle, it will be evident that the same approach for microalloyed gold could beapplied to silver. The developer of PureGold (Table 1) has also developed PureSilver,a 99.3% silver micro-alloyed material and this is being marketed in the USA. It isclaimed (3) to be easy to work with as it does not require annealing, is very tarnishresistant, casts well and is harder than sterling silver. The patent (29) coversmicroalloyed gold, silver, palladium and platinum materials.

144 Jewelry Technology Forum

In the case of silver, an alloy of at least 80.0% silver is claimed containing the rareearth gadolinium in the range 50 -15,000 ppm, with optional further additions ofalkaline earth metals, silicon, aluminium and boron in the range 50 - 15,000 ppm intotal, with hardness values of HV130 or higher and a Young’s modulus of not lessthan 7,000 kg.mm-2. A further embodiment claims an alloy of at least 99.45% silver with 50 - 5,000 ppm ofGd plus further optional additions of the same metals in the range 50 -5,000 ppm intotal. Hardness values of at least HV 120 and a Young’s modulus of 7,000 kg.mm-2

are claimed, with hardness rising to at least HV140 with 50% cold-work.These materials are claimed to be heat treatable. Gadolinium is claimed to be themost effective hardening element. The small additions do not affect the colour. Thefurther optional additions listed above have a synergistic effect, but of these calciumis preferable.

Microalloying of Platinum

Microalloying of platinum should also be possible. Of course, oxide dispersionstrengthened platinum (such as ZGS platinum from Johnson Matthey) for industrialapplications has been on the market for many years. In a patent (30) from Japan, a hard, high purity platinum alloy containing 10 -100ppm of cerium is claimed, that has a good hardness, lustre and tarnish resistancesuitable for jewellery manufacture and is at least 99% purity. Vacuum or inert gas isrequired during melting to prevent loss of cerium by oxidation. The platinum-ceriumphase diagram, shown in Figure 5, shows characteristics similar to that of the gold-cerium system as described earlier for gold, i.e little solid solubility of Ce in Pt, aeutectic system between Pt and a high platinum-containing intermetallic. For ceriumcontents of 0.03 - 0.3%, hardness values of HV 61 - 102 were obtained compared toHV 40 for pure platinum and HV 120 -136 for 950 fineness platinum alloys.

Figure 5. The platinum-cerium phase diagram

145June 2005

A commercial platinum, HPP Platinum, from Johnson Matthey is claimed to haveimproved strength and contains about 250 ppm of samarium, at 999 fineness (31).A hardness of HV55 compared to HV 50 for a pure 999 platinum is published. Workby Ning at the Chinese Institute of Precious Metals on rare earth additions has shownthat rare earth additions to platinum can improve strength [see references 3 - 5 in(32)]. In reference 32, Ning and Hu show that a 0.05% cerium addition to a platinum- 15% palladium - 3.5% rhodium alloy increases room temperature tensile strengthfrom 280 MPa to 400 Mpa, with similar improvements to tensile and creep strengthat 900°C. These increases are attributed to both solid solubility and dispersionstrengthening effects.

The Ogasa patent (29) mentioned above also includes platinum materials. Thisclaims platinum alloys of at least 85.0% platinum containing gadolinium and theother optional elements listed above for silver, preferably calcium, in the range 50 -15,000 ppm in total. Cast hardness values of at least HV120 and Young’s modulusof not less than 8,000 kg.mm-2 are claimed. With 50% cold work, hardnesses of atleast HV 150 are claimed.

A further claim is for alloys of at least 99.45% platinum and additions of gadoliniumand the optional elements, preferably calcium, in the range 50 - 5,000 ppm total withhardness and Young’s modulus values as previously given.

Thus, in summary, it is evident that microalloyed platinum alloys are possible,although it not clear whether they are being commercially produced for jewelleryapplication, and that rare earths and calcium are preferred micro-alloying additions,paralleling the work on gold.

Conclusions

The metallurgical basis for strengthening and hardening gold of at least 995 finenesshas been reviewed. This shows that calcium and the rare earths to be the preferredmicroalloying additions, but other possibilities have been noted. Commercial alloysare available and jewellery is being made in these alloys. The advantages and pitfallsof such alloys for jewellery production have been summarised.

Hardening of silver and platinum by microalloying has also been reviewed and theirmetallurgy parallels that of gold. The silver materials are known to be commerciallyavailable in the USA.

Acknowledgements

Thanks are given to many colleagues in the industry who have provided informationthat has enabled this review to be written. Thanks also to World Gold Council for theirsupport and permission to publish.

146 Jewelry Technology Forum

References

1. C.W.Corti, “Metallurgy of microalloyed 24 ct golds”, Proc. Santa Fe Symposium, 1999, p379 - 402

2. C.W.Corti, “Metallurgy of microalloyed 24 ct golds”, Gold Bulletin, 32 (2), 1999, p39 - 47

3. J.Bernadin, Private communication, 2004

4. N.Uchiyama, Patent WO 95/07367, Mitsubishi Materials Corporation, Japan, 1993

5. Japanese patent JP 07265112A2, Mitsubishi Materials Corporation, Japan, 1993

6. Japanese patent JP 7090425, Tanaka KK, Japan, 1993

7. M.Du Toit, “The development of a 24 carat gold alloy with increased hardness”, Proc Santa

Fe Symposium, 1997, p381-394

8. A.Nishio, “The development of High Strength Pure Gold”, Gold Technology, No 19, July

1996,p11-14 and S.Takahashi, N.Uchiyama & A.Nishio, “Design opportunities through

innovative materials”, Gold Technology, No 23, April 1998, p12-17

9. Anon, “The move to develop stronger gold”, Report in Europa Star magazine, November

1996, p81

10. Kazuo Ogasa, Japanese patent WO 96/31632, Three O Company, Japan, 1995

11. H.McDermott, Titan Metals, U.K., Private communication, 1995

12. O.Caloni, UnoAErre, Italy, Private communication, 1999

13. V.Faccenda, Italy, Private communication, 1998

14. J.Bernadin, “Jewellery manufacturing with the new high carat golds”, Gold Technology, No 30,

Winter 2000, p17 - 21 and “Fabricating with high karat gold microalloys”, AJM Magazine,

June, 2001, p xxx

15. C.W.Corti, Strong 24 carat golds: The metallurgy of microalloying”, Gold Technology, No 33,

Winter 2001, p27-36

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