Christopher W. Corti
CoreGold Technology Consultancy, London, UK
Christopher W. Corti is now retired from World Gold
Council, but he works for them now as a Consultant. He is
also Consultant to the Goldsmiths Company, in London
(the ancient association who operates the London Assay
Office). The consultancy company of C. W. Corti is called
‘CoreGold Technology Consultancy’. He has organized
seminars on metallurgy of precious alloys and on jewellery
technology for the World Gold Council all over the world.
This paper focuses on microalloyed gold and examines the
metallurgy - the theoretical basis for hardening - and
discusses some candidate alloying elements, which could
form the basis of microalloyed 24 ct golds. Published
information on the compositions and properties of actual
microalloyed 24 ct golds is discussed. The scope for
adapting the microalloying approach to 22 ct and other
carat golds as well as to platinum and silver is also
discussed in terms of current developments. The
advantages and disadvantages of such materials for
jewellery application are considered.
Microalloying of High Carat Gold,
Platinum and Silver
Introduction
Pure gold, platinum and silver, like all pure metals, are relatively soft with low yield
points and this has several drawbacks in the fabrication of 24ct gold, pure platinum
and pure silver jewellery, limiting design possibilities as well as making such
jewellery prone to scratching and wear. This problem has traditionally been
overcome by alloying to increase hardness and strength and has led to the use of
the carat golds, sterling silver and 950 platinum and the lower finenesses of platinum
and 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. These
two markets account for around 40% ot total gold jewellery fabrication and the
relative softness of both 22 and 24 ct gold is seen as a weakness: the development
of 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 late
1980s, hallmarkable as 24 ct, overcame many of the deficiencies of 24 ct gold, with
good strength and hardness, but has not met with much commercial success for
several reasons (1,2). In recent years, however, there have been a number of
hardened (or ‘improved strength’) 24 ct gold materials developed, with finenesses of
99.5% or higher, some in commercial production, where improved hardness and
strength 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 limited
penetration of the market. Very recently, microalloyed silver and platinum materials,
with increased strength and hardness, have been developed and are being
commercialised (3).
Until relatively recently, the little published on such microalloyed precious metals
was mainly in patents and there was little understanding of the metallurgical basis for
such materials. The work on microalloyed gold was reviewed by the author (1) at the
1999 Santa Fe Symposium and attempted to explain the theoretical basis for the
microalloying of gold. Since then, some further information has been published and
interest in microalloyed silver and platinum has emerged. It is, thus, an appropriate
time to review progress and our understanding and to widen the scope to include
the new platinum and silver materials.
This paper initially focuses on microalloyed gold, which serves as a model precious
metal 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
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• The metallurgy - the theoretical basis for hardening - and possible candidate
alloying elements, which could form the basis of microalloyed 24 ct golds.
• Published information on the compositions and practical aspects of jewellery
manufacture in actual microalloyed 24 ct golds.
Progress in adapting the microalloying approach to 22 ct and other high carat golds
as well as to platinum and silver is also discussed in terms of current developments.
The advantages and disadvantages of such materials for jewellery application are
considered.
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 are
in the market place, particularly in Japan, Figure 1. All have virtually the same melting
point, colour and density as normal pure gold. These are listed, with their
mechanical 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 for
normal commercial pure gold, cold working results in significant hardness increases and
that some materials can be further hardened by low temperature ageing heat treatments.
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Table 1. Improved Strength 24 ct Golds
Cold
Annealed
Worked Strength, Ductility,
Material Manufacturer Purity Hardness,
Comments
%
Hardness, MPa
HV
HV
High
Mitsubishi,
99.9%
55
123
500
2
Castable
Strength
Japan
Pure Gold
TH Gold
Tokuriki
99.9%
Honten, Japan
35 - 40
Hard 24
Carat
Mintek,
S. Africa
99.5%
32
PureGold
Three O Co,
Japan
99.7%
63
Uno-A-Erre Uno-A-Erre,
99.5+%
24ct Gold
Italy
90 - 100
100
Aged:
131 - 142
106
Aged:
145-176
-
-
Castable
-
-
Age
Hardenable
-
-
Castable,
Hardenable,
Chain
-
ca.130
-
-
DiAurum 24
Titan, UK
99.7%
60
(as cast)
95
-
-
Pure Gold
-
99.9%
30
50
190-380
Anneal:40
C.W.: 1
22ct Yellow
(5.5 Ag 2.8 Cu)
-
91.7%
52
100-138
220-440
Anneal:27
Castable
C.W.: 3
150
190 - 225
Aged:
230
Anneal:40
Castable,
C.W.: 3
520-900
Age
Aged:
Hardenable
15
18ct Yellow
(12.5 Ag 12.5 Cu)
-
75.0%
Castable
Perhaps not surprisingly, the highest hardnesses are achieved in the lower purity
golds of 99.5 - 99.7% fineness. Most can be cast, but the best hardness values are
achieved in the wrought condition, often coupled with ageing heat treatments. From
a practical standpoint, as far as published information tells us, these materials cannot
be simply remelted and recycled without loss of strength (8), as the hardening
microalloying additions lose their effect on remelting. As we shall see later, this is due
to the oxidation of the microalloying metals on melting.
The superior strength of these materials has a beneficial effect in manufacturing
jewellery in that certain processes can be done that are difficult with normal 24 ct
gold (8, 14,15). For example, findings such as lobster claws and some difficult chain
designs become feasible.
When compared to standard yellow carat golds, Table 1, it can be seen that the
improved strength, microalloyed 24 ct golds approach the hardness of 22 ct gold in both
annealed and cold worked conditions, but are still some way behind those of 18 ct gold.
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It is surprising that such improvements in strength and hardness can be achieved in
gold with alloying additions of only 0.5% wt. or lower. Such small additions can be
described as microalloying. It is instructive, therefore, to examine how such property
improvements are possible in microalloyed gold in terms hardening mechanisms in
gold alloys. As will be apparent, this exercise will have relevance to silver and
platinum too.
Basic Mechanisms Of Hardening Gold
There are several mechanisms for hardening pure metals and more than one can be
utilised 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 original
papers on this topic (1,2, 15). In carat golds, all mechanisms of hardening may be
utilised. As we shall see, hardening by microalloying is accomplished primarily by
dispersion hardening. There is some evidence in the literature of substantial
hardening 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 this
rose to HB 80 after cold working by 80% reduction. Tensile strength was about 190
N/mm2 compared to about 75 N/mm2 for pure gold. Hill (17) studied mixtures of gold
powder 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 to
HV 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 dispersion
hardening 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 atom
weight 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 must
differentiate alloy compositions given in terms of weight percent - the relative weights
of 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 with
gold-copper alloys. An alloy of 50% gold atoms and 50 % copper atoms, i.e. 1 gold
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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 microalloying
In the development of improved strength 24 carat golds, we are looking at total
alloying 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 are
approaching those values typically used to control grain size, such as cobalt or
iridium in carat golds. As gold is a low stacking fault metal (stacking faults are a type
of crystal lattice defect), control of grain size alone or in combination with cold work
will not yield significant hardening in pure gold, so such small additions cannot work
through grain size control only.
Significant solid solution hardening by such small weight additions is only possible
if the alloying metal is very light, i.e. it has a low density and has a small atomic
weight compared to gold. If we examine the Periodic Table, the light metals that
might 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
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
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 of
gold. 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 and
to about HV80 in the cold worked condition. So maybe a gold-lithium alloy could
provide 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 2.41
at.% calcium which is quite small - only 1 atom in 40 - and, therefore, would not be
expected to provide much solid solution strengthening. However, reference to the
phase diagram, Figure 2, shows that there is virtually no solid solution of calcium in
gold and that there is a eutectic comprising 2 phases, gold and an intermetallic
compound, Au5Ca, which has a high gold content. If this phase is finely dispersed
in the microstructure, then we have the basis of a possible alloy system that could
provide improved properties through dispersion hardening.
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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) in
the 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 gold
and to form eutectics and intermetallic compounds with gold. Table 3 lists some
relevant features of their phase diagrams with gold. Some rare earth metals have
been omitted for brevity.
Table 3 Features of Gold - Rare Earth Phase Diagrams
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?
7.7 at %*
Au4Lu
84.8 (890)
Age-hardenable?
Lutetium
* 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. Figure
3 shows the phase diagram for gold-cerium.
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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 in
excess of 0.5%, but a very low solubility as the temperature falls, it is possible that
they may be amenable to age-hardening treatment with the precipitation of fine
particles 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 done
by the Degussa company in Germany on other candidate alloy systems which
included the heavy rare earths. The basis for selection was the possibility of second
phase precipitation as the alloy containing a 1% wt alloying addition in solution was
cooled from 800°C to 400°C. From this, a table of probable hardening effectiveness
was constructed, Table 4.
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Table 4. Candidate Alloy Systems and Probable Hardening Effect,
[from Gafner, reference (18)]
System
Au - Ti
Au - Rh
Au - Ru
Au - Zr
Au - Tb
Au - Dy
Au - Ho
Au - Er
Solubil’y
800°C
1.2
0.6
1.0
2.0
1.2
1.9
3.2
4.8
Solubil’y
400°C
0.4
0.2
0
0.3
0.3
0.3
0.4
0.4
Fraction*
wt%
0.6
0.4
1.0
0.7
0.7
0.7
0.6
0.6
Ratio,
atom. wt.
4.1
1.9
2.0
2.2
1.2
1.2
1.2
1.2
Fraction* at %
2.5
0.8
2.0
1.5
0.8
0.8
0.7
0.7
Fraction
harden. phase
12.5
0.8
2.0
7.5
5.6
5.6
4.9
3.5
*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 the
1% alloying addition precipitating in atomic percent multiplied by the number of
atoms of the alloying addition in the precipitating intermetallic compound) was taken
as an indication of hardening effectiveness. The reason for developing the 990 goldtitanium alloy is obvious from this table. The promise of the rare earths and zirconium
should also be noted.
However, in this work, a 1 wt% alloying addition was being evaluated. If we consider
only a 0.5 wt% addition of rare earth, then from the solubility data at 400°C in Table
4, we cannot expect much hardening phase to precipitate on annealing solutionised
material at 400°C.
Fortunately, Degussa carried out some tests (18) on gold-rare earth alloys at alloying
levels of 1 wt% and lower. Cast alloys were annealed at 800°C for 1 hour. They were
also cold rolled up to 95% deformation and subjected to age hardening treatments
at a range of temperatures. Table 5 shows the hardness values attained for gold
alloys containing 0.5% alloy or less.
Table 5. Hardnesses of Gold - Rare Earth Alloys (from reference 19)
Alloy Composition,
wt%
Au - 0.3 Gd
Au - 0.5 Gd
Au - 0.5 Tb
Au - 0.5 Dy
Au - 0.3 Y
Au - 0.4 Y
Au - 0.5 Y
Hardness,
As cast,
HV
44
34
44
70
35
32
61
Hardness,
Annealed
HV
30
48
30
29
24
34
38
* Approximate values taken from graphs.
140
Hardness*,
95% C.W.
HV
130
115
110
120
110
120
145
Hardness*,
Aged 300°C
HV
63
85
67
75
45
174
C.W. - cold worked
Jewelry Technology Forum
From this work, it can be seen that the annealed hardness is little different from normal
pure gold, although cold worked material is much harder and in the range of the
improved strength 24 carat materials (Table 1). Age hardening heat treatments are not
very effective at these low concentrations with the exception of the 0.5% gold-yttrium
alloy (and yttrium is not strictly a rare earth metal), confirming the view expressed
earlier in that consideration of the solubility data, Table 4, of the heavy rare earths
suggested little age hardening was possible at these low alloying levels. Whether
alloys of gold with the light rare earths show good properties is not known. It is difficult
to comment on the results for gold-yttrium alloys as there is no published gold-yttrium
phase diagram (20), but recent work by Ning (21) indicates that it is similar to the
heavy rare earths with some solid solubility (about 2%) of yttrium in gold.
To summarise, the mechanism of hardening by microalloying would appear to be
based on some form of dispersion (precipitation) hardening by intermetallic phases
of high gold content in alloy systems that form eutectics at high gold contents and
where 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 is
now appropriate to compare this with what is known about the microalloyed golds
that 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 claim
gold alloys of 99% purity or higher containing 200 - 2000 ppm of one or more of
the following elements: calcium, beryllium, germanium and boron. From other
sources (22), it is clear that calcium is the principal hardening metal in High
Strength Pure Gold. Examination of the phase diagrams for gold-beryllium, goldgermanium and gold-boron shows similarities with the diagram for gold-calcium,
so similar effects on microstructure and properties are anticipated. The patent
also includes further additions of 10 -1000 ppm of one or more of many metals
including magnesium, aluminium and cobalt and /or 10-1000 ppm of rare earth
metals and yttrium. The hardness values for over 50 alloys quoted in their patent
lie typically in the range HV 100 - 140 which is consistent with the claimed
properties for High Strength Pure Gold.
In a further patent (5), an alloy of 99% gold or higher is claimed containing 5002000 ppm calcium and 1-50 ppm carbon. The role of carbon is not clear, but may
harden interstitially or preferentially segregate with some calcium to grain
boundaries.
2. PureGold - Three O Company
In their patent (10), an age hardenable alloy of 99.7% gold with a hardness
comparable to an 18 carat gold is claimed containing 50 ppm or more gadolinium
and optionally a third metal - calcium, aluminium or silicon - the total being in the
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range 100-3000 ppm. For an alloy containing gadolinium and calcium, a
maximum hardness after a combination of working and ageing of HV 176 is
described. The optimum ageing temperature is 250°C.
3. Hard 24 ct Gold - Mintek
At the time of the original review, there was no published information on this
development. Since then, a patent (23) and paper (24) have provided an
understanding of this alloy. As the authors explain (24), this alloy was developed
to overcome the problems associated with the other microalloyed golds that use
calcium and/or the rare earths, i.e. the loss of strength on remelting, due to loss
of the microalloying metals,and the expense and difficulties in making them. Hard
24 ct Gold can be made and processed on conventional equipment and is
amenable to remelting without significant loss of strength. The age-hardenable
alloy is of 995 fineness and contains 0.2% cobalt and 0.3% antimony. It is based
on precipitation hardening by a gold-antimony intermetallic, AuSb2, with the
cobalt retarding recrystallisation. The gold-antimony phase diagram is a eutectic
system at the gold-rich end and shows the features described in the previous
section. As shown in Table 1, this alloy can achieve hardnesses of HV 100 in the
cold-worked condition and up to HV 142 in the cold-worked and aged condition.
4. Other Golds
From private discussions, I am aware of the use of calcium in combination with
other alloying metals in some of the other golds listed in Table 3. In a patent from
Tanaka KK, Japan (6), an alloy for precision casting is claimed containing small
amounts of hafnium and rare earth metals.
In some reports (8,9), the cold working of the surface during finishing plays an
important role in hardening the surface.
5. Other literature
Doped pure gold wires are used extensively in the electronics industry for
bonding. In a recent paper (25), Lichtenberger and colleagues doped high purity
gold (5-9’s purity) with 3-30 ppm of aluminium, calcium, copper, silver and/or
platinum. They showed that most dopants strengthened the wire during extrusion
(beryllium had the largest effect) but only calcium and beryllium had significant
strengthening effects after annealing. This is explained on the basis of the atom
size difference in the gold lattice: Calcium atoms are about 30% larger and
beryllium atoms are about 30% smaller than gold. There will be a tendency for
calcium atoms to sit on grain boundaries and pin them.
Various patents for improved strength gold bonding wires cite additions of
bismuth, 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 additions
to gold bonding wire materials showed calcium to be more effective in increasing
stored energy, strength and ductility of cold-worked material than palladium and
this is attributed to grain boundary segregation and dislocation loop generation.
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Practical aspects of making jewellery
The practical attributes of microalloyed 24 ct golds for jewellery making have been
fully 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 small
workshop situations by conventional melting in air
• As with normal 24 ct gold, soldering must be consistent with national hallmarking
laws. In some countries, use of lower (22 ct) solders is allowed although there
may be limits on the amount of solder used.
However, another factor is the temperature of soldering, which can lead to a loss
of work hardening in the vicinity of the joint. Laser welding may overcome both
problems. There are also very low melting point 22 ct gold solders. One
solderpaste 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, as
noted earlier. The Mintek alloy is an exception here. However, remelting should
allow 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 and
other high carat golds. However, significant improvements to 22 ct golds have been
described (24, 27) using conventional alloying approaches. Van der Lingen and coworkers 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 a
cobalt-containing alloy, but with up to 1% boron included. However, as will be seen in
reference 29, the microalloying approach can be effective in 22 ct and lower carat golds
too, using gadolinium and calcium additions.
Microalloying of Silver
In principle, it will be evident that the same approach for microalloyed gold could be
applied 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 is
claimed (3) to be easy to work with as it does not require annealing, is very tarnish
resistant, casts well and is harder than sterling silver. The patent (29) covers
microalloyed gold, silver, palladium and platinum materials.
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In the case of silver, an alloy of at least 80.0% silver is claimed containing the rare
earth gadolinium in the range 50 -15,000 ppm, with optional further additions of
alkaline earth metals, silicon, aluminium and boron in the range 50 - 15,000 ppm in
total, with hardness values of HV130 or higher and a Young’s modulus of not less
than 7,000 kg.mm-2.
A further embodiment claims an alloy of at least 99.45% silver with 50 - 5,000 ppm of
Gd plus further optional additions of the same metals in the range 50 -5,000 ppm in
total. 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 the
most effective hardening element. The small additions do not affect the colour. The
further optional additions listed above have a synergistic effect, but of these calcium
is preferable.
Microalloying of Platinum
Microalloying of platinum should also be possible. Of course, oxide dispersion
strengthened platinum (such as ZGS platinum from Johnson Matthey) for industrial
applications has been on the market for many years.
In a patent (30) from Japan, a hard, high purity platinum alloy containing 10 -100
ppm of cerium is claimed, that has a good hardness, lustre and tarnish resistance
suitable for jewellery manufacture and is at least 99% purity. Vacuum or inert gas is
required during melting to prevent loss of cerium by oxidation. The platinum-cerium
phase diagram, shown in Figure 5, shows characteristics similar to that of the goldcerium system as described earlier for gold, i.e little solid solubility of Ce in Pt, a
eutectic system between Pt and a high platinum-containing intermetallic. For cerium
contents of 0.03 - 0.3%, hardness values of HV 61 - 102 were obtained compared to
HV 40 for pure platinum and HV 120 -136 for 950 fineness platinum alloys.
Figure 5. The platinum-cerium phase diagram
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A commercial platinum, HPP Platinum, from Johnson Matthey is claimed to have
improved 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. Work
by Ning at the Chinese Institute of Precious Metals on rare earth additions has shown
that 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 strength
from 280 MPa to 400 Mpa, with similar improvements to tensile and creep strength
at 900°C. These increases are attributed to both solid solubility and dispersion
strengthening effects.
The Ogasa patent (29) mentioned above also includes platinum materials. This
claims platinum alloys of at least 85.0% platinum containing gadolinium and the
other 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 modulus
of not less than 8,000 kg.mm-2 are claimed. With 50% cold work, hardnesses of at
least HV 150 are claimed.
A further claim is for alloys of at least 99.45% platinum and additions of gadolinium
and the optional elements, preferably calcium, in the range 50 - 5,000 ppm total with
hardness 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 jewellery
application, 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 fineness
has been reviewed. This shows that calcium and the rare earths to be the preferred
microalloying additions, but other possibilities have been noted. Commercial alloys
are available and jewellery is being made in these alloys. The advantages and pitfalls
of such alloys for jewellery production have been summarised.
Hardening of silver and platinum by microalloying has also been reviewed and their
metallurgy parallels that of gold. The silver materials are known to be commercially
available in the USA.
Acknowledgements
Thanks are given to many colleagues in the industry who have provided information
that has enabled this review to be written. Thanks also to World Gold Council for their
support and permission to publish.
June 2005
145
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June 2005
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