GB2352452A

GB2352452A - A gold alloy and a process for the manufacture thereof

Info

Publication number: GB2352452A
Application number: GB9914833A
Authority: GB
Inventors: Peter Rotheram; Stewart Grice
Original assignee: Cookson Precious Metals Ltd
Current assignee: Cookson Precious Metals Ltd
Priority date: 1999-06-24
Filing date: 1999-06-24
Publication date: 2001-01-31
Also published as: GB9914833D0; DE60014251D1; EP1065288B1; EP1065288A1; ATE278044T1

Abstract

An age-hardened Au-Ag-Cu-Zn quaternary alloy, particularly a 9-carat gold alloy, suitable for use in the manufacture of jewellery is provided, the alloy comprising: <SL> <LI>Gold - from 35 to 40 wt.% <LI>Silver - from 13 to 20 wt.% <LI>Copper - from 35 to 45 wt.% <LI>Zinc - from 4 to 10 wt.% </SL> wherein the alloy has a Vickers Hardness (H<SB>v</SB>) of / 200.

Description

2352452 A aold alloy and a vrocess for the manufacture thereo The present

invention relates to a gold metal alloy and a process for the manufacture thereof. The gold alloy is particularly, though not exclusively, suitable for use in the manufacture of articles of jewellery.

In the United Kingdom, gold jewellery is classified and legally controlled by a gold standard known as the carat, which is measure of the purity (or fineness) of the metal. Accordingly, gold jewellery is legally controlled, depending on the gold content, at 24-carat (pure gold), 22-carat (91.7 wt.% minimum), 18-carat (75.0 wt%), 14-carat (58.5 wt.%) and 9-carat (37.5 wt.%).

Pure gold metal is extremely soft and consequently does not generally provide a good or extended wear life for manufactured jewellery articles. It is therefore desirable to include alloying elements to provide an alloy with improved hardness and wear resistance, while still retaining a colour and lustrous finish that are associated with pure gold. Accordingly, gold alloys for manufactured jewellery articles are typically based on the ternary Au-Ag-Cu or quaternary Au-Ag-Cu-Zn alloy systems. The presence of zinc in the alloy in amounts in excess of 1 wt.% modifies the colour and workability of the alloy. These ternary and quaternary alloys are intrinsically harder than pure gold and, depending on composition, may be further heat-treated to modify the mechanical properties.

The improved hardness may result in ternary and quaternary alloys being less tolerant to cold working compared with pure gold. However, these same factors can lead to increased advantageous properties when used during the final fabrication stages of article production to give increased strength, hardness and resistance to wear.

Until recently, the study of gold has concentrated on alloys that have low intrinsic hardness values and which are easily cold-worked, i.e. formed/shaped at a temperature below the alloy recrystallization temperature.

The present invention aims to provide gold alloys having improved mechanical properties over the prior art alloys. This allows for the fabrication of jewellery articles using less alloy material compared with the prior art, while providing the same or enhanced strength and wear characteristics once heattreated.

Accordingly, in a first aspect the present invention provides an age-hardened Au-Ag-Cu-Zn quaternary alloy suitable for use in the manufacture of jewellery, the alloy comprising:

Gold - from 35 to 40 wt.% Silver - from 13 to 20 wt.% Copper - from 35 to 45 wt.% Zinc - from 4 to 10 wt.% wherein the alloy has a Vickers hardness (H.) of 2! 200.

Vickers hardness testing may be carried out using conventional testing equipment using a 5 kg load.

The alloy preferably comprises from 37 to 38 wt.% gold, more preferably approximately 37.5 wt.% gold.

If the alloy is diamond polished, and light from a tungsten filament using, for example, an EEL instrument is incident on the polished surface, then the surface reflectivity is preferably in the range of from 70 to 80%. The surface reflectivity is based on comparing incident and reflected light, but taking no account of colour.

For certain applications, for example investment casting alloys, the gold alloy may further comprise:

Silicon from 0.01 to 0.5 wt.% Boron from 0.001 to 0.15 wt.% In a second aspect of the present invention there is provided an age- hardened 9-carat yellow gold alloy suitable for use in the manufacture of jewellery, the alloy consisting essentially of:

Gold approximately 37.5 wt.% Silver from 13 to 20 wt.% Zinc from 4 to 10 wt.% and the balance copper, together with unavoidable impurities, wherein the alloy has a Vickers hardness (H,) o f 2t 2 0 0.

For investment casting applications, the age hardened 9 carat yellow gold alloy advantageously consists essentially of:

Gold - approximately 37.5 wtA Silver - from 13 to 20 wt.% Zinc - from 4 to 10 wt.% Silicon - from 0.01 to 0.5 wt.% Boron from 0.001 to 0.15 wt.% and the balance copper, together with unavoidable impurities. 5 In both the first and second aspects, the alloy preferably has a Vickers hardness (Hj for a 5 kg load in the range of from 220 to 300, more preferably from 250 to 300, still more preferably from 270 to 300. In both aspects, the alloy has a phase microstructure which includes the ordered compound AuCU3While not wishing to be bound by theory, it is thought that this compound may contribute to the improved final hardness of the material. The presence of zinc in the alloys according to the present invention is thought to reduce the height and breadth of the immicibility (two-phase) region resulting in reduced precipitation of ((a, (Au-Ag) + q2 (Au-Cu)) and AuCu3. This may improve workability through softening in both the solution-treated and aged states.

In both the first and second aspects, the alloy preferably comprises from 15 to 20 wt.% silver, more preferably from 17 to 20 wt.%.

In both the first and second aspects, the alloy preferably comprises from 5 to 10 wt.% zinc, more preferably from 8 to 10 wt.%.

In both the first and second aspects, the alloy preferably comprises from 37 to 43 wt.% copper, more preferably from 38 to 40 wt.% copper.

In a third aspect of the present invention there is provided an age-hardenable 9 carat yellow gold alloy consisting essentially of:

Gold approximately 37.5 wt.% Silver from 14.0 to 16.0 wt.% Zinc from 8 to 9 wt.% Copper from 38 to 40 wt.% together with unavoidable impurities.

In the third aspect, the age-hardenable 9 carat yellow gold alloy preferably consists essentially of:

Gold approximately 37.5 wt.% Silver approximately 15.0 wt.% Zinc approximately 8.3 wt.% Copper approximately 39.2 wt.% together with unavoidable impurities.

A particularly preferred gold alloy according to the third aspect of the present invention is a 9-carat yellow gold alloy consisting essentially of:

Gold - 37.50 wt.% Silver - 15.00 wt.% Copper - 39.17 wt.% Zinc - 8.33 wt.% together with unavoidable impurities.

The alloy composition according to the third aspect may be provided in a solution treated state or may be heat-treated to effect age-hardening. By this process, the aged alloy may achieve a Vickers hardness (H,) of _> 200. Preferably, the alloy in age-hardened condition has a H, in the range of from 220 to 300, more preferably from 250 to 300, still more preferably from 270 to 300. The alloy in age-hardened condition preferably has a phase microstructure which includes - 6 the ordered compound AuCU3Prior to ageing, the alloy composition according to the third aspect, in annealed condition, will generally have a Vickers hardness (H,) of from 80 to 150.

It will be appreciated that the alloys according the first, second and third aspects may contain unavoidable impurities, although, in total, these will generally not exceed 1 wt.% of the composition, and preferably not more than 0.5 wt.% of the composition, more preferably not more than 0.25 wt.% of the composition.

The alloy according to the first, second and third aspects is agehardenable. The alloy also exhibits reversible hardening behaviour. Accordingly, the alloy may be solution treated for maximum ductility and then aged for maximum hardness. Should a jewellery manufacturer, for example, then wish to re-soften the alloy workpiece to carry out further work, the workpiece may be re-solution treated, worked, and re-aged with substantially no detrimental effects. This is an important feature of the present invention because reversible hardening behaviour enables optimum use over the full range of jewellery manufacturing techniques. Optimum hardenability can be achieved using a specific set of heat- treatments, in particular a so lution- treatment, followed by a low temperature ageing operation.

In contrast to certain prior art gold alloys, the alloys according to the present invention do not require the presence of grain refiners, such as, for example, Co, Ir or Ru, to achieve the desired mechanical properties. Nor are elements, such as Fe, required to facilitate the reversibility of the hardening reaction.

The density of the gold alloys according to the present invention is typically in the range of from about 10.9 g/cm3 to about 11.5 g/cm', more typically from about 11.0 g/cm' to about 11.4 g/cm', still more typically approximately 11.2 g/CM3.

The melting temperature of the gold alloys according to the present invention is typically in the range of from about 8400C to about 900C, more typically from about 8600C to about 8800C.

The grain size, i.e. the statistical grain diameter in a random cross-section, of the gold alloys according to the present invention can vary depending upon the application, but will generally be in the range of from about 5 pm to about 60 pm, more typically from about 10 pm to about 30 pm.

The present invention further provides an article of jewellery in which at least a portion thereof is formed from an alloy as herein described. Such articles include, for example, rings, earings, bracelets, necklaces and the like. The alloy may also be used in other decorative applications, for example objet d'art, goblets, cups, medals and badges.

The present invention also provides a process for the manufacture of an age-hardened Au-Ag-Cu-Zn alloy suitable for use in the manufacture of jewellery, which process comprises:

(a) providing an alloy composition comprising:

Gold from 35 to 40 wt.% 8 Silver from 13 to 20 wt.% Copper from 35 to 45 wt.% Zinc from 4 to 10 wt.%; (b) solution treating the alloy composition at a temperature of up to the solidus temperature; (c) cooling the solut ion- treated alloy from step (b) to ambient temperature; and (d) age-hardening the cooled alloy from step (c) at a temperature in the range of from 200 to 400C, followed by cooling to ambient temperature, wherein the age-hardening is performed for a time sufficient to achieve a Vickers hardness (Hv) of ! 200 in the final alloy.

The solution treatment in step (b) is preferably performed at a temperature up to approximately 860C, more preferably in the range of from 500 to 860C, more preferably from 700 to 800'C. It will be appreciated that the time spent at the solution treatment temperature will depend on the size of the workpiece. However, the solution treatment will typically be performed for at least 5 minutes, more typically for from 10 to 60 minutes.

Cooling in step (c) is preferably be effected by a quench from the so lut ion- treatment temperature to ambient temperature to thereby substantially retain a solid solution. In this manner it is possible to obtain alloys, after step (d), having a Vickers hardness (Hv) in the range of from 220 to 300, preferably from 250 to 300, more preferably from 270 to 300. The quenching medium will typically comprise water.

Alternatively, cooling in step (c) may be effected by slowly cooling the alloy in air from the so lut i on- treatment temperature to ambient temperature.

This is a less rapid cooling than quenching and the ultimate alloy hardness is consequently not as high.

Nonetheless, it is till possible to obtain a H, of 200 or more. Accordingly, the alloys according to the present invention may still be hardened without a %%pure" solution- treatment, i.e. the workpiece may not have been quenched at.a sufficiently rapid rate.

Although the hdrdness values achieved may not be as high as those exhibited by rapidly quenched workpieces, they will generally still be acceptable to manufacturing jewellers.

It will be appreciated that the time spent at the age-hardening temperature will depend on the size of the workpiece and the desired hardness. However, age hardening in step (d) will typically be carried out for from 5 to 60 minutes, more typically from 10 to 60 minutes, still more typically from 30 to 60 minutes.

It will also be appreciated that over-ageing of the alloy may result in a decrease in hardness.

Age-hardening in step (d) is preferably performed at a temperature in the range of from 275 to 4000C, more preferably from 275 to 350'C, still more preferably from 280 to 3500C. In general, alloys according to the present invention exhibit a reasonably flat hardening response over this temperature range. Because manufacturing jewellers do not generally have precise temperature control, an intermediate temperature of, for example, 3100C may be used to allow for any inaccuracies in the manufacturers process control.

Cooling in step (d) may be effected by slowly cooling the alloy in air from the age-hardening temperature to ambient temperature or, alternatively, quenching the alloy form the age-hardening temperature to ambient temperature using water, for example, as the quenching medium.

The final alloy after step (d) preferably has a phase microstructure which includes the ordered compound AuCU3 The bulk alloy composition in step (a) of the process may be as herein described with reference to the first, second and/or third aspects of the present invention. Advantageously, the alloy composition consists essentially of:

Gold approximately 37.5 wt.% Silver from 13 to 20 wt.% Zinc from 4 to 10 wt.% and the balance copper, together with unavoidable impurities.

In general, the alloy composition will first be cold-worked prior to the solution- treatment in step (b). For example, the alloy composition may be formed and/or shaped into an article of jewellery prior to the solutiontreatment in step (b). However, the alloy composition may be cold-worked after the solution-treatment in steps (b) and (c) and prior to the age-hardening treatment in step (d).

The present invention also provides a gold alloy whenever produced by a process as herein described.

The present invention will be further described below with reference to the following Examples and Comparative Examples, and the following drawings, provided by way of example, in which:

Figure 1 is a graph of Vickers Hardness (Hj vs ageing temperature for an alloy according to the present invention; Figure 2 is an optical micrograph of an alloy according to the invention which has been cold worked (70%), heated to 7000C for 10 mins and then quenched; Figure 3 is an optical micrograph of an alloy according to the invention which has been cold worked (70%), heated to 7000C for 10 mins and then slowly cooled; Figure 4 is an optical micrograph of an alloy according to the invention which has been cold worked (70%), heated to 7000C for 10 mins, and then quenched, followed by ageing at 3100C for 30 mins; Figure 5 is an optical micrograph of an alloy according to the invention which has been cold worked (70%), heated to 7000C for 10 mins, and then slowly cooled, followed by ageing at 3100C for 30 mins; Figure 6 is a graph of Vickers Hardness (H,) vs % Reduction of Thickness for a solution treated alloy according to the invention prior to ageing; Figure 7 is a graph of Ultimate Tensile Strength/0.2% Proof Stress vs % Reduction of Area for a solution treated alloy according to the invention prior to ageing; Figure 8 is a graph of Elongation after fracture vs % Reduction of Area for a solution treated alloy according to the invention prior to ageing; Figure 9 is a graph of Youngs Modulus vs % Reduction of Area for a solution treated alloy according to the invention prior to ageing; and Figure 10 is an XRD trace for an aged alloy according to the present invention.

Examples 1 to--3 and Conwarative Examiples 4 and 5 Chemical compositions for three gold alloys according to the present invention (Ex 1, Ex 2 and Ex 3) and two prior art gold alloys (C Ex 4 and C Ex 5) are provided in Table 1 below. It will be appreciated that the alloys may contain unavoidable impurities, although, in total, these will generally not exceed 1 wt.% of the composition. Comparative Example 4 relates to a typical 9-carat red gold alloy, while Comparative Example 5 relates to a commercially available 9-carat yellow alloy known in the United Kingdom as 9DF or 375/DF.

The alloys were subjected to the following heat treatment. First, samples of the alloys were solution-treated at approximately 7000C for about ten minutes, and then quenched into water from red heat.

Vickers Hardness measurements were carried out on the solution- treated samples using a standard Vickers Pyramid Hardness Testing apparatus and a 5 kg load.

Next, the alloys were age-hardened by heating the solution-treated samples at a temperature of approximately 3100C for about 30 minutes. This was then followed by slow cooling in air to ambient temperature (approximately 20 to 250C). Vickers Hardness measurements were subsequently carried out on the age-hardened samples.

It can be seen f rom the Table 1 that for Comparative Example 4, there is no hardening on ageing. For Comparative Example 5, there is a moderate degree of hardening on ageing. Examples 1, 2 and 3, however, exhibit significant hardening on ageing. This hardening behaviour is also reversible.

Table 1

Alloy AU Ag Cu Zn Anneal Aged Hv Hv Ex 1 37.5 15.0 42.5 5.0 133 283 Ex 2 37.5 15.0 39.17 8.33 119 277 Ex 3 37.5 17.0 39.67 5.83 144 280 C Ex 4 37.5 5.0 57.5 0 85 85 1 C Ex 5 37.5 10.0 45.0 7.5 110 203 Examiples 4 and 5 For these examples, the alloy composition according to Example 2 above was used. Samples of the alloy were first cold worked (70%). In this process the alloy, in soft condition, is reduced in thickness or area by 70%, so that its final size is 30% of the original size. One set of samples (Example 4) was then solution-treated at approximately 7000C for about ten minutes and then quenched from red heat into water. The samples were then age-hardened by heating to temperatures of approximately 275, 300, 310, 325 and 3500C for about 30 minutes. This was then followed by slow cooling in air to the ambient temperature. Separate samples (Example 5) of the cold-worked alloy were subjected to the same heat treatment as the samples of Example 4, except, instead of quenching the samples after being held at the solut ion- treatment temperature, the samples were allowed to slowly cool in air to the ambient temperature. The results of Vickers Hardness (HJ testing on the samples according to Example 4 and Example 5 are provided below in Table 2. The ageing temperature was 3100C. It can be seen that the quenched sample had a lower hardness after quenching compared with the slowly cooled sample, but a higher hardness after ageing. The hardness of the slowly cooled sample after ageing is still acceptable for many applications.

Table 2

Condition Hardness (HJ Hardness (HJ prior to ageing after ageing at 310C Example 4 120 285 Quenched Example 5 165 225 Slow Cooled The difference in Hardness (HJ for Example 4 above compared with Example 2 in Table 1 can be attributed to experimental error.

Figure 1 is a graph of Vickers Hardness (HJ vs the ageing temperature for the samples according to Example 4 and Example 5. It can be seen that the hardening response is fairly flat over the temperature range 275 to 350C.

The microstructures for the samples according to Examples 4 and 5 are shown in Figures 2 to 5.

Figure 2 is an optical micrograph of a sample according to Example 4. The alloy was cold worked (70%), heated to 700C for 10 mins and then quenched. A single-phase solid solution of a (Au-Ag- Cu) appears to be present.

Figure 3 is an optical micrograph of a sample according to Example 5. The alloy was cold worked (70%), heated to 700C for 10 mins and then slowly cooled. A multi-phase structure appears to be present with areas of a and (a, (Au-Ag) + a2 (Au-Cu)) bulk is precipitate.

Figure 4 is an optical micrograph of a sample according to Example 4. The alloy was cold worked (70%), heated to 700C for 10 mins, and then quenched, followed by ageing at 3100C for 30 mins. There is evidence of minimal bulk precipitate (a, + cn) at the grain boundaries with perhaps micro-precipitation of these phases present but not visible using optical microscopy.

Figure 5 is an optical micrograph of a sample according to Example 5. The alloy was cold worked (70%), heated to 7000C for 10 mins, then slowly cooled, followed by ageing at 310C for 30 mins.

Again, there is evidence of further minimal bulk precipitate (a, + %) at the grain boundaries and perhaps micro-precipitation of these phases. However, the degree will be reduced compared with the Figure 4.

Figures 6 to 9 illustrate some other mechanical properties of samples of the alloy according to Example 4 prior to ageing. These f igures demonstrate that the alloy according to the present invention performs in a similar manner to conventional gold alloys in the solution treated state. 5 9-carat gold alloys are generally accepted to harden via a precipitation hardening mechanism associated with the Ag-Cu binary system. While not wishing to be bound by theoryt it is speculatively suggested that hardening may at least be the result of precipitation-- of (a, + a2) during the initial stages and further decomposition of q2 into ordered AuCU3 at higher temperatures and longer times. XRD analysis of quenched and aged samples according to the present invention indicates the presence of the ordered compound AuCU3 as a major phase, present in an amount of over 10 %. This is shown in Figure 10, which is an XRD trace for an aged alloy according to Example 2.

Again, while not wishing to be bound by theory, the increase in hardness of the slowly cooled sample (Example 5) over the quenched sample (Example 4) prior to hardening may be the result of some micro precipitated silver-rich a,, and then decomposition of small amounts of a2 into ordered AuCu3. The onset of precipitation of the modulated phases appears to be at approximately 650C, although is dependent on the composition. The time interval between cooling from this temperature to the minimum decomposition temperature may also yield some ordered AuCu3.

However, this is likely to be substantially reduced when compared to an aged sample since the rate of ordering for AuCU3 is though to be time dependent.

Claims

CLAIMS:

1. An age-hardened Au-Ag-Cu-Zn quaternary alloy suitable for use in the manufacture of jewellery, the alloy comprising:

Gold - from 35 to 40 wt.% Silver - from 13 to 20 wt.% Copper - from 35 to 45 wt.% Zinc - from 4 to 10 wt.% wherein the alloy has a Vickers Hardness (H,) of L> 200.

2. An alloy as claimed in claim 1 comprising from 37 to 38 wt.% gold.

3. An alloy as claimed in claim 2 comprising approximately 37.5 wt.% gold.

4. An alloy as claimed in any one of the preceding claims having a surface reflectivity when exposed to tungsten filament light in the range of from 70 to 80%.

5. An alloy as claimed in any one of the preceding claims further comprising:

Silicon from 0.01 to 0.5 wt.% Boron from 0.001 to 0.15 wt.%

6. An age-hardened 9 carat yellow gold alloy suitable for use in the manufacture of jewellery, the alloy consisting essentially of:

18 - Gold approximately 37.5 wtA Silver from 13 to 20 wt.% Zinc from 4 to 10 wt.% and the balance copper, together with unavoidable impurities, wherein the alloy has a Vickers Hardness (H,) o f! 2 0 0.

7. An age-hardened 9 carat yellow gold alloy suitable for use in the manufacture of jewellery, the alloy consisting essentially of:

Gold - approximately 37.5 wt.% Silver - from 13 to 20 wt.% Zinc - from 4 to 10 wt.% Silicon - from 0.01 to 0.5 wt.% Boron - from 0.001 to 0.15 wt.% and the balance copper, together with unavoidable impurities, wherein the alloy has a Vickers Hardness (Hj o f 2 2 0 0.

8. An alloy as claimed in any one of the preceding claims, wherein the alloy has a Vickers Hardness (Hj in the range of from 220 to 300, preferably from 250 to 300, more preferably from 270 to 300.

9. An alloy as claimed in any one of the preceding claims, wherein the alloy has a phase microstructure which includes the ordered compound AuCu,.

10. An alloy as claimed in any one of the preceding claims which comprises from 15 to 20 wt.% silver.

11. An alloy as claimed in claim 10 which comprises from 17 to 20 wt.% silver.

12. An alloy as claimed in any one of the preceding claims which comprises from 5 to 10 wt.% zinc.

13. An alloy as claimed in claim 12 which comprises from 8 to 10 wt.% zinc.

14. An alloy as claimed in any one of the preceding claims which comprises from 37 to 43 wt.% copper.

15. An alloy as claimed in claim 14 which comprises from 38 to 40--,;it.% copper.

16. An age-hardenable 9 carat yellow gold alloy consisting essentially of: 15 Gold - approximately 37.5 wt.% Silver - from 14.0 to 16.0 wt.% Zinc from 8 to 9 wt.% Copper - from 38 to 40 wt.% together with unavoidable impurities.

17. An age-hardenable 9 carat yellow gold alloy as claimed claim 16 and consisting essentially of: 25 Gold - approximately 37.5 wt.% Silver approximately 15.0 wt.% Zinc - approximately 8.3 wt.% Copper approximately 39.2 wt.% together with unavoidable impurities.

18. An age-hardenable 9 carat yellow gold alloy consisting essentially of: 35 Gold approximately 37.5 wt.% Silver from 14.0 to 16.0 wt.% Zinc - from 8 to 9 wt.% Copper - from 38 to 40 wt.% Silicon - from 0.01 to 0.5 wt.% Boron - from 0.001 to 0.15 wt.% together with unavoidable impurities.

19. An article of jewellery, objet d'art, goblet, cup, medal or badge in -which at least a portion thereof is formed from an alloy as claimed in any one of the preceding claims.

20. A process for the manufacture of an age- hardenable Au-Ag-Cu-Zn alloy suitable for use in the manufacture of jewellery, which process comprises:

(a) providing an alloy composition comprising:

Gold - from 35 to 40 wt.% Silver - from 13 to 20 wt.% Copper - from 35 to 45 wt.% zinc - from 4 to 10 wt.%; (b) solution treating the alloy composition at a temperature of up to the solidus temperature; (c) cooling the solution-treated alloy from step (b) to ambient temperature; and (d) age-hardening the cooled alloy from step (c) at a temperature in the range of from 200 to 4000C, followed by cooling to ambient temperature, wherein the age-hardening is performed for a time sufficient to achieve a Vickers hardness (Hv) of 2t 200 in the final alloy.

21. A process as claimed in claim 20, wherein the solution treatment in step (b) is performed at a temperature in the range of from 500 to 860C, preferably from 700 to 8000C. 5

22. A process as claimed in claim 20 or claim 21, wherein the solution treatment in step (b) is performed for at least 1 minute.

23. A process as claimed in claim 22, wherein the solution treatment in step (b) is performed for from 5 to 60 minutes, preferably from 10 to 60 minutes.

24. A process as claimed in any one of claims 20 to 23, wherein cooling in step (c) is effected by a quench from the so lut ion- treatment temperature to ambient temperature to thereby substantially retain a solid solution.

25. A process as claimed in claim 24, wherein agehardening in step (b) is carried out for a time sufficient to achieve a Vickers hardness (Hv) in the range of from 220 to 300, preferably from 250 to 300, more preferably from 270 to 300.

26. A process as claimed in any one of claims 20 to 23, wherein cooling in step (c) is effected by slowly cooling the alloy in air from the solution-treatment temperature to ambient temperature.

27. A process as claimed in any one of claims 20 to 26, wherein agehardening in step (d) is carried out for from 5 to 60 minutes.

28. A process as claimed in claim 27, wherein agehardening in step (d) is carried out for from 10 to 60 minutes, preferably from 30 to 60 minutes.

29. A process as claimed in any one of claims 20 to 28, wherein age-hardening in step (d) is performed at a temperature in the range of from 275 to 400C.

30. A process as claimed in any one of claims 20 to 29, wherein cooling in step (d) is effected by slowly cooling the alloy in air from the agehardening temperature to ambient temperature or by quenching the alloy from the- 'age-hardening temperature to ambient temperature.

31. A process as claimed in any one of claims 20 to 30, wherein the final alloy in step (d) has a phase microstructure which includes the ordered compound AuCU3.

32. A process as claimed in any one of claims 20 to 31, wherein the alloy composition further comprises:

Silicon from 0.01 to 0.5 wt.% Boron from 0.001 to 0.15 wt.%

33. A process as claimed in any one of claims 20 to 31, wherein the alloy composition consists essentially of:

34. A process as claimed in any one of claims 20 to 31, wherein the alloy composition consists essentially of:

Gold - approximately 37.5 wt.% Silver - from 13 to 20 wt.% Zinc - from 4 to 10 wt.% Silicon - from 0.01 to 0.5 wt.% Boron - from 0.001 to 0.15 wt.% and the balance copper, together with unavoidable impurities.

35. A process as claimed in any one of claims 20 to 34, wherein the alloy composition is cold-worked prior to the so lut i on- treatment in step (b).

36. A process as claimed in any one of claims 20 to 35, wherein the alloy composition is formed or shaped into an article of jewellery prior to the solution treatment in step (b).

37. A gold alloy whenever produced by a process as claimed in any one of claims 20 to 36.

38. An article of jewellery comprising a gold alloy as claimed in claim 37.

39. A gold alloy substantially as hereinbefore described with reference to any one of the Examples 1 to 5.

40. A process for the production a gold alloy substantially as hereinbefore described with reference to any one of Examples 1 to 5. 35