A gold alloy and a process for the manufacture thereof

Abstract: An age-hardened Au-Ag-Cu-Zn quaternary alloy suitable for use in the manufacture of jewellery, the alloy comprising: Gold - from 35 to 40wt.% Silver - from 13 to 20wt.% Copper - from 35 to 45wt.% Zinc - from 4 to 10wt.% and optionally Silicon - from 0.01 to 0.5wt.% Boron - from 0.001 to 0.15wt.% wherein the alloy has a Vickers Hardness (H v ) of ≥ 200. The process involves solution treating the alloy at a temperature of up to the solidus temperature, cooling to ambient temperature and age-hardening in the range of from 200 to 400°C.

Description: 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. US-A-2,042,155 relates to a gold alloy which consists of 33 to 50% Au, 10 to 18% Ag, 30 to 40% Cu, 1 to 5% Ni, and 5 to 10% Zn. FR-A-2, 305, 503 relates to a 9 carat gold alloy with 3.5 to 9.0 % Ag. 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 heat-treated. Accordingly, in a first aspect the present invention provides an age-hardened 9 carat yellow gold alloy suitable for use in the manufacture of jewellery, the alloy consisting of: Gold approximately 37.5 wt.% Silver from 13 to 20 wt.% Zinc from 4 to 10 wt.% and optionally 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 (H v ) of ≥ 200. Vickers hardness testing may be carried out using conventional testing equipment using a 5 kg load. 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 comprise: Silicon from 0.01 to 0.5 wt.% Boron from 0.001 to 0.15 wt.% As already mentioned, for investment casting applications, the age-hardened 9 carat yellow gold alloy advantageously consists 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. The alloy preferably has a Vickers hardness (H v ) 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. The alloy has a phase microstructure which includes the ordered compound AuCu 3 . While 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 ((α 1 (Au-Ag) + α 2 (Au-Cu)) and AuCu 3 . This may improve workability through softening in both the solution-treated and aged states. The alloy preferably comprises from 15 to 20 wt.% silver, more preferably from 17 to 20 wt.%. The alloy preferably comprises from 5 to 10 wt.% zinc, more preferably from 8 to 10 wt.%. The alloy preferably comprises from 37 to 43 wt.% copper, more preferably from 38 to 40 wt.% copper. In a second aspect of the present invention there is provided an age-hardenable 9 carat yellow gold alloy consisting 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.% and optionally Silicon from 0.01 to 0.5 wt.% Boron from 0.001 to 0.15 wt.% the sum constituting 100 wt.%, together with unavoidable impurities. In the second aspect, the age-hardenable 9 carat yellow gold alloy preferably consists of: Gold approximately 37.5 wt.% Silver approximately 15.0 wt.% Zinc approximately 8.3 wt.% Copper approximately 39.2 wt.% the sum constituting 100 wt.%, together with unavoidable impurities. A particularly preferred gold alloy according to the second aspect of the present invention is a 9-carat yellow gold alloy consisting of: Gold 37.50 wt.% Silver 15.00 wt.% Copper 39.17 wt.% Zinc 8.33 wt.% the sum constituting 100 wt.%, together with unavoidable impurities. The alloy composition according to the second 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 v ) of ≥ 200. Preferably, the alloy in age-hardened condition has a H v 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 the ordered compound AuCu 3 . Prior to ageing, the alloy composition according to the second aspect, in annealed condition, will generally have a Vickers hardness (H v ) of from 80 to 150. It will be appreciated that the alloys according the first and second 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 and second aspects is age-hardenable. 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 resoften 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 solution-treatment, followed by a low temperature ageing operation. While one or more grain refiners such as, for example, Co, Ir or Ru, may be included in the alloys according to the present invention, they are not an essential requirement to achieve the desired mechanical properties, in contrast to certain prior art gold alloys. 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/cm 3 to about 11.5 g/cm 3 , more typically from about 11.0 g/cm 3 to about 11.4 g/cm 3 , still more typically approximately 11.2 g/cm 3 . The melting temperature of the gold alloys according to the present invention is typically in the range of from about 840°C to about 900°C, more typically from about 860°C to about 880°C. 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 µm to about 60 µm, more typically from about 10 µm to about 30 µm. 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 a 9 carat yellow gold alloy suitable for use in the manufacture of jewellery, which process comprises: (a) providing an alloy composition consisting of Gold approximately 37.5 wt.% Silver from 13 to 20 wt.% Zinc from 4 to 10 wt.% and optionally Silicon from 0.01 to 0.5 wt.% Boron from 0.001 to 0.15 wt.% and the balance copper, together with unavoidable impurities; (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 400°C, 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 860°C, more preferably in the range of from 500 to 860°C, 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 solution-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 solution-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 still possible to obtain a H v 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 hardness 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 400°C, more preferably from 275 to 350°C, still more preferably from 280 to 350°C. 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, 310°C 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 AuCu 3 . The bulk alloy composition in step (a) of the process may be as herein described with reference to the first and/or second aspects of the present invention. Advantageously, the alloy composition consists 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 solution-treatment 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). After undergoing the ageing heat treatment, the gold alloy according to the present invention will typically exhibit the following tensile properties: 0.2% proof stress from 700 to 760 N/mm 2 , more typically from 730 to 740 N/mm 2 , still more typically approximately 735 N/mm 2 ; and ultimate tensile strength from 850 to 950 N/mm 2 , more typically from 900 to 910 N/mm 2 , still more typically approximately 905 N/mm 2 . These properties are comparable to those expected from hard rolled 9ct yellow alloys. A further advantage associated with the gold alloy according to the present invention is that it has at least as great and in some cases greater resistance to stress corrosion cracking than conventional 9 ct yellow alloys in all conditions. Furthermore, when the alloy is in the annealed condition, it shows far greater resistance to stress corrosion cracking than conventional 9ct yellow alloys. 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 (H v ) 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 700°C 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 700°C 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 700°C for 10 mins, and then quenched, followed by ageing at 310°C for 30 mins; Figure 5 is an optical micrograph of an alloy according to the invention which has been cold worked (70%), heated to 700°C for 10 mins, and then slowly cooled, followed by ageing at 310°C for 30 mins; Figure 6 is a graph of Vickers Hardness (H v ) 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 Comparative Examples 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 700°C 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 310°C for about 30 minutes. This was then followed by slow cooling in air to ambient temperature (approximately 20 to 25°C). Vickers Hardness measurements were subsequently carried out on the age-hardened samples. It can be seen from 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. Alloy Au Ag Cu Zn Anneal Hv Aged 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 C Ex 5 37.5 10.0 45.0 7.5 110 203 Examples 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 700°C 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 350°C 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 solution-treatment temperature, the samples were allowed to slowly cool in air to the ambient temperature. The results of Vickers Hardness (H v ) testing on the samples according to Example 4 and Example 5 are provided below in Table 2. The ageing temperature was 310°C. 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. Condition Hardness (H v ) prior to ageing Hardness (H v ) after ageing at 310°C Example 4 Quenched 120 285 Example 5 Slow Cooled 165 225 The difference in Hardness (H v ) 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 (H v ) 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 350°C. 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 700°C for 10 mins and then quenched. A single-phase solid solution of α (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 700°C for 10 mins and then slowly cooled. A multi-phase structure appears to be present with areas of α and (α 1 (Au-Ag) + α 2 (Au-Cu)) bulk precipitate. Figure 4 is an optical micrograph of a sample according to Example 4. The alloy was cold worked (70%), heated to 700°C for 10 mins, and then quenched, followed by ageing at 310°C for 30 mins. There is evidence of minimal bulk precipitate ( α 1 + α 2 ) 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 700°C for 10 mins, then slowly cooled, followed by ageing at 310°C for 30 mins. Again, there is evidence of further minimal bulk precipitate ( α 1 + α 2 ) 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 figures demonstrate that the alloy according to the present invention performs in a similar manner to conventional gold alloys in the solution treated state. 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 theory, it is speculatively suggested that hardening may at least be the result of precipitation of ( α 1 + α 2 ) during the initial stages and further decomposition of α 2 into ordered AuCu 3 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 AuCu 3 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 α 1 , and then decomposition of small amounts of α 2 into ordered AuCu 3 . The onset of precipitation of the modulated phases appears to be at approximately 650°C, although is dependent on the composition. The time interval between cooling from this temperature to the minimum decomposition temperature may also yield some ordered AuCu 3 . However, this is likely to be substantially reduced when compared to an aged sample since the rate of ordering for AuCu 3 is though to be time dependent.

Claims: An age-hardened 9 carat yellow gold alloy suitable for use in the manufacture of jewellery, the alloy consisting of: Gold approximately 37.5 wt.% Silver from 13 to 20 wt.% Zinc from 4 to 10 wt.% and optionally 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 (H v ) of ≥ 200. An alloy as claimed in claim 1, wherein the alloy has a Vickers Hardness (H v ) in the range of from 220 to 300 An alloy as claimed in claim 2, wherein the alloy has a Vickers Hardness (H v ) in the range of from 250 to 300. An alloy as claimed in claim 3, wherein the alloy has a Vickers Hardness (H v ) in the range of from 270 to 300. An alloy as claimed in any one of the preceding claims, wherein the alloy has a phase microstructure which includes the ordered compound AuCu 3 . An alloy as claimed in any one of the preceding claims which comprises from 15 to 20 wt.% silver. An alloy as claimed in claim 6 which comprises from 17 to 20 wt.% silver. An alloy as claimed in any one of the preceding claims which comprises from 5 to 10 wt.% zinc. An alloy as claimed in claim 8 which comprises from 8 to 10 wt.% zinc. An alloy as claimed in any one of the preceding claims which comprises from 37 to 43 wt.% copper. An alloy as claimed in claim 10 which comprises from 38 to 40 wt.% copper. An age-hardenable 9 carat yellow gold alloy consisting 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.% and optionally Silicon from 0.01 to 0.5 wt.% Boron from 0.001 to 0.15 wt.% the sum constituting 100 wt.%, together with unavoidable impurities. An age-hardenable 9 carat yellow gold alloy as claimed in claim 12 and consisting of: Gold approximately 37.5 wt.% Silver approximately 15.0 wt.% Zinc approximately 8.3 wt.% Copper approximately 39.2 wt.% the sum constituting 100 wt.%, together with unavoidable impurities. 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 defined in any one of the preceding claims. A process for the manufacture of a 9 carat yellow gold alloy suitable for use in the manufacture of jewellery, which process comprises: (a) providing an alloy composition consisting of Gold approximately 37.5 wt.% Silver from 13 to 20 wt.% Zinc from 4 to 10 wt.% and optionally Silicon from 0.01 to 0.5 wt.% Boron from 0.001 to 0.15 wt.% and the balance copper, together with unavoidable impurities, (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 400°C, 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. A process as claimed in claim 15, wherein the solution treatment in step (b) is performed at a temperature in the range of from 500 to 860°C. A process as claimed in claim 16, wherein the solution treatment in step (b) is performed at a temperature in the range of from 700 to 800°C. A process as claimed in any one of claims 15 to 17, wherein the solution treatment in step (b) is performed for at least 1 minute, and wherein age-hardening in step (d) is carried out for from 5 to 60 minutes. A process as claimed in claim 18, wherein the solution treatment in step (b) is performed for from 5 to 60 minutes, and wherein age-hardening in step (d) is carried out for from 10 to 60 minutes. A process as claimed in claim 19, wherein the solution treatment in step (b) is performed for from 10 to 60 minutes, and wherein age-hardening in step (d) is carried out for from 30 to 60 minutes. A process as claimed in any one of claims 15 to 20, wherein the alloy composition is formed or shaped into an article of jewellery prior to the solution-treatment in step (b).

Inventors: GRICE STEWART, ROTHERAM PETER, GRICE, STEWART, ROTHERAM, PETER
Applicants: COOKSON PRECIOUS METALS LTD, COOKSON PRECIOUS METALS LTD.
Application year: 2000
Main classification: C22C5
IPC codes: C22C5, C22F1, A44C27, C22C9