EP1913167A1

EP1913167A1 - Platinum alloy and method of production thereof

Info

Publication number: EP1913167A1
Application number: EP05774929A
Authority: EP
Inventors: Peter Tews
Original assignee: Degussa GmbH; Allgemeine Gold und Silberscheideanstalt AG
Current assignee: Allgemeine Gold und Silberscheideanstalt AG; Evonik Operations GmbH
Priority date: 2005-08-03
Filing date: 2005-08-04
Publication date: 2008-04-23
Also published as: WO2007014576A1; IL189218A0; CA2618216A1; DE202005021635U1; CN101263237A; JP2009503261A

Abstract

The disclosure relates to a platinum alloy comprising (a) 47.5 to 54.99 wt.% of platinum, (b) 1.5 to 10 wt.% of cobalt, and (c) 35.01 to 51 wt.% of copper. The platinum alloys have excellent mechanical and optical properties for use in the production of ornamental articles such as rings, necklaces, earrings, watch bands, watch bodies and other jewelry. The disclosure further relates to a method of preparing the platinum alloys and their use in the production of ornamental articles. Still further the disclosure relates to ornamental articles comprising the platinum alloy and a method of production thereof.

Description

PLATINUM ALLOY AND METHOD OF PRODUCTION THEREOF

FIELD OF THE INVENTION

The present invention relates to a platinum alloy and a method of production thereof. In particular, the present invention relates to_. platinum alloys that are suitable for the fabrication of ornamental articles such as rings, necklaces, bracelets, earrings, watch bands, watch bodies and other jewelry. Furthermore, the present invention relates to an ornamental article made from the. platinum alloy and a method of production thereof.

BACKGROUND OF THE INVENTION

Platinum is a precious metal and is relatively expensive. In recent years platinum has come into increasing prominence as a metal used for jewelry fabrication. Platinum for fine jewelry is commonly sold in high concentrations of over 85 percent by weight.

Pure platinum metal (PtIOOO) is soft and does not have the mechanical strength for most jewelry applications. For this reason, in most jewelry applications various kinds of platinum alloys are employed. Platinum alloys are desirable for their neutral color when combined with gems, they are hypo- allergenic, they have high tensile strength, and a pleasurable heft due to their high- density.

The jewelry industry uses three main classes of platinum alloys. These classes are Pt950, Pt900 and Pt850. These alloys have a platinum content of 95, 90 and 85 wt.%, respectively. Commercially available alloys frequently used in the fabrication of jewelry include Pt/Ir 900/100 (90 wt.% platinum and 10 wt.% iridium), PtCu950 (95 wt.% of platinum and 5 wt.% of copper) and PtCo950 (95 wt.% of platinum and 5 wt.% of cobalt).

Various high platinum content jewelry materials are known in the art. The term "high platinum content" as used herein refers to platinum alloys having a platinum content equal or greater than 85 wt.%.

For example, U.S. Patent. 4,165,983 describes an alloy for fabricating jewelry containing at least 95 wt.% platinum, 1.5 to 3.5 wt.% gallium, and a balance of at least one of indium, gold, palladium, silver, copper, cobalt, nickel, ruthenium, indium and rhodium. U.S. Patent 5,846,352 describes a heat-treated platinum-gallium alloy for fabricating jewelry containing 1 to 9 wt.% gallium and a small amount of palladium. Japanese published patent application JP 61-133340 describes an alloy for fabricating jewelry consisting of 84.to 96 wt.% platinum, 1 to 10 wt.% gallium, 0.5 to 10 wt.% copper, and 0.01 to 5 wt.% cobalt. Japanese published patent application JP 61-034133 describes an alloy for fabricating jewelry containing 84 to 96 wt.% platinum, 0.5 to 10 wt.% cofoalt, 0.5 to 10 wt.% copper and 0.01 to 0.5 Y, B, CaB mischmetal.

Although such alloys have satisfactory mechanical and optical properties that make them suitable for jewelry fabrication, these alloys are expensive to produce due to their high platinum content.

There are also a number of low platinum content jewelry materials known in the art. The term "low platinum content" as used herein refers to platinum alloys having a platinum content smaller than 85 wt.%.

U.S. Patent No. 6,048,492 describes a platinum alloy composition for use in jewelry products containing about 58.5 wt.% of platinum, 26.5 to 36.5 wt.% of palladium and 5 to 15 wt.% of either iridium, copper or ruthenium. U.S. Patent

2,279,763 describes a ductile platinum alloy containing 10 to 80 wt.% of platinum,

12 to 90 wt.% of palladium, and 1 to 15 wt.% of ruthenium. WO2004/059019 Al describes PT-base bulk solidifying amorphous alloys utilizing platinum and other ingredients and requiring the presence of phosphorous. The amorphous alloys described in this document are prepared by quenching the molten alloy from above the melt temperature to ambient temperatures and thus achieving a substantially (i.e. non-crystalline) structure of the alloy and requiring more than 50% to be in the amorphous state.

A disadvantage of known low platinum content jewelry materials is that they often have inferior mechanical and physical properties compared to the high platinum content jewelry materials. In particular, the castability of known low platinum content jewelry materials is not as good as that of high platinum content alloys. Also, the color of known low platinum content jewelry materials differs from the typical "platinum color" of Pt950 alloys that is desired by most customers of fine jewelry. Hence, low platinum content jewelry materials are often rejected by customers for aesthetical reasons. In fact, it is very difficult to produce a low platinum content jewelry material that combines both the mechanical strength and workability as well as the optical properties of high platinum content materials.

Due to the potential improvements in properties and performance of such alloys, there is a need for additional alloys suitable for use in jewelry and art applications. Accordingly, it would be desirable to provide a platinum alloy composition suitable for jewelry that is less expensive than the platinum that is presently available, yet still provides a platinum jewelry item with desirable technological and optical properties.

SUMMARY OF THE INVENTION

According to the present invention there is provided an improved low platinum content alloy composition that contains 47.50 to 54.99 wt.% of platinum, 1.5 to 10 wt; % of cobalt and 35.01 to 51 wt.% of copper. The alloys according to the present - A -

invention are particularly well suited for the fabrication of ornamental articles, such as rings, necklaces, earrings, watch bands, watch bodies and other jewelry.

Surprisingly, it was found that despite of their relatively low platinum content the alloys of the present invention exhibit excellent mechanical and optical properties that make them extremely suitable for the manufacturing of ornamental products such as jewelry of any kind. Due to the lower density of the alloys of the present invention it is possible to manufacture thinner, lighter constructions and castings at considerable less cost than with high platinum content alloys (e.g. Pt850, Pt900, Pt950). The platinum alloys of the present invention have a lower melting range compared to known low platinum content alloys as described in, for example, U.S. Patent 6,048,492. Due to their relatively low melting temperature they cast easier than previously known platinum alloys and are more energy efficient. This, lower temperature alloy also allows a lower mold temperature, decreasing defect rate due to shrinkage porosity, investment cracking, inclusions, and contaminations that occur more readily at highly-elevated temperatures.

The alloys according to the present invention are particularly well suited for the fabrication of jewelry due to their improved hardness, workability, castability, deformability, wear and abrasion properties, and resistance to corrosion. The platinum alloy composition of the invention appears and looks no different than 95 percent platinum, but is substantially lighter, less dense, and thus, less expensive, to produce. In fact, the platinum alloy composition of the present invention has essentially the same color and appearance as PtCu950 alloy.

The invention further relates to a method of preparing the alloys of the present invention by formulating and mixing the components of the alloy in the specified amounts and melting them together.

The alloy may be formed into a desired shape. Such operations are many and include casting or fabricating. Some examples of fabrication can be by rolling of the alloy into a sheet, drawing a wire, molding, casting, forging, stamping or constructing the object or shape useful as a jewelry component.

Accordingly, the invention also relates to a method of manufacturing an ornamental article, which comprises formulating one of the platinum alloys described above and then utilizing the alloy as a component of jewelry.

Further, the invention also relates to the use of such alloys in the production of ornamental articles such as jewelry. Still further, the invention relates to ornamental articles comprising such alloys.

Accordingly, it is an object of the invention to provide an improved low platinum content platinum alloy composition.

Still another object of the invention is to provide an improved platinum alloy composition that is suitable for use in jewelry for the mass commercial market.

A further object of the invention is to provide an improved platinum alloy composition which is substantially lighter and less dense than conventional platinum alloy compositions.

Still further it is an object of the invention to provide platinum alloys which may be cast more readily than known platinum alloys.

Still other objects and advantages of the invention will in part be obvious, , and will in part be apparent from the following description.

DETAILED DESCRIPTION

The platinum alloy compositions of the invention include platinum in an amount of 47.5 to 54.99 wt.%. The platinum content of the alloy compositions of the invention is significantly lower than that of conventional Pt850, Pt900 and Pt950 platinum alloys commonly used in the jewelry industry.

According to one embodiment of the invention the platinum alloy comprises:

47.5 to 54.99 wt.% of platinum; 1.5 to 10 wt.% of cobalt; and 35.01 to 51 wt.% of copper.

Preferably, the platinum content of this alloy is from 48.5 to 52.5 wt.% , in particular from 49 to 51 wt.%, based on the total alloy composition. According to another embodiment of the invention, the platinum content of this alloy is preferably from 52.5 to 54.5 wt.%. If the platinum content of the alloy is smaller than 47.5 wt.% the workability and stampability of the alloy decrease significantly and the alloy loses its platinum-like color. If the platinum content of the alloy is greater than 63 wt.%, the costs for the production of the alloy increase significantly while, at the same time, the mechanical and chemical properties of the alloy do not improve significantly.

Preferably, the cobalt content of the alloys of the present invention is from 1.5 to 10 wt.%, in particular 2 to 8 wt.% or 3.5 to 5.5 wt.%, based on the total alloy composition. If the cobalt content of the alloy is smaller than 1.5 wt.%, the mechanical properties and the workability of the alloy decrease significantly and the alloy loses its platinum-like color. If the cobalt content of the alloy is greater than 10 wt.% the alloy becomes too hard.

Preferably, any balance in the alloys of the present invention is made up by copper. The platinum alloys of the present invention may further comprise 0.001 to

2 wt.% of at least one first metal selected from the group consisting of indium and ruthenium. A combination of these elements may also be added, so long as the total amount does not exceed 2 wt.% of the alloy composition. Indium and/or ruthenium can be added as metal hardeners in order to improve the hardness of the alloy, with iridium being the preferred hardener since it offers gradual hardness improvements over a wide range of concentrations, with no deterioration of alloy properties.

The platinum alloys of the present invention may further comprise 0.001 to 2 wt.% of at least one second metal selected from the group consisting of indium and gallium. A combination of these elements may also be added, so long as the total amount does not exceed 2 wt.% of the alloy composition. Indium and gallium may be added to improve the precipitation hardening of the alloy. The platinum alloys of the present invention may further comprise palladium in an amount of 0.001 to 5 wt.%, preferably 0.25 to 2.5 wt.%. An addition of palladium is useful in order to vary the color of the alloy.

The platinum alloys of the present invention may further comprise silicon in an amount of 0.001 to 0.5 wt.%, preferably in an amount of 0.1 to 0.3 wt.%. It was found that an addition of silicon in the specified amount improves the casting properties of the alloy and results in a smoother surface of the casted article. This effect is particularly desirable when the alloy of the present invention is used for the manufacture of an ornamental article where excellent casting properties are

'-required. It was, found that silicon in the amounts specified is soluble in the Pt-Co-

Cu alloys of the present invention and results in the aforementioned effect. In contrast thereto, addition of silicon to high content platinum alloys generally results in inhomogeneous low melting phases and is thus undesirable.

The platinum alloys of the present invention may further comprise zirconium in an amount of 0.001 to 0.5 wt.%, preferably in an amount of 0.1 to 0.3 wt.%. It was found that an addition of zirconium in the specified amount improves the workability of the alloys.

According to a preferred embodiment of the present invention, the total amount of other elements present in the platinum alloy besides Pt, Cu, and Co does not exceed about 10 wt.%, preferably about 7.5 wt.%, even more preferably about

5 wt.% , and most preferably about 4 wt.%, based on the total weight of the platinum alloy.

The platinum alloy of the present invention may be present in a crystalline state or in an amorphous state. Preferably, the platinum alloy of the present invention is present in a substantially crystalline state. The term "substantially crystalline state" as used herein means that the platinum alloy is greater than fifty percent crystalline by volume. Preferably, the platinum alloy is at least about ninety percent crystalline by volume, more preferably at least ninety five percent crystalline by volume and most preferably about one hundred percent crystalline by volume. While phosphorous can be used as an additive in some platinum alloys to make the alloy more brittle and/or amorphous, the addition of phosphorous to the alloy of the present invention is not particularly preferred since it is not desired to produce an amorphous alloy. Therefore, if phosphorous is added to the platinum alloy of the present invention, such an addition should be done in moderate amounts. Preferably, the amount of phosphorous contained in the platinum alloy of the present invention is less than 4.2 wt.%, more preferably less than 3.4 wt.%, even more preferably less than 2.3 wt.% and most preferably less than about 1.5 wt.% of phosphorous, based on the total alloy composition. According to another preferred embodiment the platinum alloy of the present invention contains less than about 2.0 wt.% and more preferably less than about 1 wt.% of phosphorous, based on the total alloy composition.

Advantageously, the alloy can include any one of a number of property enhancing agents, including a deoxidizing agent, grain reducing agent, a viscosity decreasing agent or a color variation agent. The number and amount of the other additives may vary depending on the desired mechanical properties of the alloy and can readily be determined by a person of ordinary skill in the art by means of routine experiments.

According to another embodiment of the invention, the platinum alloy consists essentially of, apart from impurities, 48 to 53 wt.% of platinum,

3.5 to 6.5 wt.% of cobalt, and 40.5 to 48.5 wt.% of copper, wherein copper may be substituted by one or more of the following elements in the amounts specified: 0.001 to 2 wt.% of at least one of said first metals; 0.001 to 2 wt. % of at least one of said second-metals; 0.001 to 5 wt. % of palladium; 0.001 to 0.5 wt.% of silicon; and/or 0.001 to 0.5 wt.% of zirconium

The alloys of the present invention exhibit excellent mechanical and physical properties such as tensile strength, Vickers hardness and elongation at break. The tensile strength of the platinum alloys of the present invention is in the range of 450 to 800 N/mm². The Vickers hardness of the platinum alloys of the present invention, measured at softened state, is in the range of 130 to 210 HVlO. The elongation at break of the platinum alloys of the present invention is at least about 20 %.

A further advantage of the present, invention is that the color tone of the platinum alloy corresponds essentially to the platinum white color tone of a PtCu950 alloy, which is aesthetically very appealing.

The alloys of the present invention may be prepared by conventional alloying methods that are well known in the art. The preparation of the alloy generally includes the step of melting platinum, cobalt and copper and any other component in the specified amounts. The method may further include the step of hardening the alloy by cold working or heat treatment.

The method may include the steps of annealing and then quenching the alloy, before hardening the alloy.

The alloys are usually cast from melts under a shielding gas and then shaped. After shaping, they may be subjected to heat treatment, possibly under a shielding gas, to improve their mechanical properties.

In order to prepare the platinum alloy composition of the invention, a high temperature melting process is carried out. This can achieved using induction melting equipment, as is well known in the art. At all times, extreme care should be exercised in order to limit metal contamination, as platinum is easily contaminated by many elements routinely present in the environment. Such care can be achieved by melting the metals in either a vacuum or an inert gas atmosphere, by preventing contact with other metals, and by preventing mixing with silica-based products. The platinum alloy is preferably melted and blended together by induction heating in appropriate crucibles for platinum alloys. After melting, the alloy can be poured through water to create grain-shot and can then be dried, weighed and used for casting.

For the preparation of the alloy of the present invention, the components of the inventive composition are preferably melted in a silica crucible (for small, fast melts) or a zirconium oxide (for large, slow melts) crucible in an induction oven. It is preferred to use a vacuum or inert gas in the melting process and to place all components of the alloy composition in the crucible at the same time. In the melting of the alloy, the molten metals should preferably be "turned" (utilizing medium to low frequency induction fields) in order to obtain an appropriate mixing of the metals.

Following the melting step, the resulting alloy nugget elements can be subjected to cold rolling and/or annealing in order to improve mechanical qualities of the mix. Thereafter, the mixed metal composition can optionally be re-melted as before, and a shot or plate be produced.

The preparation of the platinum alloys of the present invention can further comprise an annealing step. Annealing can be carried out either in a furnace or with a torch, as is well known in the art. The annealing temperature depends on the platinum content and the melting point of the alloy and will readily be determined by a person of ordinary skill by routine experiments. Preferably, the annealing is done in a furnace that is atmosphere controlled with shielding gas.

The shielding gas can be any of the non-oxidizing inert gasses, such as argon, nitrogen, or mixtures thereof; anti-oxidizing gasses such as hydrogen, carbon monoxide, or "forming" or "cracked ammonia" gas (nitrogen with a few percent of hydrogen). The piece can also be protected from oxidation by enveloping them with commercially available heat-treating wraps.

The alloys can be used for a wide variety of jewelry cqmponents, such as rings, clasps, spring parts, even compression-spring settings for gemstones, and the like.

Furthermore, the alloys can be repeatedly annealed and heat-treated/age- hardened, if desired..

As used herein, the term "age-hardening" is essentially synonymous with the term "precipitation hardening" which results from the formation of tiny particles of a new constituent (phase) within a solid solution. The presence of these particles create stress within the alloy and increase its yield strength and hardness. See, B. A. Rogers, "The Nature of Metals", p.320 (Iowa State University Press, 1964); H. W. Polock, "Materials Science and Metallurgy", p. 266 (Reston Pub. Inc. 1981) and "The Metals Handbook", pp.1-2 (Am. Soc'y Metals, 1986). In their annealed/softened state the alloys can be worked by standard jewelry-making techniques: they can be rolled, drawn, soldered to, shaped, bent, stamped, etc. These alloys can be applied to a variety of designs for springs, gemstone mountings in rings, pendants, bracelets, chains, precious metal art objects, and the like. It should be noted that in designing for structure of the jewelry or art object, the smallest cross-sectional area and shape of a component is taken into account. It is possible to adapt the design of the alloy to almost any configuration. The basic forms of these designs can vary, from simple sheet, to ring-shapes and more complex helixes, v-shapes, and the like. Objects can be wire, sheet, springs of all types, pendants, chain-links, brooches, and a multitude of others. Standard jewelry soldering techniques can be applied and repairs requiring heat can be carried out. The alloys can be shaped, bent, built onto, annealed, and when the piece is done, the spring power and hardness can be regained by heat-treatment. The ornamental product can be made by casting. The hardness of the alloys may also be further increased by heat treatment. The heat treatment may be carried out in a range of from 300 to 950 ⁰C with a suitable value being in the range of from 600 to 950 ⁰C, and typically of the order, of 800 ⁰C. The alloys can be softened by standard annealing procedures, typically at about 1000 ⁰C to 1030 ⁰C, or higher.

The alloys may be used in the form of wire, sheet or other manufactured article and may be given intricate shapes and forms due to their great hardness combined with great ductility. The alloys according to the present invention can be used, for example, in the fabrication of wedding bands. Such weddings bands are generally produced by sawing blanks from tubes and then further working the blanks by suitable measures, such as milling, drawing, forging, and polishing.

Other jewelry articles that can be manufactured from the alloys according to the present invention include, for example, rings, necklaces, bracelets, earrings, bangles, stickpins, watch bands, watch bodies, wristwatches, tooth picks as well as other decorative articles such as ballpoint pens, letter openers, pocket knife handles, and the like.

The following Examples are provided to illustrate certain aspects of the invention and it is understood that such an Example does not limit the scope of the invention as defined in the appended claims.

EXAMPLE

An alloy of the composition as specified in the following table was weighted and molten under vacuum in a zirconia crucible in a vacuum induction furnace at a temperature of 1480 to 1500 ⁰C to obtain a homogeneous melt. The alloy was cast into a steel mold to form blocks having a dimension of 2O x 140 mm. In the following table, the physical properties of the alloy specimens thus formed are specified. The melting range was determined by measuring the cooling curve of the alloy with a Degussa resistance furnace HRl/Pt/PtRHlO equipped with a Linseis thermo element and a temperature-time-plotter L250. The Vickers hardness was determined according to DIN 50133 using a Wolpert V-Testor 4521 instrument. The tensile strength, elongation at break and yield stress were determined according to DIN 50145 using a Zwick ZOlO instrument. The color was determined visually.

COMPARISON EXAMPLE

A commercially available Pt/Cu 950/50 alloy was weighted and molten under vacuum in a zirconia crucible in a vacuum induction furnace to obtain a homogeneous melt. The alloy was cast into a steel mold to form blocks having a dimension of 20 x 140 mm. The physical properties of the alloy specimens thus formed were tested as described above and are specified in the following table.

TABLE

EXAMPLE l EXAMPLE 2 COMPARATIVE EXAMPLE

Composition Pt 50 wt.% Pt 50 wt.% Pt 95 wt.% Cu 45 wt.% Cu 44.5 wt.% Cu 5 wt.% Co 5 wt.% Co 5 wt.% Si 0.25 wt.% Zr 0.25 wt.%

Density 12.7 12.7 20.3 Color platinum white platinum white platinum white Melting Range 1330 -1360 1330 -1360 1730 -1745 Castability very good excellent fair Workability good good good

Hardness [HV] softened state 130 130 110 20 % cold rolled 230 230 185 40 % cold rolled 260 260 210 60 % cold rolled 280 280 235

Tensile Strength

[N/mm²] softened state 600 600 320 60 % cold rolled - 1000 - 1000 - 800

Yield Stress 350 350 130

[N/mm²]

Elongation at >30 >30 > 30 Break [%]

The experimental results indicate that the alloy according to the present invention exhibits superior casting, wear and abrasion properties when compared to a conventional Pt/Cu 950/50 alloy. The results of Example 1 further indicate that the castability of the alloy of the present invention is enhanced by addition of small amounts of silicon. Furthermore, the experimental results indicate that the forming properties and the color tone of the alloy according to the present invention are comparable to those of a conventional Pt/Cu 950/50 alloy. The alloy according to the present invention was found to be an excellent material for the manufacture of jewelry articles such as rings, bracelets or necklaces.

The principle of the invention and the best mode contemplated for applying that principle have been described. It is to be understood that the foregoing is illustrative only and that other means and techniques can be employed without departing from the true scope of the invention defined in the following claims.

Claims

CLAIMS1. A platinum alloy comprising:47.50 to 54.99 wt.% of platinum,

1.5 to 10 wt.% of cobalt, and 35.01 to 51 wt.% of copper.

2. The platinum alloy of claim 1, wherein said alloy has a substantially crystalline structure.

3. The platinum alloy of claim 1, wherein said alloy comprises 48.5 to 52.5 wt.% of platinum.

4. The platinum alloy of claim 1, wherein said alloy comprises 49to 51 wt.% of platinum.

5. The platinum alloy of claim 1, wherein said alloy comprises 52.5 to 54.5 wt.% of platinum.

6. The platinum alloy of any one of claims 1 to 5, wherein said alloy comprises 2.0 to 8.0 wt.% of cobalt.

7. The platinum alloy of any one of claims 1 to 6, wherein said alloy comprises 3.5 to 5.5 wt.% of cobalt.

8. The platinum alloy of any one of claims 1 to 7, wherein said alloy further comprises 0.001 to 2 wt.% of at least one first metal selected from the group consisting of indium and ruthenium.

9. The platinum alloy of any one of claims 1 to 8, wherein said alloy further comprises 0.001 to 2 wt.% of at least one second metal selected from the group consisting of indium and gallium.

10. The platinum alloy of any one of claims 1 to 9, wherein said alloy further comprises 0.001 to 5 wt.% of palladium.

11. The platinum alloy of any one of claims 1 to 10, wherein said alloy further comprises 0.001 to 0.5 wt.% of silicon.

12. The platinum alloy of claim 11, wherein said alloy comprises 0.1 to 0.3 wt:% of silicon.

13. The platinum alloy of any one of claims 1 to 12, wherein said alloy further comprises 0.001 to 0.5 wt.% of zirconium.

14. The platinum alloy of claim 13, wherein said alloy comprises 0.1 to 0.3 wt.% of zirconium.

15. The platinum alloy of any one of claims 1 to 14, wherein the total amount of other elements present in said platinum alloy besides Pt, Cu, and Co does not exceed 10 wt.%, based on the total weight of said platinum alloy.

16. The platinum alloy of any one of claims 1 to 15, wherein the total amount of other elements present. in said platinum alloy besides Pt, Cu, and Co does not exceed 7.5 wt.%, based on the total weight of said platinum alloy.

17. The platinum alloy of any one of claims 1 to 16, wherein the total amount of phosphorous present in said platinum alloy is less than about 2.0 wt.% based on the total weight of said platinum alloy.

18. The platinum alloy of any one of claims 1 to 17, consisting essentially of:

48 to 53 wt.% of platinum, 3.5 to 6.5 wt.% of cobalt, and 40.5 to 48.5 wt. % of copper,

wherein copper may be substituted by one or more of the following elements in the amounts specified:

0.001 to 2 wt.% of at least one of said first metals;

0.001 to 2 wt.% of at least one of said second metals;

0.001 to 5 wt.% of palladium;

0.001 to 0.5 wt.% of silicon; and/or

0.001 to 0.5 wt.% of zirconium

19. The platinum alloy of any one of claims 1 to 18, wherein the tensile strength of said alloy is in the range of 450 to 800 N/mm².

20. The platinum alloy of any one of claims 1 to 19, wherein the Vickers hardness of said alloy, measured at soft state, is in the range of 130 to 210 HVlO.

21. The platinum alloy of any one of claims 1 to 20, wherein the elongation at break of said alloy is at least 20 %.

22. The platinum alloy of any one of claims 1 to 21, wherein the color tone of said alloy corresponds essentially to the platinum white color tone of a PtCu950 alloy.

23. A method of preparing an alloy according to any one of claims 1 to 22, which comprises (a) blending the components of the alloy; and (b) melting the alloy.

24. A platinum-colored material for ornamental purposes comprising a platinum alloy according to any one of claims 1 to 22.

25. An ornamental article comprising the platinum alloy of any one of claims 1 to 22.

26. The ornamental article of claim 25, wherein said ornamental article is a ring, a necklace, an earring, a watch band, a watch body or other jewelry.

27. A method of fabricating the ornamental article of claim 25 or 26, which comprises forming the ornamental article from an alloy according to any one of claims 1 to 22.

28. The method of claim 27, wherein the alloy is casted into the shape of the ornamental article.

29. The use of a platinum alloy according to any one of claims 1 to 22 for the manufacture of an ornamental article such as a ring, a necklace, an earring, a watch band, a watch body or other jewelry.