BACKGROUND OF THE INVENTION

This invention relates to a platinum alloy which has a sufficiently high platinum content to comply with hallmarking or other prescribed requirements for certifying the fineness of the platinum.

Platinum hallmarking standards vary from country to country but the general standards are 800, 900 and 950 fineness. The last mentioned fineness (95 weight percent platinum) is the most common hallmarking standard used.

Hallmarked platinum, with a fineness of at least 800, has long been regarded as an item of value and investment. In the case of jewellery, while this view does apply to some extent, a perceived drawback is that the jewellery, over a period of time, depreciates in value due to the loss of the relatively soft platinum caused by wear, and may lose its shape due to the softness of the metal.

Various attempts have been made to produce hallmarkable platinum having alloying additions which increase the hardness of the metal. These additions may however have the effect of rendering the alloy unworkable to a large extent. This makes the manufacture of jewellery from such alloys difficult.

Alloying increases the hardness of platinum and commercial alloys are generally in the range of 40 to 160 Vickers hardness. Is this respect reference is made to Figure 1 (Reference - Johnson and Mathey) of the attached drawings which depicts curves of hardness vs percentage alloying elements for commercial jewellery alloys. These alloys can be increased by hardness from 60 to 80 points on the Vickers scale by cold working.

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An increased alloy hardness is beneficial because it generally results in improved wear resistance and a stronger, more durable, alloy. As the hardness increases the ductility generally decreases and so a balance between optimum strength and ductility must be achieved.

SUMMARY OF THE INVENTION

The invention is concerned with platinum alloys which can be worked in a softened state and which can subsequently be hardened by a heat treatment process. The importance of this is that ductility is retained but higher hardnesses and strength are obtained. The higher hardness should result in improved wear resistance.

The invention provides a platinum alloy which contains at least 80 wt% platinum and at least a binary addition of from 0.001 to 6.1 wt% selected from the following: titanium, zirconium, vanadium, tin, germanium and gallium.

The alloy may include a ternary addition selected from the following: iridium, ruthenium, palladium, gold, rhodium, silver and zirconium, the ternary addition being present in an amount (wt%) which maintains the phase field which is created by the binary addition.

The ternary addition may be added for any desired property or properties in the resulting alloy, such as colour, casting properties, wear resistance, springiness, resistivity, melting point, thermal conductivity, initial hardness, ductility and malleability.

In one form of the invention the binary addition is approximately 2 wt% and is selected from titanium and germanium.

In a second form of the invention the binary addition is from 3 to 4 wt% zirconium.

In a third form of the invention the binary addition is from 5 to 6 wt% tin.

In a fourth form of the invention the binary addition is from 3.8 to 6 wt% gallium.

In a preferred form of the invention the binary addition is approximately 2 wt% titanium and the ternary addition is approximately 3 wt% and is selected from iridium, ruthenium, palladium, gold, rhodium and silver.

Other suitable alloys are provided by:

binary addition ternary addition approximately 1 wt% titanium approximately 2 wt% zirconium approximately 3 wt% zirconium approximately 2 wt% palladium approximately 3 wt% vanadium approximately 11.9 wt% iridium approximately 3.8 wt% tin approximately 0.8 wt% palladium approximately 5 wt% gallium approximately 0.5 wt% palladium

An important aspect of the invention is that the aforementioned compositions can be hardened by heat treatment or cold working. In each alloy system the annealed hardness varies but a heat treatment hardness increase of more than 10 Vickers is observed.

Hardening as a consequence of a heat treatment process was not observed in the following systems: 3Zr-12Pd-85Pt 3.2V-1.8Pd-95Pt 1.5Sn-2.5Pd-96Pt 2.4Ni-2.5Pd-95.1Pt

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- 1Ti-3Zr-96Pt

2Ti-3Zr-95Pt 2Ti-2Zr-96Pt 1.5Tl-2Zr-96.5Pt 2Ti-3Cu-95Pt - 2Ti-3Co-95Pt

2Ti-3W-95Pt

Although hardening is seen in certain binary systems for set amounts, the effect is not always witnessed in ternary extensions of these systems as the ternary alloy has different influences on the phase field and stability of the second phase forming. The platinum-titanium ternaries were the most successful alloys.

The invention also provides a method of preparing an alloy which includes the steps of melting platinum and at least a binary addition selected from the following: titanium, zirconium, vanadium, tin, germanium and gallium, to form an alloy containing at least 80 wt% platinum and from 0,001 to 6,1 wt% of the binary addition, and hardening the alloy by cold working or heat treatment.

The method may include the step of adding a ternary addition selected from the following: iridium, ruthenium, palladium, gold, rhodium, silver and zirconium, the ternary addition being present in an amount (wt%) which maintains the phase field which is created by the binary addition.

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

The invention also provides a method of making an artefact which includes the steps of forming the artefact from an alloy of the aforementioned kind, and hardening the formed artefact by heat treatment or cold working.

If the hardness is high eg above 200 Vickers, obtained for example with germanium and gallium alloys, the artefact may be made by casting. The hardnesses of these alloys may also be further increased by heat treatment.

The heat treatment may be carried out in a range of from 300°C to 950°C with a suitable value being in the range of from 600°C to 950°C, and typically of the order of 800°C. The alloys can be softened by standard annealing procedures, typically at about 1000°C to 1030X, or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of examples with reference to the accompanying drawings in which:

Figure 1 is a set of curves of hardness vs percentage alloy elements in platinum, for various commercially available jewellery alloys • this Figure has been referred to in the preamble to this specification and is not further described herein;

Figure 2 shows the effect of heat treatment cycles on the hardness of platinum- vanadium alloys;

Figure 3 shows the effect of heat treatment cycles on the hardness of platinum- zirconium alloys; Figure 4 shows the hardening in Pt 2wt% Ti alloys with 3 wt% ternary additions; and

Figure 5 shows the hardening of some alloys in the 1-5 wt% titanium-platinum system.

DESCRIPTION OF PREFERRED EMBODIMENTS

Various alloys were produced by button arc melting under an argon environment by melting the required quantities of the various ingredients. The alloys were melted a number of times to ensure thorough mixing and were cooled within

minutes on a water cooled copper hearth.

The results are summarised hereinafter for a range of binary alloys (Part A); ternary alloys (Part B); and further particular alloys (Part C).

Part A: Range of Binary Alloys

Investigation into Binary Alloys of 98 wt% platinum

98 wt% Platinum 2 wt% X. where X=Cu. Mo. Ni. Co. Mn. Sn. Fe. Mo. Cr. Ta. W. Zr.

Si. Ge. V. Ga. In. Ti

Alloys of 98 wt% platinum and a 2 wt% alloying addition were made and annealed at 1050 ^β C for 15 minutes, and water quenched. The alloy additions consisted of copper, magnesium, nickel, cobalt, manganese, tin, iron, molybdenum, chromium, tantalum, tungsten, zirconium, silicon, germanium, vanadium, gallium, indium and titantium. Each sample was heat treated at 600°C for 10 minutes and water quenched and its hardness was then measured. Two further heat treatment cycles at 800°C for 10 minutes and at 800°C for 30 minutes were conducted and the hardness was measured.

The results suggest that the 2 wt% additions of titanium, germanium, and indium show hardening after heat treatment.

2 wt% additions of the following elements showed no, or no significant, hardening: nickel, silicon, vanadium, chrome, manganese, iron, cobalt, magnesium, copper, gallium, zirconium, molybdenum, tin, tantalum, tungsten, silver and gold.

98 wt% Platinum 2 wt% Titanium

Extensive work has shown that, in addition to the annealed hardness of approximately 170-180 Vickers and heat treated hardness of 260-280 Vickers, with a combination of cold working and heat treating, a hardness in the region of 400- 430 Vickers, which is extremely high, is feasible. Heat treatments in the region of 600°C to 900°C showed that a better hardening could be observed at a higher temperature. Transmission electron microscopy, Differential Thermal Analysis as well as a variety of other techniques have been used to gain aπ understanding of the hardening mechanism.

97 wt% Platinum 3wt% X. X=Mn. Cr

The annealed hardnesses of 97 wt% platinum 3 wt% manganese, and 97 wt% platinum 3 wt% chromium, were observed to be about 108, and 142, Vickers hardness respectively. A heat treatment at 800°C for 20 minutes followed by a water quench resulted in hardnesses of approximately 132 and 104 Vickers. Further heat treatment cycles did not appear to enhance the hardening effect.

Investigation into Binary Alloys of 96 wt% platinum 96 wt% Platinum 4 wt% X. where X=Cu. Ni. Mn. Sn, Fβ. Mo. Cr. Ta. W, Zr. Ge. V. Ga.

Ti. Aq. Au

Alloys of 96 wt% platinum and a 4 wt% alloying addition were made and annealed at 1050 ^C C for 15 minutes and water quenched. The alloying additions consisted of copper, nickel, cobalt, manganese, tin, iron, molybdenum, chrome, tantalum, tungsten, zirconium, germanium, vanadium, gallium, titanium, silver and gold. Each sample was heat treated at 800°C for 10 minutes and water quenched, and its hardness was then measured. A further heat treatment cycle at 800°C for 10 minutes was conducted and the hardness was measured.

The results suggest that the 4 wt% additions of gallium, zirconium and tin show hardening after heat treatment. On the other hand 4 wt% additions of the following elements showed little or no hardening: nickel, titanium, vanadium, chrome, maganeεe, iron, copper, cobalt, germanium, molybdenum, tantalum, tungsten, silver and gold.

Further Binary Alloys

Table 1 reflects hardnesses for various binary alloys subjected to heat treatments, as indicated.

Part B: Ternary Alloys

Platinum-Titanium Ternaries

Investigations show that if 8 wt% of palladium is added to 90 wt% platinum- 2 wt% titanium, the hardening effect previously observed in the platinum-2 wt% titanium system is inhibited. However 95 wt% platinum, 2 wt% titanium and 3 wt% X where X=iridium, ruthenium, rhodium, gold, silver or palladium showed hardening (Figure 4). Additions of 3 wt% copper, tungsten or cobalt appeared to inhibit or suppress the hardening effect.

Optimisations of temperatures for the iridium and palladium alloys have been conducted over a temperature range of from 600X to 900°C. With cold working and heat treating both the iridium and palladium ternary additions could achieve hardnesses of the order of 400 Vickers.

The preceding work identified certain compositions that can be hardened by means of heat treatment or cold working. The temperature and time parameters can vary in each case. Although one can normally deduce phase changes on the

basis of phase diagrams, in the case of platinum systems only limited work has been done and there are many areas of uncertainty. This makes it extremely difficult to predict with certainty that any phase transformations will occur, let alone what they are. It was for this reason that so many different systems were explored.

It is recognised that, in general, the addition of from 1 to 5 wt% of an alloying addition results in an increase in hardness because of solution strengthening. Heat treating these alloys can result in an even higher hardness as a consequence of precipitation hardening, ordering, or any other phase transformation. Heat treating however does not always increase hardness and can even cause softening (eg 2 or 4 wt% additions of chromium, iron, copper, tungsten, manganese, molybdenum, etc). Thus the identification of alloys which, after annealing, are sufficiently ductile to be worked, and which can subsequently be hardened by heat treatment, is not readily predictable and remains of substantial importance to a number of industries and in particular to the jewellery industry.

Observations on Ternary Additions

Binary phase diagrams represent what phases (structure and composition determined) are present in a materials system at various temperatures, compositions and pressures. When a third element is added, the binary phase diagram no longer represents phases present at equilibrium conditions. The presence of the third alloying element can change the critical range and location of phase fields in the binary system. Critical temperatures for the phase field to exist can be altered and the phase field can be contracted or expanded depending on the nature of the element. One cannot with certainty predict the influence of the third element.

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In steels for example some elements stabilise the austenite phase (eg manganese, nitrogen, nickel) whereas other elements stabilise the ferrite phase rather (eg chromium). Thus by changing the alloying elements the occurence of the austenite or ferrite phase is promoted. The influence of an element is generally observed through experimental procedures.

Hardening as a consequence of heat treatment, shown in binary platinum systems, could be as a result of another phase forming. The addition of a ternary element can result in the hardening phase either being stabilised with the phase field still present within that temperature and composition range, or it can compact the phase field and place the alloy in a different phase region where hardening cannot occur. It is very complicated and difficult to predict the influence of the ternary element. If however the ternary element forms a solid solution with platinum the likelihood of maintaining the hardening phase fields is higher. The lower the addition the greater the probability of maintaining the phase fields. At higher compositions additional phases may form between the ternary element and platinum, or between the ternary element and the secondary element. The addition of iridium for example is highly likely to maintain the hardening effect, especially at lower compositions (eg 3wt%).

Part C: Particular Platinum Alloys

The following specific examples are given for the indicated binary additions:

Titanium

Additions of 0,5-5 wt% titanium to platinum increased the bulk hardness of platinum and were observed to harden the alloy after heat treatment. The 2 wt% titanium-platinum alloy specifically had an annealed hardness of about 170-180 Vickers hardness, a heat treatment induced hardness of about 260-280 Vickers

hardness and a hardness of about 400-430 Vickers as a result of cold working and heat treating.

Figure 5 includes curves which illustrate the hardening of some alloys in the 1-5 wt% titanium-platinum system.

3 wt% ternary additions of palladium, iridium, rhodium, ruthenium, gold and silver to a 95 wt% platinum 2 wt% titanium alloy produced an alloy comparable in hardness to the 98 wt% ρlatinum-2 wt% titanium alloy. Ternary additions of copper, cobalt and tungsten to the 95 wt% platinum- 2 wt% titanium alloy did not appear to show a similar hardening effect.

Hardening as a consequence of heat treating was shown in the following ternary systems (wt%):

1.9Ti-16lr-82.1Pt - 1Ti-2Zr-97Pt

2Ti-3Pd-95Pt

2Ti-3lr-95Pt

2Ti-3Ru-95Pt

2Ti-3Rr>95Pt - 2Ti-3Au-95Pt

2Ti-3Ag-95Pt

Heat treatment did not produce hardening in the following systems (wt%): 1Ti-3Zr-96Pt - 2Ti-3Zr-95Pt

2Ti-2Zr-96Pt 1.5Ti-2Zr-96.5Pt 2Ti-3Cu-95Pt 2Ti-3Co-95Pt

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- 2Ti-3W-95Pt

Zirconium

Additions of from 1 to 5 wt% zirconium to platinum were noted to increase the hardness of platinum. Samples were annealed at about 1000°C (set 1050°C) for

15 minutes and water quenched. The annealed hardnesses are given in Figure

3. The samples were then heat treated at about 800°C for 10 minutes and water quenched. Slight increases in hardness were observed but they were within experimental error. Further heat treatments at about 800°C for varying times increased the hardnesses of the alloys (Figure 3). The addition of 3 wt% zirconium resulted in the most marked increase in hardness, the alloy having an annealed hardness of about 288 Vickers and a heat treated hardness of from 349-

360. The alloys generally softened after a total heat treatment of about 44 minutes. These results have been successfully obtained a number of additional times.

A combination of cold work and heat treating could increase this hardness even more.

Hardening as a consequence of heat treating was shown for the ternary system

(wt%): 3Zr-2Pd-95Pt, but did not result for the following system (wt%): 3Zr-12Pd-85Pt.

Vanadium

Additions of from 1 to 5 wt% vanadium to platinum were noted to increase the hardness of platinum. Samples were annealed at about 1000°C (set 1G50°C) for 15 minutes and water quenched. The annealed hardnesses are given in Figure 2. The samples were then heat treated at about 600°C for 10 minutes and water

quenched. Slight increases in hardness were observed but they were within experimental error. Further heat treatments at about 800"C for varying times increased the hardnesses of the alloys (Figure 2). The addition of 3 wt% vanadium resulted in the most marked increase in hardness, the alloy having an annealed hardness of about 157 Vickers and, with ongoing heat treatment, a hardness of up to 233 Vickers. A combination of cold work and heat treating will increase this hardness even more.

The results of the hardness tests are summarized in Table 2.

Hardening as a consequence of heat treating was also shown in the following ternary system (wt%): 3V-11.9lr-85.1Pt, but was not observed in the system: 3.2V-1.8Pd-95Pt.

Indium

None of the indium 1-5 wt% additions showed significant hardening, see Table 3.

Tin

Additions of tin to platinum increased the bulk hardness of the alloy which showed a hardening effect with heat treatment. The annealed hardness of a 96.6 wt% platinum-3.4 wt% tin alloy was about 144 Vickers and the heat treated hardness was of the order of 163 Vickers. The annealed hardness of a 95.2 wt % platinum-4.8 wt% tin alloy was in the region of 170 Vickers and the heat treated hardness was about 179 Vickers. 5.4-5.5 wt% additions of tin also showed significant hardening.

Hardening as a result of heat treating was observed in: 3.8Sn-0.8Pd-95.4Pt but not in: 1.5Sn-2.5Pd-96Pt

Magnesium

Additions of magnesium to platinum increased the bulk hardness of the alloy which showed a hardening effect with heat treatment. The annealed hardness of a 99.2 wt % platiπum-0.8 wt% magnesium alloy (weighed out 95 wt% piatinum-5 wt% magnesium) was 140-149 Vickers and the heat treated hardness was 170-179 Vickers.

Germanium

Additions of germanium to platinum increased the bulk hardness of the alloy which showed a hardening effect with heat treatment. The annealed hardness of a 98 wt% ρlatinum-2 wt% germanium alloy (weighed out) was about 265 Vickers and the heat treated hardness was of the order of 305 Vickers.

Gallium

Additions of gallium to platinum increased the bulk hardness of the alloy which showed a hardening effect with heat treatment. The annealed hardness of a 96 wt% platinum-4 wt% gallium alloy was about 232 Vickers and the heat treated hardness was about 341 Vickers. A later investigation also observed hardening but only up to 280 Vickers. Additions of 4.4, 5.2 and 6.1 wt% also showed increases in hardness.

Hardening as a consequence of heat treatment was observed in: 5Ga-0.5Pd- 94.5Pt.

The aforementioned alloys, after annealing, exhibit adequate ductility to enable them to be used, for example, in the manufacture of items of jewellery and similar artefacts but, after heat treatment, have hardnesses which are increased and hence exhibit improved wear resistance and durability, and greater strength.

Conclusion

The indium and tin systems showed slight hardening.

The titanium and zirconium systems showed significant hardness increases (2 wt% Ti annealed 170-180, hardened 260-280; 3 wt% Zr annealed 288, hardened

349-360).

The vanadium (3 wt%) and magnesium (2 wt%) systems showed hardness increases.

The gallium and germanium additions showed hardness increases but their annealed hardnesses suggest that they may not be workable. The annealed hardness of 4 wt% gallium is about 232 and hardened 290-341. The annealed hardness of 2 wt% germanium is about 265 and hardened 271-305. These alloys could possibly be used in their as-cast states.

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TABLE 1