FIELD OF THE INVENTION

The present invention relates to AgCuGe alloys of high silver content, high hardness and thermal stability and resistance to elution of copper when in contact with human sweat or other moisture. Articles of the alloy may be of silver sheet or may be castings, e.g. investment castings. It also relates to a homogeneous copper/boron alloy useful in the production of silver alloys containing relatively large amounts of boron, but reducing or avoiding the formation of boron hard spots.

BACKGROUND TO THE INVENTION

Patent GB-B-2255348 (Rateau, Albert and Johns; Metaleurop Recherche) discloses a novel silver alloy that maintains the properties of hardness and lustre inherent in Ag-Cu alloys while reducing problems resulting from the tendency of the copper content to oxidise. The alloys are ternary Ag-Cu-Ge alloys containing at least 92.5 wt% Ag, 0.5-3 wt% Ge and the balance, apart from impurities, copper. The alloys are stainless in ambient air during conventional production, transformation and finishing operations, are easily deformable when cold, easily brazed and do not give rise to significant shrinkage on casting. They also exhibit superior ductility and tensile strength. Germanium was stated to exert a protective function that was responsible for the advantageous combination of properties exhibited by the new alloys and was in solid solution in both the silver and the copper phases. The microstructure of the alloy was said to be constituted by two phases, a solid solution of germanium and copper in silver surrounded by a filamentous solid solution of germanium and silver in copper which itself contains a few intermetallic CuGe phase dispersoids. The germanium in the copper-rich phase was said to inhibit surface oxidation of that phase by forming a thin GeO and/or GeCk protective coating which prevented the appearance of firestain during brazing and flame annealing. Furthermore, the development of tarnish was appreciably delayed by the addition of germanium, the surface turned slightly yellow rather than black and tarnish products were easily removed by ordinary tap water. EP-B-0729398 (Johns) discloses a silver/germanium alloy which comprised a silver content of at least 77 wt % and a germanium content of between 0.4 and 7%, the remainder principally being copper apart from any impurities, which alloy contained elemental boron as a grain refiner at a concentration of greater than 0 ppm and less than 20ppm. The boron content of the alloy could be achieved by providing the boron in a master copper/boron alloy having 2 wt % elemental boron CuB2, see US 2964397 (Weil) which discloses that effective alloying of up to 10% by weight of boron into copper can be achieved by heating an intimate mixture of the granulated or powdered elements to 1500°C, advantageously to 1650-1700°C). It was reported that such low concentrations of boron surprisingly provided excellent grain refining in a silver/germanium alloy, imparting greater strength and ductility to the alloy compared with a silver/germanium alloy without boron. Nevertheless, WO 2006/113847 (Davis etal. , Stern Leach Company et al.) reported that introducing boron into a precious metal alloy using CuB2 introduces hard spots into the products which are believed to be non-equilibrium phase 1B22 particles that form in copper saturated with boron when cooled from the liquid phase to the solid phase. Davis et al. explain that hard spots are frequently not detected until after the precious metal jewellery alloy is polished and inspected resulting in needless expense for the processing of ultimately unsatisfactory products.

An example in US 9222150 (Johns) explains that a master alloy is made by melting together 79wt% Cu, 18wt% Ge and 3wt% of a Cu/B alloy containing 2wt% boron. The Cu is melted together with the Cu/B master alloy. High temperatures can be used because there are no other elements to damage. The temperature is then lowered and the germanium is added just above the Ge melting point. Melting is therefore in descending order of melting temperatures i.e. copper/copper-boron master alloy/germanium. The resulting master alloy comprises, apart from impurities, and with a 50% boron loss on melting, about 82wt% Cu, about 18wt% Ge and about 0.03wt% boron, together with any impurities. There is then added 72 g of the above master alloy and 928 g of 9999 purity fine silver which when melted together just above the melting point of the fine silver (e.g. at about 960-1200°C.) with a 50% boron loss gives a desired silver/copper/germanium ternary alloy of composition about 92.8wt% Ag, 5.90wt% Cu, 1.30wt% Ge and about 11 ppm boron. The master alloy is weighed and placed in a crucible for melting and the fine silver is weighed and placed in the crucible, which is then heated to melt the silver and the master alloy under a protective cover of natural gas to prevent unnecessary oxidation. Silver has a known affinity for oxygen, which affinity increases with temperature. When exposed to air, molten silver will absorb about 22 times its volume of oxygen. Like silver, copper also has a great affinity for oxygen, typically forming copper oxide. Thus, in forming or re-melting sterling silver and other silver- copper alloys, care must be taken to prevent oxidation. When the mixture becomes molten, it may be stirred e.g. with a carbon rod and poured through a tundish into water, so that the silver becomes solidified into shot-like granules or pellets of diameter about 3-6 mm which is a form in which sterling silver is typically sold. The resulting alloy granules could be used in investment casting using traditional methods and was cast at a temperature of 950-980°C. and at a flask temperature of not more than 676°C. under a protective atmosphere. The investment material which was of relatively low thermal conductivity provided for slow cooling of the cast pieces. Investment casting with air- cooling for 15-25 minutes followed by quenching of the investment flask in water after 15-25 minutes gave a cast piece having a Vickers hardness of about 70 which was approximately the same hardness as sterling silver. The products exhibited excellent tarnish and firestain resistance and had a fine grain structure due to their boron content. It was found that a harder cast piece could be produced by allowing the flask to cool in air to room temperature, the piece when removed from the flask having a Vickers hardness of about 90-100 HV. Contrary to experience with Sterling silver, where necessary, the hardness could be increased even further by precipitation hardening e.g. by placing castings or a whole tree in an oven set to about 300°C. for 20-45 minutes to give heat-treated castings of approaching 125 HV. The germanium content was towards the upper limit of that then considered desirable in a 0.925 type alloy. However, the specification is entirely silent about hard spots or about cracking of investment cast products. It will also be understood by skilled readers that although the theoretical possibility of employing master alloys had been disclosed, no such master alloy for AgCuGe products has in fact since been fully developed and commercialised. US 2010/239454 discloses an alloy, comprising: from 93.5 wt % to 95.5 wt % of silver; from 0.5 to 3 wt % of germanium; 1-40 parts per million of boron; and the remainder, apart from impurities, being copper; the weight ratio of copper to germanium being from 4:1 to 3:1; and the alloy being resistant to the development of porosity and brittleness, the development of hot short cracking defects when investment cast, the development of cracks or shattering on annealing and quenching and the development of cracks and sagging when heated for joining or torch annealing. In an example, a ternary silver-copper-germanium alloy (Ag=94.5 wt %, Ge=1.2 wt %, Cu=4.1 wt %, B=0.0008 wt % (8 ppm) is prepared by melting silver, copper and germanium together at 1050°C. under an atmosphere of nitrogen, and adding the boron as a copper-boron master alloy at the last possible moment. The molten mixture is then continuously cast into strip.

GB-A-2485374 discloses a silver-copper-germanium-silicon-boron alloy suitable for investment casting e.g. of rings that comprises (by weight): at least 77 % silver and 0.2-3 % germanium. Silicon is present in an amount to give a clean silvery appearance to the casting, e.g. 10 ppm and to avoid cracking and other processing problems and boron is present in an amount to impart grain refinement, e.g. up to 40 ppm. The alloy can either be zinc free or comprise 0.2-1.0 % zinc.

Various alloys in accordance with the above patents have been produced under the trademark Argentium. One group of Sterling-type alloys comprises Ag 93.5wt%, Ge 0.7-1.1 wt%, balance copper. A second group of Britannia-type alloys comprises Ag 96 wt%, Ge 0.7-1.1 wt%, balance copper. For the alloys of both groups, small amounts of silicon may be added when the alloy is to be formed into an investment-cast product, and 10-60 ppm boron is added as grain refiner. Other elements suggested in the prior art are not added to these alloys.

SUMMARY OF THE INVENTION

In one aspect, the invention provides an alloy comprising at least 94wt% silver, l-3wt% germanium, 0.5-1 wt% silicon in the case of a casting alloy and the balance a homogeneous copper alloy formed from a melt of copper with 2 wt% copper boride in an amount of at 10-20 wt% based on the weight of the copper, the alloy being of high hardness and thermal stability and being resistant to elution of copper when in contact with human sweat or other moisture. A group of these alloys may comprise 94-95.5wt% silver, in a preferred embodiment 94wt% silver, and l-2wt% germanium. They may be in the form of rolled strip or casting grain.

The change in silver content from 93.5wt% to 94wt% gives rise to advantages that would be unexpected from the relatively small change in silver content. These are exemplified by a nominally 940 wrought silver alloy was formed by melting fine 0.999 silver casting grain (94.2g), germanium ( 1.3 g) and pieces of the 10% copper/boron alloy of Example 1 to give a nominal 60 ppm boron in the resulting alloy (possibly about 40 ppm owing to boron loss, although the germanium in the copper is believed to be relatively protected from the oxygen content in the silver):

• A surprisingly shorter melting range of Argentium 940 in comparison to Argentium

935 and standard Sterling silver, see below:

ALLOY SOLIDUS TEMP LIQUIDUS TEMP. MELTING RANGE

Argentium 935 silver 803°C/1477°F 903°C/1657°F 100°C/212°F Argentium 940 silver 860°C/1580°F 895°C/1643°F 35°C/95°F Standard Sterling silver 802°C/1475°F 899°C/1650°F 97°C/206°F

• Avoidance of heat distortion at red heat e.g. at a solder melting temperature of 720°C consequent on a binary copper-germanium eutectic that melts at 554°C. Fig. 1 is a photograph comparing the differences between standard Sterling silver, Argentium 935 and Argentium 940 at soldering temperatures. The large increase in the melting- point of Argentium 940 compared with Argentium 935 is responsible for its improved high-temperature stability. Argentium 940 also addresses the issue of heat distortion at higher temperatures which some silversmiths may have encountered and indeed struggled with for certain assemblies in the past. resistance to elution of copper when in contact with human sweat or other moisture. Although commercialised, it has been found that Ag 93.5wt% Argentium alloys may leak copper and cause “green fingers” amongst users who wear jewellery of this grade. The problem is less prevalent or entirely absent in alloys with 940 and 960wt% Ag, a copper elution sweat test based on BS EN 1811:1999 which exposes samples to synthetic sweat for one week and quantifies the elution rate of copper by spectrophotometric analysis of the liquid indicating about a 62% fall in elution rate from Argentium alloys on going from a 930 to a 940 silver content alloy and a further 7% fall bringing the overall fall relative to the 930 alloy of 69%. The elution rate is relatively stable between 940 and 960 but on going to a 970 alloy the test revealed a second sharp drop in copper elution of 92% compared to the 930 alloy.

• Increased hardness due to the melting of copper boride into most or preferably the totality of the copper used to form these alloys which not only permits relatively high amounts of boron to be incorporated resulting in harder alloys but also reduces or avoids the formation of boron hard spots which are observed at the polishing stage and can render a product unsaleable. Although the amount of boron incorporated into the resulting silver alloy or castings therefrom, e.g. if the alloy is provided in the form of casting grain, will be less than that predictable from the amount of copper boron added owing e.g. to consumption of boron in deoxidation of the silver, it is believed that the amounts incorporated will be higher than readily achievable by previous methods and during re-casting sufficient boron may remain to provide re-cast products of satisfactory strength and hardness. It is believed that 940 alloys have not been popular up to now inter alia because of inadequate hardness. However, increased hardness in these high silver alloys is desirable, especially for those of the Britannia grade or higher which because of their lower copper content are relatively soft.

• Improved ease of manufacturing granulated silver or other metal e.g. gold products, in which small spheres or other particles of silver are fused in a decorative pattern to a silver or other metal substrate so as to partly or completely cover a surface thereof in a decorative pattern. The products may be in the form of rings, broaches, bracelets, necklaces or the like. The shorter melting range of the 940 alloy facilitates granulation with clean lines, reducing or avoiding potentially unsightly bleeding of the granulating metal into the substrate metal. The invention also provides a method of making a silver alloy resistant to elution of copper when in contact with human sweat and comprising at least 94wt% silver and l-3wt% germanium, boron in an amount effective for grain refinement, balance copper, said method comprising: melting with stirring in an inert atmosphere or in vacuo at a temperature of at least 1500°C copper and copper boron containing up to 10wt% boron (e.g. l-10wt%, most usually 2wt% boron) based on the weight of the copper and recovering the resulting copper-boron composition; melting with stirring in an inert atmosphere or in vacuo at >960-1100°C (e.g. 1030-1100°C) the silver, the germanium and the copper-boron composition in an amount to provide at least a major proportion of the copper content of the resulting silver alloy and recovering solidified alloy. Preferably the copper-boron composition melted with the silver and germanium provides the entirety of the copper in the solidified alloy.

The invention further provides a homogeneous copper alloy formed from a melt of copper with 2 wt% copper boride in an amount of at 10-20 wt% based on the weight of the copper for use in the casting of alloys comprising at least 94wt% silver and l-3wt% germanium and boron in an amount effective for grain refinement, the copper-boron composition being for use in an amount to provide a major proportion of the copper content of the resulting silver alloy permitting relatively high amounts of boron to be incorporated resulting in harder alloys but also reducing or avoiding the formation of boron hard spots which are observable at the polishing stage and can render a product unsaleable.

DESCRIPTION OF PREFERRED FEATURES

The silver may be 999 fine silver having a casting temperature range of 1030- 1100°C, 961°C melting range and a density of 10.5g/cm ³ , available from Metalor, Birmingham and Cooksongold of Hatton Garden, London, and as casting grain from GSM, Cranston RI and RioGrande in the US. Copper for forming the alloys is preferably 99.9% electrolytic copper available as polished shot from Belmont Metals, Inc (NY, USA).

2 wt% Copper Boride is available from a variety of sources including Belmonet Metals, Inc and may have an iron content of 500ppm. In the proportions proposed to be used according to this invention that proportion is undesirably high, and iron is preferably <250 ppm, more preferably <100ppm. It will be appreciated that the boron content of the homogeneous copper alloy produced by melting copper and copper boride together may be less than the stoichiometrically predicted amount of copper owing to the phenomenon of “boron fade” in which boron escapes at temperatures above 900°C even in the presence of an inert atmosphere. Thus boron may be lost both during the formation of the homogeneous copper alloy at temperatures of typically 1500-1700°C, preferably about 1500°C, during the subsequent formation of the silver alloy typically at 950-1100°C, and again during any subsequent melting or re-casting e.g. for the production of articles from casting grain. It should also be noted that in addition to acting as a grain refiner, boron is also a deoxidant, and dissolved oxygen in molten silver may also give rise to boron fade. However, the relatively high amounts of boron incorporated into the homogeneous copper alloy impart higher contents of boron into the resulting silver alloys both initially and during subsequent remelting which improve hardness and grain refinement while reducing or avoiding hard spots.

Germanium is available in high purity e.g. from ABSCO Limited of Haverhill, Suffolk. Preferably the alloy comprises 1.1-1.2 wt% germanium.

Silicon may be added in amounts of e.g. 10 ppm up to 0.2 wt% and may be added as a CuSi alloy containing e.g. 10-30wt% Si. In an alloy in which germanium is present e.g. in at least equal amounts it is fully compatible with the germanium so that the two elements (which are both metalloids in Group IV of the periodic table) form single phase(s) and the tendency of the silicon to migrate to grain boundaries is reduced or eliminated. In consequence the advantages flowing from incorporation of silicon in terms of deoxidation and forming bright castings can be obtained and cracking and other problems associated with conventional silicon-containing silver alloys do not appear or are significantly alleviated. Amounts of silicon in embodiments of the alloy may be 0.01- 0.1 wt% e.g. 0.05-0.08 wt% with a reference value of 0.07 wt% (700 ppm). In embodiments the wt% silicon is 20% of the weight% of germanium, e.g. about 10% of the weight of the germanium.

Melting of the copper and copper boron is preferably at 1500-1700°C. One or both meltings are conveniently in an induction furnace, in which when the metals become molten, the electromagnetic field produces inductive stirring. Melting is preferably in vacuo. The resulting alloy may be formed into conveniently sized pieces of cast strip or into casting grain.

The resulting silver alloy may be solidified as an ingot and rolled into strip. Alternatively, the silver, the germanium and the copper-boron composition may be melted with 0.5-1 wt% silicon e.g. 0.7 wt% silicon for 940 and 960 alloys and possibly 0.5 wt% for the 970 alloys, the alloy is poured into a mould and is recovered as a casting or in the form of casting grain.

As previously explained, silver contents in the present Ag-Ge-Cu alloys may range from 940-970 with alloys e.g. having silver contents at or close to 940, 960 or 970, the increased hardness of the 960 and 970 alloys owing to the higher boron contents achievable increasing the range of products for which these alloys may be suitable.

The invention is now further described in the following Examples.

Example 1.

An alloy of copper and boron for forming into silver alloys was formed by melting high purity electrolytic copper with 10% by weight of finely divided particles of jewellery grade copper boride CuB2 from Belmonet Metals, Inc (NY, USA) and containing 2 wt% copper in vacuo in an Indutherm VTC 200V/Ti tilt casting machine at 1500°C. The copper boride had a relatively low iron content of < lOOppm. Particles containing boron which initially appeared on the surface of the molten copper ceased to be apparent at about 1500°C, the copper and boron becoming intimately mixed through eddy currents which vigorously stirred the melt. The molten materials were held in the casting chamber at the casting temperature for at least 30 seconds. The resulting alloy was then cast as an ingot which was cleaned with a Scotch-Brite abrasive wheel. The cleaned ingot was and rolled into thin strip nominally containing 2000ppm boron (not allowing for boron loss or fade during melting) which was then cut into small pieces for formation of jewellery alloys.

In an alternative procedure, the same starting materials were heated in a crucible in the same furnace as before, and after melting the CuB alloy was passed through an apertured casting ladle to form fine granules which fall into a stirred bath of water and become solidified and cooled. The cast granules were removed from the bath and dried.

Example 2.

The procedures of Example 1 were repeated except that the proportion of copper boride was increased to 15%, giving cut small pieces or granules of boron content nominally 3000 ppm, but assayed at 2470 ppm consequential on boron loss during melting.

Example 3.

The procedures of Example 1 were repeated except that the proportion of copper boride was increased to 20%, giving cut small pieces or granules of boron content nominally 0.4 wt% (not allowing for boron loss during melting).

Example 4

A Britannia grade wrought alloy was formed by melting fine 0.999 silver casting grain (96.2g), germanium (1.2g) and pieces of the copper/boron alloy (2.6g) of Example 1 (10% variant) in the Indutherm VTC 200V/Ti tilt casting machine in vacuo at 1100°C. The alloy was cast into an ingot which was also cleaned with a Scotch-Brite abrasive wheel and rolled into strip of thickness below 4 mm containing substantially greater amounts of boron (nominally 52ppm before any boron loss) exhibiting substantially greater hardness than previous alloys made with copper and small amounts of CuB2. On polishing, the alloy was found to be free of hard spots and is suitable for forming e.g. into rings.

The above procedure was repeated with pieces of the copper/boron alloy (2.6g) of Example 2 (15% variant) and Example 3 (20% variant) with similar results.

A ring formed from casting grain of the elements described above (15% variant) by re-melting comprised Ag 96.14 wt%, Cu 2.76wt%, Ge 1.03 wt%, Si 0.07 wt% and B 10 ppm.

Example 5

A Britannia grade casting alloy was formed by melting fine 0.999 silver casting grain (96.2g), germanium (l.lg), finely divided silicon (0.7g) and pieces of the above 10% copper/boron alloy (2.6g) in the Indutherm VTC 200V/Ti tilt casting machine in vacuo at 1100°C. The alloy was poured into a mould and allowed to solidify to give a moulded product that contained substantially greater amounts of boron exhibiting substantially greater hardness than previous moulded products made with copper and small amounts of CuB2. On polishing, the product was found to be free of hard spots.

Example 6

A nominally 940 wrought silver alloy was formed by melting fine 0.999 silver casting grain (94g), germanium (1.2g) and pieces of the 10% copper/boron alloy of Example 1 (4.8g) in the Indutherm VTC 200V/Ti tilt casting machine in vacuo at 1100°C. The alloy was cast into an ingot which was also cleaned with a Scotch-Brite abrasive wheel and rolled into strip containing substantially greater amounts of boron exhibiting substantially greater hardness than previous alloys made with copper and small amounts of CUB ₂ . On polishing, the alloy was found to be free of hard spots.

Example 7

A ring was cast at 1050°C from fine 0.999 silver casting grain, the copper/boron alloy of Example 2, germanium and silicon and on assay was found to contain 96.14% Ag, 2.76% Cu, 1.03% Ge, 0.07% Si and 10 ppm boron. It was free from hard spots and had a hardness as cast of 60HV, comparable to that of conventional Britannia silver as cast, which on heating for 90 minutes in a furnace at 300°C increased to 98HV, being of a hardness even more suitable for jewellery articles and easy to polish.

A sample of the same cast material was subjected to two-stage hardening firstly by annealing at 730°C for 40 minutes followed by quenching and secondly by a hardening stage at 300°C for 1 hour, and exhibited a hardness of 120HV.

A skilled person requested to evaluate an alloy sample of this composition commented that one of the most remarkable changes in the 960 alloy as it stands now is that it requires no annealing before the hardening of as cast items which is a convenience and time saver. For tension setting stones e.g. into rings the alloy proved reliable with good resilience whereas earlier versions of the alloy proved precariously close to borderline. Furthermore, with previous alloys it was necessary to burnish castings before final finishing to achieve the best possible polish, whereas with the present alloy rings as cast finished beautifully and it was unnecessary to incorporate a burnishing step.