Silver ternary alloy

Abstract

A firestain and tarnish-resistant ternary alloy of silver, copper and germanium contains from more than 93.5 wt % to 95.5 wt % Ag, from 0.5 to 3 wt % Ge and the remainder, apart from incidental ingredients (if any), impurities and grain refiner, copper. In order to further protect an article made from the alloy, it may be surface treated with an alkanethiol, alkyl thioglycollate, dialkyl sulphide or dialkyl disulphide.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a ternary alloy of silver, copper and germanium, to finished or semi-finished shaped articles made from the alloy, and to the use for the surface treatment of the alloy with alkanethiol, alkyl thioglycollate, dialkyl sulfide or dialkyl disulfide. BACKGROUND TO THE INVENTION [0002] Since ancient times it has been appreciated that unalloyed ‘fine’ silver is too soft to withstand normal usage, and it has been the practice to add a proportion of a base metal to increase hardness and strength. In the UK, legislation that has existed since the fourteenth century specifies a minimum silver content of articles for sale at 92.5% (the Sterling standard) or 96% (the Britannia standard), but does not specify the base metal constituents. Experience convinced early silversmiths that copper was the most suitable of the metals available to them. Modern silver-sheet manufacturers generally adhere to this composition, although sometimes a...

AI Technical Summary

Benefits of technology

[0015] Although GB-B-2255348 discloses a range of silver content from Sterling to Britannia grades, as previously explained a skilled person would not contemplate using a silver content above the legal minimum for the intended grade because of the cost of the silver. He is not given by the disclosure of that patent or that of the subsequent patents relating to the ternary alloys any incentive to adopt intermediate silver contents. However, it has now been found that silver-copper-germanium ternary alloys having silver contents between those of the Sterling and Britannia grades possess valuable properties that facilitate both casting, welding and other thermal treatments used in the manufacture of silver articles.

[0016] In particular, the applicants have become aware of the desirability of reducing or avoiding the formation and / or melting of the above mentioned binary copper-germanium eutectic which melts at 554° C. During the production of e.g. 925 Argentium silver alloys, the formation of this phase can be avoided by careful control of the casting conditions since under equilibrium cooling conditions the crystallisation is complete at below 640° C. However, this binary phase can create problems during subsequent thermal treatment of the alloys, e.g. using brazing alloys which typically have melting points in the range 680-750° and torch annealing which typically involves heating a workpiece to a dull red heat at 700-750° C. On heating the workpiece to or beyond these temperatures incipient melting occurs with a small amount of material corresponding to this binary phase becoming molten while the bulk remains stable. When the workpiece returns to ambient temperature, porosity develops where the alloy has liquefied. This contributes brittleness and e.g. as noted in GB-B-2255348 there is a tendency for the alloy to sag when heated for joining or annealing operations. Although the use of the boron grain refiner of U.S. Pat. No. 6,168,071 and EP-B-0729398 significantly reduces the pitting and sagging consequent on formation and melting of the binary eutectic, the formation and melting of that eutectic is, as previously mentioned, not elimated and there is still scope for the further development of the ternary alloy to improve its pitting and sagging properties. By increasing the silver content above the level for Sterling but less than that for Britannia it is possible to produce an alloy in which the above binary eutectic either does not form or gives rise to reduced problems in subsequent heat treatment. This provides alloys with a much greater inherent stability under thermal processing. The germanium addition prevents the reduction in hardness that would be seen in a silver-copper alloy of this composition. The alloy also shows resistance to tarnishing, even under very arduous test conditions.

Problems solved by technology

In particular, the applicants have become aware of the desirability of reducing or avoiding the formation and / or melting of the above mentioned binary copper-germanium eutectic which melts at 554° C. During the production of e.g. 925 Argentium silver alloys, the formation of this phase can be avoided by careful control of the casting conditions since under equilibrium cooling conditions the crystallisation is complete at below 640° C. However, this binary phase can create problems during subsequent thermal treatment of the alloys, e.g. using brazing alloys which typically have melting points in the range 680-750° and torch annealing which typically involves heating a workpiece to a dull red heat at 700-750° C. On heating the workpiece to or beyond these temperatures incipient melting occurs with a small amount of material corresponding to this binary phase becoming molten while the bulk remains stable.

This contributes brittleness and e.g. as noted in GB-B-2255348 there is a tendency for the alloy to sag when heated for joining or annealing operations.

Above 4:1 the alloy is more likely to exhibit firestain, whereas below 3:1 the high germanium content gives rise to formability problems.

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