GB2426250A

GB2426250A - Silver alloys

Info

Publication number: GB2426250A
Application number: GB0510243A
Authority: GB
Inventors: Peter Gamon Johns
Original assignee: Middlesex Silver Co Ltd
Current assignee: Middlesex Silver Co Ltd
Priority date: 2005-05-20
Filing date: 2005-05-20
Publication date: 2006-11-22
Also published as: CN101218361A; GB0510243D0

Abstract

A silver alloy comprises 92.5 - 97 wt% Ag, 1-4.5 wt% Cu, 0.4-4 wt% Zn, 0.8-1.5 wt% Ge, 0 to 0.2 wt% Si, In or Sn and 0-0.2 wt% Mn, the balance being boron as grain refiner, incidental ingredients and impurities. Alternatively silicon may be present from 0.05 - 2 wt%. The said alloy preferably comprises boron as grain refiner added as copper boride or as a boron hydride, e.g. sodium borohydride. A further group of alloys comprises a ternary alloy of silver, copper and germanium containing 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, the grain refiner being sodium borohydride or another boron hydride.

Description

METAL ALLOY MANUFACTURING

FIELD OF THE INVENTION

This invention relates to a method of manufacturing precious metal alloy, and to precious metal products made of the above alloy. It also relates to a method for making a master alloy for making the above metal alloys.

BACKGROUND TO THE INVENTION

Many attempts have been made to produce silver alloys that are resistant to tarnish and/or firestain.

In all but the largest manufacturing companies, most of the annealing and soldering required to assemble finished or semi-finished articles is carried out with the flame of an air-gas blowtorch. The oxidising or reducing nature of the flame and the temperature of the articles are controlled only by the skill of the silversmith. Pure silver allows oxygen to pass easily through it, particularly when the silver is heated to above * red heat. Silver does not oxidise in air, but the copper in a silver/copper alloy is oxidised :.: "20 to cuprous or cupric oxide. Pickling of the oxidised surface of the article in hot dilute S. sulphuric acid removes the superficial but not the deeper seated copper oxide so that the : *.* surface consists of fine or unalloyed silver covering a layer of silver/copper oxide

SASS

: mixture. The pure silver is easily permeated during further heating, allowing copper located deeper below the surface to become oxidised. Successive annealing, cold working and pickling produces a surface that exhibits the pure lustre of silver when * : "* lightly polished but with heavier polishing reveals dark and disfiguring stains known as fire-stain' or fire'. Soldering operations are much more productive of deep fire-stain because of the higher temperatures involved. When the depth of the fire-stain exceeds about 0. 025mm (0.010 inches) the alloy is additionally prone to cracking and difficult to solder because an oxide surface is not wetted by solder so that a proper metallurgical bond is not formed.

US-A-381 1876 (Harigawa et al., K. K. Suwa Seikosha) alleges that Sn, In and Zn synergistically reduce tarnish of silver alloys. It describes and claims alloys consisting essentially of 4-10 wt% Sn, 0.5 - 12 wt% In, and 0.1 - 5 wt% Zn, the remainder being silver. It also alleges that mechanical strength and tarnish resistance may be further increased by adding Ti, Zr, Be, Cr, Si, Al, Ge andlor Sb which protect the surface of silver alloys by oxidizing preferentially and forming stable oxides.

Amounts of such additional elements less than 0.001 wt% are ineffective. If more than 1 wt% Ti, Zr, Be, Cr or Si is added, the alloy is said to become brittle and insoluble components are said to form that interfere with polishing. Additions of 0.00 1 - 5 wt% Al, Ge and Sb are said to promote tarnish resistance without reducing workability. The alloy is said not to suffer from firestain because of the absence of copper but this property is not confirmed by torch annealing experiments carried out by the present applicants and furthermore the alloy is soft.

US-A-4973446 (Bernhard et al., United Precious Metal Refining) discloses a silver alloy composition of the Sn, In, Zn type that also contains copper and boron. It comprises 89-93.5 wt% Ag, 0.01-2 wt% Si, about 0.001- 2 wt% B, about 0.5-5 wt% Zn, about 0.5-6 wt% Cu, about 0.25-2 wt% Sn, and about 0.01-1.25 wt% In. Silicon is * added as a de-oxidant. Boron is added to reduce the surface tension of the molten alloy, :.: * 20 and to allow it to blend homogeneously. Zinc is added to reduce the melting point of the S...

alloy, to add whiteness, to act as a copper substitute, to act as a dcoxidant, and to : ***. improve fluidity of the alloy. Copper is added as a conventional hardening agent for S. *:. silver, as well as acting as the main carrying agent for the other materials. Tin is added to improve tarnish resistance, and for its hardening effect. Indium is added as a grain- :.:.25 refining agent, and to improve the wetability of the alloy. Silver must be present in the * : * necessary minimal percentage to qualify as either coin silver or sterling silver. In the experience of the present inventors, although tarnish resistance is exhibited to some extent, together with some firestain reduction on investment casting, firestain resistance on soldering or annealing is not obtained because of the copper content.

Further allegedly anti-tarnish silver alloys of the Zn, Cu, In, Sn family are disclosed in US 2004/02 19055 (Croce), the alloy having at least 85 wt% Ag and the balance also including Fe. Boron may be present as an optional ingredient US 6726877 (Eccles) discloses an allegedly fire scale resistant, work hardenable jewellery silver alloy composition comprising at least 86 wt% Ag, 0.5 - 7.5 wt% Cu, 0.07 -6 wt% of a mixture of Zn and Si wherein 0.02-2 wt % Si and 0.01 - 2. 0 wt% Ge are present. The alloy may also include rheology modifying and other additives to aid in improving the castability and/or wetting performance of the molten alloy. For example, about up to 3.5% by weight of a modif'ing additive selected from In, B or a mixture thereof may be added to the alloy to provide grain refinement and/or provide greater wettability of the molten alloy. The compositions may be formed by the addition of a master alloy to fine silver, the master alloy comprising e. g. 52.5 - 99.85 wt % Cu, 0.1 wt % Zn and 0.05 - 12.5 wt% Ge. Experiments by the present applicants have not confirmed fire resistance of available embodiments of the alloy, especially during torch annealing.

Patent GB-B-2255348 (Rateau, Albert and Johns; Metaleurop Recherche) discloses a 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% * 20 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 I...

significant shrinkage on casting. They also exhibit superior ductility and tensile strength. Germanium exerts a protective function that is responsible for the :. 25 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 is constituted by two phases, a solid solution of germanium and copper in silver surrounded by a filamentous solid solution of germanium and silver and copper. The germanium in the copper-rich phase inhibits surface oxidation of that phase by forming a thin GeO and/or Ge02 protective coating that prevents firestain during brazing and flame annealing. Furthermore the development of tarnish is appreciably delayed by the addition of germanium, the surface turning slightly yellow rather than black and tarnish products being easily removed by ordinary tap water. The alloy is useful inter a/ia in jewellery and silversmithing.

US-A-6168071 (Johns) describes and claims inter alia a silver/germanium alloy having an Ag content of at least 77% by weight, a Ge content of between 0.5 and 3% by weight, the remainder being copper apart from any impurities, which alloy contains boron as a grain refiner at a concentration of up to about 20 parts per million. It further discloses providing the boron content by a master CU/B alloy having a boron content of about 2 percent by weight. The boron in the copper/boron alloy is elemental boron. Providing the boron content within a Cu/B alloy is stated to overcome the problem of handling pure boron, which typically is a lightweight powder. Such copper/boron master alloys are said to be commonly available, and for example Belmont Metals Inc offers on its website a copper-based master alloy containing 2 wt% boron with any of As, Be, Cd, Cr, Fe, Li, Mg, Ni, P, Si, Te, Ti, Zn or Zr.

Silver alloy according to the teaching of GB-B-2255348 and EP-B-0729398 is now commercially available in Europe and in the USA under the trade name Argentium, and the word "Argentium" as used herein refers to these alloys. The 925 * grade Argentium alloy comprises 92.5 wt% (minimum) Ag, 1.1-1. 3 wt% Ge, 6 ppm B, :.: 20 the balance being copper and impurities. The alloy shows excellent resistance to tarnishing even under very arduous conditions. A passive layer is formed by the : *.. germanium, which significantly slows the formation of copper suiphide, the main cause S. Si of tarnishing on conventional silver alloys. Even in a hydrogen suiphide atmosphere the degree and depth of tarnish is significantly less compared to a conventional silver alloy :.:. 25 or a silver plated item. The same mechanism that creates the tarnish resistance also * : " results in the formation of a passive layer which significantly reduces the depth of fire- staining' or the fire layer' that is produced in this alloy when torch annealing in air.

Trials have shown that Argentium is substantially free from firestain, which reduces the amount of polishing that the alloy requires and can give rise to considerable cost savings. As previously explained, other commercial alloys develop firestain on simple torch annealing, and it has been found that in practice just over 1% Ge in a silver jewellery or silversmithing alloy is desirable to avoid firestain even in subsequently tested alloys containing relatively low copper levels and even with zinc.

Despite the advantages of existing Argentium alloy grades, there is a need for further improvement of the alloy with respect to its stability under thermal processing and in particular to its resistance to pitting andlor sagging when heated for the purposes of annealing or joining. There is also a need for alloys that combine these favourable properties with hardness and resistance to tarnishing.

W02004/106567 discloses 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 C 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 : : * 20 phase becoming molten while the bulk remains stable. When the workpiece returns to *a.

* 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 S. %q when heated for joining or annealing operations. Although the use of the boron grain refiner of US-A-6168071 and EP-B- 0729398 significantly reduces the pitting and :.: * 25 sagging consequent on formation and melting of the binary eutectic, the formation and * melting of that eutectic is, as previously mentioned, not eliminated 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.

The invention of WO 2003/1 06567 therefore provides a ternary alloy of silver, copper and germanium containing 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, impurities and grain refiner, copper. A typical alloy that has been found to be suitable contains about 94.5 wt% Ag, about 4.3 wt% Cu and about 1.2 wt% Ge. In the above alloy the weight ratio of Cu to Ge is about 3.6: 1 whereas in the existing 925 grade Argentium the ratio can be from 5.8: 1(1.1 wt% Ge) to 4.8: 1(1.3 wt% Ge). The applicants suggested that it is the reduction in the Cu: Ge weight ratio that is responsible for the reduced thermal processing problems, the CuGe eutectic either not forming or forming in a significantly reduced amount during post-melt thermal processing. In particular the ratio is preferably from 4: ito 3: 1, more preferably about 3.5: 1. Above 4: 1 the alloy is more likely to exhibit firestain, whereas below 3: 1 the high germanium content gives rise to formability problems. In the above alloy, preferred Ag contents ranged from about 94.0 to about 95.5 wt%, lower values being preferred for reducing the expense of the silver.

: SUMMARY OF THE INVENTION 20

In one aspect, the invention provides a silver alloy comprising 92.5 -97 wt% Ag, 1-4.5 wt% Cu, 0.4-4 wt% Zn, 0.8-1.5 wt% Ge, 0 to 0.2 wt% Si, In or Sn and 0-0.2 . .1 wt% Mn, the balance being boron as grain refiner incidental ingredients or impurities.

In a further aspect the invention provides a silver alloy comprising 92.5 - 97 wt% Ag, 1-3 wt% Cu, 1-4 wt% Zn, 0.8-1.5 wt% Ge, 0 to 0.2 wt% Si, In or Sn and 0-2 wt% Mn, the balance being boron as grain refiner added as an alkali metal borohydride, incidental ingredients or impurities.

The invention further provides casting grain as aforesaid containing silicon in an amount effective to produce an as-cast silvery appearance and inhibit mold reactions in articles made by investment casting. Such reactions are in general nor detrimental to the properties of finished products, but require processing for their removal and can be disconcerting for those new to the use of the present alloys. The invention therefore provides silver alloy casting grain comprising 92.5 97 wt% Ag, 1-4.5 wt% Cu, 0.4-4 wt% Zn, 0.8-1.5 wt% Ge, 0.05-2 wt% Si, 0 to 0.2 wt% In or Sn and 0-0.2 wt% Mn, the balance being boron as grain refiner, incidental ingredients and impurities.

In a further aspect, the invention relates to the use in AgCuGe silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt% and preferably 1-3 wt%, especially 1-1.5 wt% of 1-3 wt% Zn to reduce or prevent pitting or sagging on heating the alloy and/or to increase annealed hardness.

In a further aspect, the invention relates to the use in AgCuGe silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt% and preferably 1-3 wt%, especially 1-1.5 wt% of 1-3 wt% Zn e.g. 2-3 wt% Zn to reduce or prevent pitting or sagging on heating the alloy and up to 0.2 wt% of Sn, In or Si or a mixture thereof to reduce or prevent zinc oxide formation on heating the alloy. * S.

In a yet further aspect, the invention provides a ternary alloy of silver, copper and germanium containing 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 SI..

grain refiner, copper, the grain refiner being sodium borohydride or another boron hydride. * . * . S

S

DESCRiPTION OF PREFERRED FEATURES

It has been realized that a good way to reduce sag in AgCuGe alloys when heated for soldering and annealing is to remove some of the copper. Ideally the copper needs to be below 3%. Increasing the silver content is effective to a degree, but at 95% and above, the alloys were very soft after annealing or soldering (although they could be hardened by precipitation).

It has also been noted that when the silver content of the alloy was above 95.5%, firestain surprisingly reappeared. It has now been found that if the content is raised above 92.5% and about 1 - 2% Zn is added, sagging can be reduced or eliminated, and surprisingly a better annealed hardness is obtained using higher silver in combination with zinc, rather than replacing the copper with silver or zinc alone. This is significant as high silver or high zinc silver alloys are normally too soft to be used as general-purpose alloys. Higher levels of zinc (above 2%) result in heavy zinc oxide when annealing or soldering in air but it has been observed that a small amount of Sn, In or Si (>0.2) will suppress the zinc oxide formation. The resulting alloy also has superior corrosion resistance to attack by acids. The invention therefore contemplates alloys containing Sn, In or Si in an amount effective to suppress zinc oxide formation during torch annealing. Surprisingly, the presence of zinc has not proved detrimental to the ability of AgCuGe based silver alloys to precipitation harden using the methods described below.

Grain refining AgCuGe-based silver jewellery or silversmithing alloys using sodium borohydride can also improve hardness by about 10 HV. An optimum combination of Ag, Zn and sodium borohydride can produces a sterling silver alloy with * : 0 improved annealed hardness and superior mechanical properties. Furthermore, sodium * from e.g. sodium borohydride could make the alloy useful as an electrical contact S..

: * material. Sodium at ppm levels has arc-quenching properties, as also does germanium. S..

* If desired, the germanium or copper content may be substituted, in part, by one or more incidental ingredient elements selected from Al, Ba, Be, Cd, Co, Cr, Er, Ga, Mg, Ni, Pb, Pd, Pt, Ti, V, Y, Yb and Zr, provided the effect of germanium in terms of S.....

* providing firestain and tarnish resistance is not unduly affected. The weight ratio of germanium to incidental ingredient elements may range from 100: 0 to 80: 20, preferably from 100: 0 to 60: 40. The term "incidental ingredients" permits the ingredient to have ancillary functionality within the alloy e.g. to improve colour or as- moulded appearance, and includes the previously mentioned metals or metalloids Si, Zn, Sn or In in amounts appropriate for "deox".

Silicon, in particular, may be added to silver alloys for casting grain e. g. in an amount of up to 0.5 wt % more usually 0.1-0.2 wt%, and is conveniently provided in the form of a copper-silicon master alloy containing e.g. about 10 wt% Si. When incorporated e.g. into casting grain of a silver-copper-germanium ternary alloy it can provide bright investment castings immediately on removal from the mold. It may be added to casting grain e.g. before investment casting or it may be incorporated into the silver at the time of first melting to form an alloy.

Boron is incorporated into precious metal alloys as an oxygen scavenger or in the case of silver alloys additionally or alternatively as a grain refiner. It may be added as a Cu/B master alloy, to molten silver alloy by bubbling a gaseous borane e.g. diborane into the alloy in admixture with a non-reactive gas such as argon, by introducing into the alloy a borane which is solid at ambient temperatures e.g. decaborane B10H14 (m.p 100 C, b.p. 213 C), or by adding an alkylated borane e.g. triethylborane or tri-n-butyl borane, although the latter reagents are spontaneously combustible and require care in handling. Preferably, however, the boron is added as a metal borohydride, e.g. a borohydride of an alkali metal, a pseudo-alkali metal or an alkaline earth metal, e.g. lithium borohydride. Sodium borohydride is especially : .. 20 preferred because it is widely available commercially and can be obtained in the form of * :* relatively large pellets, which are convenient to handle during precious metal melting S...

operations. * IS * S S S...

*. The boron is advantageously solid e.g. a metal borohydride or a higher borane : 25 such as decaborane, and is in the form of pellets or granules which are advantageously S wrapped in a layer or foil of silver and plunged as a group into the molten metal.

S..... * .

Boron can be added to the other molten components both on first melting and at intervals during casting to make up for boron loss if the alloy is held in the molten state for a period of time, as in a continuous casting process for grain. This facility is not available when using a copper/boron master alloy because adding boron changes the copper content and hence the overall proportions of the various constituents in the alloy.

It has surprisingly been found that when adding a borane or borohydride that more than 20 ppm can be incorporated into a silver or other precious metal alloy without the development of boron hard spots. This is advantageous because boron is rapidly lost from molten silver, according to one experiment the content of boron in molten silver decaying with a half-life of about 2 minutes. The mechanism for this decay is not clear, but it may be an oxidative process. It is therefore desirable to incorporate more than 20 ppm boron into an alloy as first cast i.e. before investment casting or before rolling into strip, and amounts of e. g. up to 50 ppm, typically up to 80 ppm, and in some instances up to 800 or even 1000 ppm may be incorporated. Thus there could be produced according to the present method silver casting grain containing about 40 ppm. boron. Owing to boron loss during subsequent re-melting and investment casting, casting to form strip, rod or wire, strip rolling or other downstream processes, the boron content of finished pieces may be closer to the 1-20 ppm characteristic of the prior art, but the ability to achieve relatively high initial boron concentrations means that improved consistency may be achieved during the manufacturing stages and in the final finished products. Furthermore higher boron content is desirable for master alloys which will be melted with precious metal to make casting grain and then further melted to form rod, wire, or investment casting.

: :20 A surprising difference in properties exists between conventional Sterling silver . alloys and other Ag-Cu binary alloys on the one hand and Ag-Cu-Ge silver alloys on the * ** S * other hand, in which gradual cooling of the binary Sterling-type alloys results in coarse :.. precipitates and little precipitation hardening, whereas gradual cooling of Ag-Cu-Ge * alloys (including those containing the further additives and incidental ingredients set out : . ? above) results in fine precipitates and useful precipitation hardening, particularly where the silver alloy contains an effective amount of grain refiner. Furthermore, the addition S.....

* of germanium to sterling silver changes the thermal conductivity of the silver alloy, compared to standard sterling silver. The International Annealed Copper Scale (IACS) is a measure of conductivity in metals. On this scale the value of copper is 100%, pure silver is 106%, and standard sterling silver 96%, while a sterling alloy containing 1.1% germanium has a conductivity of 56%. The significance of this is that the Argentium sterling and other germanium-containing silver alloys do not dissipate heat as quickly as standard sterling silver or their non-germaniumcontaining equivalents, a piece will take longer to cool, and precipitation hardening to a commercially useful level (preferably to Vickers hardness 110 or above, more preferably to 115 or above) can take place during natural air cooling or during slow controlled air cooling.

The ability of the present silver alloys containing 1 wt% copper or above and optionally zinc as well as germanium to precipitation harden enables the copper content of the alloy to be reduced. Even though an alloy of lower copper content may be relatively soft as cast, reheating at a low temperature e.g. 200-300 C may bring the hardness up to the level of normal sterling silver or better. This is a significant advantage because the copper content is actually the most detrimental part of the alloy from the standpoint of corrosion resistance, but in a standard sterling alloy less copper means unacceptably low hardness. If the copper content is reduced, the silver content may simply be increased or there may be incorporated zinc e.g. in an amount of 1-2 wt%. Other possibilities include increasing the germanium content or adding zinc or another alloying element. Silver alloy of Ag 973 parts per thousand and containing about 1.0 wt% Ge, balance copper, has been successfully precipitation hardened by gradual air cooling from an annealing temperature, and it is believed that Ag-Cu-Ge alloys with silver content above this level are also precipitation hardenable. * ** * S

The benefit of not having to quench to achieve the hardening affect is a major advantage of silver alloys that can be made from the present master alloys. There are : * : very few times in practical production that a silversmith can safely quench a piece of *. nearly finished work. The risk of distortion and damage to soldered joints when * 25 quenching from a high temperature would make the process not commercially viable. In fact standard sterling can also be precipitation hardened but only on subsequent ****** * quenching and this is one reason why precipitation hardening is not used for sterling silver.

1-low the invention may be put into effect will now be further described with reference to the following Examples:

Example 1

A ternary silver-copper-germanium alloy (Ag = 94.7 wt%, Ge = 1.2 wt%, Cu = 3.9 wt% Si = 0.2 wt% (added as a CuJSi master alloy), is prepared by melting silver, copper, germanium and master alloy together in a crucible by means of a gas-fired furnace which becomes heated to a pour temperature of about 2000 F. The melt is covered with graphite to protect it against atmospheric oxidation and in addition a hydrogen gas protective flame is provided. Stirring is by hand using graphite stirring rods. When the above ingredients have become liquid, pellets of sodium borohydride to give up to 100 ppm boron e.g. 80 ppm are packaged or wrapped in pure silver foil of thickness e.g. about 0.15mm. The foil wrapper holds the pellets of sodium borohydride in a single group and impedes individual pellets becoming separated and floating the surface of the melt. The wrapped pellets are placed into the hollow cupped end of a graphite stirring rod and plunged beneath the surface of the melt which at this stage is covered with a ceramic fibre blanket to quench the resulting flame from decomposition of the borohydride. The hydrogen burns off over a period of about 1-2 minutes with a L:2O stirring action being applied, after which evolution of hydrogen ceases and the boron * content is substantially incorporated into the melt together with at least some of the S...

: sodium which is believed innocuous to properties of the resulting alloy. *..

* After boron addition, the crucible pivots to permits the molten alloy to be poured * : . 25 into a tundish whose bottom is formed with fine holes. The molten silver pours into the tundish and runs through the holes in streams which break into fine pellets which fall S.....

into a stirred bath of water and become solidified and cooled. The cast pellets are removed from the bath and dried.

The resulting alloy granules are used in investment casting using traditional methods and using a calcium sulphate bonded investment, and are cast at a temperature of 95 0-980 C and at a flask temperature of not more than 676 C under a protective atmosphere. The investment material, which is of relatively low thermal conductivity, provides for slow cooling of the cast pieces. Investment casting with air- cooling for 15- minutes followed by quenching of the investment flask in water after 15minutes gives a cast piece having an expected Vickers hardness of about 70, which is approximately the same hardness as sterling silver. The resulting casting has a matt silvery finish when removed from the mold, and an even finer grain structure than when Cu/B master alloy is used, due e.g. to the relatively high boron content permitted by the sodiumborohydride and the energetic dispersion of the boron into the molten silver as the borohydride decomposition reaction proceeds. The alloy can be polished easily, is free from boron hard spots, and gives products that exhibit excellent tarnish and firestain resistance. Precipitation hardening to expected hardness values of e.g. about Vickers can be achieved by subsequent torch annealing, quenching and reheating in an oven at about 300 C.

However, a harder cast piece can be produced by allowing the flask to cool in air to room temperature, the piece when removed from the flask having an expected Vickers hardness of about 110 which is similar to the value that can be achieved by the torch anneal/quenchlreheat method. Contrary to experience with Sterling silver, where necessary, the hardness can be increased even further by precipitation hardening e.g. by .:20 placing castings or a whole tree in an oven set to about 300 C for 20-45 minutes to give heat-treated castings of an expected hardness approaching 125 Vickers.

:.:::. Example2

S

A ternary silver-copper-germanium alloy (Ag = 94.7 wt%, Ge = 1.2 wt%, Cu 4.1 wt%) is prepared by melting silver, copper and germanium and master alloy S.....

* together and adding sodium borohydride as described in Example 1 and is formed into sheet. Pieces of the sheet are brazed together to form shaped articles by passage through a brazing furnace and are simultaneously annealed. Precipitation hardening develops without a quenching step by controlled gradual air-cooling in the downstream cooling region of the furnace. For this purpose, it is desirable that the material should spend at least about 8-30 minutes in the temperature range 200-300 C which is most favourable for precipitation hardening. Articles that have been brazed in a furnace in this way and gradually cooled can achieve hardness of 110-115 Vickers. Exceptionally small grain size and good firestain and tarnish resistance is obtained because of the sodium borohydride addition.

Example 3

Alloys were prepared with the compositions and boron contents indicated in Table I below using CuB master alloys the source of boron.

Table!

Precip. Precip. Annealed Sample B Ag% Ge% Cu% Hardened* Hardened* hardness ID ppm (air-cooled) HV (quenched) HV (air-cooled) HV Sterling 92.7 0 0 7.3 86 / 75 2.1 95.44 1.5 4 3.06 108 115 67 2.2** 96 1.55 Yes 2.45 107 110 64 2.3** 96 2 Yes 2 110 106 63 2.4** 97.30 1 Yes 1.7 93 99 40 28 28 2.5** 98.66 1.2 Yes 0.14 No precipitation No precipitation 28 *1* hardening hardening I.. * **

: Precipitation hardening (air cooled) - sample annealed, air cooled, then heated at ** * 300 C for 45 minutes. Precipitation hardening (quenched) sample annealed, quenched, then : * *heated at 300 C for 45 minutes.

j5.:** No final assay results available. Table shows alloy make-up before melting.

Further improvements in hardness and greater ease in polishing are obtained by increasing the boron content using sodium borohydride in place of CuB master alloy, melting following the procedure set out in Example 1.

Example 4

Zinc containing alloys according to the invention are prepared as set out in Table II below and their hardnesses are measured. In the above table, boron is added as CuB master alloy; a further improvement is obtained using lithium borohydride as described above.

Example 5

Zinc and manganese containing alloys according to the invention are prepared as set out in Table III below and their hardnesses are measured. In the table III, boron is added as CuB master alloy; a further improvement is obtained using lithium borohydride as described above.

Example 6

:, 20 Corrosion of the Copper Phase in Silver Alloys: I...

I pe,.

It is well-known that sterling silver tarnishes faster than pure silver because of : :. the copper content. French Vinaigrette Dressing (pH level 3.5) test has been used to attack the copper phase in a selection of silver alloys as set out in Table IV, containing * 25 different quantities of copper. Samples of silver alloys were partially immersed in * French Vinaigrette Dressing for 24 hours. A round blob' of vinaigrette was placed on * I* I the samples approximately 1cm above the surface of the liquid. In this test, standard sterling Sterilite B and the ternary alloy all exhibited discoloration, whereas the alloys 4.4 and 4.7 exhibited substantially no discoloration. ..

it S S e, a & S. S I.* * *.* a. S a * a S S 5 a

Table II

Precip. Hardened* Precip. Hardened* (quenched) Annealed hardness Zinc alloys Ag% Ge% B ppm Cu% Zn% (air-cooled) HV HV (air-cooled) HV 3.l** 95 1.5 4-8 2.5 1 109 114 74 3.2** 93.2 1.3 4-8 4.8 0.7 113 117 56 33** 93.2 1.1 0 5.2 0.5 101 115 64 34** 92.7 1.3 4-8 4 2 113 117 72 * S.. ** S *Se * S 5 S * S * .. S ** * * ** * . *.. . S S * * * . * S * S S S *.* S * . S

Table III

Precip. Annealed Annealed Manganese I B Ag% Ge% Cu% Zn% Mn% Sn% Hardened* hardness hardness Zinc alloys ppm (quenched) HV (air-cooled) HV (quenched) HV 4.1** 93.2 1 4-8 4.6 1 0.2 77 4.2** 93.2 1 4-8 3.6 2 0.2 82.6 51.6 43** 93.2 1 4-8 2.6 3 0.2 70.2 51 44** 94.2 1 4-8 2.6 2 0.2 76 48 45** 200 Grams MN4 0.2 117 76 49 4.6** 93.2 1 4-8 1.6 4 0.2 58.5 49 47** 95.2 1 4-8 1.6 2 0.2 99 53 42 4.8** 94.2 1 4-8 2.6 2.2 115 64 47 (No Mn) 49** 200 Grams MN7 0.2 68 49 * Precipitation hardening (air cooled) - sample annealed, air cooled, then heated at 300 C for 45 minutes.

Precipitation hardening (quenched) - sample annealed, quenched, then heated at 300 C for 45 minutes.

** No final assay results available. Table shows alloy make-up before melting.

* *** *S S **e S. * S S S * * S. * ** S S *S * . *5S S S S S S :. . * *.5 **S **S

Table IV

Alloy Ag% Ge% B ppm Cu% Zn% Mn% Sn% Si% Standard sterling 92.5 7.5 B' 92.7 0.42 5.13 0.9 0.8 0.05 AgCuGe ternary alloy 93.2 1.2 4 ppm 5.6 4.4 94.2 1 4-8ppm 2.6 2 0.2 4.7 95.2 1 4-8ppm 1.6 2 0.2

Claims

1. A silver alloy comprising comprising 92.5-97 wt% Ag, 1-4.5 wt% Cu, 0.44 wt% Zn, 0.8-1.5 wt% Ge, 0 to 0.2 wt% Si, In or Sn and 0-0.2 wt% Mn, the balance being boron as grain refiner, incidental ingredients and impurities.

2. The alloy of claim 1, wherein Ag is 93.5 - 95.5 wt%.

3. The alloy of claim 1, wherein Ag is about 94 wt%.

4. The alloy of any preceding claim, wherein Cu is about 2.5 - 3 wt%.

5. The alloy of any preceding claim, wherein Ge is I - 1.5 wt%.

6. The alloy of any preceding claim, wherein Zn is 1 - 3 wt%.

7. The alloy of any preceding claim comprising boron as grain refiner added as copper boride.

8. The alloy of any preceding claim, comprising boron as grain refiner added as a boron hydride. S... * * S...

9. The alloy of claim 8, comprising boron as grain refiner added as sodium borohydride. : 25

10. A ternary alloy of silver, copper and germanium containing from more than 93.5 wt% to 95.5 wt% Ag, from 0.5 to 3 wt% Ge and the remainder, apart from incidental S.....

imgredients (if any), impurities and grain refiner, copper, the grain refiner being sodium borohydride or another boron hydride.

11. The alloy of any of claim 10, containing from 1.0 to 1.5 wt% Ge.

12. Silver alloy casting grain comprising 92.5 -97 wt% Ag, 1-4.5 wt% Cu, 0.4-4 wt% Zn, 0.8-1.5 wt% Ge, 0.05-2 wt% Si, 0 to 0.2 wt% In or Sn and 00.2 wt% Mn, the balance being boron as grain refiner, incidental ingredients and impurities.

13. The castng grain of claim 12, wherein Ag is 93.5 - 95.5 wt%.

14. The casting grain of claim 13, wherein Ag is about 94 wt%.

15. The casting grain of any of claims 12-14, wherein Cu is about 2.5 - 3 wt%.

16. The casting grain of any of claims 12-15, wherein Ge is 1 - 1.5 wt%.

17. The casting grain of any of claims 12-16, wherein Zn is I - 3 wt%.

18. The casting grain of any of claims 12-17, comprising boron as grain refiner added as an alkali metal borohydride.

19. Use in AgCuGe silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt% of 1-3 wt% Zn to reduce or prevent pitting or sagging on heating the alloy. * *1 * * *

20. Use in AgCuGe silver alloys of silver content 93.5-95.Swt% and having a Ge S...

content of 0.8-3 wt% of 1-3 wt% Zn to reduce or prevent pitting or sagging on heating * : :* the alloy and to increase annealed hardness. 25

* ,

21. Use in AgCuGe silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt% of 1-3 wt% Zn to reduce or S....

* prevent pitting or sagging on heating the alloy and up to 0.2 wt% of Sn, In or Si or a mixture thereof to reduce or prevent zinc oxide formation on heating the alloy.

22. Use in AgCuGe silver alloys of silver content corresponding to at least coinage or Sterling standard and having a Ge content of 0.8-3 wt% of 2-3 wt% Zn to reduce or prevent pitting or sagging on heating the alloy and up to 0.2 wt% of Sn, In or Si or a mixture thereof to reduce or prevent zinc oxide formation on heating the alloy. * ** * S e S.. S *SS* * 4 S.-. * S. * . I I...

I S.. * I * . 4 Sea

S

*1.1SI

S I