Au-based bulk metallic glass with high tarnish resistance

Abstract

A bulk metallic glass comprises: at least 45 at% Au; one or more of Ag and Pd; Si; and Ge. Bulk metallic glasses are also known as bulk-solidifying amorphous alloys. The bulk metallic glasses of the present disclosure are gold-based bulk metallic glasses. Such bulk metallic glasses are quaternary or higher-order gold-based alloys including Si, Ge and at least one of Ag and Pd, which can be extended to higher-order alloys by the addition of further alloying elements.

Description

[0001] AU-BASED BULK METALLIC GLASS WITH HIGH TARNISH RESISTANCE

[0002] The invention relates to a bulk metallic glass. In particular, the invention relates to a goldbased bulk metallic glass comprising at least 45 at% gold (Au).

[0003] Background

[0004] Gold-based bulk metallic glasses are of interest for luxury goods notably for their high hardness that confers excellent scratch and wear resistance. They have over twice the hardness of conventional crystalline alloys with similar gold content. In the absence of crystallization, bulk metallic glasses also show many processing advantages; namely minimal shrinkage on casting and the ability to be shaped like plastics, as well as by methods such as additive manufacturing.

[0005] Bulk metallic glasses are hardened cooled liquids, which solidify without crystallising. The resulting solid bulk metallic glasses exhibit a high flow stress, and no long-range order.

[0006] The properties of bulk metallic glasses that make them desirable for jewellery include low casting temperatures, excellent scratch resistance, thermoplastic formability, and a reduced need for fast cooling during casting compared to metallic glasses in general.

[0007] Disadvantages include a lack of tarnish resistance, difficulty in attaining precious-metal content sufficient for hallmark compliance, and an inability to be cold-worked because their plastic deformation is by the propagation of shear bands.

[0008] Gold (Au) based bulk metallic glasses have been tested previously, with some known compositions containing Si and Cu. These previously tested compositions have suffered from rapid tarnishing and are not suited for applications in jewellery.

[0009] For these bulk metallic glasses containing Au, Si and Cu, it has been proposed in the literature that during casting, a layer of silica (SiO2) forms on the surfaces that are in contact with air. This SiO2 layer acts as an effective barrier to tarnishing. Unfortunately, this protective SiO2 layer is readily removed, both locally by scratching, and more generally by subsequent polishing. Once removed, there is rapid tarnishing, a process that starts when silicon within the bulk metallic glass oxidises at room temperature, leading to the formation of amorphous SiO2 at the surface. This oxidation of the silicon is greatly accelerated, in effect catalysed, in the presence of copper. Then, the copper within the metallic glass diffuses out through the SiO2 layer while SiO2, CU2O, and metallic nanocrystals (comprising Au and Cu) form below the SiO2 layer and grow into the bulk material, leaving an Au-rich, Si-depleted zone near the surface of the bulk metallic glass. Tarnishing occurs in air, and in that case the copper diffusing outwards oxidises to CU2O or CU2S on the surface. Tarnishing occurs in contact with an aqueous solution; testing relevant for medical and dental applications is mostly in simulated body fluid or artificial saliva at body temperature (310 K, 37°C). In these cases, the copper is leached out into the solution and corrosion products form on the surface. Whether under dry or wet conditions, the effect is rapid tarnishing, facilitated by the interplay between copper and silicon and by fast diffusion of copper.

[0010] It is desirable to provide a gold-based bulk metallic glass with a higher level of tarnish resistance than has been exhibited by known compositions.

[0011] Summary of the Invention

[0012] According to a first aspect of the present disclosure there is provided a bulk metallic glass, comprising: at least 45 at% Au; one or more of Ag and Pd;

[0013] Si; and Ge.

[0014] Bulk metallic glasses are also known as bulk-solidifying amorphous alloys. The bulk metallic glasses of the present disclosure are preferably substantially amorphous. The bulk metallic glasses of the present disclosure are preferably at least 80% amorphous by volume, particularly preferably at least 90% amorphous by volume or at least 95% amorphous by volume. In particularly preferred embodiments of the present disclosure the bulk metallic glass is 100% amorphous by volume.

[0015] Although some alloys with compositions similar to those of the present disclosure may be found in the prior art, in the prior art these compositions have only been known for crystalline alloys, and not for bulk metallic glasses. Bulk metallic glasses can be readily identified by the skilled person, and told apart from crystalline alloys, due to their amorphous nature. It is well known in the art that X-ray diffraction (XRD) analysis can be used to show whether an alloy is amorphous or crystalline. Differential scanning calorimetry (DSC) is also usable to measure the width of the supercooled liquid region (ATX) and the reduced glass-transition temperature (Trg) of an alloy, which are known to be key indicators of glass-forming ability. Bulk metallic glasses exhibit significantly higher (for example 2-3 times higher) mechanical hardness than crystalline alloys having similar compositions, as well as being significantly more scratch-resistant. Bulk metallic glasses also exhibit advantageous processing characteristics such as thermoplastic formability, which are not found with crystalline alloys. Bulk metallic glasses are much less common than crystalline alloys, and form less readily in most compositions, so unless an alloy is identifiably amorphous in nature, it would typically be presumed to relate to a crystalline alloy and not to a bulk metallic glass.

[0016] The bulk metallic glasses of the present disclosure are gold-based, or Au-based, bulk metallic glasses. Such bulk metallic glasses are quaternary or higher-order gold-based alloys including Si, Ge and at least one of Ag and Pd, which can be extended to higher- order alloys by the addition of further alloying elements.

[0017] The present inventors have carried out a systematic review of all potential gold-based glass-forming systems (illustrated in Figure 1a-1c), using a ‘metallic glass by design’ strategy developed by them and based on the earlier work by Li et al., “How many bulk metallic glasses are there?” ACS Combinatorial Science, vol. 19, p. 687-693, 2017. The inventors’ review used a series of a-priori parameters (Figures 1a & 1b), followed by thermodynamic assessment using CALPHAD (Figure 1c).

[0018] Although it has been ignored in all recent work on gold-based bulk metallic glasses for jewellery applications, the present inventors identified Au-Ge-Si as a ternary system of interest based on several indications of glass-forming ability.

[0019] Early work (H. S. Chen and D. Turnbull, “Formation and stability of amorphous alloys of Au- Ge-Si” Acta Metallurgica, vol. 18, p. 261-263, 1970) identified AuyyGeuSig (at.%) as the best glass-forming composition. Later, Thompson et al. investigated the crystal nucleation kinetics in amorphous (Au.CujyyGeuSig (at.%) (C. V. Thompson, A. L. Greer and F.

[0020] Spaepen, “Crystal nucleation in amorphous (Auioo-yCuy)77SigGei4 alloys,” Acta Metallurgica, vol. 31 , p. 1883-1894, 1983), but this work does not appear to have been followed up over the past 40 years.

[0021] The key effort to deliver Au-based bulk-solidifying amorphous alloys was made by Schroers et al. in US Patent no. 8501087 focused on the alloy composition Ag4gAg5.5Pd2.3Cu26.9Sii6.3 (at%). In US8501087, the authors list many potential alloying components, but state that “the most preferred combination of components for Au-based quaternary alloys of the current invention are: Au, Cu, Ag and Si; Au, Cu, Si and P; Au, Cu, Pd and Si; and Au, Cu, Si, and Be. The most preferred combinations for five component Au-based alloys of the current invention are: Au, Cu, Pd, Ag and Si; Au, Cu, Ag, Si and P; Au, Cu, Pd, Si and P; Au, Cu, Ag, Si and Be; and Au, Cu, Pd, Si and Be”. Although no experimental data are provided, the example compositions identified in US8501087 are: Au4gCu26.9Ag5.5 Pd2.3 Sii6.31 Au47Cu29.8Ag4Pd2.5Sii6.71

[0022] Au48.2Cu27Ag5.5Pd2.3Sii3Be4, Au47Cu28.8Ag4Pd2.5Sii6.7Zr1, Au48Cu3oAgsSii7, AussCusoSi P?, AussCusoSilsBe?, Au6iCui6.7Ag4Pd2.3 She, and Au33Cu44.7Ag4Pd2.3 She-

[0023] Despite testing many compositions with the aim of delivering Au-based bulk metallic glasses for jewellery applications, the authors of US8501087 did not use germanium as an ingredient in any of their example compositions. US8501087 does not disclose any bulk metallic glass compositions that contain germanium, and the reader of US8501087 is instead taught that alloy systems containing Si, P and Be have been found to be best for Au-based bulk metallic glasses.

[0024] In the present invention, the inventors have found that the addition of at least one of silver (Ag) or palladium (Pd) advantageously leads to a dramatic improvement in the glassforming ability of alloys containing Au, Ge and Si. Silver and palladium appear to improve the glass-forming ability of such compositions by increasing the stability of the glass (raising the glass-transition temperature Tg) and improving its stability against crystallization. The addition of Ag and / or Pd, and preferably also Cu, to the Au-Ge-Si alloying system improves the glass-forming ability to such an extent that the material becomes a bulk glass former in which the solid alloy is entirely amorphous.

[0025] The inventors have further found that the presence of germanium in gold-based bulk metallic glasses improves the tarnish resistance of bulk metallic glasses across a wide range of compositions. Compared to the previously known compositions such as those disclosed in US8501087, all samples tested within the scope of the claims show improved tarnish resistance, albeit with different degrees of glass-forming ability.

[0026] By incorporating Ge, and Ag and / or Pd, into the same gold-and-silicon containing bulk metallic glass, the present invention provides a highly advantageous combination of glassforming ability and tarnish resistance. These properties exceed those seen for Au-based bulk metallic glasses known in the prior art, and the improved tarnish resistance added to the hardness, processability and scratch-resistance of the bulk metallic glass, makes materials within the scope of the claims exceedingly promising for use in jewellery.

[0027] Despite the longstanding commercial interest in the preparation of gold-based metallic glasses for jewellery, and the extensive efforts made for example by the authors of US8501087, the present inventors have found that these properties are achieved with a non-obvious combination of ingredients that has previously been ignored. The bulk metallic glass may have the composition, or formula, Aux(AgyPdz)wCuaGe6Sic, and preferably in this composition, x > 45, w > 0, a > 0, b > 0, c > 0.

[0028] The gold content of the bulk metallic glass may be varied to control the karatage of the resulting gold-based bulk metallic glass, for example to result in 14 karat or 18 karat gold alloys which satisfy the 14 karat and 18 karat gold alloy hallmarks respectively. The Aubased bulk metallic glass preferably contains at least 49 at% Au, or at least 52 at% Au, or at least 58.5 wt% Au, or at least 75 wt% Au. The bulk metallic glass may preferably have a minimum Au content which is more than 75 wt%.

[0029] In the bulk metallic glass composition Aux(AgyPdz)H,CuaGefiSic, preferably the Au content is x > 49 at%, or x > 52 at%, or x > 58.5 wt%, or x > 75 wt% Au.

[0030] The bulk metallic glass may contain up to 20 at% Si, preferably between 5 at% and 15 at%, particularly preferably between 7 at% and 12 at% Si, or between 9 at% and 11 at%.

[0031] In the bulk metallic glass composition Aux(AgyPdz)H,CuaGefiSic, preferably the Si content is c < 20 at%, preferably in which 5 at% < c < 15 at%, particularly preferably in which 7 at% < c < 12 at% or 9 at% < c < 11 at%.

[0032] The bulk metallic glass may contain up to 25 at% Ge, preferably less than 20 at% Ge. While glasses may be formable with germanium contents of above 20 at%, such compositions may be less commercially desirable for jewellery applications.

[0033] In the bulk metallic glass composition Aux(AgyPdz)wCuaGe6Sic, preferably the Ge content is b < 25 at%, particularly preferably b < 20 at% Ge.

[0034] The bulk metallic glass may preferably contain at least 7 at.% Ge. The present inventors have found that germanium contents of at least 7 at% or 7.5 at% Ge advantageously lead to a dramatic improvement in tarnish resistance compared to similar compositions containing less or no germanium. Preferably the bulk metallic glass comprises at least 7.5 at% Ge, particularly preferably between 7 at% and 15 at% Ge.

[0035] In the bulk metallic glass composition Aux(AgyPdz)wCuaGe6Sic, preferably the Ge content is b > 7 at.% preferably b > 7.5 at%, particularly preferably 7 at% < b < 15 at%.

[0036] The bulk metallic glass preferably contains less than 5 at.% Pd, preferably up to 4.5 at%, or up to 4 at%, particularly preferably between 2 at% and 5 at% Pd. The inventors have found that Pd contents below 5 at% advantageously improve the glass-forming ability of the bulk metallic glass composition, but that Pd contents of 5 at% and above lead to crystallinity in the solid, and promote the formation of undesirable silicides.

[0037] In the bulk metallic glass composition Aux(AgyPdz)wCuaGesSic, preferably the Pd content is z < 5 at.%, preferably z < 4.5 at%, or z < 4 at%, particularly preferably 2 at% < z < 5 at%.

[0038] The bulk metallic glass may preferably contain up to 15 at.% Ag, preferably between 2 at% and 12 at%, or between 5 at% and 10 at% Ag.

[0039] In the bulk metallic glass composition Aux(AgyPdz)wCuaGesSic, preferably the Ag content y < 15 at.%, particularly preferably 2 at% < y < 12 at%, or 5 at% < y < 10 at%.

[0040] The bulk metallic glass may preferably contain a combined Pd and Ag content of up to 15 at.%, or up to 13 at%, or up to 10 at%. Preferably the bulk metallic glass may contain a combined Pd and Ag content of between 5 at% and 12 at%, or between 7 at% and 10 at%.

[0041] In the bulk metallic glass composition Aux(AgyPdz)wCuaGesSic, preferably the combined Ag and / or Pd content w < 15 at%, preferably 5 at% < w < 12 at%, or 7 at% < w < 10 at%.

[0042] In preferred embodiments, the bulk metallic glass comprises Cu. The presence of copper in the composition in addition to the Ag and / or Pd significantly enhances the glass-forming ability of the alloy.

[0043] The bulk metallic glass may contain up to 25 at.% Cu. Preferably the bulk metallic glass comprises between 5 at% and 23 at% Cu, particularly preferably between 8 at% and 18 at% Cu.

[0044] In the bulk metallic glass composition Aux(AgyPdz)wCuaGesSic, preferably the Cu content is 0 < a < 25 at.%, preferably 5 at% < a < 23 at% Cu, particularly preferably 8 at% < a < 18 at%.

[0045] The bulk metallic glass may preferably contain a combined Ag, Pd and Cu content of up to 32 at%, or up to 30 at%. Preferably the bulk metallic glass may contain a combined Pd, Ag and Cu content of between 20 at% and 32 at%, or between 23 at% and 30 at%.

[0046] In the bulk metallic glass composition Aux(AgyPdz)wCuaGesSic, preferably the combined Ag and / or Pd and optionally Cu content (w+a) < 32 at%, preferably 20 at% < (w+a) < 32 at%, or 23 at% < (w+a) < 30 at%. In some preferred embodiments, the atomic ratio of Ge / Si (the ratio of the atomic percentages of Ge and Si) in the bulk metallic glass is greater than 0.5.

[0047] The atomic ratio of Ge / Si (the ratio of the atomic percentages of Ge and Si) in the bulk metallic glass may preferably be greater than 1 . In other words, the atomic content of Ge in the composition may be greater than the atomic content of Si.

[0048] Particularly preferably the atomic ratio of Ge / Si may be between 0.5 and 1 .5. The inventors have found that compositions in this range may advantageously provide the best balance between the properties of glass-forming-ability and tarnish-resistance.

[0049] The atomic ratio of Ge / Si in the bulk metallic glass may preferably be less than or equal to 2.5, or less than or equal to 2. The present inventors have found that Ge / Si ratios higher than this lead to a decrease and eventual loss of the glass-forming ability of the alloy.

[0050] In the bulk metallic glass composition Aux(AgyPdz)wCuaGesSic, preferably blc is greater than 1 , particularly preferably b / c is less than or equal to 2.5, or less than or equal to 2.

[0051] The bulk metallic glass may preferably have a combined Ge and Si content of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%. The present inventors have found that such levels of Ge and Si typically lead to excellent tarnish resistance. These levels of Ge and Si exceed the quantities used in prior-art glass compositions. The quantities used in the prior-art compositions were limited because of the negative effect of these components on glass-forming ability. In the present invention, however, the addition of Ag and / or Pd counteracts this effect to provide a tarnish-resistant Au-based bulk metallic glass.

[0052] In the bulk metallic glass composition Aux(AgyPdz)wCuaGesSic, preferably 10 < (b+c) < 30 at%, preferably 17 < (b+c) < 30 at%, particularly preferably 18 < (b+c) < 23 at%.

[0053] The bulk metallic glass may preferably have a metalloid content of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%. Both Ge and Si are metalloids, so in a composition containing no other metalloid ingredients, the metalloid content is equivalent to the combined Ge and Si content of the composition. In some preferred embodiments, other metalloids such as gallium, Ga, may constitute a portion of the metalloid content.

[0054] The bulk metallic glass may contain additional additives in addition to the Au, Ag and / or Pd, optional Cu, Ge and Si. The bulk metallic glass may comprise one or more of Al, Ga, Sn, In and Ca. These elements may possibly be used as alloying additions to the composition. Should one or more of these elements be incorporated as additives, the bulk metallic glass preferably comprises a total of less than 5 at% of the one or more of Al, Ga, Sn, In and Ca.

[0055] For example, the bulk metallic glass may have the composition Aux(AgyPdz)wCuaGesSicZd, in which Z is one or more of Al, Ga, Sn, In and Ca. The additive Z content may preferably be d < 5 at%.

[0056] In a preferred embodiment, the bulk metallic glass may comprise Ga. In such an embodiment, the Ga may partially substitute for either Ge or Si in the metalloid component of the alloy composition. The bulk metallic glass may have a metalloid content, or a combined Ga, Ge and Si content, of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%.

[0057] Bulk metallic glasses which additionally comprise Ga may have the composition Aux(AgyPdz)wCuaGesSicGae. The bulk metallic glass may preferably have a combined Ga, Ge and Si content, (b+c+e), of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%. Preferably e < 5 at%.

[0058] The bulk metallic glass may optionally comprise one or more of P, Pt, Ni, Co. These additives may be incorporated into the composition due to their similarity to the metallic components of the bulk metallic glass. Should one or more of these elements be incorporated as additives, the bulk metallic glass preferably comprises a total of less than 5 at% of the one or more of P, Pt, Ni, Co. The present inventors have found that incorporating more than 5 at% of these additives into the composition leads to a dramatic decrease in glass-forming ability.

[0059] For example, the bulk metallic glass may have the composition Aux(AgyPdz)wCuaGesSicPe, in which p is one or more of P, Pt, Ni, Co. Preferably the additive p content is e < 5 at%.

[0060] The bulk metallic glass may be 14K gold, or 18K gold. The bulk metallic glasses according to the disclosure may advantageously satisfy the 14-karat and / or 18-karat gold alloy hallmark.

[0061] The bulk metallic glass may be cast into bulk objects with a diameter of up to 3 mm or more, or up to 4 mm or more, or up to 5 mm or more. Method of Forming the Bulk Metallic Glass

[0062] According to a second aspect, the present disclosure may also provide a method of forming a gold-based bulk metallic glass according to the first aspect, or the third aspect. The method may include forming an alloy melt having the ingredients (alloying components) described above, and then cooling the alloy melt from above its melting temperature to a temperature below its glass-transition temperature Tgto form a bulk metallic glass containing or consisting of an amorphous glass phase.

[0063] The alloy melt may be cooled to a temperature below its glass-transition temperature Tgat a cooling rate which prevents formation of a crystalline phase. The alloy melt may be cooled at a cooling rate between 10 K / s and 1000 K / s.

[0064] The method may comprise the step of casting the bulk metallic glass into bulk objects with a diameter of up to 3 mm or more, or up to 4 mm or more, or up to 5 mm or more. As the casting diameter affects the cooling rate of the alloy melt as it solidifies, larger casting diameters are more prone to the formation of crystalline phases in the solid alloy.

[0065] The alloy melt may comprise at least 45 at% Au; one or more of Ag and Pd; Si; and Ge. The preferred quantities of each component, and the optional additional ingredients of the composition, are discussed above and below in relation to the bulk metallic glass.

[0066] The alloy melt may be cooled and solidified using conventional techniques for the formation of bulk metallic glasses, which are well-known to the person skilled in the art.

[0067] Third Aspect

[0068] According to a third aspect of the present disclosure, there is provided a bulk metallic glass, having the composition Aux(AgyPdz)wCuaGesSic, wherein x > 45, w > 0, a > 0, b > 0, c > 0.

[0069] The bulk metallic glass preferably comprises Ag and / or Pd. Thus, y > 0 and z > 0, but w > 0.

[0070] The Au content is preferably x > 49 at%, or x > 52 at%, or x > 58.5 wt%, or x > 75 wt% Au.

[0071] The Si content is preferably c < 20 at%. In preferred embodiments 5 at% < c < 15 at%, particularly preferably 7 at% < c < 12 at%.

[0072] The Ge content may be b < 25 at%, preferably b < 20 at% Ge. The Ge content may be b > 7 at.% preferably b > 7.5 at%, particularly preferably 7 at% < b < 15 at%.

[0073] The Pd content may be z < 5 at.%. Preferably z < 4.5 at%, or z < 4 at%, particularly preferably 2 at% < z < 5 at%.

[0074] The Ag content may be y < 15 at.%, preferably 2 at% < y < 12 at%, or 5 at% < y < 10 at%.

[0075] The Cu content may be 0 < a < 25 at.%, preferably 5 at% < a < 23 at% Cu, particularly preferably 8 at% < a < 18 at%.

[0076] Preferably the combined Ag and / or Pd and optionally Cu content is (w+a) < 32 at%, preferably 20 at% < (w+a) < 32 at%, or 23 at% < (w+a) < 30 at%.

[0077] The atomic ratio of Ge / Si in the bulk metallic glass, b / c, may be greater than 0.5, or greater than 1. The atomic ratio of Ge / Si in the bulk metallic glass, b / c, may be between 0.5 and

[0078] 1 .5. The atomic ratio of Ge / Si in the bulk metallic glass, b / c, may be less than or equal to

[0079] 2.5, or less than or equal to 2.

[0080] The bulk metallic glass may have a combined Ge and Si content, (b+c), of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%.

[0081] The bulk metallic glass may optionally contain an additional additive Z, so that the bulk metallic glass has the composition Aux(AgyPdz)wCuaGesSicZd, in which Z is one or more of Al, Ga, Sn, In and Ca. The additive Z content may be d < 5 at%.

[0082] The bulk metallic glass may additionally comprise Ga, so that the bulk metallic glass has the composition Aux(AgyPdz)wCuaGesSicGae.

[0083] The bulk metallic glass may have a combined Ga, Ge and Si content, (b+c+e), of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%.

[0084] The bulk metallic glass may optionally contain an additional additive p, so that the bulk metallic glass has the composition Aux(AgyPdz)wCuaGesSicPe, in which is one or more of P, Pt, Ni, Co. The additive Z content is preferably d < 5 at%.

[0085] The bulk metallic glass may be 14K gold, or 18K gold.

[0086] The bulk metallic glass may be cast with a cross-section of 5 mm or less, or 4 mm or less, or 3 mm or less. Brief Description of the Drawings

[0087] The invention will now be described, by way of example only, by reference to the following Figures, in which:

[0088] Figures 1a & 1b are charts of a-priori parameters considered by the inventors during assessment of the promise of potential glass-forming alloy systems based on one of four precious metals;

[0089] Figure 1c illustrates the inventors’ thermodynamic assessment of glass-forming systems using CALPHAD; the estimated width of the glass-forming composition range increases with the relative depression of the liquidus temperature;

[0090] Figure 2a shows how the width of the supercooled liquid region (ATX) and the reduced glass-transition temperature (Trg) vary with the copper content of the of Auyy-xCuxGeuSig bulk metallic glass.

[0091] Figure 2b shows how the width of the supercooled liquid region (ATX) and the reduced glass-transition temperature (Trg) vary with the silver content of the Au54AgxCu23-xGei4Sig bulk metallic glass.

[0092] Figure 3a shows differential scanning calorimetry (DSC) traces for glasses of compositions Ag49AgsPd2Cu23Gei3Si8 (2Pd) and Ag49Ag5PdsCu2oGei3Si8 (5Pd);

[0093] Figure 3b shows X-ray diffraction (XRD) traces for the alloy compositions of Figure 3a;

[0094] Figure 4a shows DSC traces for four alloy compositions containing different quantities of Ge and Si;

[0095] Figure 4b shows XRD traces for the four alloy compositions of Figure 4a;

[0096] Figure 4c plots key parameters for glass-forming ability as a function of composition (Ge / Si ratio);

[0097] Figure 5 illustrates XRD traces for Au49Ag5Pd2Cu25Ge?Sii2 (at.%) cast with different crosssection diameters;

[0098] Figure 6a is a ternary projection of Au-Cu-Si, showing the relative liquidus depression;

[0099] Figure 6b is a ternary projection of Au-Ge-Si, showing the relative liquidus depression; Figure 7 compares ion-release rates during incubation in artificial saliva at 310 K for 7 days of Au54Cu23Gei4Sig, Au54AgsCui8Gei4Si9, and Au49Ag5.5Pd2.3Cu26.9Sii6.3i

[0100] Figure 8a shows a scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) and energy-dispersive X-ray spectroscopy (EDX) scan for a sample of Au54AgsCui8Gei4Si9 incubated in artificial sweat at 310 K for 7 days;

[0101] Figure 8b shows a STEM HAADF and EDX scan for a sample of Au49Ag5.5Pd2.3Cu26.9Sii6.3 incubated in artificial sweat at 310 K for 28 days;

[0102] Figure 9 shows STEM EDX analysis of Au54Cu23Gei4Sig after 7 days in artificial saliva at 310 K, followed by 6 months in air at room temperature;

[0103] Figure 10 compares the sub-surface tarnishing depth for a different metallic glass compositions incubated in artificial saliva at 310 K for 7 days;

[0104] Figure 11 shows DSC and XRD traces of the compositions Au54AgxCu23-xGei4Sig (x = 0, 5, 10, 15);

[0105] Figure 12 shows DSC and XRD traces of a variety of compositions tested by the inventors.

[0106] Detailed Description

[0107] Elements of each alloy were weighed out and then arc-melted to form buttons. These buttons were then cast into thin ribbons using the melt-spinning technique. The thin nature of the ribbons (<100 pm) means that high cooling rates can be achieved such that marginal glass-formers can be cast fully amorphous. A conical casting with a maximum diameter of 6 mm was also cast via suction casting into a massive Cu mould (Figure 5) to assess the bulk glass-forming ability of this alloy series.

[0108] In order to study the glass-forming abilities and tarnish-resistance of Au-based bulk metallic glasses containing Si, Ge and one or both of Ag and Pd, the inventors prepared a selection of alloys having the compositions set out below in Table 1 .

[0109] The alloys were prepared using conventional methods, by preparing an alloy melt with the desired ratio of ingredients, and casting the alloy melt into amorphous solid samples.

[0110] Table 1 All compositions analysed in this work. XRD measurements were performed on as- cast ribbons.

[0111] The XRD analysis shows that all-but-one alloy cast as a ribbon is fully amorphous. The DSC was used to measure values of Trg and ATx, seen as key indicators of glass-forming ability. By altering the Ge / Si ratio and adding Ag and Pd, the inventors have found that it is possible to elevate the values of Trg and ATx to the levels of those of other known bulk glass-formers. Compositions containing Ag, Pd, Ge and Si were found to form bulk metallic glasses with values of Trg and ATx that were higher than the Au54Cu23Ge14Si9 composition known from the prior art. In order to provide alloys suitable for use in applications such as jewellery, the requirement of glass-forming ability must be balanced against the need for sufficient tarnish resistance, and the inventors have found that compositions within the scope of the claims exhibit a highly desirable balance of these properties.

[0112] Glass-Forming Ability Figure 2 shows the variation with composition of two parameters: (i) the temperature range (DTX) over which the samples of interest are in the supercooled liquid state upon heating at the rates typical of conventional DSC; and (ii) the reduced glass-transition temperature (Trg, defined as the ratio of the glass-transition temperature to the liquidus temperature of the alloy). The glass-forming ability of the composition is expected to increase with an increase in each of these parameters. Figure 2a - variation with the copper content of the of Au?7- xCuxGeuSig composition reported in C. V. Thompson, A. L. Greer and F. Spaepen, “Crystal nucleation in amorphous (Au o-yCuy^ySigGeu alloys,” Acta Metallurgica, vol. 31 , p. 1883- 1894, 1983. Figure 2b - variation with the silver content of the Au54AgxCu23-xGei4Sig bulk metallic glass for compositions within the scope of the claims; this shows that around 5 at.% Ag can be added to dramatically improve glass-forming ability.

[0113] Figures 3a and 3b illustrate the effect of Pd content on the crystallinity of the solid cast samples. Figure 3a compares DSC traces of compositions Ag4gAgsPd2Cu23Gei3Si8 (containing 2 at% Pd) and Ag4gAg5PdsCu2oGei3Si8 (containing 5 at% Pd). Figure 3b shows XRD traces for these same alloys, and shows that crystallinity occurs in the solid material at higher Pd content. Large additions of Pd promotes the formation of silicides, so the inventors have found that it is preferable for amorphous glass compositions that the Pd content remains below 5 at%.

[0114] The inventors have found that the addition of Ag and / or Pd, and preferably both Ag and Pd, to compositions containing Au-Ge-Si significantly improves the glass-forming ability of Au- Ge-Si to make it a bulk glass former.

[0115] DSC traces (Figure 4a) and XRD traces (Figure 4b) show amorphous character across different compositions. The compositions featured in Figures 3a, 3b, 4a and 4b are the same as those featured in Figures 11 and 12.

[0116] A plot of key parameters for glass-forming ability (GFA) as a function of composition (Figure 4c) shows that there is a general decrease in GFA as the proportion of Ge relative to Si increases.

[0117] Glass-forming ability (GFA) was assessed across a different range of alloys with varying Ge:Si ratio and overall metalloid content. While all of the compositions tested are glassy, there appears to be a general decrease in glass-forming ability (GFA) with increasing Ge:Si ratio (Fig. 4c).

[0118] A sample of Au49Ag5Pd2Cu25Ge?Sii2 (at.%) was cast as a cone with maximal diameter of 6 mm. The X-ray diffraction results in Figure 5 show that as the diameter of the cast sample increases, sharp peaks appear, indicating that the sample has begun to crystallize on cooling.

[0119] The Au49Ag5Pd2Cu25Ge?Sii2 alloy had the highest glass-forming ability of the compositions in Table 1. The XRD patterns in Figure 5 suggest that this alloy can be cast fully glassy in diameters up to 3 mm. Further alloy development to optimise the composition of the Aubased bulk metallic glass is expected to increase the critical casting diameter. Ternary projections of Au-Cu-Si (Figure 6a) and Au-Ge-Si (Figure 6b) were computed using the SSOI5 database and ThermoCalc Version 2023b. These ternary projections are part of the CALPHAD assessment which uses the liquidus depression as a measure of likelihood of glass-forming ability. A value of relative liquidus depression dT > 0.2 is an indication of glass-forming ability. The red line indicates the limit imposed by hallmarking for 18 K gold.

[0120] To assess the potential range of GFA as a function of Au-Ge-Si ratio, Figure 6b shows thermodynamic modelling using CALPHAD. This modelling shows that the potential glassforming region extends beyond the limit of 18 K gold, so that it is possible for the bulk metallic glass of the present invention to contain sufficient gold to be classed as an 18 K gold alloy.

[0121] In summary, the inventors’ experimental results show that:

[0122] • Addition of copper, silver and palladium provide the tested compositions with high glass-forming ability which leads to a range of bulk and commercially desirable alloys.

[0123] • Addition of 0-5 at.% Pd, 0-15 at.% Ag, and 0-25 at.% Cu may be preferable in order to provide different compositions across the glass-forming region of this alloy system.

[0124] • A combination of thermodynamic modelling and experimental results suggests that glass-forming ability is possible across the entire Ge-Si phase space, up to 20% Si across to around 25% Ge.

[0125] Tarnish Resistance

[0126] Figure 7 illustrates comparative ion-release rates for incubation of Au54Cu23Gei4Sig, Au54AgsCui8Gei4Si9, and the composition Au49Ag5.5Pd2.3Cu26.9Sii6.3 known from J. Schroers, B. Lohwongwatana, W. L. Johnson and A. Peker, “Gold based bulk metallic glass,” Applied Physics Letters, vol. 87, 061912, 2005.

[0127] To assess tarnish resistance, samples were polished and immersed in artificial saliva (DIN EN 10271) for seven days at 310 K (37°C). Samples showed a variation in tarnish resistance, depending on their composition. For many of these samples, the ion-release rates for copper are lower than for compositions without Cu (Figure 7). The lowered ion-release rates are a good indicator of improved tarnish resistance.

[0128] Figures 8a and 8b show STEM HAADF & EDX images of samples incubated at 310 K in simulated body fluid (artificial sweat): Au54AgsCui8Gei4Si9 for 7 days (Figure 8a), and Au49Ag5.5Pd2.3Cu26.9Sii6.3 for 28 days (Figure 8b). The surface of each sample is on the righthand side. Previous compositions, without Ge, show a corrosion scale that penetrates around 400 nm into the material. It is a combination of crystalline ccpAu (with some presence of the other metallic elements in the alloy) and SiO2. Previous studies on gallium-containing compositions have shown that attempting to improve tarnish resistance by increasing Ga / Sn content ultimately leads to an overall decrease and loss of GFA (O. Gross et al., “Development of novel 18-karat, premium-white gold bulk metallic glasses with improved tarnishing resistance,” Materials Design, vol. 140, p. 495- 504, 2018). It was suggested that a protective oxide layer forms on the surface (N. Neuber et al., “The role of Ga addition on the thermodynamics, kinetics, and tarnishing properties of the Au-Ag-Pd-Cu-Si bulk metallic glass forming system,” Acta Materialia, vol. 165, p. 315- 326, 2019).

[0129] Figure 9 shows STEM EDX analysis of a cross-section through a Au54Cu23Gei4Sig bulk metallic glass sample after 7 days in artificial saliva at 310 K, followed by 6 months in air at room temperature. For this glass containing Ge, there is no evidence for a protective oxide scale at the surface of the sample (top of main image), suggesting that there must be ion release and dissolution from the surface. From imaging the tarnish layer, it is clear that and Ge does not oxidise, but preferentially segregates into the residual glass. Although this composition does not contain Ag or Pd, the inventors consider it likely that the addition of Ge changes the chemical potential of Si, thereby changing the kinetics of oxidation.

[0130] Figure 10 compares focused ion-beam scanning electron microscopy (FIB-SEM) images of a variety of samples to characterise the sub-surface tarnishing depth for a variety of different compositions after incubation in artificial saliva for 7 days at 310 K. All samples were imaged at the same working distance and magnification. These results show that the addition of >7 at.% Ge leads to a dramatic improvement in tarnish resistance. Compared to the previously known compositions, all samples show improved tarnish resistance, albeit with different degrees of glass-forming ability.

[0131] Figure 11 contains DSC and XRD traces of the compositions Aus4AgxCu23-xGei4Sig (x = 0, 5, 10, 15), showing that all samples are amorphous, and that the addition of 5 at.% leads to an increase in stability of the supercooled liquid against crystallization (higher ATX).

[0132] Figure 12 contains DSC and XRD traces of other compositions listed in Table 1 and shown in Figure 1. All samples except Au54PdsAg5Cui8Gei2Si6 are amorphous. The addition of 5 at.% Pd leads to the formation of a crystalline fraction, indicated by XRD peaks consistent with known palladium silicides.

[0133] The results obtained by the inventors in this study suggest that adding Ag and Pd to Aubased bulk metallic glass compositions has a deleterious effect on tarnish resistance. However, the inventors have found that these ingredients compensate for this downside as they are highly beneficial for glass-forming ability. Increasing Si content improves glass- forming ability, but appears to decrease tarnish-resistance. Increasing Ge content appears to reduce the glass-forming ability of the alloy, but significantly improves tarnish resistance.

[0134] By incorporating Si and both Ge and Ag / Pd into Au-based alloy compositions, the inventors have found that it is possible to provide a bulk metallic glass which combines the benefits of good glass-forming ability with desirably high tarnish resistance.

[0135] The project leading to this application has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 695487).

[0136] Preferred Aspects

[0137] Preferred aspects of the invention are defined in the following numbered clauses:

[0138] 1. A bulk metallic glass, comprising: at least 45 at% Au; one or more of Ag and Pd;

[0139] Si; and

[0140] Ge.

[0141] 2. A bulk metallic glass according to clause 1 , in which the bulk metallic glass has the composition Aux(AgyPdz)wCuaGesSic, wherein x > 45, w > 0, a > 0, b > 0, c > 0.

[0142] 3. A bulk metallic glass according to clause 1 or 2, in which the bulk metallic glass contains at least 49 at% Au, or at least 52 at% Au, or at least 58.5 wt% Au, or at least 75 wt% Au.

[0143] 4. A bulk metallic glass according to clause 1, 2 or 3, in which the bulk metallic glass contains up to 20 at% Si, preferably between 5 at% and 15 at%, particularly preferably between 7 at% and 12 at% Si.

[0144] 5. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass contains up to 25 at% Ge, preferably less than 20 at% Ge.

[0145] 6. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass contains at least 7 at.% Ge, preferably at least 7.5 at% Ge, particularly preferably between 7 at% and 15 at% Ge.

[0146] 7. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass contains less than 5 at.% Pd, preferably up to 4.5 at%, or up to 4 at%, particularly preferably between 2 at% and 5 at% Pd. 8. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass contains up to 15 at.% Ag, preferably between 2 at% and 12 at%, or between 5 at% and 10 at% Ag.

[0147] 9. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass comprises a combined Pd and Ag content of up to 15 at.%, or up to 13 at%, or up to 10 at%.

[0148] 10. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass comprises a combined Pd and Ag content of between 5 at% and 12 at%, or between 7 at% and 10 at%.

[0149] 11. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass comprises Cu.

[0150] 12. A bulk metallic glass according to clause 11 , in which the bulk metallic glass contains up to 25 at.% Cu, preferably between 5 at% and 23 at% Cu, particularly preferably between 8 at% and 18 at% Cu.

[0151] 13. A bulk metallic glass according to clause 11 or 12, in which the bulk metallic glass contains a combined Ag, Pd and Cu content of up to 32 at.%, or up to 30 at%.

[0152] 14. A bulk metallic glass according to clause 13, in which the bulk metallic glass contains a combined Pd and Ag content of between 20 at% and 32 at%, or between 23 at% and 30 at%.

[0153] 15. A bulk metallic glass according to any preceding clause, in which the atomic ratio of Ge / Si in the bulk metallic glass is greater than 0.5, preferably wherein the atomic ratio of Ge / Si is greater than 1 , particularly preferably wherein the atomic ratio of Ge / Si is between 0.5 and 1.5.

[0154] 16. A bulk metallic glass according to any preceding clause, in which the atomic ratio of Ge / Si in the bulk metallic glass is less than or equal to 2.5, or less than or equal to 2.

[0155] 17. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass has a combined Ge and Si content of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%.

[0156] 18. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass comprises one or more of Al, Ga, Sn, In and Ca.

[0157] 19. A bulk metallic glass according to clause 18, in which the bulk metallic glass comprises less than 5 at% of one or more of Al, Ga, Sn, In and Ca.

[0158] 20. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass comprises Ga, and in which the bulk metallic glass has a combined Ga, Ge and Si content of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%. 21. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass comprises one or more of P, Pt, Ni, Co.

[0159] 22. A bulk metallic glass according to clause 20, in which the bulk metallic glass comprises less than 5 at% of one or more of P, Pt, Ni, Co.

[0160] 23. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass is 14K gold.

[0161] 24. A bulk metallic glass according to any of clauses 1 to 22, in which the bulk metallic glass is 18K gold.

[0162] 25. A bulk metallic glass according to any preceding clause, in which the bulk metallic glass is cast with a cross-section of 5 mm or less, or 4 mm or less, or 3 mm or less.

[0163] 26. A bulk metallic glass, optionally according to any preceding clause, having the formula Aux(AgyPdz)wCuaGesSic, wherein x > 45, w > 0, a > 0, b > 0, c > 0.

[0164] 27. A bulk metallic glass according to clause 26, in which the Au content is x > 49 at%, or x > 52 at%, or x > 58.5 wt%, or x > 75 wt% Au.

[0165] 28. A bulk metallic glass according to clause 26 or 27, in which the Si content is c < 20 at%, preferably in which 5 at% < c < 15 at%, particularly preferably 7 at% < c < 12 at%.

[0166] 29. A bulk metallic glass according to clause 26, 27 or 28, in which the Ge content is b < 25 at%, preferably b < 20 at% Ge.

[0167] 30. A bulk metallic glass according to any of clauses 26 to 29, in which the Ge content is b > 7 at.% preferably b > 7.5 at%, particularly preferably 7 at% < b < 15 at%.

[0168] 31. A bulk metallic glass according to any of clauses 26 to 30, in which the Pd content is z < 5 at.%, preferably z < 4.5 at%, or z < 4 at%, particularly preferably 2 at% < z < 5 at%.

[0169] 32. A bulk metallic glass according to any of clauses 26 to 31, in which the Ag content y < 15 at.%, preferably 2 at% < y < 12 at%, or 5 at% < y < 10 at%.

[0170] 33. A bulk metallic glass according to any of clauses 26 to 32, in which the combined Ag and / or Pd content w < 15 at%, preferably 5 at% < w < 12 at%, or between 7 at% < w < 10 at%.

[0171] 34. A bulk metallic glass according to any of clauses 26 to 33, in which the Cu content is 0 < a < 25 at.%, preferably 5 at% < a < 23 at% Cu, particularly preferably 8 at% < a < 18 at%.

[0172] 35. A bulk metallic glass according to any of clauses 26 to 34, in which the atomic ratio of Ge / Si in the bulk metallic glass, blc, is greater than 0.5, preferably wherein the atomic ratio of Ge / Si is greater than 1 , particularly preferably wherein the atomic ratio of Ge / Si is between 0.5 and 1.5 A bulk metallic glass according to any of clauses 26 to 35, in which the atomic ratio of Ge / Si in the bulk metallic glass, blc, is less than or equal to 2.5, or less than or equal to 2. A bulk metallic glass according to any of clauses 26 to 36, in which the bulk metallic glass has a combined Ge and Si content, (b+c), of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%. A bulk metallic glass according to any of clauses 26 to 37, in which the bulk metallic glass additionally contains an additive Z, so that the bulk metallic glass has the formula Aux(AgyPdz)wCuaGesSicZd, in which Z is one or more of Al, Ga, Sn, In and Ca. A bulk metallic glass according to clause 38, in which the additive Z content is d < 5 at%. A bulk metallic glass according to any of clauses 26 to 39, in which the bulk metallic glass additionally contains an additive p, so that the bulk metallic glass has the formula Aux(AgyPdz)wCuaGesSic e, in which is one or more of P, Pt, Ni, Co. A bulk metallic glass according to clause 40, in which the additive p content is e < 5 at%. A bulk metallic glass according to any of clauses 26 to 41 , in which the bulk metallic glass additionally comprises Ga, so that the bulk metallic glass has the formula Aux(AgyPdz)wCuaGefiSicGae. A bulk metallic glass according to clause 42, in which the bulk metallic glass has a combined Ga, Ge and Si content, (b+c+e), of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%. A bulk metallic glass according to any of clauses 26 to 43, in which the bulk metallic glass is 14K gold. A bulk metallic glass according to any of clauses 26 to 43, in which the bulk metallic glass is 18K gold. A bulk metallic glass according to any of clauses 26 to 45, in which the bulk metallic glass is cast with a cross-section of 5 mm or less, or 4 mm or less, or 3 mm or less.

Claims

Claims1. A bulk metallic glass, comprising: at least 45 at% Au; one or more of Ag and Pd;Si; and Ge.

2. A bulk metallic glass according to claim 1 , in which the bulk metallic glass has the composition Aux(AgyPdz)wCuaGesSic, wherein x > 45, w > 0, a > 0, b > 0, c > 0.

3. A bulk metallic glass according to claim 1 or 2, in which the bulk metallic glass contains at least 49 at% Au, or at least 52 at% Au, or at least 58.5 wt% Au, or at least 75 wt% Au.

4. A bulk metallic glass according to claim 1, 2 or 3, in which the bulk metallic glass contains up to 20 at% Si, preferably between 5 at% and 15 at%, particularly preferably between 7 at% and 12 at% Si.

5. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass contains up to 25 at% Ge, preferably less than 20 at% Ge.

6. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass contains at least 7 at.% Ge, preferably at least 7.5 at% Ge, particularly preferably between 7 at% and 15 at% Ge.

7. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass contains less than 5 at.% Pd, preferably up to 4.5 at%, or up to 4 at%, particularly preferably between 2 at% and 5 at% Pd.

8. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass contains up to 15 at.% Ag, preferably between 2 at% and 12 at%, or between 5 at% and 10 at% Ag.

9. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass comprises a combined Pd and Ag content of up to 15 at.%, or up to 13 at%, orup to 10 at%.

10. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass comprises a combined Pd and Ag content of between 5 at% and 12 at%, or between 7 at% and 10 at%.11 . A bulk metallic glass according to any preceding claim, in which the bulk metallic glass comprises Cu.

12. A bulk metallic glass according to claim 11 , in which the bulk metallic glass contains up to 25 at.% Cu, preferably between 5 at% and 23 at% Cu, particularly preferably between 8 at% and 18 at% Cu.

13. A bulk metallic glass according to claim 11 or 12, in which the bulk metallic glass contains a combined Ag, Pd and Cu content of up to 32 at.%, or up to 30 at%.

14. A bulk metallic glass according to claim 13, in which the bulk metallic glass contains a combined Pd and Ag content of between 20 at% and 32 at%, or between 23 at% and 30 at%.

15. A bulk metallic glass according to any preceding claim, in which the atomic ratio of Ge / Si in the bulk metallic glass is greater than 0.5, preferably wherein the atomic ratio of Ge / Si is greater than 1 , particularly preferably wherein the atomic ratio of Ge / Si is between 0.5 and 1.5.

16. A bulk metallic glass according to any preceding claim, in which the atomic ratio of Ge / Si in the bulk metallic glass is less than or equal to 2.5, or less than or equal to 2.

17. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass has a combined Ge and Si content of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%.

18. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass comprises one or more of Al, Ga, Sn, In and Ca.

19. A bulk metallic glass according to claim 18, in which the bulk metallic glass comprises less than 5 at% of one or more of Al, Ga, Sn, In and Ca.

20. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass comprises Ga, and in which the bulk metallic glass has a combined Ga, Ge and Si content of 10-30 at%, preferably 17-30 at%, particularly preferably 18-23 at%.

21. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass comprises one or more of P, Pt, Ni, Co.

22. A bulk metallic glass according to claim 20, in which the bulk metallic glass comprises less than 5 at% of one or more of P, Pt, Ni, Co.

23. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass is 14K gold.

24. A bulk metallic glass according to any of claims 1 to 22, in which the bulk metallic glass is 18K gold.

25. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass is cast with a cross-section of 5 mm or less, or 4 mm or less, or 3 mm or less.

26. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass is substantially amorphous.

27. A bulk metallic glass according to any preceding claim, in which the bulk metallic glass is at least 80% amorphous by volume, preferably at least 90% amorphous by volume or at least 95% amorphous by volume.

28. A bulk metallic glass according to any of claims 1 to 25, in which the bulk metallic glass is 100% amorphous by volume.

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