Amorphous alloy

Abstract

The invention provides an amorphous alloy. In one embodiment, the amorphous alloy is composed, in addition to impurities, of: i) 52.55 to 75.13 at.% of Au; ii) from 11.74 at.% to 15.55 at.% of Ge; iii) from 8.13 at.% to 10.77 at.% of Si; iv) 5-21.13 at.% of at least one element selected from the group consisting of Ag, Bi, Pd, and Pt.

Description

TECHNICAL FIELD

[0001] The present invention relates to a gold-based bulk metallic glass with improved tarnish resistance. The present invention also relates to an article of jewelry or a piece of jewelry comprising at least one component made of said alloy. BACKGROUND

[0002] Bulk metallic glasses, also known as amorphous alloys, are alloys that solidify into a metastable amorphous solid at sufficiently high cooling rates. In order to achieve cooling rates below the critical cooling rate during casting, the thickness of the cast will be limited by the critical casting thickness d c limit. In the supercooled liquid region (ΔΤ x ) of the metallic glass, i.e. in the temperature range between the glass transition temperature (Τ g ) and the crystallization temperature (Τ x ), it behaves like a plastic and deforms when pressed. Metallic glasses can have very different properties than traditional crystalline metallic alloys. In jewelry applications metallic gold has several advantages like near-net-shaping, high hardness and strength; and significantly higher whiteness than traditional 18K gold alloys.

[0003] US Patent No. 9,695,494 B2 reports a gold-based metallic glass with the chemical formula Au 49 Ag 5.5 Pd 2.3 Cu 26.9 Si 16.3 . The approach is based on the ternary eutectic Au-Cu-Si system. The gold content of this alloy is as high as 76 wt. % and therefore can be labeled as 18K gold. The alloy has a high hardness of up to 360 Hv and a high yield strength of up to 1100 MPa. The alloy after polishing can be rated as “excellent white” (yellow index of 17.8). However, the tarnish resistance of this alloy is poor. It severely tarnishes to “beige” within a few days in a wear test. The tarnish mechanism in Au-Cu-Si based metallic glasses was later identified to be due to the formation of amorphous silicon oxide dendrites and their intrusion into the base metal, followed by the formation of a copper oxide layer that forms indentations on the surface. Thus, the alloy ends up with a reddish hue. From this mechanism it can be concluded that the presence of Cu and Si are the main culprits for the tarnish. One approach to improve the tarnish resistance is to reduce the Cu / Si ratio. International Publication No. 2018 / 001564 later reports two variants of Au-Cu-Si based bulk metallic glasses. Part of the Cu is replaced by Ga and Sn. In a wear test it was concluded that the tarnish resistance of Au 51.6 Ag 5.8 Pd 2.4 Cu 20.2 Si 13.3 Ga 6.7 is better than that of Au 49 Ag5.5 Pd 2.3 Cu 26.9 Si 16.3 Another approach is to study alternative alloy systems that do not use Cu or Si.

[0004] A glass-liquid transition was found in the ternary eutectic system Au-Ge-Si (see Chen, H.S. et al., "Evidence of a Glass-Liquid Transition in a Gold-Germanium-Silicon Alloy", J. Chem. Phys. 48(6), 2560-2571 (1968), incorporated herein in its entirety). The alloy Au 77 Ge 13.65 Si 9.45 The average thickness of the splat films produced was about 20 microns. The glass transition temperature was 290-295 K. The crystallization temperature was about 304 K. This low glass transition temperature and low critical casting thickness make it unsuitable for use in jewelry production and applications. SUMMARY

[0005] The present invention provides an amorphous alloy. In one embodiment, the amorphous alloy consists essentially of: i) 52.55 - 80.12 at. % Au; ii) 11.74 - 15.55 at. % Ge; iii) 8.13 - 10.77 at. % Si; and iv) 5 - 21.13 at. % of at least one element selected from Ag, Bi, Pd or Pt.

[0006] In another embodiment, the amorphous alloy consists essentially of: i) 52.55 - 75.13 at. % Au; ii) 11.74 - 15.55 at. % Ge; iii) 8.13 - 10.77 at. % Si; and iv) 5 - 21.13 at. % of at least one element selected from Ag, Bi, Pd or Pt.

[0007] The present invention also provides a method of making the amorphous alloy of the present invention. In one embodiment, the elements are added to a crucible in the following order, from first to last: i) Pd-Ag-Au-Bi-Ge-Si; ii) Pt-Ag-Au-Bi-Ge-Si; iii) Pd / Pt-Ag-Au-Bi-Ge-Si.

[0008] The present invention further provides a jewelry item comprising at least one component made of the amorphous alloy of the present invention.

[0009] The present invention relates to Au-based bulk-solidifying amorphous alloys.

[0010] It is an object of the present invention to prepare the Au-based alloys in Au-Ge-Si ternary alloys.

[0011] It is another object of the present invention to extend the Au-Ge-Si system to higher alloys by adding one or more elements selected from the group consisting of Ag, Pd, Pt and Bi.

[0012] It is another object of the present invention to improve the glass forming ability of the Au-based alloys compared to Au-Ge-Si ternary alloys.

[0013] It is another object of the present invention to improve the thermal stability of the Au-based alloys compared to ternary Au-Ge-Si alloys.

[0014] It is another object of the present invention to improve the tarnish resistance of the Au-based alloys compared to Au-Cu-Si based bulk metallic glasses.

[0015] It is another object of the present invention to make the Au-based alloys suitable for jewelry applications. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is the X-ray diffraction pattern of cast alloys 5 to 8 at the critical casting thickness.

[0017] Figure 2 is the differential scanning calorimetry (DSC) pattern of cast alloys 5 to 8.

[0018] Figure 3A is a 3 mm diameter as-cast rod of alloy 7.

[0019] Figure 3B is Figure 3A is the shape of the rod in after being bent into a U-shape at 328 K (10x magnification). DETAILED DESCRIPTION

[0020] The following terms will be used to describe the present invention. Where not specifically defined herein, the terms used to describe the present invention shall be construed in accordance with the commonly understood meaning by one of ordinary skill in the art.

[0021] Herein, the term "BMG" refers to bulk metallic glass.

[0022] Herein, the term "at.%" refers to atomic percent.

[0023] Herein, the term "d c " refers to critical casting thickness.

[0024] Herein, the term "T g" refers to the glass transition temperature.

[0025] The term "T x " refers to the crystallization temperature.

[0026] The term "ΔT x " refers to the supercooled liquid region, i.e., the difference between T g and T x .

[0027] The term "T1" refers to the liquidus temperature.

[0028] The term "YI" refers to the yellowness index.

[0029] The term "XRD" refers to X-ray diffraction.

[0030] The term "DSC" refers to differential scanning calorimetry.

[0031] The term "ΔE * " refers to the color difference between two colors. ΔE* is calculated by the following equation: where L * , a * , and b * are the axes in CIELAB coordinates.

[0032] The present disclosure provides an amorphous alloy. In one embodiment, the amorphous alloy consists essentially of: i) 52.55 - 80.12 at. % Au; ii) 11.74 - 15.55 at. % Ge; iii) 8.13 - 10.77 at. % Si; and iv) 5 - 21.13 at. % of at least one element selected from Ag, Bi, Pd, or Pt.

[0033] An amorphous alloy is provided. In one embodiment, the amorphous alloy consists of, except for impurities, i) 52.55-75.13 at. % Au; ii) 11.74-15.55 at. % Ge; iii) 8.13-10.77 at. % Si; and iv) 5-21.13 at. % of at least one element selected from Ag, Bi, Pd, or Pt. In one embodiment, the impurities are any elements that do not significantly affect the intended performance resulting from the combination of elements from (i) to (iv). In one embodiment, the intended performance is glass forming ability. In one embodiment, the minimum critical casting thickness of the glass forming ability is 0.5 mm. In one embodiment, the minimum glass transition temperature of the glass forming ability is 315 K. In one embodiment, the intended performance is tarnish resistance of the amorphous alloy. In one embodiment, the tarnish resistance refers to a ΔΕ* less than 21 or a ΔΥΙ less than 46 after 14 days of immersion in artificial sweat at 37 ± 2 °C.

[0034] In one embodiment, the at least one element is selected from the group consisting of: i) 5-10 at. % Ag; ii) 0.01-8.69 at. % Bi; iii) 0.01-2.44 at. % Pd; iv) 0.01-2.44 at. % Pt; and v) 0.01-2.44 at. % total of Pd and Pt.

[0035] In one embodiment, the amorphous alloy consists essentially of: 69.4 at. % Au; 13.65 at. % Ge; 9.45 at. % Si; and 7.5 at. % Ag.

[0036] In one embodiment, the amorphous alloy consists essentially of: 68.2 at. % Au; 13.65 at. % Ge; 9.45 at. % Si; 7.5 at. % Ag; and 1.2 at. % Pd.

[0037] In one embodiment, the amorphous alloy consists essentially of: 65.83 at. % Au; 13.65 at. % Ge; 7.5 at. % Ag; 1.2 at. % Pd; and 2.37 at. % Bi.

[0038] In one embodiment, the amorphous alloy consists essentially of: 65.83 at. % Au; 13.65 at. % Ge; 9.45 at. % Si; 7.5 at. % Ag; 1.2 at. % Pd; and 2.37 at. % Bi.

[0039] In one embodiment, the amorphous alloy consists essentially of: 63.46 at. % Au; 13.65 at. % Ge; 9.45 at. % Si; 7.5 at. % Ag; 1.2 at. % Pd; and 4.74 at. % Bi.

[0040] In one embodiment, the amorphous alloy consists essentially of one or more of the following sets of elements: i) 63.68 - 73.59 at. % Au, 12.65 - 15.55 at. % Ge, 8.76 - 10.77 at. % Si, and 5 - 10 at. % Ag; ii) 61.28 - 72.99 at. % Au, 12.65 - 15.55 at. % Ge, 8.76 - 10.77 at. % Si, 5 - 10 at. % Ag, and 0.6 - 2.4 at. % Pd; iii) 52.59 - 71.49 at. % Au, 12.65 - 15.55 at. % Ge, 8.76 - 10.77 at. % Si, 5 - 10 at. % Ag, 0.6 - 2.4 at. % Pd, and 1.5 - 8.69 at. % Bi; iv) 66.85 - 71.85 at. % Au, 12.65 - 14.65 at. % Ge, 9 - 10 at. % Si, and 6.5 - 8.5 at. % Ag; v) 64.45 - 70.95 at. % Au, 12.65 - 14.65 at. % Ge, 9 - 10 at. % Si, 6.5 - 8.5 at. % Ag, and 0.9 - 2.4 at. % Pd; vi) 58.95 - 69.15 at. % Au, 12.65 - 14.65 at. % Ge, 9 - 10 at. % Si, 6.5 - 8.5 at. % Ag, 0.9 - 2.4 at. % Pd, and 1.8 - 5.5 at. % Bi.

[0041] In one embodiment, the Au content in the amorphous alloy of the present invention is selected from any one of 52.55, 53.00, 54.00, 55.00, 56.00, 57.00, 58.00, 59.00, 60.00, 61.00, 62.00, 63.00, 64.00, 65.00, 66.00, 67.00, 68.00, 69.00, 70.00, 71.00, 72.00, 73.00, 74.00, 75.0, or 75.13 at. %. In one embodiment, the Au is further selected from any one of 76.00, 77.00, 78.00, 79.00, 80.00, or 80.12.

[0042] In one embodiment, the Ge composition in the amorphous alloy of the present invention is selected from any one of 11.74, 12.00, 13.00, 14.00, 15.00, or 15.55 at. %.

[0043] In one embodiment, the Si composition in the amorphous alloy of the present invention is selected from any one of 8.13, 8.50, 9.00, 9.50, 10.00, 10.50, or 10.77 at. %.

[0044] In one embodiment, the Ag, Bi, Pd, or Pt composition in the amorphous alloy of the present invention is selected from any one of 5.00, 6.00, 7.00, 8.00, 9.00, 10.00, 11.00, 12.00, 13.00, 14.00, 15.00, 16.00, 17.00, 18.00, 19.00, 20.00, 21.00, 21.13 at. %.

[0045] In one embodiment, the amorphous alloy comprises one or more of the following properties: i) a minimum critical casting thickness of 0.5 mm; ii) a minimum glass transition temperature of 315 K; iii) a minimum undercooled liquid region of 10 K; iv) improved thermal stability compared to Au-Ge-Si ternary alloys; v) improved glass forming ability compared to Au-Ge-Si ternary alloys; vi) improved resistance to tarnishing compared to Au-Cu-Si based bulk metallic glasses (BMGs).

[0046] In one embodiment, the amorphous alloy further comprises one or more of the following properties: i) a minimum Vickers hardness of 200 HV; ii) a minimum compressive strength of 480 MPa.

[0047] The present invention also provides a method of making the amorphous alloy of the present invention. In one embodiment, the method comprises the step of adding the elements to a crucible in the following order, from first to last: i) Pd-Ag-Au-Bi-Ge-Si; ii) Pt-Ag-Au-Bi-Ge-Si; iii) Pd / Pt-Ag-Au-Bi-Ge-Si.

[0048] In one embodiment, Pd / Pt-Ag-Au-Bi-Ge-Si in (iii) means the step of adding the elements to the crucible in any one of the following order, from first to last: Pd and Pt are added simultaneously, followed by Ag, Au, Bi, Ge, and Si.

[0049] The present invention further provides an ornament comprising at least one component made of the amorphous alloy of the present invention.

[0050] In one embodiment, the accessory is a piece of jewelry or a piece of furniture.

[0051] In one embodiment, the at least one component is formed by thermoplastic molding or molding.

[0052] In one embodiment, the jewelry alloy has a glass forming ability compared to Au 76.9 Ge 13.65 Si 9.45 improved. Table 1 lists alloy 1 and alloy 4 from previous literature for comparison purposes, while alloys 5 to 8 of the present invention were prepared according to the following conditions. The alloys were prepared from individual elements with purity > 99.99%. The individual elements were melted in an induction furnace with a graphite crucible. The following order of elements can be maintained in the crucible (from bottom to top): Pd / Pt - Ag - Au - Bi - Ge - Si, as this can reduce the formation of palladium and / or platinum silicides or other intermetallics that can lead to crystallization. To achieve rapid cooling, the melt was cast into a copper mold. The weight percentage of Au for all example alloys was more than 75%, so they can be hallmarked as 18K gold.

[0053] Table 1 shows the composition of the example alloys of the present invention and the composition of the alloys used for comparison.

[0054] The amorphous structure of alloys 5 to 8 was verified by X-ray diffractometer (Rigaku SmartLab 9KW) using Cu K α radiation. The diffraction patterns are shown in Figure 1 Table 2 lists the d c and thermal behavior of alloys 1 to 8. The critical casting thickness of alloys 5 to 8 ranges from 2 to 7 mm. The example of the present invention shows a significant improvement in the critical casting thickness compared to the 20 pm of the flake thickness produced by alloy 1. In addition, it is highly recommended to add Bi to the system. When Bi is added, the d c can even increase to 7 mm. This indicates that the addition of Bi leads to a significant improvement in the glass forming ability in the Au-Ge-Si system.

[0055] Table 2. Critical thickness (d c ) and thermal behavior of selected alloys. The data of alloys 1 to 4 are taken from previous literature.

[0056] In one embodiment, the jewelry alloy has improved thermal stability. All thermal properties were measured by differential scanning calorimetry (Mettler Toledo DSC3) at a heating rate of 20 K / min. Temperatures are also listed in Table 2. The T g of alloy 1 was 290-295 K, ΔT x was 11 K. The T g of the present examples (alloys 5 to 8) ranges from 317 to 324 K, while ΔT x is as high as 26 K. The higher T g and larger ΔT x indicate a higher stability of the supercooled liquid and also a higher likelihood of plastic deformation for jewelry production. The introduction of Bi into the system does not significantly decrease ΔT x or T g but increases the glass forming ability of the bulk metallic glass.

[0057] In one embodiment, the jewelry alloy has a hardness suitable for jewelry applications. In one embodiment, the jewelry alloy has a Vickers hardness (load 0.2 kg) of at least 200 HV. Table 3 shows the Vickers hardness of as-cast alloys 5 to 8.

[0058] Table 3. Vickers hardness (0.2 kg) of as-cast alloys of the present invention. Alloy Hardness (HV 0.2) 5 (Invention) 200 6 (Invention) 211 7 (Invention) 215 8 (Invention) 208

[0059] In one embodiment, the jewelry alloy has a strength suitable for jewelry applications. In one embodiment, the jewelry alloy has a compressive strength of at least 480 MPa. Rod-shaped samples with a diameter of 8 mm (aspect ratio 1 :2) were prepared from alloy 7 for compressive strength testing. The compressive strength of alloy 7 was 487 MPa.

[0060] In one embodiment, the jewelry alloy has improved tarnish resistance. Tarnish resistance testing was performed according to ISO 10271. 8x8x1 mm plate samples were prepared from alloys 2-7. The sample surfaces were sanded to remove oxide residues formed during casting prior to testing. The samples were then immersed in an artificial sweat solution and incubated at 37±2°C for 14 days. The degree of tarnish can be expressed in terms of ΔE* and ΔYI compared to their original polished state. ΔE* describes the total color change. YI is a number calculated from spectrophotometric data that indicates a change in color from colorless to yellow. All color measurements were expressed in CIELAB coordinates and YI was calculated according to ASTM D1925. The sample alloys of the present invention exhibited a lustrous white color that was as attractive as polished. All sample alloys can be rated as "superior white" since their YI was less than 13. Table 4 lists all color measurements. From the Δa* and Δb* values, it can be seen that the metallic glasses of the Au-Cu-Si system (alloys 2-4) tarnished to a red hue, while the metallic glasses of the present invention (alloys 5-8) tarnished to a yellow hue. Alloys 3 and 4, which partially replaced Cu with Sn and Ga, respectively, were considered to have improved tarnish resistance over alloy 2. In this experiment, alloys 3 and 4 exhibited a 2.0-4.1% reduction in ΔE* and ΔYI, showing a slight improvement in tarnish resistance. However, the alloys of the present invention exhibited at least a 22% reduction in ΔE* and ΔYI. The tarnish resistance of the disclosed alloys of the present invention was significantly improved over previous inventions. The high tarnish resistance makes the present invention more suitable for jewelry applications.

[0061] Table 4. Tarnish resistance performance of selected alloys. Alloy ΔL* Δa* Δb* ΔE* ΔYI 2 (Comparative) -15.35 +15.05 +16.80 27.28 +56.68 3 (Comparative) -12.25 +12.35 +19.54 26.16 +55.54 4 (Comparative) -14.09 +9.61 +20.06 26.57 +55.25 5 (Invention) -1.65 +2.45 +20.20 20.41 +41.00 6 (Invention) -2.20 +3.69 +17.32 17.85 +37.62 7 (Invention) 8 (Invention) -5.64 +6.09 +18.73 20.49 +44.30

[0062] In one embodiment, the jewelry alloy is suitable for jewelry fabrication. Previous studies have suggested (Schroers, J. "The superplastic forming of bulk metallic glasses," The Journal of the Minerals, Metals & Materials Society. 57(5), 35-39 (2005)) that one possible method of fabricating metallic glasses is through hot plastic forming. Amorphous feedstock material can be prepared in thicknesses up to 3 mm, in pellet or other simple geometric shapes. Upon application of pressure, at a temperature within the material's undercooled liquid region, the feedstock material can be pressed into a pre-determined shape mold. Plastic deformation can be observed in alloys 5-8 within their respective ΔT x In the example of Figure 3, plastic deformation of a cast rod of alloy 7 is shown. The rod was bent at 328 K, which is 5 K above its T In the example of Figure 3, plastic deformation of a cast rod of alloy 7 is shown. The rod was bent at 328 K, which is 5 K above its T g The rod was bent without any signs of fatigue.

Claims

1. An amorphous alloy, characterized in that: The amorphous alloy, excluding impurities, consists of the following components: i.52.55-75.13at.%Au; ii.11.74-15.55at.%Ge; iii. 8.13-10.77 at.% Si; and iv.5-21.13at.% of at least one element selected from Ag, Bi, Pd or Pt.

2. The amorphous alloy according to claim 1, characterized in that: The at least one element is selected from the following group: i.5-10 at.% Ag; ii.0.01-8.69at.%Bi; iii. 0.01-2.44 at.% Pd; iv. 0.01-2.44 at.% Pt; and v.0.01-2.44at.% total of Pd and Pt.

3. The amorphous alloy according to claim 1, characterized in that: The amorphous alloy comprises: 69.4 at.% Au; 13.65 at.% Ge; 9.45 at.% Si; and 7.5 at.% Ag.

4. The amorphous alloy according to claim 1, characterized in that: The amorphous alloy comprises: 68.2 at.% Au; 13.65 at.% Ge; 9.45 at.% Si; 7.5 at.% Ag; and 1.2 at.% Pd.

5. The amorphous alloy according to claim 1, characterized in that: The amorphous alloy comprises: 65.83 at.% Au; 13.65 at.% Ge; 9.45 at.% Si; 7.5 at.% Ag; 1.2 at.% Pd; and 2.37 at.% Bi.

6. The amorphous alloy according to claim 1, characterized in that: The amorphous alloy comprises: 63.46 at.% Au; 13.65 at.% Ge; 9.45 at.% Si; 7.5 at.% Ag; 1.2 at.% Pd; and 4.74 at.% Bi.

7. The amorphous alloy according to claim 1, characterized in that: The amorphous alloy comprises: 63.68-73.59 at.% Au; 12.65-15.55 at.% Ge; 8.76-10.77 at.% Si; and 5-10 at.% Ag.

8. The amorphous alloy according to claim 1, characterized in that: The amorphous alloy comprises: 61.28-72.99 at.% Au; 12.65-15.55 at.% Ge; 8.76-10.77 at.% Si; 5-10 at.% Ag; and 0.6-2.4 at.% Pd.

9. The amorphous alloy according to claim 1, characterized in that: The amorphous alloy comprises: 52.59-71.49 at.% Au; 12.65-15.55 at.% Ge; 8.76-10.77 at.% Si; 5-10 at.% Ag; 0.6-2.4 at.% Pd; and 1.5-8.69 at.% Bi.

10. The amorphous alloy according to claim 1, characterized in that: The amorphous alloy comprises: 66.85-71.85 at.% Au; 12.65-14.65 at.% Ge; 9-10 at.% Si; and 6.5-8.5 at.% Ag.

11. The amorphous alloy according to claim 1, characterized in that: The amorphous alloy comprises: 64.45-70.95 at.% Au; 12.65-14.65 at.% Ge; 9-10 at.% Si; 6.5-8.5 at.% Ag; and 0.9-2.4 at.% Pd.

12. The amorphous alloy according to claim 1, characterized in that: The amorphous alloy comprises: 58.95-69.15 at.% Au; 12.65-14.65 at.% Ge; 9-10 at.% Si; 6.5-8.5 at.% Ag; 0.9-2.4 at.% Pd; and 1.8-5.5 at.% Bi.

13. The amorphous alloy according to claim 1, characterized in that: The amorphous alloy has one or more of the following properties: i. The minimum critical casting thickness is 0.5 mm; ii. The minimum glass transition temperature is 315K; iii. The minimum subcooled liquid region is 10K.

14. A method for manufacturing the amorphous alloy of claim 1, comprising the step of adding elements to a crucible in the following order from first to last: Pd-Ag-Au-Bi-Ge-Si.

15. A method for manufacturing the amorphous alloy of claim 1, comprising the step of adding elements to a crucible in the following order from first to last: Pt-Ag-Au-Bi-Ge-Si.

16. A method for manufacturing the amorphous alloy of claim 1, comprising the step of adding elements to a crucible in the following order from first to last: Pd and Pt added simultaneously, followed by -Ag-Au-Bi-Ge-Si.

17. An ornament comprising at least one component made of the amorphous alloy of claim 1.

18. The ornament according to claim 17, characterized in that: The ornament is a piece of jewelry or a decorative item.

19. The ornament according to claim 17, characterized in that: The at least one component is formed by thermoplastic molding or molding.

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