High strength, Anti-tarnish silver alloy composition

Abstract

The present invention discloses a high strength, anti-tarnish silver alloy composition comprising silver in an amount of 50-99.9wt% and process thereof.

Description

[0001] HIGH STRENGTH, ANTI-TARNISH SILVER ALLOY COMPOSITION

[0002] FIELD OF INVENTION:

[0003] The present invention relates to a high strength, anti-tarnish silver alloy composition comprising silver in an amount of 50-99.9wt% and process thereof.

[0004] BACKGROUND AND PRIOR ART:

[0005] Silver, akin to gold, is classified as a precious metal and is characterized by its remarkable softness, ductility, and malleability. It possesses a striking white metallic sheen and can achieve a high degree of polish. Additionally, silver demonstrates excellent electrical and thermal conductivity, reflectivity, and notable resistance to corrosion. These inherent properties render silver an advantageous material for a diverse array of industrial uses. Beyond its role in coinage, silver is utilized in the creation of jewelry, high-end tableware, and utensils. Furthermore, it is employed in medical instruments, dentistry, optical devices, electrical and electronic applications, photographic materials, conductive surfaces in voltaic cells, as well as in various formed, extruded, and molded components for multiple industrial purposes.

[0006] When exposed to atmospheric sulfur, a common environmental contaminant, silver undergoes a chemical reaction that results in the formation of silver sulfide (Ag2S). This process can lead to a dark discoloration on the surface of silver items, which becomes more pronounced as the layer of silver sulfide thickens. Continued exposure to sulfur can result in further color changes, producing brown and blue hues that detract from the metal's original luster.

[0007] The formation of silver sulfide is particularly undesirable because it alters the inherent properties of silver. The refractive index, hardness, electrical conductivity, and UV-visible reflectance of silver sulfide differ significantly from those of pure silver. As a result, the presence of this tarnish not only affects the aesthetic appeal of silver items but also diminishes their functional performance in applications where conductivity and reflectivity are critical.

[0008] Silver tarnishes when it is exposed continuously to Sulphur containing compounds, which will affect the aesthetically pleasing appearance of silver. Along with Sulphur, moisture, oxygen and chlorides can accentuate the process of tarnishing. The common uses of silver are for jewellery and article making industries. The aesthetic features of silver jewelry including the color, visible light reflectance etc., are greatly diminished by tarnishing and therefore to prevent tarnishing, the existing methods were CVD, PVD coatings of Nitrides on the surface of silver alloy, or alloying silver with high oxidizing elements such as Germanium, Silicon, Yttrium, cobalt, etc. Similarly, the addition of alloying elements shall decrease the workability of the materials. Since we do not have any control over the Sulphur content in ambient atmosphere, silver is modified by different alloying and coatings to retain the colour and lustre for longer time. Over the century, a large volume of research has been carried out on developing tarnish free fine silver (97-99.9wt.% pure) and Sterling silver (92.5 wt.% pure & remaining Cu and other alloying elements). Different methods have been made use, including PVD, CVD etc to provide coating over silver. However, this results in compromising mechanical properties like hardness, tensile strength, scratch resistance, ductility etc.

[0009] Alloying Ag with oxide forming elements is economical and industrially viable and has large benefits. Research in this area has identified several oxide-forming elements, including chromium, silicon, magnesium, manganese, zinc, and tin, as well as titanium, zirconium, indium, and antimony, which can inhibit tarnishing while preserving the mechanical properties and malleability of silver. For instance, elements such as germanium (found in Argentium silver) and indium can influence various factors, including the composition of the alloying elements, which may result in solid solutions, intermetallic compounds, or intermediate phases that impact oxidation, the thickness and structure of the oxides formed on the surface, and their stability. Furthermore, it is essential that these alloying elements do not compromise the workability, color, or lustre of silver and its alloys.

[0010] US20150030872A1 discloses the method of imparting tarnish protection on silver and also with colour appearance to silver and its alloys. It discloses a multiple barrier layer coating for the protection for the silver through various steps. Dispersing nanoparticles (silver, diamond, corundum, silica nanoparticles) in slurry with solvent uniformly over the object by sonicating in an ultrasonic bath and applying thin film of oxides like AI2O3 or Ti O2 nitrides, oxy-nitrides through PVD, CVD or sol-gel technique and finally applying protective layer conformal coating (polymer coating) followed by annealing.

[0011] CN109943745A discloses an anti-tarnishing silver alloy composed of silver (89.0- 99.0%), aluminum (0.01-10%), iron (0.01-11%), copper (0.01-6%), titanium (0.01-10%), zirconium (0.01-11%), indium (0.01-6%), Al-Ti-B alloy (0-8%), and cobalt (0.01-5%). The production process involves melting these components in electric arc furnaces, casting them into ingots, and subjecting them to homogenization heat treatment in a protective atmosphere. This method produces a silver alloy through quenching, refining crystallite size and enhancing antioxidant properties, sulfuration resistance, and corrosion resistance.

[0012] CN110318968 A discloses a type of silver alloy for jewellery that resists tarnishing, along with its preparation method, which falls under the category of alloy materials. The jewellery is made from an anti -tarnishing silver alloy containing the following components by weight percentage: Cu 1-8%; Ni 0.5-4%; Zr 0-0.5%; Al 0.2-2%; Mo 0.2-1%, with the remainder being Ag. This silver alloy is created using a powder metallurgy technique, involving pre-sintering and plasma discharge sintering. This method effectively reduces the loss of elements seen in traditional melting processes, resulting in less oxidation and higher consistency during sintering. US5021214A discloses a silver alloy of high discoloration resistance, and improvement in colour maintenance of Ag alloys. US’214 disclose that additional content of Cu further improves mechanical properties of the products. It is disclosed in US’214 that the addition of Copper in the Ag alloys above 3.0% by weight would improve the mechanical properties, more specifically hardness of the product but at the same time the product exhibits no good discoloration resistance against sulphides.

[0013] US20140003992 discloses anti tarnish sterling silver alloy composition comprising titanium and optionally palladium or niobium, among other metals.

[0014] US20080166260 discloses silver alloy compositions containing Ag, Zn, In, Sn, Al, Mg, Ti and copper that have an improved resistance to sulphidation and formation of fire stain.

[0015] US10876189B2 discloses an age-hardenable sterling silver alloy containing 0.7 to 1.9% by weight palladium, and a sum total of from 2.5 to 6.8% by weight of zinc and indium, wherein zinc and indium are both present in the age hardenable sterling silver alloy, to 0 to 0.25% of germanium or silicon, 0 to 3% of copper;and 0 to 2% of tin or gallium., with improved resistance to tarnishing, mainly used for the realization of valuable articles such as jewellery, silverware, coins and medals. The inventors of the present invention recognized a significant need in the art for a silver alloy composition that not only possesses high hardness but also effectively resists silver tarnishing. Their goal was to enhance both the mechanical and reflective properties of the alloy while minimizing the number of alloying elements used in its composition, which is hitherto not practised in the art. .

[0016] SUMMARY OF THE INVENTION:

[0017] In accordance with the above, the primary objective of the present invention is to provide high strength, anti-tarnish silver alloy composition comprising of alloying elements Al, Cu, Zn, Ti, Zr, and Mg. Another object of the present invention is to provide high strength, anti -tarnish silver alloy composition which shows high hardness, high elasticity, good grain refinement, high tensile strength.

[0018] Yet another object of the present invention is to provide silver alloys which could be used to make quality silver jewellery / article that can sustain the colour and lustre for longer period.

[0019] In an aspect, the present invention discloses high strength, anti-tarnish silver alloy composition comprising of at least 50% by weight of silver with two or more alloying elements selected from; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0020] In an aspect, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 50 to 99.9% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0021] In an aspect, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 75% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.01 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0022] In another aspect, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 80% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.01 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0023] In another aspect, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 92.5% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0024] In another aspect, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 92.5% by weight of silver with the alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; and the balance being copper.

[0025] In yet another aspect, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 92.5% by weight of silver with alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; and the balance being copper.

[0026] In an aspect, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 97% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0027] In another aspect, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 99% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper. In another aspect, the present invention provides a process for preparing high strength, anti-tarnish silver alloy composition comprising; a. Melting silver of at least 50wt.%, followed by adding two or more alloying elements selected from Copper, Aluminium, Zinc, Titanium, Zirconium, Magnesium in suitable amount in under vacuum ; and b. Cold rolling the silver alloy to obtain the desired alloy.

[0028] In accordance with the above, the process steps a) comprises of melting wherein silver of at least 50wt% was melted along with two or more alloying elements selected from Cu, Al, Zn, Ti, Zr, Mg.

[0029] The step b) comprises of cold rolling wherein Silver alloy obtained in step (a) were cold-rolled with reduction in size to obtain the desired alloy.

[0030] In an alternate aspect, in the process of the present invention the alloying elements may be added through the master alloy comprising Ag-5Ti, Ag-5Zr. Cu-5Zn, Al- 5Mg, Cu-5Ti, Cu-2Ti, Cu-5Zr, Cu-5Zn, Zn-5Ti.

[0031] In an aspect, the alloy obtained by the process of the present invention is optionally subjected to Passivation Heat Treatment (PHT) at a temperature ranging between 400 to 600°C for 30 to 120 minutes in a muffle furnace, followed by cooling to obtain the desired alloy.

[0032] Accordingly, the Passivation heat treatment is carried out to achieve the oxide layer on the top surface of the alloy through the diffusion of alloying elements present in it. Passivation treatment is carried out at 400 to 600°C for 30 to 120 minutes in an ambient atmosphere or by passing oxygen.

[0033] After passivation treatment the alloy samples are cooled to obtain the desired alloy. The passivation heat treatment in the present invention promotes oxide layer formation on the silver alloy's surface through diffusion of alloying elements thereby improving its anti-tarnish property.

[0034] The addition of master alloys during the process for preparing the silver alloy composition in the present invention eliminates hot tearing of the alloy during solidification, improves workability, eliminates scaling of the cold rolled sheets during annealing, avoids segregation and floating, and improves the grain structure.

[0035] Further, the combined addition of alloying elements selected from aluminium (Al), zinc (Zn), titanium (Ti), zirconium (Zr), magnesium (Mg) through master alloy significantly enhances the properties of silver alloys. This is achieved through the formation of protective oxide layers, which contribute to solid solution strengthening, refinement of the grain structure, and ultimately result in increased tensile strength, hardness, and tarnish resistance of the cast silver alloy. Additionally, these elements facilitate precipitation strengthening in both cast and wrought forms of the alloy. These oxide layers serve to inhibit the interaction with sulphur, thereby significantly enhancing the resistance of the surface to tarnishing over extended periods.

[0036] In an aspect, the selection of alloying elements of the present invention into the silver alloy was based on their lower free energy values and a Pilling Bedworth Ratio greater than 1.

[0037] DESCRIPTION OF THE DRAWINGS

[0038] Fig.l: Depicts the Optical Microscopic images of (a) TR3 (b)TR4 (c)TR5 (d)TR7 (e)TR8 (f) TR9 (g) TRIO (h)TRl l (i) TR12 (j) TR14 (k)TR15.

[0039] Fig.l: Depicts the EDX Elemental mapping of TR5 after PHT (a) SE Image of analysed region (b) Silver(c) Copper (d) Aluminium (e) Zinc (f) Oxygen. Fig.3: Depicts the EDX Elemental mapping of TR9 after PHT (a) SE Image of analysed region (b) Silver(c) Copper (d) Aluminium (e) Zinc (f) Zirconium (g) Titanium (h) Oxygen.

[0040] Fig.4: Depicts the Vickers hardness of (a) present silver alloys of 92.5wt% (b) comparative Vickers hardness of 97wt. % and 99wt. % present silver alloys and of conventional silver (CS3) (c) comparative Vickers hardness of 97wt. % and 99wt. % present silver alloys and pure silver (PS).

[0041] Fig 5: Depicts the comparative Stress versus strain curve of (a) present silver alloys and conventional silver (CS3) (b) present 97wt. % and 99wt. % silver alloys and pure silver (PS).

[0042] Fig.6: Depicts the Accelerated tarnish test done on Set 1 samples of the present invention (a) before PHT (b) after PHT at 450°C for 1 hour.

[0043] Fig.7: Depicts the Accelerated tarnish test done on Set 2 samples of the present invention (a) before PHT (b) after PHT at 450°C for 1 hour.

[0044] Fig.8: Depicts the Accelerated tarnish test done on Set 3 samples of the present invention (a) before PHT (b) after PHT at 450°C for 1 hour.

[0045] Fig.9: Depicts the Accelerated tarnish test done on Set 4 samples of the present invention (a) before PHT (b) after PHT at 450°C for 1 hour.

[0046] Fig.10: Depicts the Accelerated tarnish test done on 75 wt%. pure Ag and 80 wt. % & pure Ag and samples before PHT.

[0047] Fig.ll: Depicts the Accelerated tarnish test done on 75wt. % pure Ag and 80wt. % pure Ag samples after PHT at 450°C for 1 hour. Fig.12: Depicts the UV- Visible reflectance of samples of the present invention (a) before tarnish test (b) after tarnish test.

[0048] Fig.13: Depicts the Accelerated tarnish test done on conventional silver alloy composition with varying copper amount (a) Before PHT and (b) After PHT.

[0049] Fig.14: Depicts the Vickers hardness of conventional silver alloy composition with varying copper amount.

[0050] Fig.15: Depicts the Stress versus strain curve of conventional silver alloy composition with varying copper amount.

[0051] DISCLSOURE OF THE INVENTION:

[0052] The present invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects therefore maybe fully understood and appreciated.

[0053] The present invention addresses the need of a silver alloy that possesses antitarnish characteristics, along with enhanced strength and hardness, while preserving the colour and lustre of silver jewellery or items crafted from such alloys.

[0054] The pure silver (PS) as described in the entire specification means and relates to the silver content of 99.1 to 99.9%.

[0055] The conventional silver alloy (CS) as described in the entire specification means and relates to the silver of 50-99% and balance copper.

[0056] The inventors of the present invention surprisingly found that adding alloying elements Copper (Cu), Zinc (Zn), Aluminium (Al), Zirconium (Zr), Magnesium (Mg), Titanium (Ti) to silver of at least 50% by weight enhances the grain structure of silver alloys in their as-cast state, resulting in materials with increased strength, hardness, and superior tensile strength.

[0057] In an embodiment, the present invention discloses high strength, anti-tarnish silver alloy composition comprising of at least 50% by weight of silver with two or more alloying elements selected from; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0058] In an embodiment, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 50 to 99.9% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0059] In an embodiment, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 75% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.01 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0060] In an embodiment, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 80% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.01 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0061] In an embodiment, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 92.5% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0062] In another embodiment, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 92.5% by weight of silver with the alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; and the balance being copper. In yet another embodiment, the present invention discloses high strength, antitarnish silver alloy composition comprising 92.5% by weight of silver with alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; and the balance being copper.

[0063] In an embodiment, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 97% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0064] In another embodiment, the present invention discloses high strength, anti-tarnish silver alloy composition comprising 99% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

[0065] In another embodiment, the present invention provides a process for preparing high strength, anti-tarnish silver alloy composition comprising; a. Melting silver of at least 50wt.%, followed by adding two or more alloying elements selected from Copper, Aluminium, Zinc, Titanium, Zirconium, Magnesium in suitable amount in under vacuum ; and b. Cold rolling the silver alloy to obtain the desired alloy.

[0066] In accordance with the above, the process steps a) comprises of melting wherein silver of at least 50wt% was melted along with two or more alloying elements selected from Cu, Al, Zn, Ti, Zr, Mg.

[0067] The step b) comprises of cold rolling wherein Silver alloy obtained in step (a) were cold-rolled with reduction in size to obtain the desired alloy.

[0068] In an alternate embodiment, in the process of the present invention the alloying elements may be added through the master alloy comprising Ag-5Ti, Ag-5Zr. Cu- 5Zn, Al-5Mg, Cu-5Ti, Cu-2Ti, Cu-5Zr, Cu-5Zn, Zn-5Ti.

[0069] In an embodiment, the alloy obtained by the process of the present invention is optionally subjected to Passivation Heat Treatment (PHT) at a temperature ranging between 400 to 600°C for 30 to 120 minutes in a muffle furnace, followed by cooling to obtain the desired alloy.

[0070] Accordingly, the Passivation heat treatment which is carried out to achieve the oxide layer on the top surface of the alloy through the diffusion of alloying elements present in it.

[0071] The addition of master alloys during the process for preparing the silver alloy composition in the present invention eliminates hot tearing of the alloy during solidification, improves workability, eliminates scaling of the cold rolled sheets during annealing, avoids segregation and floating, and improves the grain structure. Further, the combined addition of alloying elements selected from aluminium (Al), zinc (Zn), titanium (Ti), zirconium (Zr), magnesium (Mg) through master alloy significantly enhances the properties of silver alloys. This is achieved through the formation of protective oxide layers, which contribute to solid solution strengthening, refinement of the grain structure, and ultimately result in increased tensile strength, hardness, and tarnish resistance of the cast silver alloy. Additionally, these elements facilitate precipitation strengthening in both cast and wrought forms of the alloy. These oxide layers serve to inhibit the interaction with sulphur, thereby significantly enhancing the resistance of the surface to tarnishing over extended periods.

[0072] In an embodiment, the selection of aluminium (Al) and zinc (Zn) into the silver alloy of the present invention was based on their lower free energy values and a Pilling Bedworth Ratio greater than 1 as depicted in Table 4 below. The aluminium and zinc low free energy values allows their addition into the silver alloy of the present invention with minimal energy, resulting in a stable, uniform alloy.

[0073] Aluminium increases tensile strength, whereas zinc enhances ductility, and magnesium contributes to solid solution strengthening, facilitating the shaping of the material without the risk of cracking.

[0074] In yet another embodiment, the Pilling Bedworth Ratio (PBR) for aluminium and zinc relative to silver is greater than 1, indicating that these elements expand during oxidation, forming a protective oxide layer that enhances the corrosion resistance and durability of the silver alloy while maintaining its appearance.

[0075] In an embodiment, the present invention provides a silver alloy composition that combines two or more elements selected from Copper, Aluminium, Zinc, Titanium, Zirconium, Magnesium, with silver content levels of 92.5 wt. %, 97 wt. %, and 99 wt. %, 75 wt. %, and 80 wt. % as detailed in Table 2. In another embodiment, micro concentrations of Ti, Zr, and Mg were added to prevent CuO formation from AI2O3, ZnO, TiCh, ZrCh, and MgO. This enhances tarnish resistance, as CuO is an unstable oxide in the presence of sulfur.

[0076] In yet another embodiment, the micro concentrations of Ti, Zr, and Mg may be added using Ag-5Ti, Ag-5Zr, and Al-5Mg master alloys.

[0077] In an embodiment, adding alloying elements such as titanium (Ti), zirconium (Zr), and magnesium (Mg) in alloy labelled below as TRIO and TR11 significantly strengthens the solid solution, refines grain structure, and improves tensile strength, hardness, and tarnish resistance, due to prevention of copper oxide formation as shown in Figure 1.

[0078] In an embodiment, the silver alloy of the present invention shows good grain refinement and thus high hardness which strengthens the soft silver and improves the range of its applications from low hardness articles to high.

[0079] In an embodiment, the silver alloy labelled as TR5 show aluminium (Al) particle segregation in the secondary phase, which is important as oxidized aluminium forms a protective layer against oxidation from copper oxide (CuO) and fire stains. The uniform distribution of zinc (Zn) particles oxidizes into zinc oxide (ZnO), providing a barrier against tarnishing from silver sulphide. Additionally, the presence of titanium (Ti) and zirconium (Zr) in the alloy labelled as TR9 enhances the performance of the silver alloy (See Figures 2 and 3).

[0080] In another embodiment, the silver alloy composition of the present invention shows resistance to sulfidization of the silver present on the surface of the alloy, even when exposed to an environment with elevated levels of Sulphur.

[0081] In an embodiment, the Vickers hardness of the present silver alloys having 92.5wt. % silver was compared with the conventional silver (CS). Accordingly, as shown in Fig 4a the silver alloys labelled as TR7, TR8, TR9, TRIO and TR 11 of the present invention showed Vickers hardness in the range of 105-132Hv, whereas, conventional silver alloy has the Vickers hardness of 97.3 Hv

[0082] In an embodiment, the Vickers hardness of various silver alloys was compared to corresponding conventional sterling silver slloy (CS3) and pure silver. As depicted in Fig 4b, the silver alloy labelled as TR12 (97.0wt.% silver alloy), TR13(97.0wt.% silver alloy), TR14(99.0 wt.% silver alloy) and TR15(99.0wt.% silver alloy) showed the Vickers hardness in the range of 72-82Hv, whereas, conventional sterling silver alloy (CS3) has the Vickers hardness of 97.3 Hv and Pure Silver has the hardness of 61.4 Hv as depicted in Figure 4c.

[0083] In an embodiment, the silver alloys of the present invention comprising silver in the range of 92.5 -99.0wt. % (TR7 to TRI 5) exhibited high hardness.

[0084] In an embodiment, the tensile strength, yield stress and elongation was measured and compared with conventional sterling silver alloy (CS3). The silver alloys labelled as TRI 2, TRI 3, TRI 4, TRI 5 exhibited high yield stress as compared with conventional silver or conventional sterling silver alloy (CS3) depicted in Fig 5 and Table 5 below.

[0085] In an embodiment, the silver alloys of the present invention show high tensile strength indicating that they resist plastic deformation across a broad spectrum of stress values and possess a high degree of durability against breakage.

[0086] In an embodiment, an accelerated tarnish test was performed to assess the tarnishing resistance of present silver alloys before passivation heat treatment and after passivation heat treatment as depicted in the figures Figs.6, 7, 8 and 9. As depicted in the said Figs, the present silver alloys exhibited improved resistance to tarnish as compared to corresponding conventional silver alloys. In an embodiment, the reflectance characteristics of silver alloy samples of the present invention were analysed across the UV-Vis spectrum before and after a tarnish test simulating environmental exposure. It was observed that the silver alloy samples resistant to tarnishing maintained high reflectance due to specific alloying elements that inhibit Ag2S layer formation. Consequently, these alloys preserve optical properties better than pure silver, making them suitable for applications requiring visual appeal and durability. Figure 10a and 10b and table 6 shows the essential role of material composition in influencing the reflectance characteristics of silver and its alloys when exposed to tarnishing agents. Accordingly, the silver alloy TR9, TRIO and TR12 show reflectance in the range of 38.0-43.0% as compared to pure silver (PS) which shows the reflectance of only 19.5%.

[0087] In an embodiment, the silver alloy composition of the present invention shows improved mechanical properties such as increased tensile strength, hardness, over the conventional silver alloy as well as improved tarnish resistance compared to pure silver.

[0088] In an embodiment, the present invention relates to the use of alloying metals selected from the group consisting of aluminium, zinc, zirconium, magnesium, titanium in appropriate proportions which are compatible with at least 50wt. % by weight of silver and other conventional alloying metal such as copper to impart solid solution strengthening / hardening of the alloy. The alloy of the present invention have high hardness, resistance, tensile strength suitable for making jewellery, coinage, silverwares and decorative items.

[0089] In an embodiment, the properties of the silver alloy composition of the present invention were compared with the conventional silver alloy composition with varying copper amount as described in table 1 below. Accordingly, as shown in table 1, the conventional silver alloy composition consists of silver in the range of 50% to 98% and the balance being copper. In another embodiment, the conventional silver alloy compositions with varying copper amount were subjected to a tarnish test both before and after passivation heat treatment to evaluate their tarnish resistance. The said silver alloy with varying copper amount were found to be prone to rapid tarnish formation, both before passivation and after the treatment, as illustrated in Figures 13(a) and 13(b).

[0090] In yet another embodiment, the Vickers hardness of the conventional silver alloy composition with varying copper amount was calculated and progressive increase in hardness was observed with rising copper content, with 30% copper exhibiting the highest value of 148.8 Hv as depicted in figure 14.

[0091] However, this increase in hardness correlates with reduced tarnish resistance, implying that an improvement in hardness adversely affects tarnish resistance with high-copper amount.

[0092] In an embodiment, the tensile strength, yield stress, and elongation of conventional silver alloy composition with varying copper amount were measured. The results indicated that increasing the copper content in the silver alloy led to a corresponding increase in tensile strength, as presented in Table 7 of Example 12 and illustrated in Figure 15 below.

[0093] In contrast, the enhancement in tensile strength was associated with a decrease in tarnish resistance, indicating that improved tensile strength adversely affects tarnish resistance with high-copper amount.

[0094] In an embodiment, the present invention advantageously provides a silver alloy composition with said alloying metals that exhibits anti-tarnish properties with high reflectance along with improved hardness, tensile strength and mechanical properties which the conventional sterling silver alloy, pure silver and the conventional silver alloy composition with varying copper amounts have failed to achieve as illustrated above and in the Figs.

[0095] The invention is described in greater detail with reference to the examples which are intended to be purely illustrative and not limited to the present invention and are provided in order to be favouring their being understood by a person skilled in the art.

[0096] Example 1: Conventional silver alloy composition

[0097] Table 1: Composition of Conventional Silver Alloy.

[0098] Example 2: Composition of the present silver alloy. Based on alloying elements added, the alloys were divided into four different sets.

[0099] Set 1 is Ag(92.5wt%)— Cu-Zn-Al alloy,

[0100] Set 2 is Ag(92.5wt%)— Cu-Zn-Al-Ti-Zr alloy, Set 3 is Ag(92.5wt%)-Cu-Zn-Al-Ti-Zr-Mg alloy,

[0101] Set 4 is Ag(97wt%, 99wt% , 75wt.% and 80wt.%)-Cu-Zn-Al-Ti-Zr-Mg alloy.

[0102] Table 2: Composition of Silver alloys prepared.

[0103] Example3: Composition of Master alloys, pure silver alloys and sterling silver alloys. Table.3 Composition of Master alloys, pure silver alloys and sterling silver alloys.

[0104] Example 4: Parameter for selecting the Alloying elements.

[0105] Table 4. The free energy G, kJ / mol and Pilling Bedworth Ratio of various elements.

[0106] Example 5: Preparation of Silver Alloy Composition: Different compositions of silver alloys were prepared and characterized. The compositions of the alloy in wt.% is shown in Table 2 above. Aluminium, Zinc, Titanium and Zirconium form a complete solid solution in Ag. Ti, Zr, and Mg were added in micro concentration using Ag-5Ti, Ag-5Zr, Al-5Mg master alloys. The Process comprises the following steps:

[0107] A. Melting - The silver alloys were prepared by melting silver of at least 50wt.%, followed by adding two or more alloying elements selected from Copper, Aluminium, Zinc, Titanium, Zirconium, Magnesium in vacuum Cold rolling - The Silver alloy obtained in step A was cold for the formation of the desired alloy.

[0108] Example 6: Passivation heat treatment

[0109] The cold rolled silver alloys obtained in example 4 may be subjected to passivation heat treatment.

[0110] Passivation heat treatment is carried out to achieve the oxide layer on the top surface of the alloy through the diffusion of alloying elements present in it. Accordingly, the cold rolled silver alloy is heated at 400 to 600°C for 30 to 120 minutes in an ambient atmosphere or by passing oxygen. After passivation treatment the alloy samples are cooled to yield the silver alloy of desired properties.

[0111] Example 7: Phase and Microstructural analysis-

[0112] The optical micrographs of samples are shown in Fig. l. There was a marginal grain refinement of silver when Ti, Zr, and Mg were added and no grain refinement was observed for TR3, TR4 and TR5 alloys. The grains were observed coarse for TR3, TR4 and TR5 alloys.

[0113] In Fig.2 and Fig.3, SEM elemental mapping after PHT was done on TR5 and TR9 alloys respectively. Both TR5 and TR9 alloys exhibited the segregation of Al particles on the secondary phase which lead to the oxidation of Al to form a protective layer and hence prevented formation of CuO and was thus fire stain. Zn particles were distributed evenly, which ensures that they get oxidized and will give ZnO over the entire surface, and further prevent sulphidization of silver. Even distribution of Ti and Zr also contributed to both tarnish resistance and grain refinement.

[0114] Example 8: Hardness analysis

[0115] Fig.4 shows the hardness measurements of 97wt.%, 99wt.%, and sterling silver alloys along with 99.99% pure silver. Pure silver is a soft metal, having a hardness of 61.4 Hv, which is not an appropriate hardness for a jewellery metal. As shown in Fig 4a the silver alloys labelled as TR7, TR8, TR9, TRIO and TR 11 showed Vickers hardness in the range of 105-132Hv, whereas, corresponding conventional silver including pure silver has the Vickers hardness of 61.4 Hv. As depicted in Fig 4b, the silver alloy labelled as TR12 (97wt.% silver alloy), TR13(97.0wt.% silver alloy), TR14(99wt.%silver alloy) and TR15(99.0wt.% silver alloy) showed the Vickers hardness in the range of 72-82Hv, whereas, corresponding conventional silver including pure silver has the Vickers hardness of 61.4 Hv.

[0116] Example 9: Study of tensile strength

[0117] Fig.5 shows tensile stress strain graphs of developed tarnish resistant alloys of the present invention. The samples were prepared by ASTM D638 type 5 standards. Slow strain rate of .0001m / s was applied on samples. All the samples showed improved tensile strength, high yield stress, and elongation than corresponding conventional silver and pure silver and they exhibited long range plastic deformation before breaking. The parameters were derived from the stress-strain graph illustrated in Fig. 5 and subsequently presented in Table 5 below. Table.5Parameters calculated from Stress strain graph shown in Fig.5

[0118] Example 10: Accelerated Tarnish test-

[0119] The accelerated tarnish test was carried out using a beaker containing a solution of 20ml Ammonium Sulphide with 20% concentration and 5ml of IN Sulphuric acid which produces sulphide gas (i.e., tarnishing environment). This tarnish test method was grouped under the category of accelerated tarnish test. If the sample could withstand this sulphide environment without tarnishing for a period of 10 minutes, then it was inferred that it could survive prolonged exposure in the normal atmosphere. Samples were placed above the beaker containing the solution with the support of a wire mesh. Finally, the entire arrangement was kept inside the desiccators with a closed lid. The test was performed for 2 minutes, 5 minutes, and 10 minutes. The results of the accelerated tarnish test conducted on all the samples are shown in Fig.6, 7, 8 and 9. The accelerated tarnish test for corresponding conventional silver and pure Silver (99.9%pure) was also conducted and compared with the newly developed alloy (Fig.6). The silver alloys of the present invention showed good tarnish resistance in comparison to the corresponding conventional silver and pure silver (99.9% Pure) after the passivation heat treatment.

[0120] Example 11: UV-Visible reflectance-

[0121] Fig.10 shows UV-Visible reflectance of samples before and after tarnish test. It was observed that all the samples have reflectance comparable to that of pure silver before the tarnish test. After tarnish test pure silver of 99.9% reduces its reflectance due to the Ag2S layer formed, but the developed tarnish resistant alloys maintained high reflectance. The percentage reflectance of the samples following the accelerated tarnish test at a wavelength of 700 nm was measured and presented in Table 6.

[0122] Table.6. The % Reflectance of samples after accelerated tarnish test, at 700nm wavelength.

[0123] Example 12: Study of tensile strength of the conventional silver alloy composition. Table 7: Parameters calculated from Stress strain graph shown in Fig.15

[0124] Conclusion: The silver alloy of the present invention has improved hardness along with improved anti -tarnish activity in terms of high reflectance on exposure to environmental conditions.

[0125] Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

Claims

im,1. A high strength, anti-tarnish silver alloy composition comprising of at least 50% by weight of silver with two or more alloying elements selected from; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

2. The high strength, anti-tarnish silver alloy composition as claimed in claim1, wherein said silver alloy comprises 50 to 99.9% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

3. The high strength, anti -tarnish silver alloy composition as claimed in claim1, wherein said silver alloy comprises 75% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.01 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

4. The high strength, anti-tarnish silver alloy composition as claimed in claim1, wherein said silver alloy comprises 80% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.01 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

5. The high strength, anti -tarnish silver alloy composition as claimed in claim 1, wherein said silver alloy comprises 92.5% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

6. The high strength, anti-tarnish silver alloy composition as claimed in claim5, wherein said silver alloy comprises 92.5% by weight of silver with alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; and the balance being copper7. The high strength, anti-tarnish silver alloy composition as claimed in claim5, wherein said silver alloy comprises 92.5% by weight of silver with alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; and the balance being copper.

8. The high strength, anti-tarnish silver alloy composition as claimed in claim1, wherein said silver alloy comprises 97% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

9. The high strength, anti-tarnish silver alloy composition as claimed in claim1, wherein said silver alloy comprises 99% by weight of silver with two or more alloying elements consisting of; i. 0.01 to 3.5% by weight of zinc; ii. 0.01 to 3.5% by weight of aluminium; iii. 0.01 to 1.5% by weight of titanium; iv. 0.005 to 1.5% by weight of zirconium; v. 0.001 to 1.5% by weight of magnesium; and the balance being copper.

10. A high strength, anti -tarnish silver alloy composition as claimed in claim 1 prepared by the process comprising; a. Melting silver of at least 50wt.%, followed by adding two or more alloying elements selected from Copper, Aluminium, Zinc,Titanium, Zirconium, Magnesium in suitable amount in vacuum; and b. Cold rolling the silver alloy to obtain the desired alloy.

11. The high strength, anti-tarnish silver alloy composition as claimed in claim 10, wherein the alloying elements may be introduced through the master alloys selected from Ag-5Ti, Ag-5Zr. Cu-5Zn, Al-5Mg, Cu-5Ti, Cu-2Ti, Cu-5Zr, Cu-5Zn, Zn-5Ti.

12. The high strength, anti-tarnish silver alloy composition as claimed in any one of the preceding claims 1 to 11 comprising (i) hardness in the range of 105-132Hv; (ii) high reflectance in the range of 38-42%; and with high tensile strength.

13. The high strength, anti -tarnish silver alloy composition as claimed in any one of the preceding claims for making jewellery, coinage, silverwares and decorative items.

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