Claims:1. A method for improving tarnish resistance property of silver, the method comprising:
melting and casting a silver slab comprising an alloy composition in weight percentage (wt%) of:
Germanium (Ge) of 0.45 wt% to 0.60 wt%;
Zinc (Zn) of 3.0 wt% to 7.8wt %;
Tin (Sn) of 0.5 wt% to 2.8 wt%;
Manganese (Mn) up to 0.2 wt% and
the balance being Silver (Ag) along with incidental elements;
homogenizing the silver slab at a temperature ranging from 420°C to 560°C for a time period of 15 to 30 minutes;
subjecting the silver slab to a forming process to form a silver foil of pre-determined thickness;
annealing the silver foil at a temperature ranging from 420°C to 560 °C for a time period of 5 to 30 minutes.

2. The method as claimed in claim 1, wherein the melting of the alloy composition is carried out in a graphite crucible with a lid placed in a furnace at a predetermined temperature before casting the silver slab.

3. The method as claimed in claim 2, wherein the predetermined temperature for melting the alloy composition ranges from 1000ºC to 1060ºC.

4. The method as claimed in claim 2, wherein the furnace is at least one of a vacuum induction furnace and a muffle furnace.

5. The method as claimed in claim 1, wherein the homogenization of the silver slab is carried out in at least one of a tubular furnace and muffle furnace under inert gas atmosphere.

6. The method as claimed in claim 5, wherein the inert gas is at least one of argon and cracked ammonia.

7. The method as claimed in claim 1, wherein the forming process is a cold rolling process.

8. The method as claimed in claim 1, wherein the pre-determined thickness of the silver foil ranges from 60 micron to 100 micron.

9. A silver alloy with improved tarnish resistance property comprising:

Germanium (Ge) of 0.45 wt% to 0.60 wt%;
Zinc (Zn) of 3.0 wt% to 7.8wt %;
Tin (Sn) of 0.5 wt% to 2.8 wt%;
Manganese (Mn) up to 0.2 wt% and
the balance being Silver (Ag) along with incidental elements.

10. The silver alloy as claimed in claim 9, wherein the silver alloy retains about 84% to 86% of the lustre, after undergoing an accelerated tarnish test for up to 6 minutes.

, Description:TECHNICAL FIELD

Present disclosure in general relates to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure relates to a silver alloy with improved tarnish resistance property. Further, embodiments of the disclosure disclose a method for improving the tarnish resistance property of silver.

BACKGROUND OF THE DISCLOSURE

Silver is a soft, white, lustrous transition metal that exhibits highest electrical conductivity, thermal conductivity and reflectivity of any metal. Silver is found all around the world and exists in earth’s crust in native form as well as in combined form. Silver metal has been valued as a precious metal and has been widely used in jewellery industry, electrical and electronics industry, medical industry, photography and in various other high technologies. Silver may be easily alloyed with other alloying elements to improve its properties based on requirement. As an example when copper is alloyed with silver, its strength and workability may be improved. Conventionally, though silver is a precious metal, it has a strong tendency to react with sulphur present in atmosphere thereby forming a black silver sulphide layer on surface overtime, leading to discolouring of silver. This is known as tarnishing of silver.
Researchers have been working on methods to improve the tarnish resistance property of silver for quite sometime and several techniques have been developed to improve tarnish resistance of silver. Micro alloying is one such method in which the alloy addition may be <0.5 wt% to boost the tarnish resistance and retain the malleability of silver, where micro alloying is performed by introducing at least one of aluminum (Al), antimony (Sb), cadmium (Cd), gallium (Ga), germanium (Ge), indium (In), lithium (Li), manganese (Mn), magnesium (Mg), Silicon (Si), tin (Sn), titanium (TI) and Zinc (Zn), into molten silver. But the effective tarnish resistance may be less by such methods of micro alloying. Most of the researchers have worked on sterling silver wherein, the alloying additions are done along with silver and copper which increases the strength of the silver alloy significantly. But it impacts the malleability of the silver. Additionally, most commonly used alloying elements added to silver to combat the tarnishing are Ge, Zn, In, Sn, Ga, Mn and Si. These alloying elements in addition to pure silver provides a synergistic effect against tarnishing. But again, choosing a right proportion of alloying elements addition may be another limitation.
The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior arts.
SUMMARY OF THE DISCLOSURE

One or more shortcomings of the prior art are overcome by method as disclosed and additional advantages are provided through the method as described in the present disclosure.

Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.

In one non-limiting embodiment of the present disclosure, a method for improving tarnish resistance property of silver is disclosed. The method includes steps of melting and casting a silver slab. The silver slab comprises an alloy composition in weight percentage (wt%) of: Germanium (Ge) of 0.45 wt% to 0.60 wt%, Zinc (Zn) of 3.0 wt% to 7.8 wt %, Tin (Sn) of 0.5 wt% to 2.8 wt%, Manganese (Mn) up to 0.2 wt% and the balance being Silver (Ag) along with incidental elements. Further to casting, the silver slab is subjected to homogenization treatment. The homogenization of the casted silver slab is carried out at a temperature ranging from 420°C to 560°C for a time period of 15 to 30 minutes. The silver slab is then subjected to a forming process to form a silver foil of pre-determined thickness. This silver foil is then subjected to annealing. The annealing is carried out at a temperature ranging from 420°C to 560 °C for a time period of 5 to 30 minutes. Thus a silver foil with an improved tarnish resistance property is obtained.

In an embodiment, the melting of the alloy composition is carried out in a graphite crucible with a lid placed in a furnace at a predetermined temperature before casting the silver slab.

In an embodiment, the predetermined temperature for melting the alloy composition ranges from 1000ºC to 1060ºC.

In an embodiment, the furnace is at least one of a vacuum induction furnace and a muffle furnace.

In an embodiment, homogenization of the silver slab is carried out in at least one of a tubular furnace and a muffle furnace under inert gas atmosphere.

In an embodiment, the inert gas is at least one of argon and cracked ammonia.

In an embodiment, the forming process is a cold rolling process.

In an embodiment, the pre-determined thickness of the silver foil ranges from 60 micron to 100 micron.

In another non-limiting embodiment, a silver alloy with improved tarnish resistance property is disclosed. The silver alloy comprises the composition in weight percentage (wt%) of: Germanium (Ge) of 0.45 wt% to 0.60 wt%, Zinc (Zn) of 3.0 wt% to 7.8wt %, Tin (Sn) of 0.5 wt% to 2.8 wt%, Manganese (Mn) up to 0.2 wt% and the balance being Silver (Ag) along with incidental elements.

In an embodiment, the silver alloy retains about 84% to 86% of the lustre in comparison to pure silver, after undergoing an accelerated tarnish test for up to 6 minutes.

It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined together to form a further embodiment of the disclosure.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

The novel features and characteristics of the disclosure are set forth in the appended description. The disclosure itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiments when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:

Figure.1 is a flowchart illustrating a method for improving tarnish resistance property of silver, according to an exemplary embodiment of the present disclosure.

Figures.2a, 2b and 2c illustrates a graphical representations of color value measurement of pure silver and silver alloy of the present disclosure in rolled and polished condition before tarnish test, according to an exemplary embodiment of the present disclosure.
Figures. 3a and 3b illustrates a graphical representations of the color value measurement of pure silver and silver alloy of the present disclosure in rolled and polished condition after tarnish test, according to an exemplary embodiment of the present disclosure.

Figures. 4a, 4b and 4c illustrates a graphical representations of the hardness measurement of the pure silver compared with silver alloy, in accordance with some embodiments of the present disclosure.

Figures. 5a and 5b illustrates a schematic view of samples of pure silver and silver alloy of the present disclosure after tarnish test, according to an exemplary embodiment of the present disclosure.

The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the methods illustrated herein may be employed without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION

The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent methods do not depart from the scope of the disclosure. The novel features which are believed to be characteristic of the disclosure, as to method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.

In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

While the disclosure is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the scope of the disclosure.

The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusions, such that a method that comprises a list of acts does not include only those acts but may include other acts not expressly listed or inherent to such method. In other words, one or more acts in a method proceeded by “comprises… a” does not, without more constraints, preclude the existence of other acts or additional acts in the method.
Embodiments of the present disclosure discloses a method for improving tarnish resistance property of silver and a silver alloy thereof. Silver has been valued as a precious metal and may be widely used in applications such as jewellery, manufacturing items such as cutlery, idols, glassware and like to impart aesthetics. Silver has a high reflectance property which allows it to be used for making mirrors as well. Additionally, silver also has high malleability which allows it to be rolled into thin foils. Further, silver has a strong tendency to react with sulphur present in atmosphere thereby forming a black silver sulphide layer on surface, leading to discolouring of silver called as tarnishing of silver. Many researchers have been working on methods to improve the tarnish resistance property of silver. Micro alloying is one of the methods in which the alloy addition may be <0.5 wt% to boost the tarnish resistance and retain the malleability of silver, where micro alloying is performed by introducing at least one of aluminum (Al), antimony (Sb), cadmium (Cd), gallium (Ga), germanium (Ge), indium (In), lithium (Li), manganese (Mn), magnesium (Mg), Silicon (Si), tin (Sn), titanium (TI) and Zinc (Zn) into molten silver. But the effective tarnish resistance may be reduced/mitigated by the method of micro alloying. Most of the researchers have been working on sterling silver wherein, the alloying addition are done along with silver and copper which increases the strength of the silver significantly. However, such techniques impacts the malleability of the silver. Most commonly used alloying elements added to silver to combat the tarnishing are Ge, Zn, In, Sn, Ga, Mn and Si. These alloying elements in addition to pure silver provides a synergistic effect against tarnishing. However, choosing a right proportion of alloying elements addition may be a limitation.

According to various embodiments of the disclosure, a method for improving tarnish resistance property of silver and a silver alloy is disclosed. The method comprising addition of alloying elements to pure silver. The alloying elements are 99.9% pure and are added in weight percentage (%) of: Germanium (Ge) of 0.45 wt% to 0.60 wt%, Zinc (Zn) of 3.0 wt% to 7.8 wt %, Tin (Sn) of 0.5 wt% to 2.8 wt%, Manganese (Mn) up to 0.2 wt% and the balance being Silver (Ag) along with incidental elements. Further, the method steps includes melting of the alloy composition in a graphite crucible. The graphite crucible containing the alloy composition is placed in a furnace and heated to a predetermined temperature to melt the alloy composition. The predetermined temperature for melting the alloy composition ranges from 1000ºC to 1060ºC. The melted alloy composition is then casted into a silver slab. In an embodiment, the silver slab is subjected to a homogenization treatment to impart compositional uniformity. The homogenization treatment of the silver slab is carried out in a furnace for a time period of 15 to 30 minutes at a temperature ranging from 420°C to 560°C. During homogenization, inert gas atmosphere is maintained. Further, the silver slab is subjected to a forming process to reduce the silver slab into thin silver foil of predetermined thickness. Lastly, the silver foil is subjected to annealing treatment at a predetermined temperature for a predetermined time period. The annealing treatment reduces the hardness of the silver foil making is malleable for manufacturing.

In another embodiment, a silver alloy with improved tarnish resistance property is disclosed which is produced by the method of the present disclosure. The silver alloy comprises an alloy composition in weight percentage (wt%) of: Germanium (Ge) of 0.45 wt% to 0.60 wt%, Zinc (Zn) of 3.0 wt% to 7.8wt %, Tin (Sn) of 0.5 wt% to 2.8 wt%, Manganese (Mn) up to 0.2 wt% and the balance being Silver (Ag) along with incidental elements.

Henceforth, the present disclosure is explained with the help of figures for a method of improving tarnish resistance property of silver. However, such exemplary embodiments should not be construed as limitations of the present disclosure since the method may be used on other types of alloy composition where such need arises. A person skilled in the art can envisage various such embodiments without deviating from scope of the present disclosure.

Figure.1 in an exemplary embodiment of the present disclosure which illustrates a flowchart depicting the method for improving tarnish resistance property of silver. The method is described with reference to the flowchart blocks and is as below. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks may be combined in any order to implement the method. Additionally, individual blocks may be deleted from the methods without departing from the scope of the subject matter described herein. The method is particularly applicable to silver alloys, and it may also be extended to other type of alloys as well.
The method for improving tarnish resistance property of silver, according to the present disclosure consists of melting an alloy composition and casting to a silver slab. The melting of the alloy composition is carried out in a graphite crucible closed with a lid, placed in a furnace and heated at the temperature ranging from 1000ºC to 1060ºC . The furnace may be at least one of a vacuum induction furnace and a muffle furnace. The composition of the alloys in weight percentage (wt%) includes Germanium (Ge) of 0.45 wt% to 0.60 wt%, Zinc (Zn) of 3.0 wt% to 7.8wt %, Tin (Sn) of 0.5 wt% to 2.8 wt%, Manganese (Mn) up to 0.2 wt% and the balance being Silver (Ag) along with incidental elements as indicated by block 100.

At block 101, the casted silver slab is subjected to a homogenization treatment. Homogenization helps to mitigate the segregation of alloying elements in silver alloy. Homogenization also removes the inevitably occurring chemical inhomogeneity to ensure microstructural homogeneity. In an embodiment, the homogenization treatment is carried out in a furnace at a temperature ranging from 420°C to 560°C for a time period of 15 to 30 minutes under inert atmosphere. In an embodiment, the silver slab is heated in a furnace which may be at least one of a tubular furnace and a muffle furnace. Further, the inert atmosphere and the inert gas may be at least one of argon and cracked ammonia.

Now at block 102, the silver slab after homogenising step further subjected to a forming process to reduce the silver slab into a silver foil. During forming process, the thickness of the silver foil may range from 60 micron to 100 micron. In an embodiment, the forming process is at least one of a cold rolling process. The cold rolling process may be a multi-pass rolling operation along with intermediate annealing to obtain the silver foil.
At block 103, the silver foil post forming is further subjected to annealing treatment. Annealing is a heat treatment process that changes the physical properties and sometimes also the chemical properties of a material. Annealing treatment helps in releasing mechanical stresses induced during the forming process which is a cold working process. Further, annealing also helps to improve ductility and malleability of silver alloy thereby making subsequent operations easier. In an embodiment, the annealing treatment is carried out at predetermined temperature which may range from 420°C to 560°C for a time period of 5 to 30 minutes. Thus a silver foil is obtained which shows improved tarnish resistance property in comparison to pure silver as indicated by block 104.
Example:

Further embodiments of the present disclosure will be now described with an example of a particular composition of the silver alloy. Experiments have been carried out for a specific composition of the silver alloy formed by using the method of the present disclosure. Results have been compared on various fronts to showcase the improvement of tarnish resistance property the silver alloy compared to pure silver. The composition of the silver alloy for which the tests are carried out is as shown in below table 1.
S. No Ge (%) Zn (%) Sn (%) Mn (%) Ag(%)
Silver Alloy 0.5 4.0 1.0 0.2 94.3

Table - 1

In an embodiment of the present disclosure, various experiments have been carried out on pure silver and the silver alloy composition as mentioned in Table – 1. For conducting the experiment for measuring the tarnish resistance property and hardness value, the specimen of pre-determined dimensions and thickness is prepared by the method of the present disclosure. After the experiments, test results have been compared. In subsequent paragraphs of the disclosure, the method of carrying out the experiment and test results in conjunction with the figures is disclosed.

In an embodiment, the pure silver may be 99.9% pure silver.

Referring to Figures. 2a, 2b and 2c which shows a graphical representation of the color value measurement of pure silver and silver alloy of the present disclosure in rolled and polished condition before tarnish test. In an embodiment, the color measurement is carried out by a Spectrophotometer, where the spectrophotometer is a color measurement device which is used to capture the color values of the silver and silver alloy. When a color is expressed in CIELAB, L* defines lightness (100 is white, 0 is black), a* denotes the red/green value (positive value of a is red, negative value of a is green) and b* the yellow/blue value (positive value of b is yellow, negative value of b is blue). In the illustrative embodiment of Figure 2a, L* comparison between the pure silver and silver alloy which gives information about lightness part of the color, where higher the value, higher lightness of the sample. Silver alloy lightness value is close to pure silver and difference in value is due to effect of addition of the alloy. Similarly Figures 2b and 2c shows respectively the a* and b* value comparison between the silver and silver alloy.

Referring to Figures. 3a and 3b which shows a graphical representation of the color value measurement of pure silver and silver alloy of the present disclosure in rolled and polished condition after tarnish test. It is evident from figure 3a that pure silver is able to retain only about 54% to 56% lustre/shine property after 6 minutes of accelerated tarnish test. Whereas figure 3b shows that the silver alloy of the present disclosure is able to retain about 84% to 86% lustre/shine property even after 6 minutes of accelerated tarnish test. This comparison shows that the silver alloy is much more resistant to tarnishing as compared to pure silver or non-alloyed silver.

Experimental Steps/procedure:
In an embodiment, the tarnish test is carried out on a rolled pure silver and silver alloy samples. The pure silver and silver alloy are cut into small samples of predetermined dimensions and thickness. These samples were polished to a mirror finish using a buffer wheel. The as prepared samples were exposed to the sulfur containing atmosphere. This tarnish test method is grouped under the category of accelerated tarnish test. If the sample can withstand in this sulphide environment without tarnishing for a period of 6 minutes then the sample can retain its lustre/shine properties over prolonged exposure in normal atmosphere. Both pure silver and silver alloy samples are subjected to the tarnish test and the results are shown in figure 2 and figure 3.
Referring to figure 4a, 4b and 4c which shows a graphical representation of the hardness measurement of the pure silver compared with silver alloy of the present disclosure in as cast, rolled and annealed condition. It is evident from figures 4a, 4b and 4c that the silver alloy of present disclosure shows a higher hardness value as compared to pure silver in a similar cast, rolled and annealed condition. Thus, the silver alloy has higher tensile strength and wear resistance properties compared to pure silver.

In an embodiment, the hardness measurement is done using vickersTM hardness tester with a load of 100 g for 10 sec of dwell time.

Now referring to Figure. 5a and 5b, which shows a schematic view of pure silver and silver alloy of the present disclosure after tarnish test. It is evident from the figure that pure silver acquired a black surface layer after tarnish test whereas the silver alloy retains its lustre/shine property up to 84% to 86%.

It should be understood that the experiments are carried out for a particular composition of the silver alloy and the results brought out in the previous paragraphs are for the composition shown in Table – 1. However, this composition should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the silver alloy as well.

In an embodiment, the silver alloy of the present disclosure retains its lustre/shine property even after prolonged exposure to sulphide environment without tarnishing in comparison with pure silver.

In an embodiment, the alloying method of the present disclosure to mitigate tarnishing is cost effective.

In an embodiment, the silver alloy retains the properties of silver such as mirror finish, conductivity properties, malleability and like.
Equivalents:

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope being indicated by the following claims.