Claims:WE CLAIM:
1. A gold alloy comprising gold (Au) ranging from about 91.6 wt% to 99.9 wt%, titanium (Ti) ranging from about 0.1 wt% to 0.9 wt%, and optionally one additional metal ranging from about 7.3 wt% to 8.3 wt%, wherein the Ti is capable of grain refinement and forming nano-sized precipitate with Au.

2. The gold alloy as claimed in claim 1, wherein said alloy is a 22 Karat (22KT) gold alloy comprising gold (Au) ranging from about 91.6 wt% to 91.8 wt%, titanium (Ti) ranging from about 0.1 wt% to 0.9 wt%, and at least one additional metal ranging from about 7.3 wt% to 8.3 wt%, wherein the Ti is capable of grain refinement and forming nano-sized precipitate with Au.

3. The gold alloy as claimed in claim 1, wherein said alloy is a 24 Karat (24KT) gold alloy comprising gold (Au) ranging from about 99.1 wt.% to 99.9 wt%, titanium (Ti) ranging from about 0.9 wt% to 0.1 wt%, wherein the Ti is capable of grain refinement and forming nano-size precipitate with Au.

4. The gold alloy as claimed in claim 1, wherein the nano-sized precipitate of Ti and Au is formed in the Au matrix of the gold alloy; and wherein the nano-sized precipitate is Au4Ti.

5. The gold alloy as claimed in claim 1, wherein the additional metal is selected from a group comprising copper (Cu), silver (Ag), zinc (Zn), nickel (Ni) and combinations thereof.

6. The gold alloy as claimed in any of the claims 1-5, wherein the 22 Karat gold alloy or 24 Karat gold alloy comprises nano-sized precipitates of Au4Ti in the Au matrix.

7. The gold alloy as claimed in claim 1, claim 2 or claim 5, wherein the 22 Karat gold alloy comprises Au at 91.7 wt%, Ti at 0.5 wt%, copper at 5.8 wt%, and silver at 2.0 wt%.

8. The gold alloy as claimed in any of the claims 1-7, wherein the alloy possesses a hardness of about 120 HV to 200 HV, preferably 140 to 185 HV on the Vickers scale.

9. A method of preparing the gold alloy as claimed in claim 1, the method comprising:
a) combining a granulated master alloy with a molten gold alloy to obtain a gold alloy containing Ti,
wherein the master alloy comprises copper (Cu) or gold (Au) ranging from about 91.7 wt% to 99.9 wt%, and titanium (Ti) ranging from about 0.1 wt% to 8.3 wt%;
b) refining grain size of the gold alloy obtained in step (a) by heterogeneous nucleation;
c) casting the grain refined gold alloy; and
d) carrying out precipitation strengthening of the casted and grain refined gold alloy to obtain the gold alloy as claimed in claim 1.

10. The method of preparing the gold alloy as claimed in claim 9, wherein the gold alloy is a 22KT gold alloy or a 24KT gold alloy.

11. The method of preparing the gold alloy as claimed in claim 9, wherein the master alloy is Cu-xTi or Au-xTi, with ‘x’ ranging from 0.1 to 8.3; wherein the step a) comprises addition of Ti to the molten gold alloy through said master alloy; and wherein the precipitation strengthening of step d) results in in-situ formation of nano-sized precipitate of Au4Ti in the Au matrix.

12. The method of preparing gold alloy as claimed in any of the claims 9-11, wherein the master alloy is combined in the range of about 0.1 wt to 8.3 wt % with the molten gold alloy ranging from about 91.7 wt % to 99.9 wt % to obtain the 22 Karat or 24 Karat gold alloys containing Ti.

13. The method of preparing the gold alloy as claimed in any of the claims 9-12, wherein the step a) is carried out at a melting temperature ranging from about 1100 °C to 1400 °C.

14. The method of preparing the gold alloy as claimed in any of the claims 9-13, wherein the heterogeneous nucleation comprises Au4Ti of the gold alloy to readily undergo peritectic reaction resulting in a fine equiaxed grain structure with a grain size of about 60 µm.

15. The method of preparing the gold alloy as claimed in any of the claims 9-14, wherein the casting is performed at a temperature ranging from about 1100 °C to 1400 °C.

16. The method of preparing the gold alloy as claimed in any of the claims 9-15, wherein the precipitation strengthening comprises steps of solutionising and age hardening, wherein the grain refined and casted gold alloy is solutionised at a temperature ranging from about 700 °C to 900 °C for a time-period of about 10 minutes to 60 minutes, and the solutionised gold alloy is subjected to age hardening at a temperature ranging from about from 450°C to 650°C for a time-period of about 15 minutes to 60 minutes.

17. The method of preparing the gold alloy as claimed in any of the claims 9-16, wherein the solutionised gold alloy is subjected to rapid quenching in water before subjecting to age hardening.

18. A master alloy for adding titanium (Ti) to gold to obtain the gold alloy as claimed in claim 1, said master alloy comprising copper (Cu) or gold (Au) ranging from about 91.7 wt% to 99.9 wt%, and titanium (Ti) ranging from about 0.1 wt% to 8.3 wt%.

19. The master alloy as claimed in claim 18, wherein the master alloy is Cu-xTi or Au-xTi, with ‘x’ ranging from 0.1 to 8.3.

20. An article comprising the gold alloy as claimed in any of the claims 1-8, or the gold alloy prepared according to the method of any of the claims 9-17.

21. The article as claimed in claim 20, wherein the article is selected from a group comprising jewelry, coinage, watch case, and articles for dental application, metallurgy application, electronic application, plasmonic application and functional coatings or any combinations thereof.
, Description:TECHNICAL FIELD
The present invention relates to the field of metallurgy and material sciences. More particularly, the present disclosure relates to alloy development and provides gold material with improved hardness and method of preparation thereof. Advantageously, the present invention ensures that the developed gold material has improved hardness and also retains its luster, color and purity.

BACKGROUND OF THE DISCLOSURE
The value of gold was rooted in its relative rarity, easy handling and minting, easy smelting and fabrication, resistance to corrosion and other chemical reactions (nobility) and its distinctive color. As a precious metal, gold has been used for coinage, jewelry, and other arts from a long time. The purity of gold present in the alloyed system is termed as Karat/Carat. Pure gold (24-Karat or 24KT) is very soft and cannot be used for making ornament and therefore needs alloying to achieve reasonable hardness. However, alloying lessens the purity of gold and also lightens its original rich yellow color.

There is a huge market for 22 karat (22KT) gold ornament because of its color, which is near to pure gold (24KT). 22KT gold alloy has moderate strength, high ductility, malleability and has good tarnish resistance. On the other hand, the hardness of 22KT gold alloy is lower which is a constraint in applications including manufacturing of light weight jewelry. Accordingly, 18KT gold alloy is preferred for light weight jewelry/studded jewelry which has higher strength and hardness to hold the precious stones. However, needless to say, the purity is a disadvantage since 18KT gold alloy has lesser gold purity compared to 22KT gold.

The main disadvantage of gold jewelry, especially 22KT gold, is its low hardness even after alloying. Copper (Cu) is usually alloyed to increase the hardness of the gold by solid solution strengthening and Silver (Ag) is added to reduce the red colour imparted by Copper. However, increasing the concentration of Copper and Silver increases the strength but decreases the karatage of gold (purity). In addition to Cu and Ag, several other alloying elements have been tried for strengthening the 22KT namely Zn, Ni, Pt, Pd, Mn, Cr and rare earth metals. Nonetheless, elements such as Ni are toxic and may cause irritation when the jewelry comes in contact with the skin. Further, other elements such as Zn, Pt, Pd, Mn and Cr lighten the golden yellow colour of the material and decrease the tarnish resistance. Rare earth elements like Gd and Ce tries to strengthen the pure gold by different strengthening mechanisms such as grain size, solid solution, precipitation and dispersion strengthening. However, the segregation of rare earth oxide in the inter-dendritic region of the castings causes poor deformability of the gold. In addition, employment of rare earth metals as alloying components is not a cost-effective approach/solution.

Accordingly, it will be of great importance and lucrative to the industry if gold material, especially 22KT gold and 24KT gold having improved hardness but also retaining the purity, colour and luster is provided. Further, it will be highly beneficial if improving/enhancing the hardness of gold is carried out in a simple, efficient and cost-effective manner without requiring work hardening procedures in applications such as manufacturing of light weight jewelries and studded jewelries. The present disclosure tries to address the said existing need of the art.

OBJECTS OF THE DISCLOSURE
The primary object of the present disclosure is to enhance the hardness of gold alloys without affecting the parameters including, but not limiting to purity, colour, workability, tarnish resistance, wear and corrosion resistance.

It is another object of the present disclosure to enhance the hardness/strength of gold alloys by designing a method involving grain size refinement and age hardening (also known as precipitation hardening).

It is another object of the present disclosure to provide metal alloys/master alloys that dissolve properly in gold without affecting deformation behavior caused due to formation of hard aggregates/agglomerates.

It is another object of the present disclosure to achieve addition of titanium (Ti) in the gold alloy to enhance the hardness.

It is another object of the present disclosure to achieve nano-sized precipitates of Au4Ti in the Au matrix of gold alloy.

It is another object of the present disclosure to provide nucleation of Au4Ti particles to a-Au through peritectic reaction to achieve less agglomeration.

It is another object of the present disclosure to provide Au4Ti in the blocky shape particles of average 10µm size having body centered tetragonal (BCT) structure.

It is another object of the present disclosure to provide fine equiaxed grain structure that increases the hardness and strength of the gold alloy, preferably 22KT and 24KT gold alloys, through heterogeneous nucleation.

It is another object of the present invention to add additional metal (alloying elements) selected from a group comprising but limiting to copper (Cu), silver (Ag), Zinc (Zn) and nickel (Ni) to the gold, especially 22KT gold to increase the hardness/strength.

It is another object of the present disclosure to provide simple and efficient method for grain refinement.

It is another object of the present disclosure to achieve cost effective and high grain refining efficiency in comparison to simple addition/admixing of elemental titanium (Ti) or other refractory, rare earth elements which does not provide the desired hardness/strength along with retaining parameters such as purity, colour, workability, tarnish resistance, wear and corrosion resistance.

It is another object of the present disclosure to eliminate surface peeling effect due to cold rolling of gold alloy, due to fine grain structure.

It is another object of the present disclosure to show good age hardening behavior at both cast and deformed/extruded conditions even at lower addition level of titanium (Ti).

It is another object of the present disclosure to attain biocompatibility, tarnish resistance and non-toxicity of gold alloy by addition of Ti.

It is another object of the present disclosure to meet light weight jewellery and studded 22KT or 24KT gold jewellery constraint of lower hardness.

It is another object of the present disclosure to enhance the strength of gold alloy, preferably 22KT or 24KT gold alloy, to hold precious stones in studded jewelry.

STATEMENT OF THE DISCLOSURE
The present disclosure relates to a gold alloy comprising gold (Au) ranging from about 91.6 wt% to 99.9 wt%, titanium (Ti) ranging from about 0.1 wt% to 0.9 wt%, and optionally one additional metal ranging from about 7.3 wt% to 8.3 wt%, wherein the Ti is capable of grain refinement and forming nano-sized precipitate with Au.

The present disclosure further relates to a method of preparing the gold alloy as described above, the method comprising:
a) combining a granulated master alloy with a molten gold alloy to obtain a gold alloy containing Ti,
wherein the master alloy comprises copper (Cu) or gold (Au) ranging from about 91.7 wt% to 99.9 wt%, and titanium (Ti) ranging from about 0.1 wt% to 8.3 wt%;
b) refining grain size of the gold alloy obtained in step (a) by heterogeneous nucleation;
c) casting the grain refined gold alloy; and
d) carrying out precipitation strengthening of the casted and grain refined gold alloy to obtain the gold alloy as described above.

The present disclosure also provides a master alloy for adding titanium (Ti) to gold to obtain the gold alloy as described above, said master alloy comprising copper (Cu) or gold (Au) ranging from about 91.7 wt% to 99.9 wt%, and titanium (Ti) ranging from about 0.1 wt% to 8.3 wt%.

The present disclosure further relates to articles comprising the gold alloy as described above, or the gold alloy prepared according to the method as described above.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 depicts a flow-chart indicating the method steps of preparing the gold alloy of the present disclosure.

Figure 2 depicts the optical micrograph of grain refined and extruded 22KT gold product which shows fine equiaxed grain structure/morphology obtained according to the present method.

Figure 3 depicts the articles (bangles) manufactured by 22KT gold alloy of the present disclosure.

Figure 4 depicts the TEM bright field image of 22 KT gold alloy of the present disclosure showing nano scale precipitates of Au4Ti in the Au matrix at peak aging time (30 minutes) during age hardening treatment.

Figure 5 depicts the SEM photomicrograph of Au-6wt.% Ti master alloy showing ?-Au dendrites in stoichiometric Au4Ti matrix.

Figure 6 depicts the EDX spectrum (mapping on the complete region shown in Fig. 4) taken on Au-6wt.% Ti master alloy which shows Ti and Au peaks confirming the presence of ?-Au and stoichiometric Au4Ti.

DETAILED DESCRIPTION OF THE DISCLOSURE
As used herein, the phrase ‘molten gold alloy’ or ‘gold melt’ employed in the method of the present disclosure refers to gold alloy containing gold and optional alloying metals selected from a group comprising but not limiting to copper (Cu), silver (Ag), zinc (Zn), nickel (Ni) and combinations thereof. Further, said ‘molten gold alloy’ or ‘gold melt’ may or may not contain titanium (Ti). Preferably, said ‘molten gold alloy’ or ‘gold melt’ does not contain any Ti and the Ti is introduced by the method described in the present disclosure. In an exemplary embodiment, ‘molten gold alloy’ or ‘gold melt’ employed in the method of the present disclosure refers to 22 Karat or 24 Karat gold alloy comprising features/components as described above.

As used herein, the phrase ‘nano-sized precipitates of titanium and gold’ or ‘nano-sized precipitates of Au4Ti refers that the titanium and gold / Au4Ti particles are intermetallic particles and form nano-sized precipitates in the gold (Au) matrix of the gold alloy of the present disclosure. In an exemplary embodiment of the present disclosure, ‘nano-sized precipitates of titanium and gold’ or ‘nano-sized precipitates of Au4Ti refers that the titanium and gold / Au4Ti particles are intermetallic particles and form nano-sized precipitates in the gold (Au) matrix of the 22KT or 24KT gold alloy.

As used herein, the phrase ‘equiaxed grain size/structure’ refers to equiaxed grains/crystals that have axes of approximately the same length. Further, equiaxed grain structure/morphology is an indication of increased hardness and strength of the 22KT or 24KT gold alloy described in the present disclosure.

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. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” or “containing” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The present invention tries to address the existing concern of gold materials, especially 22KT or 24KT gold alloys which do not possess enhanced hardness, and even if efforts are directed towards achieving the same, the same is at the cost of compromising on characteristics such as purity, colour and luster of the gold.

Accordingly, the present disclosure primarily relates to a gold alloy product comprising enhanced hardness and strength. Further, the present disclosure provides grain refinement and precipitation strengthening based methods for preparing gold material to achieve the product having enhanced hardness and strength.

In an embodiment, while the hardness of the gold alloy of the present disclosure is improved, the characteristics such as purity, color, workability, tarnish resistance, wear and corrosion resistance is retained.

According to a primary aspect of the present invention, a gold alloy is provided wherein the enhanced hardness is provided by the addition of titanium (Ti).

Particularly, the present disclosure provides a gold alloy comprising gold (Au) ranging from about 91.6 wt% to 99.9 wt%, titanium (Ti) ranging from about 0.1 wt% to 0.9 wt%, and optionally one additional metal ranging from about 7.3 wt% to 8.3 wt%, wherein the Ti is capable of grain refinement and forming nano-sized precipitate with Au.

In an embodiment, the alloy of the present disclosure is a 22KT or 24KT gold alloy and comprises titanium in the range of 0.1 wt% to 0.9 wt %.

In another embodiment of the present disclosure, the titanium is present as nano-sized precipitates of titanium and gold in the Au matrix of the gold alloy. In an exemplary embodiment of the present disclosure, the titanium is present as nano-sized precipitates of titanium and gold in the Au matrix of the 22KT or 24KT gold alloy.

In yet another embodiment, the gold alloy of the present disclosure comprises nano-sized precipitates of Au4Ti in the Au matrix. In an exemplary embodiment of the present disclosure, the 22KT or 24KT gold alloy comprises nano-sized precipitates of Au4Ti in the Au matrix.

In still another embodiment of the present disclosure, the Ti is added to the gold alloy through a master alloy.

In still another embodiment of the present disclosure, the Ti is added to the 22KT or 24KT gold alloy through CuxTi intermetallic particles using master alloy Cu-xTi (x = 0.1 to 8.3).

In still another embodiment of the present disclosure, the Ti is added to the 22KT or 24KT gold alloy through AuxTi intermetallic particles using master alloy Au-xTi (x = 0.1 to 8.3).

In an embodiment, the 22 KT gold alloy product of the present disclosure comprises gold (Au) ranging from about 91.6 wt% to 91.8 wt%, titanium (Ti) ranging from about 0.1 wt% to 0.9 wt% and at least one additional metal ranging from about 7.3 wt% to 8.3 wt%, wherein the Ti is capable of grain refinement and forms nano-sized precipitates with Au in the Au matrix of the 22KT gold alloy.

In another embodiment, other alloying metals (additional metals) including but not limiting to copper (Cu), silver (Ag), zinc (Zn) and/or nickel (Ni) are present in the 22 KT gold alloy product such that the total wt.% of the gold (Au), titanium (Ti) and additional metals amount to 100.

In an exemplary embodiment, the 22 Karat gold alloy comprises Au at 91.7 wt%, Ti at 0.5 wt%, copper at 5.8 wt%, and silver at 2.0 wt%.

In an embodiment, the 24KT gold alloy product of the present disclosure comprises gold (Au) ranging from about 99.1 wt.% to 99.9 wt%, titanium (Ti) ranging from about 0.9 wt% to 0.1 wt%, wherein the Ti is capable of grain refinement and forms nano-size precipitate with Au in the Au matrix of the 24KT gold alloy.

In another embodiment, the gold alloy product of the present disclosure is not prepared by admixing or simple addition of the alloying metals (including Ti) to the gold. Instead, the gold alloy is formed from the method which comprises Ti addition through master alloy, grain refinement and precipitation strengthening procedures as described herein, and results in said gold alloy comprising nano-sized precipitates of Au4Ti.

In an exemplary embodiment, the gold alloy, preferably the 22KT gold alloy or the 24KT gold alloy of the present disclosure possesses a hardness of about 120 HV to 200 HV on the Vickers scale. In another embodiment, the 22KT gold alloy or the 24KT gold alloy of the present disclosure possesses a hardness of about 140 to 180 HV on the Vickers scale.

The present disclosure further provides a method of preparing a gold alloy comprising nano-sized precipitates of Au4Ti as described above, said method comprising:
a) combining a granulated master alloy with a molten gold alloy to obtain a gold alloy containing Ti,
wherein the master alloy comprises copper (Cu) or gold (Au) ranging from about 91.7 wt% to 99.9 wt%, and titanium (Ti) ranging from about 0.1 wt% to 8.3 wt%;
b) refining grain size of the gold alloy obtained in step (a) by heterogeneous nucleation;
c) casting the grain refined gold alloy; and
d) carrying out precipitation strengthening of the casted and grain refined gold alloy to obtain the gold alloy as described above.

In an embodiment of the present disclosure, the preparation of gold alloy comprising nano-sized precipitates of Au4Ti is characterized by the following steps:
producing a master alloy comprising Ti;
granulation of the master alloy;
alloying/combining the master alloy with a gold alloy and grain size refinement;
casting; and
precipitation strengthening of the casted and grain refined gold alloy.

In an embodiment of the present disclosure, the above described method is employed to prepare a 22KT gold alloy or a 24KT gold alloy.

In an embodiment of the present disclosure, the above method employs a master alloy Cu-xTi or Au-xTi, with ‘x’ ranging from 0.1 to 8.3.

In another embodiment of the present disclosure, the step of alloying/combining the master alloy with a gold alloy comprises addition of Ti to the molten gold alloy through said master alloy.

In yet another embodiment of the present disclosure, the master alloy is alloyed/combined in the range of about 0.1 wt to 8.3 wt % with the molten gold alloy ranging from about 91.7 wt % to 99.9 wt % to obtain the 22KT or 24KT gold alloys containing Ti.

In still another embodiment of the present disclosure, the step of alloying/combining the master alloy with a gold alloy is carried out at a melting temperature ranging from about 1100 °C to 1400 °C.

In still another embodiment of the present disclosure, the precipitation strengthening in the above described method results in in-situ formation of nano-sized precipitate of Au4Ti in the Au matrix.

In an embodiment of the present disclosure, the heterogeneous nucleation comprises Au4Ti of the gold alloy to readily undergo peritectic reaction resulting in a fine equiaxed grain structure with a grain size of about 60 µm.

In another embodiment of the present disclosure, the casting step is performed at a temperature ranging from about 1100 °C to 1400 °C.

In yet another embodiment of the present disclosure, the precipitation strengthening in the above described method comprises steps of solutionising and age hardening, wherein the grain refined and casted gold alloy is solutionised at a temperature ranging from about 700 °C to 900 °C for a time-period of about 10 minutes to 60 minutes, and the solutionised gold alloy is subjected to age hardening at a temperature ranging from about from 450°C to 650°C for a time-period of about 15 minutes to 60 minutes.

In another embodiment of the present disclosure, the solutionised gold alloy is subjected to rapid quenching in water before subjecting to age hardening.

In an exemplary embodiment of the present disclosure, the method of preparing gold alloy, particularly 22KT gold alloy, is detailed as below:

(a) Producing a master alloy comprising Ti:
The gold-based, Au-xTi (x = 0.1 to 8.3) and copper-based, Cu-xTi (x = 0.1 to 8.3) master alloys are prepared in arc melting furnace (model AM-133 supplied by Vacuum System Technologies and Services Ltd). The maximum vacuum level achieved is 1x 10-5bar. Melting is done at 0.39 - 1.5 bar positive argon pressure. The melting temperature is set to 1800oC. Upon removing the arc, the melt solidifies rapidly in the water cooled copper hearth. To achieve homogeneity in the casting, the melting and solidification cycle is repeated several times for the same alloy.

(b) Granulation of the master alloy:
The master alloys (Au-xTi and Cu-xTi) prepared are completely brittle and can be easily granulated into required powder size (0.5 to 2 mm) using a Fritsch pulverizer. The experiments in the present disclosure also involve employing abrasively cut pieces (~10mm sized) of the master alloys. Melt granulation of Cu-xTi master alloy can also be employed. During melt granulation, the melt is protected using argon and the melt is transferred rapidly into the cold running water for granulation.

(c) Combining the master alloy with 22 KT gold alloy and grain size refinement:
Grain refinement experiment is carried out in the induction melting furnace with argon as cover gas. The grain refinement is conducted at a temperature between 1250oC-1400 oC. Pure gold (99.99%) is melted along with Cu and Ag to make 22K gold. For each experiment, about 100g of alloy can be chosen. The granulated master alloy of desired wt.% is added into the melt, once it reaches the desired temperature. In some cases, particularly for preparation of 22KT gold alloy, the master alloy (granulates/cut pieces) is added together with Au, Cu and Ag, and melted. For homogeneity, the melt is kept for 5 minutes and inductively stirred. The melt is then poured into a cylindrical graphite mould. The cast yield, the quality of the casting, the microstructure (grain structure) of the casting is evaluated and compared with the regular 22K (un-grain refined).

(d) Casting:
The melt is cast using three different types of mould: (i) rectangular graphite mould - a long face opened to atmosphere, (ii) rectangular graphite mould – short face opened to atmosphere, (iii) cylindrical tapered mould. Cylindrical mould gave better yield and smooth surface finish with minimum piping.

(e) Precipitation strengthening:
The casting obtained is annealed and quenched for homogenization and softening at about 700-9000C for about 10-60 minutes. The alloying is then cold-rolled or wire drawn for making it into sheets or wire. During cold-working, intermittent annealing is carried out for softening. At this stage, hand-made jewelries are produced. Before final polishing, the jewelry products are solutionized at about 700-900°C for about 10-60 minutes followed by rapid quenching in water. The samples are then age hardened at about 500-600 °C, for about 30 minutes. The age hardening parameter set, is the peak aging condition where the alloy reaches its highest hardness. The parameters are optimized by studying the aging kinetics of the alloy by varying the time and temperature of the treatment.

In another exemplary embodiment of the present disclosure, the method of preparing gold alloy comprising nano-sized precipitates of Au4Ti is depicted in Figure 1. The flow diagram (Figure 1) illustrates the process which involves mater alloy preparation, grain refinement and precipitation strengthening of the gold material as some of the important steps according to the present invention. The present method of preparing gold alloy (eg. 22KT gold alloy or 24KT gold alloy) strengthens the hardness of the gold alloy without affecting its colour, lusture and purity.

According to the present disclosure, a 22KT gold melt or 24KT gold melt is prepared by the addition of Ti through Au-xTi master alloy, Cu-xTi master alloy, or a combination of Au-xTi and Cu-xTi master alloys. Said inclusion of Ti through master alloys Au-xTi and/or Cu-xTi provide the benefits of being biocompatible and non-toxic unlike direct addition/admixing of Ti in molten metal which requires expensive vacuum/inert atmosphere induction melting system. Thus, the approach of the present method not only results in achieving nano-sized precipitates of Au4Ti in the final 22KT or 24KT gold melt gold alloy, but the method is also cost-effective and safe/non-toxic unlike direct mixing of Ti. Master alloys namely binary Cu-xTi of different weight percent (x= 0.1 to 8.3 wt %) and Au–xTi of different weight percent (x= 0.1 to 8.3 wt %) are employed to form Au4Ti intermetallic particles of different volume percent in the Au matrix. The Au-xTi/ Cu-xTi metal alloy is added as granules as they readily mix, at melting temperature of about 1100°C to 1400°C since the density of Au4Ti is 16.45g/cc which is near to the density of liquid gold (17.31g/cc). On holding for short period after adding the master alloy to molten gold, the intermetallic particles decompose into the melt to release Ti.

In an embodiment, during alloy melting, the master alloy is added into the molten gold and the Au4Ti intermetallic particles act as heterogeneous nucleating sites. The Au4Ti particles nucleate a-Au through peritectic reaction in the grain refinement process. Au4Ti are blocky shape particles of average 10µm size and have body centered tetragonal (BCT) structure.

Grain refinement is one of the strengthening mechanisms for metals and alloys. Grain refinement in solidifying alloy can be obtained in three different ways - (i) adding heterogeneous nucleating sites and promoting large number of nuclei for grain growth in the melt, (ii) adding large number of melt additive having low solid solubility which provides constitutional super cooling and therefore decreases the rate of grain boundary migration and the rates of grain growth once nuclei are formed, (iii) achieving grain refinement by rapidly solidifying the melt, mould vibration or agitation. It is important that alloying elements during preparation of gold alloys dissolve properly in gold and do not form hard aggregates, since such formation of hard aggregates will affect the deformation behavior and also create problems when refining the scrap. Further, specific methodologies/approaches are needed to arrive at varying purity of gold such as 22KT, 24KT, 18KT, 14KT having the desired structural and mechanical properties. For instance, to prepare a 22KT or 24KT gold alloy with desired characteristics such as enhanced hardness while retaining other important parameters necessary for jewelry-based applications, the known/conventional methods for manufacturing high karat golds including 22KT or 24KT alloys may not lead to desired results. In other words, the present approach/method described in the present disclosure for preparing gold alloys comprising nano-sized precipitates of Ti and Au in the Au matrix of the gold alloy is unique and can be employed to obtain gold alloys with varying purities (eg. 22 KT or 24 KT) and desired structural/mechanical properties.

In an embodiment of the present disclosure, the 22KT or 24KT gold is grain refined by heterogeneous nucleation method using Au4Ti to achieve a high hardness (about 120 to 200 HV, preferably 140 to 180 HV) for jewelry making. The grain refining process of the present disclosure is highly efficient, wherein the as cast grain size of about 1000µm is decreased to 60µm after the completion of grain refinement step. In addition, the grain refinement by heterogeneous nucleation also breaks the as cast dendritic structure and results in fine equiaxed grain structure which predominantly increases the hardness and strength of the alloy. The grain refining efficiency is much improved when the present method is employed in comparison to direct addition of elemental Ti or other refractory, rare earth elements at higher amounts. Further, the grain refinement of gold occurs due to heterogeneous nucleation and does not affect the purity. The Au4Ti intermetallic particles are formed after dissolution of master alloy wherein said Au4Ti particles nucleate a-Au through peritectic reaction. Since the intermetallic particles Au4Ti are in-situ prepared, they do not get oxidized and the particles get evenly distributed in the melt while readily undergoing the peritectic reaction and result in less agglomerate formation. Hence, a finer grain structure in the 22KT or 24KT gold alloy is achieved by the method of the present disclosure.

In another embodiment of the present disclosure, a 22KT gold alloy with higher amount of copper and silver can also be efficiently grain refined with master alloy without the poisoning effect of silver with the heterogeneous nucleating particle, Au4Ti.

In yet another embodiment of the present disclosure, since Ti is added into the 22KT or 24KT gold melt through Au-xTi and/or Cu-xTi master alloys, an open induction melting furnace is sufficient for melting and casting operation. The intermetallic particles (AuxTi or CuxTi) will not get oxidized like elemental Ti when added into the molten Au at high temperatures. In an exemplary embodiment, the slag formed during the process is significantly less (about 3 to 5%) and about 95% to 97% product yield is obtained in the casting step.

Since lower amount of Ti (about 0.1 to 0.9 wt %) is added during the process of the present disclosure, the workability of the 22KT or 24KT gold alloy product is not affected. Further, the Au4Ti intermetallic particles (nano-sized precipitates) are completely dissolved in the molten Au due to which the workability is fully retained. In addition, the surface peeling effect (orange peel effect) due to cold rolling of the 22KT or 24KT alloy is completely eliminated due to fine grain structure of the present gold alloy obtained during the grain refinement process.

It an exemplary embodiment of the present disclosure, the cast grain size in the 22KT gold alloy is decreased from 1000 µm to 60 µm.

In an embodiment of the present disclosure, the grain refined and casted 22KT gold alloy is tested for precipitation strengthening behavior by heat treatment (solutionising and age hardening) wherein the 22KT gold alloy is tested at both cast condition or rolled/extruded condition. It is observed that lower addition Ti (about 0.1 to 0.9 wt%) showed good age hardenable behavior at both cast and in deformed (rolled/extruded) conditions.

The AuxTi intermetallic phase formed after the addition of master alloy is the meta-stable phase in the 22KT gold alloy. After the peritectic reaction which nucleates a-Au, the Ti will be in the Au alloy as a solute. In the method of the present disclosure, coherent nano-sized AuxTi precipitates are formed in the Au matrix by suitable heat treatments (solutionising and age hardening), which is called precipitation strengthening to increase the hardness/strength. The solutionising and age hardening time and temperature is varied to obtain peak hardness in the alloy. In an exemplary embodiment of the present method, the age hardening behavior of the grain refined samples containing Ti is conducted in both as cast condition as well as in rolled and extruded conditions. Before age hardening, the samples are solutionised at a temperature between 700°C to 900°C for about 10 minutes to 60 minutes, and rapidly quenched in water. No significant grain coarsening is observed after solutionising the alloy at high temperatures.

In another exemplary embodiment of the present disclosure, the artificial ageing/age hardening is conducted for the grain refined & solutionised samples at an isothermal temperature varied from 450°C to 650°C depending upon the Ti concentration. The hardness is increased from 100 HV (solutionized sample) to 180 HV on the Vickers scale. This is due to the nano-sized Au4Ti precipitation in the Au matrix after age hardening step.

In yet another exemplary embodiment of the present disclosure, Figure 2 depicts optical micrograph of grain refined and extruded 22KT gold alloy product which shows fine grain structure. This extruded product retains its required physical properties along with the improved hardness. The formation of nano-sized precipitates of Au4Ti in the Au matrix of the 22 KT alloy with Ti due to age hardening further improves the strength and hardness of the alloys. Since the Au4Ti precipitates are nano-sized, the original (bright or dark) yellow colour and luster of 22KT gold is retained and the product of the present disclosure is useful for various applications including gold jewelry manufacturing and dental applications.

The present disclosure also provides master alloy for adding titanium (Ti) to gold to obtain a gold alloy such as 22KT gold alloy or 24KT gold alloy of the present invention. In an embodiment, said master alloy comprises copper (Cu) or gold (Au) ranging from about 91.7 wt% to 99.9 wt%, and titanium (Ti) ranging from about 0.1 wt% to 8.3 wt%. More particularly, the master alloy is Cu-xTi or Au-xTi, with ‘x’ ranging from 0.1 to 8.3.

The present disclosure further relates to articles manufactured from the 22 KT or 24 KT gold alloys described herein. The 22 KT or 24 KT gold alloys of the present disclosure and the corresponding article(s) find applications where high hardness is important without compromising on other important features including colour, purity and luster, particularly in jewelry applications. Exemplary embodiments of articles manufactured using the present 22 KT or 24 KT gold alloys include but are not limited to studded jewelry, hand-made and machine made jewelries, links, and other load bearing parts and numerous other gold based applications including dental applications, metallurgy applications, electronic applications, watch cases (eg. for premium watches), plasmonic application and functional coatings.

The present disclosure is thus successful in providing gold alloy product comprising Ti as an important alloying metal added through master alloy(s), wherein said gold alloy possess improved mechanical properties such as enhanced hardness but yet retaining the important characteristics including purity, colour/brilliance, workability, tarnish resistance, scratch resistance, wear and corrosion resistance etc. In an exemplary embodiment of the present disclosure, said alloy is a 22KT or 24KT gold alloy.

In particular, the unconventional method of preparing gold alloys, especially 22 KT and 24 KT gold alloys, by employing micro additions of Ti (eg. 0.1 wt.% to 0.9 wt.%) into the gold matrix via. Au4Ti nano-precipitates contributes to high hardness.

The present invention therefore provides gold alloys comprising Ti and a unique method to add Ti into the gold alloy melt kept at about 1100 °C to 1400 °C without the need of inert atmosphere or vacuum. The Ti is added through master alloy produced either by gold based (Au-xTi) or copper based (Cu-xTi) wherein the ‘x’ ranges from 0.1 wt.% to 8.3 wt.% Ti. In an exemplary embodiment, the master alloy is admixed as granules into the new gold alloy melt and the titanium goes into the melt as AuxTi or CuxTi (x= 2, 4 or 6) intermetallic particles. These intermetallic particle dissolves and releases the Ti into the gold alloy melt. The Ti in the said gold alloy melt refines the grain structure of the new alloy casting by heterogeneous nucleation. The resulting new alloy (as-cast condition or cold deformed) is then age hardened, where the Ti forms nano sized precipitates of Au4Ti homogeneously in the gold matrix, which increases the hardness to about 120 to 200 HV, preferably 150 to 180 HV.

The unconventional method of hardening described in the present disclosure can thus be adopted for gold purity/karatage including 22KT and 24KT.

In an embodiment, the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

EXAMPLES

EXAMPLE 1: Preparation of gold alloys comprising nano-sized AuxTi precipitates
22KT gold alloys comprising Au at 91.6 wt% to 91.8wt%, Ti at 0.1 wt% to 0.9 wt%, additional metals Cu, Ag, Zn, Co, B, Ni at 7.3 wt% to 8.3 wt% were prepared (Table 1).

Table 1: 22KT gold alloy compositions having nano size Au4Ti precipitates
Alloy code Au
(%) Cu
(%) Ag
(%) Co
(%) B
(%) Ti
(%) Zn
(%)
Alloy 1 91.75 5.80 2.35 - - 0.1 -
Alloy 2 91.75 5.80 2.25 - - 0.2 -
Alloy 3 91.75 5.80 2.15 - - 0.3 -
Alloy 4 91.75 5.55 2.2 - - 0.5 -
Alloy 5 91.75 5.425 2.075 - - 0.75 -
Alloy 6 91.75 5.3 1.95 1 -
Alloy 7 91.75 5.80 1.95 - 0.5 -
Alloy 8 91.75 5.8 1.70 - - 0.75 -
Alloy 9 91.75 5.8 1.45 - - 1 -
Alloy 10 91.75 6.05 1.70 - - 0.5 -
Alloy 11 91.75 6.3 1.45 - - 0.5 -
Alloy 12 91.75 6.75 1.0 - - 0.5 -
Alloy 13 91.75 6.9 0.85 - - 0.5 -
Alloy 14 91.75 7.25 0.5 - - 0.5 -
Alloy 15 91.75 7.75 - - - 0.5 -
Alloy 16 91.75 4.75 3.0 - - 0.5 -
Alloy 17 91.75 3.75 4.0 - - 0.5 -
Alloy 18 91.75 6.75 0.98 - .02 0.5 -
Alloy 19 91.75 5.8 1.45 0.5 - 0.5 -
Alloy 20 91.75 5.8 0.95 1.0 - 0.5 -
Alloy 21 91.75 5.8 0.45 1.5 - 0.5 -
Alloy 22 91.75 5.8 - 1.95 - 0.5 -
Alloy 23 91.75 5.50 - - - 0.5 2.25
Alloy 24 91.75 5.8 - - - 0.5 1.95
Alloy 25 91.75 5.8 0.28 - - 0.5 1.67
Alloy 26 91.75 6.37 - - - 0.5 1.38
Alloy 27 91.75 5.50 0.88 - - 0.5 1.37
Alloy 28 91.75 6.08 0.58 - - 0.5 1.09
Alloy 29 91.75 5.8 1.16 - - 0.5 0.79
Alloy 30 91.75 6.67 0.29 - - 0.5 0.79
Alloy 31 91.75 7.25 - - - 0.5 0.5
Alloy 32 91.75 6.37 0.88 - - 0.5 0.5
Alloy 33 91.75 5.50 1.75 - - 0.5 0.5
Alloy 34 91.75 7.45 - - - 0.5 0.3
Alloy 35 91.75 7.6 - - - 0.5 0.15

The following method was employed to prepare the above described 22 KT gold alloys:

The method comprises of preparing master alloys (Au based or Cu based master alloys containing Ti) as a first step. The master alloy is granulated/cut into small pieces, which is then added into the molten 22KT gold (Au-Cu-Ag) kept at 1350 to 1400oC. After the master alloy is completely mixed, the melt is poured into a graphite crucible and casted into cylindrical/billet shaped. The casting obtained is annealed at about 800oC for about 60 minutes and quenched rapidly in ice cold water. After annealing, the casting is cold-rolled or cold-wiredrawn into desired shape or product. The product is solutionized at about 800oC for about 60 minutes, and quenched rapidly in ice cold water to obtain super saturation of Ti. After solutionizing, age hardening of the product is carried out by heat treating it in a muffle furnace at 550 to 600oC for about 30 minutes. The product is air cooled by keeping it outside the furnace after the desired age hardening time is completed.

The present method adapted to prepare the gold alloys is also presented schematically in Figure 1.

EXAMPLE 2: Analysis of structural and mechanical properties
The 22KT gold alloy products prepared under Example 1 by employing Au-6wt% Ti master alloy were analyzed for their properties. Microstructural characterization was carried out using optical microscope and image analysis, and Transmission electron microscopy (TEM).

Figure 3 shows the optical micrograph of grain refined 22KT gold extruded product prepared according to the present disclosure. The microstructure shows fine equiaxed grain structure with an average grain size of 80 µm, measured using image analyser.

The TEM bright field image of 22KT gold alloy of the present disclosure is depicted in Figure 4 which shows nano scale precipitates of Au4Ti in the Au matrix at peak aging time (30 minutes) during age hardening treatment. These Au4Ti precipitates are coherent with the Au matrix and lies in the path of the moving dislocation. Their presence arrests the dislocation movement and therefore leads to the improvement in the hardness of the alloy.

The microstructure of the gold based master alloy, Au-6wt.%Ti is evaluated using Scanning Electron Microscope and the composition of the phases is measured by EDX microanalysis. Figure 5 shows the SEM photomicrograph of Au-6wt% Ti master alloy showing ?-Au dendrites in stoichiometric Au4Ti matrix. Although the composition lies exactly in the Au4Ti intermetallic region of the phase diagram, the rapid solidification (water cooled copper hearth in VAM) of the Au-6wt.%Ti master alloy causes non-equilibrium transformation that leads to the formation of ?-Au dendrites and Au2Ti (that cannot be identified in SEM) in the alloy.

The EDX microanalysis (Figure 6) was performed by mapping the whole region of Au-6wt.%Ti master alloy shown in Figure 4. The spectrum shows Ti and Au peaks confirming the presence of ?-Au and stoichiometric Au4Ti. The inset shows the atomic and mass% of the elements Au and Ti. The atomic % of Au and Ti is 89.05 and 10.95 respectively.

Mechanical properties were analyzed by Vickers hardness method at 500 g load and 10 sec. dwell time. Figure 3 depicts the articles (bangles) manufactured by the 22KT gold alloy of the present disclosure. Said articles show enhanced hardness when compared to regular 22KT gold alloy. The following table (Table 2) further provides a comparison of the hardness of regular 22KT gold versus Ti containing 22KT gold alloys of the present disclosure, wherein said Ti addition is via. master alloy resulting in nano-size Au4Ti precipitates.

Table 2: Comparison of the hardness of regular/conventional 22KT gold versus 22KT gold alloys of the present disclosure
Sl. No Hardness (HV) of Regular 22KT alloy with 100g load Hardness (HV) of 22KT alloy of the present disclosure with 100g load
Average of all trials 97.2 181.77

Thus, the gold alloys containing nano-sized Au4Ti precipitates produced according to the present method have improved structural and mechanical properties including enhanced hardness/strength and retain the original properties of gold (eg. 22KT or 24KT gold) such as color, purity, luster, workability etc. The presently defined gold alloys therefore can be immensely useful in various applications, especially in jewelry making and several other fields where good combination properties of hardness/strength along with color, purity, luster/brightness, tarnish resistance, wear & corrosion resistance is needed.