The invention relates to an alloy comprising (per 1000 parts by weight) 675 to 805 parts of gold, 190 to 245 parts of titanium, 5 to 60 parts of M1, and 0 to 20 parts of M2, wherein M1 is selected from the group consisting of palladium, chromium, vanadium and mixtures thereof, and wherein M2 is selected from the group consisting of iron, manganese, and mixtures thereof. The invention also relates to a process for preparing this alloy and to an item (watch or jewel) comprising or consisting of this alloy.
The invention relates to an alloy based on gold and titanium. This alloy exhibits enhanced corrosion and color change properties. It can be used in horology or jewelry.
Due to their properties, such as hardness or tensile strength, gold-titanium based alloys are commonly used in fields such as dentistry, ceramics, horology or jewelry.
For instance,
Ti3Au (in mole fraction) exhibits hardness values above most steels and approximately four times those of titanium.
However, specific applications, for instance horology or jewelry, require alloys having a greater gold fraction than in Ti3Au (molar fraction).
For instance,
In addition to gold and titanium, this alloy comprises at least two alloying elements, as in Au0.427Ti0.50Fe0.072B0.001 (in mole fraction) i.e., by weight, Au750Ti213Fe36B1, in which the total amount of iron and boron represents 37 ‰ by weight vs gold and titanium.
Binary Ti1-xAux alloys have been reported (
Ott et al. have discussed the control of the grain size in gold or conventional alloys (AuAgCu) during the solidification of the molten alloy or during a recrystallization step (
Even though some of the above alloys may be used in jewelry or horology, there is still an interest in improving the properties of gold alloys, especially their resistance to color change even when exposed to corrosive conditions. The invention relates to a gold-titanium alloy having these properties.
The invention relates to an alloy (three main elements) having improved properties due to the combination of specific elements. This alloy exhibits improved color change resistance (i.e. a better tarnishing behavior) especially when exposed to a corrosive environment. This improvement results from the presence of a specific alloying element (Pd, Cr, V) in addition to gold and titanium.
More specifically, the invention relates to an alloy of formula AuxTiyM1zM2j, wherein:
Formula AuxTiyM1zM2j also includes: AuxTiyM1z (j = 0); AuxTiyM1zM2jR (R is a grain-refining element) and AuxTiyM1zR (j = 0; R is a grain-refining element).
M1 is preferably palladium.
When j = 0, the alloy is of formula AuxTiyM1z, wherein:
During the preparation of the alloy, a grain-refining element can be incorporated. This grain-refining element advantageously represents 0 to 2000 ppm, preferably from 30 to 2000 ppm, more preferably from 30 to 1000 ppm, by mass relative to the total mass of gold, Ti, M1 and M2. A grain-refining element is not an alloying element.
Preferably, the alloy consists of "gold, titanium, M1, optionally M2", and includes 0 to 2000 ppm of a grain-refining element R, preferably from 30 to 2000 ppm, more preferably from 30 to 1000 ppm.
The grain-refining element R is preferably selected from the group consisting of:
The grain-refining element R is preferably selected from the group consisting of: iridium, ruthenium, boron, cobalt, barium, yttrium, zirconium, and mixtures thereof. More preferably, the grain-refining element is selected from the group consisting of: iridium, ruthenium, and mixtures thereof.
The amount of grain refiner is preferably 30 to 200 ppm by weight with respect to the total amount of gold, titanium, M1 and M2, for instance 50 to 150 ppm.
The alloying element M1 (Pd, Cr, V and mixtures thereof) allows increasing the resistance to color change.
The alloying element M2 (Fe, Mn, and mixtures thereof) allows increasing the hardness.
All ranges include the end-points. For instance, the "675 to 805" and "between 675 and 805" ranges include the 675 and 805 values.
In the alloy of formula AuxTiyM1zM2j, the amount x of gold ranges from 675 to 815 parts by weight, for instance from 740 to 815 or from 750 to 815 parts by weight. It preferably ranges from 750 to 770 parts by weight.
In the alloy of formula AuxTiyM1zM2j, the amount y of titanium ranges from 190 to 245 parts by weight. It preferably ranges from 190 to 240 parts by weight, more preferably from 200 to 220 parts by weight.
For instance, in parts by weight, x can range from 750 to 760, y from more than 200 to 220, z from 5 to 50 and j from 0 to 20, with 240 ≤ y + j ≤ 250 and x + y + j = 1000.
According to a specific embodiment, the alloy has any one of the following formula (in weight fraction): Au750-815Ti200-220M15-50M20-20, Au750-815Ti200-220M15-50M20-20R10-200ppm, wherein the amount of grain refiner R ranges from 10 to 200 ppm by weight of the total amount of "gold, titanium, M1 and M2".
According to a particular embodiment of the invention, the alloy has a molar ratio gold/titanium of from 42.17/57.83 (18 carats, based on the weight of "gold, titanium, M1 and M2") to less than 54.86/45.14 (20 carats).
When x varies from 675 to 805, the amount of gold approximately ranges from more than 16 to less than 20 carats as compared to the total amount of "gold, titanium, M1 and M2".
The alloy may comprise impurities. In general, impurities may result from the metals used to form the alloy. Advantageously, these possible impurities amount to a total of less than 1000 ppm, more preferably, less than 500 ppm, more preferably less than 250 ppm, by weight of the alloy (x + y + z + j). The alloy may comprise a total amount of impurities of less than 100 ppm.
Accordingly, the alloy may comprise more impurities than grain refiner. The benefit resulting from the presence of a grain refiner is greater than any negative effect that might result from the presence of impurities, even when the amount of impurities exceeds that of grain refiner.
In particular, impurities can include any one or more of carbon, oxygen and nitrogen. In general, the amount of oxygen may be greater than that of nitrogen, which may be greater than that of carbon.
The total amount of one or more of carbon, oxygen and nitrogen is preferably less than 1000 ppm, more preferably, less than 500 ppm, more preferably less than 300 ppm even more preferably less than 250 ppm, by weight of the alloy (x + y + z + j). However, in some cases, the total amount of one or more of oxygen and nitrogen may amount to more than 1000 ppm, preferably less than 10 000 ppm, more preferably less than 7500 ppm, even more preferably less than 5000 ppm. This large amount of oxygen and/or nitrogen may result from the process for preparing the alloy, in particular if the melted/solubilized Au, Ti, M1, M2 and R (M2 and R are optional) elements are more or less slightly exposed to air. Experimental conditions may therefore impact the amount of impurities such as oxygen, carbon and nitrogen; however, in general, metal or metalloid impurities result from the Au, Ti, M1, M2 and M (M2 and R are optional) materials used to prepare the alloy.
The total amount of impurities other than carbon, nitrogen and oxygen is preferably less than 500 ppm, more preferably less than 250 ppm, even more preferably less than 230 ppm, by weight of the alloy (x + y + z + j). These other impurities may include but not limited to hydrogen, sulfur; silicon; phosphorous; selenium; halogens; metals other than gold, titanium, M1, M2 and R; metalloids other than the grain refiner R.
According to a preferred embodiment, non-metallic and non-metalloid impurities represent less than less than 1000 ppm, more preferably, less than 500 ppm, more preferably less than 300 ppm even more preferably less than 250 ppm) while metallic impurities (metals other than gold, titanium, M1, M2 and R; metalloids other than the grain refiner R) preferably represent less than 500 ppm, more preferably less than 100 ppm, even more preferably less than 50 ppm, by weight of the alloy (x + y + z + j).
The AuxTiyM1zM2j alloy has a grain size that preferably ranges from 10 to 300 µm, more preferably from 5 to 100 µm.
The AuxTiyM1zM2j alloy preferably comprises between 0 and 50 % by volume of Ti3Au (molar fraction) precipitates (Ti3Au or Ti42.17Au57.83 by weight).
Formula Ti3Au corresponds to an alloy consisting of gold and titanium, wherein the molar ratio gold/titanium is 1/3. All other formulas, for instance AuxTiyM1zM2j, AuxTiyM1zM2jR or AuxTiyM1z, are in weight.
Ti3Au precipitates can be formed during a final heat treatment to improve the hardness of the alloy.
These Ti3Au precipitates are dispersed within the AuxTiyM1zM2j matrix.
The alloy has a hardness that preferably ranges from 200 to 650 Hv, more preferably from 400 to 650 Hv, for instance from 40 to 500 Hv after a hardening heat treatment.
Due to its hardness properties, the alloy resists to scratches. In addition, the grain refiner, improves the aesthetic properties of the alloy.
It also exhibits improved resistance to corrosion and has reduced color change properties.
The present invention also relates to a process for preparing the alloy of formula AuxTiyM1zM2j. This process comprises the following steps:
Step 1/ is an alloying step, which is preferably carried out by casting or from powder technology. It can involve mixing powders of pure elements or powder(s) of pre-alloyed elements.
The powder technology includes any technique selected from the group consisting of: metal injection, selective melting and powder fusion.
The metal injection preferably includes a sintering step while the selective melting and the powder fusion preferably include localized melting by laser or electron beam.
The powder technology is preferably any one selected from the group consisting of: MIM-like (metal injection molding-like), spark plasma sintering (SPS), binder jetting, material extrusion, material jetting, sheet lamination, vat photopolymerization, powder bed fusion (for instance LPBF i.e. laser powder bed fusion), selective laser melting (SLM), direct energy deposition and hot pressing.
It is preferably a laser or electron beam assisted additive manufacturing step.
It is preferably a selective melting technique, more preferably a selective laser melting (SLM) technique.
Step 1/ preferably consists in melting/solubilizing the Au, Ti, M1, M2 and R elements (M2 and R are optional) at a temperature TPREP that may be greater than that of the respective melting points of these elements (or mixtures thereof) and below that of their (or mixtures thereof) respective boiling points. For instance, alloying gold, titanium and iridium may be carried out at a temperature of more than 1500°C (melting point of the alloy) and less than 2856°C (boiling point of gold). The temperature TPREP may be below the melting point of Ti or R. It is preferably above the melting point of gold. Melted gold can then solubilize one or more of: titanium, M1, M2, grain refiner R.
The alloying step 1/ may include making a pre-alloy or pure elements. It may involve using powder(s) or bulk piece(s). Making a pre-alloy allows a lower casting temperature. If used, the pre-alloying step is preferably carried out in an arc furnace.
In general, pre-alloying or casting (without pre-alloying) the alloy, in particular in an arc furnace, is carried out at a temperature that preferably ranges from 1500 to 2850°C, more preferably from 1500 to 2700°C. In that case, the different elements of the alloy are preferably maintained at this temperature for 5 seconds to 30 minutes, for instance for 5 seconds to 20 minutes. For instance, it can last from 5 seconds to 5 minutes, more preferably for 10 seconds to 2 minutes.
Casting the different elements of the alloy can consist in casting the appropriate amounts of elements by any means, for instance any one of: arc furnace, induction furnace, cold crucible induction melting...
The grain refiner may also act as seeding material for the AuxTiyM1zM2j alloy as the material resulting from step 1/ has an improved microstructure as compared to alloys consisting of gold and titanium. While thermal treatments increase the grain size, the grain refiner counterbalances this effect by slowing down the grain's growth.
According to a preferred embodiment, the homogenizing step 2/ is carried out at a temperature, THOM, of from 1200 to 1400°C, more preferably between 1250 and 1350°C.
The homogenizing step 2/ is optional. The skilled person in the art is able to consider if a homogenizing step should be carried out on the material resulting of step 1/, for instance depending on the size of the cast or if it has been prepared from powder(s).
The homogenizing step 2/ is preferably carried out for 10 minutes to 12 hours, more preferably for 30 minutes to 8 hours, for instance for 1 hour to 2 hours.
For instance, homogenizing step 2/ may be carried out between 1250 and 1350°C, for 1 hour to 2 hours.
The process preferably includes a cooling stage between steps 2/ and 3/. The homogenized material is preferably rapidly cooled, by air quenching or water quenching, more preferably by air quenching.
Deforming step 3/ is preferably carried out by cold compression or by rolling compression or by hot forging, more preferably by hot forging. The deformation is preferably comprised between 20 and 80%, more preferably between 30 and 70%.
The deforming step is preferably carried out at a temperature TDEF of between 500 and 1300°C, more preferably between 700 and 1200°C.
According to a preferred embodiment, the annealing step 4/ is carried out at a temperature, TANN, of from 1100 to 1400°C, more preferably between 1250 and 1350°C.
Annealing step 4/ is preferably carried out for 5 minutes to 12 hours, more preferably for 10 minutes to 4 hours, for instance for 10 minutes to 2 hours.
For instance, annealing step 4/ may be carried out between 1250 and 1350°C, for 10 minutes to 2 hours.
Precipitation step 5/ is preferably carried out at a temperature TPRECIP of from 400 to 1000°C, more preferably between 500°C and 900°C, for instance between 600°C and 900°C.
At least one of steps 1/ to 5/ may be followed by a cooling stage to room temperature. Steps 1/ to 5/ are preferably followed by a fast cooling stage to room temperature, preferably by water or air quenching.
In general, TPREP is greater than THOM, which is greater than TDEF. On the other hand, TPRECIP is generally smaller than THOM.
The present invention also relates to an item comprising or consisting of the alloy of formula AuxTiyM1zM2j. Accordingly, this alloy can be used in order to manufacture luxury goods.
For instance, it may be a watch component comprising (or consisting of) the alloy. It may be a jewel comprising (or consisting of) the alloy.
The item comprising or consisting of the alloy of formula AuxTiyM1zM2j can be a jewel, a leather good, or a clothing accessory. It may also be a watch, a writing accessory, or a decorative item. For instance, it can be any of the followings: ring, ear ring, necklace, bracelet, pendant, watch or watch movement component (case, bezel, case back, crown, other case small parts, balance wheel, gear wheel, axis, screw...), buckle (belt, purse...), tie bar, cuff links, money clip, hair pin, pen, paper knife...
The alloy according to the invention can therefore be used in a field selected from the group consisting of: jewelry, horology, clothing, writing accessories, leather goods, and ornaments.
The invention and its advantages will become more apparent to one skilled in the art from the following figures and examples.
Alloys of formula AuxTiyM1zM2j have been prepared following different experimental conditions, as outlined in Table 1. Samples "INV" refer to alloys according to the invention while samples "REF" are reference examples.
The process for preparing the different alloys comprises the following steps:
Table 2 and
The Tuccillo-Nielsen test has been carried out in order to evaluate the degradation behavior (corrosion) of the alloys of Table 1 at the material-electrolyte interface, by a cyclic immersion system.
The equipment consists of two carrying wheels, holding the samples (alloys), which are in a continuous rotation, at a speed of 1 rpm, so the samples alternately pass from a liquid medium to a gaseous medium. The equipment tanks are connected to a thermostat with external recirculation, so as to maintain a constant temperature of no more than 37°C, during testing.
The electrolyte used is artificial sweat according to the EN 1811 standard with buffered pH at 6.50. Throughout the test, the pH is adjusted to remain at 6.50±0.2 by adding HCl or NaOH. The tested samples are embedded in cold resin and polished (1 µm grain). Then, the samples are fixed to the ends of the wheel by plastic screws. Throughout the test, degradation is monitored by color measurement according to ISO 8654 and by taking photos or by optical microscopy.
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.