EP2402467B1 - Gold alloy with improved hardness - Google Patents

Description

The invention relates to the use of a precipitate for curing an 18-carat gold alloy.

The invention also relates to a timepiece, jewelery or jewelery comprising at least one component made of such an alloy.

It is an object of the invention to provide a gold-based alloy which has improved hardness qualities compared not only to pure gold, but also to known gold-based alloys.

The main applications are watchmaking, jewelery, jewelery, and dentistry.

The hardening of gold is an ancient problem, which, since antiquity, has led to the use of alloys in order to obtain mechanical characteristics sufficient to ensure at least the holding of artifacts. Indeed, the process of hardening by plastic deformation of the material, which applies well to certain metals, applies poorly to gold since it has only very little consolidation during deformation, and more, recrystallizes at relatively low temperatures. The method of refining the grain size, theoretically allowing to raise the elastic limit of the material, is not suitable either for the gold, which has a face-centered cubic structure, hereinafter referred to as FCC, since there are enough active sliding systems for the free passage of dislocations from one grain to another.

The dissolution of alloying elements is the most commonly used method, often empirically, and provides only poor hardness, of the order of 150 to 155 HV on the Vickers scale in the best case .

Various attempts have been made, for example to refine the grain size as in the patent document EP 0 284 699 in the name of Steinemann with a binary alloy containing gold and another metal selected from aluminum, gallium or silicon, or with a similar pseudo-binary alloy still containing copper, for at most 15% of the gold concentration. Such a compound gives a centered cubic structure and a grain size less than 50 microns, which allows to obtain a certain ductility, which is not the quality sought here.

The preparation of dental alloys with increasing hardness in time and at body temperature is still known from the patent document US 5,338,378 in the name of Kyushu University, which uses an alloy with 67% to 82% gold, 18% to 33% copper, and 2% to 8% of at least one other metal taken from gallium, aluminum and zinc, this alloy undergoes a quenching operation after heating to 650-700 ° C, prior to use. Similarly a patent EP 0 978 572 in the name of Hafner GmbH describes an alloy consisting of 70% to 80% gold, 15% to 25% copper, 0% to 15% silver, and 0.1% to 5% gallium, which, unexplained, oxidizes little during a second treatment at 400 ° C following a first treatment at 800 ° C, and acquires a hardness that increases with time at room temperature.

• Le document dit D1 : JP 8 013060 A au nom de PILOT PEN, décrit l'obtention d'un alliage d'or de dureté améliorée avec plusieurs compositions possibles :
  • 15 à 19% de cuivre et 4 à 10% d'argent, et 0,3 à 1% d'aluminium et/ou de magnésium,
  • ou bien 15 à 19% de cuivre, 4 à 10% d'argent, 0,3 à 1% d'aluminium et/ou de magnésium, et 0,3 à 2% de zinc.

On incorpore à ce mélange de 0,1 à 1% de ruthénium et/ou de cobalt.
Toutefois, cette composition correspond à un or rose, ce qui n'est pas revendiqué ici.The following documents are still known:

• The document says D1: JP 8 013060 A in the name of PILOT PEN, describes obtaining a gold alloy of improved hardness with several possible compositions:
  • 15 to 19% copper and 4 to 10% silver, and 0.3 to 1% aluminum and / or magnesium,
  • or 15 to 19% copper, 4 to 10% silver, 0.3 to 1% aluminum and / or magnesium, and 0.3 to 2% zinc.

0.1 to 1% of ruthenium and / or cobalt is incorporated in this mixture.
However, this composition corresponds to a pink gold, which is not claimed here.

The document says D2: EP 0 978 572 A1 in the name of HAFNER, describes an alloy with 70 to 80% gold and 15 to 25% copper, to which is added 0.1 to 5% gallium. According to certain variants it could still contain: 0.1 to 3% of zinc, and / or 0.5 to 5% of silver, and / or 0.1 to 0.5% of silicon, and / or of 0 , 1 to 2% iron and / or 0.1 to 0.3% indium, and / or 0.1 to 0.5% aluminum and / or 0.1 to 3% tin.

The document says D3: US 5,338,378 A on behalf of OHTA MICHIO, describes a dental gold alloy with slow aging of 20 to 30 days and during which its hardness increases further. It comprises from 82 to 67% of gold, from 18 to 33% of copper, from 0 to 2% of a hardening accelerator selected from gallium and zinc. The alloy is subjected to heating at 650-700 ° C and quenched with water. In another composition, it comprises from 2 to 8% of such an accelerator, which comprises at least one metal selected from the group consisting of 1 to 4% gallium, from 0.4 to 2% of aluminum, and 1 to 5% of zinc. In another composition, it comprises 1 to 4% gallium. In another composition, it comprises from 1 to 5% of zinc.

The document says D4: Article "18 carat yellow gold alloys with increased hardness" by SUSS, RAINER, published in 2004 , describes gold alloys with improved hardness, and the influence of addition metals, both for obtaining particular physical characteristics and colors.

The document says D5: Article "Metallurgy of gold" by FISCHER-BÜHNER, published on 20.05.2010 , details the metallurgy of gold and its alloys.

It turns out that among the multiple studies carried out, it is known to select gold alloys of suitable hardness, without always understanding the physico-chemical mechanism infallibly leading to the desired hardness.
In short, the known processes are empirical, poorly controlled, and often give rise to alloys which, on the one hand have a moderate hardness, and on the other hand have a very particular coloring and very different from that of pure gold.

In the context of the invention, it is a question of developing an alloy which has properties of good surface hardness, greater than 250 HV on the Vickers scale, as well as hardness properties equivalent to the core, in comparison with the hardnesses of the order of 155 HV commonly obtained by dissolving metal alloying elements.

It is still important to be able to maintain the appearance of gold, as well as its brilliance.

• on choisit, pour son aptitude à constituer des précipités avec l'or, un second métal qui est l'aluminium;
• on choisit au moins un dit métal d'addition pour son aptitude à favoriser une structure cubique faces centrées FCC stable d'une part, et pour son aptitude à augmenter la solubilité à haute température de l'aluminium dans l'or d'autre part ;
• on crée les conditions d'insertion, dans une structure cubique faces centrées FCC résultant de la mise en solution d'un mélange d'or, d'aluminium, et dudit au moins un métal d'addition, de précipités d'aluminium avec l'or ;
• on prépare un mélange comportant en masse au moins 75% d'or, de 0,1% à 2,1% d'aluminium, et un complément constitué dudit au moins un métal d'addition, l'aluminium et ledit métal d'addition étant choisis pour obtenir au moins un précipité sélectionné d'aluminium avec l'or pour constituer un intermétallique conférant audit alliage une dureté supérieure à 250 HV ;
• on restreint la sélection dudit au moins un précipité sélectionné au seul dit précipité sélectionné d'aluminium et d'or Al₂Au₅ ;
• on met en solution ledit mélange par une mise en température entre 650°C et 700°C;
• on effectue un refroidissement brusque après ladite mise en solution sous la forme d'une trempe;
• on effectue après ledit refroidissement brusque un traitement de revenu structurant à une température comprise entre 200°C et 250°C pour donner naissance audit précipité sélectionné d'aluminium et d'or Al₂Au₅,
• on fait croître de façon contrôlée ledit précipité sélectionné d'aluminium et d'or Al₂Au₅; en maintenant ledit revenu structurant pendant une durée suffisante pour obtenir la dureté requise ;
• on effectue un refroidissement à l'ambiante,

et encore caractérisé en ce que ledit métal d'addition est choisi parmi les éléments suivants: argent, chrome, cuivre, fer, hafnium, manganèse, niobium, palladium, platine, vanadium.For this purpose the invention relates to the use of a precipitate for the hardening of an 18-carat gold alloy, characterized in that said precipitate is a precipitate of aluminum and gold Al ₂ Au ₅ for hardening a gold alloy containing at least 75% gold, from 0.1% to 2.1% aluminum, and the complement at 100%, between 22.4% and 24.0% of at least an addition metal chosen on the one hand for its ability to promote a CFC-centered cubic structure and on the other hand for its ability to increase the solubility of aluminum in gold on the other hand, said use resulting from the insertion of said precipitate of aluminum and gold Al ₂ Au ₅ in a face-centered cubic CFC structure resulting from the dissolution of said mixture consisting of gold, aluminum, said at least one metal addition , and aluminum precipitates with gold, that said insertion is carried out by a process for obtaining an 18-carat gold-based hardness alloy. improved, according to which:

• one chooses, for its aptitude to constitute precipitates with the gold, a second metal which is aluminum;
• at least one said addition metal is chosen for its ability to favor a stable face-centered cubic FCC structure on the one hand, and for its ability to increase the high-temperature solubility of aluminum in gold on the other hand ;
• the insertion conditions are created in a cubic face-centered FCC structure resulting from the dissolving of a mixture of gold, aluminum, and said at least one addition metal, of aluminum precipitates with 'gold ;
• a mixture comprising at least 75% gold, from 0.1% to 2.1% of aluminum, and a complement consisting of at least one addition metal, aluminum and said metal, are prepared; addition being selected to obtain at least one selected precipitate of aluminum with gold to form an intermetallic imparting to said alloy a hardness greater than 250 HV;
• the selection of said at least one selected precipitate is restricted to only said selected precipitate of aluminum and gold Al ₂ Au ₅ ;
• said mixture is brought into solution by bringing the temperature between 650 ° C and 700 ° C;
• an abrupt cooling is effected after said dissolution in the form of quenching;
• after said quenching, a structuring income treatment is carried out at a temperature of between 200 ° C. and 250 ° C. to give rise to said selected precipitate of aluminum and gold Al ₂ Au ₅ ,
• said selected Al ₂ Au ₅ aluminum and gold precipitate is controlled grown; maintaining said structuring income for a time sufficient to obtain the required hardness;
• cooling is carried out at ambient temperature,

and further characterized in that said addition metal is selected from the following elements: silver, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum, vanadium.

According to one characteristic of the invention, said Al ₂ Au ₅ aluminum and gold precipitate constitutes an intermetallic imparting to said alloy a hardness greater than 250 HV, to improve the structural hardening of said alloy.

According to a particular characteristic of the invention, said addition metal is silver.

According to one particular characteristic of the invention, one said additive metal is silver, and is completed by another addition metal of concentration lower than that of silver. More particularly, said other addition metal is copper.

According to a particular characteristic of the invention, said quenching is carried out by quenching with water.

According to a particular characteristic of the invention, said structuring income is carried out at least 24 hours after said sudden cooling.

The invention also relates to a timepiece, jewelery or jewelery comprising at least one component made of an 18K gold alloy with improved hardness comprising a selected precipitate of aluminum and gold Al ₂ Au ₅ thus used and inserted into a CFC-faced cubic structure resulting from dissolving a mixture of gold, aluminum, said at least one adduct, and said selected aluminum and gold precipitate Al ₂ At ₅ , said component comprising at least 75% of gold, from 1.0% to 2.1% of aluminum, and the complement at 100%, of between 22.4% and 24.0%, of minus a chosen addition metal and further characterized in that said addition metal is selected from the following elements: silver, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum, vanadium.

In the preferred embodiment of the description, concerning an 18-carat alloy, the alloy retains the specific appearance of pure gold. This alloy obtained, by its improved hardness, is resistant to scratches, and is entirely appropriate for timepieces, jewelery or jewelery, and in particular for their visible components such as glasses and watch frames, and structures jewelry, bracelets, clasps, buckles, and other items.

The invention provides a simple, reproducible method of implementation, making it possible to obtain with certainty a gold alloy with a required hardness, greater than 250 HV, with a short treatment time. The alloy obtained is directly usable without requiring additional aging.

• la figure 1 est un diagramme de phases d'un alliage pseudo-binaire Au-Ag-AI selon l'invention, dans un exemple d'alliage à 18 carats, et qui représente les différentes phases, en fonction, d'une part en abscisse de la concentration d'aluminium, c'est-à-dire du rapport entre la masse d'aluminium et la masse totale de l'alliage, et d'autre part en ordonnée de la température, ici représentée en degrés Celsius ;
• la figure 2 est un diagramme de dureté Vickers en ordonnée, en fonction du temps en abscisse, d'un alliage selon l'invention réalisé dans un domaine préférentiel A du diagramme de la figure 1 en comparaison d'un or 18 carats obtenu de manière standard.

Other features and advantages of the invention will appear on reading the description which follows, with reference to the appended drawings, in which:

• the figure 1 is a phase diagram of an Au-Ag-AI pseudo-binary alloy according to the invention, in an 18-carat alloy example, and which represents the different phases, depending, on the one hand, on the abscissa of the aluminum concentration, that is, the ratio of the aluminum mass to the total mass of the alloy, and on the other hand on the ordinate of the temperature, here represented in degrees Celsius;
• the figure 2 is a Vickers hardness chart on the ordinate, as a function of time on the abscissa, of an alloy according to the invention produced in a preferred domain A of the diagram of FIG. figure 1 compared to a standard 18-carat gold.

It is an object of the invention to provide a gold-based alloy which has improved hardness qualities compared not only to pure gold, but also to known gold-based alloys.

The invention uses a method of structural hardening, by the selection of particular elements, which are here chosen to form particular precipitates. Among the various precipitates that can form gold with other metals, under very specific physico-chemical conditions, it is a question of choosing those which can be controlled, by the implementation of an appropriate treatment, the germination and growth, to optimize the mechanical characteristics, and in particular here to improve the hardness.

In particular, the mechanical characteristic that the present invention makes it possible to improve, by the creation of a particular process, is the hardness, which concerns both the core hardness of the alloy, and the surface hardness which is very important in watches, jewelery, jewelery to resist scratches or at least to minimize the effects.

We are thus in a situation very different from most gold alloys, used in jewelery, which are most often elaborated so as to include the minimal gold title guaranteeing an appearance close to that of gold, and with the search for great formability, so as to allow rolling or stretching, hollow bodies or sheets, easy to form, and easy to weld.

The inventive step of the invention was to search for the possibility of insertion, in a cubic face-centered or FCC structure, of precipitates, and to grow them in a controlled manner, so as to obtain a hardness greater than the usual hardness. .

The invention relates more particularly to the field of gold alloys with a high gold content, and particularly 18-carat alloys, comprising at least 75% of their weight of gold.

The selection of aluminum is peculiar to the invention, because of the ability of this metal to form with gold different precipitates: Al ₂ Au ₅ , AlAu ₂ , AlAu. These three precipitates make it possible to obtain alloys with improved hardness.

The process is carried out so as to obtain the Al ₂ Au ₅ precipitate, which provides, embedded in an alloy, a normal behavior during machining or processing operations. It is therefore necessary to obtain the creation of this precipitate Al ₂ Au ₅ , and preferably to obtain the creation of this precipitate Al ₂ Au ₅ alone, because it has better properties than the other two precipitates AlAu ₂ and AlAu.

The creation of this precipitate Al ₂ Au ₅ must be obtained within an FCC structure. A binary alloy composed solely of gold and aluminum is inconvenient to develop, and is very fragile, making it unsuitable for most jewelery and jewelery jobs. It is therefore necessary to stabilize the FCC phase by the incorporation of at least one other alloying element making it possible to ensure the intermetallic solubility at high temperature, and also to ensure the longest possible FCC phase; for a range of aluminum content as wide as possible.

Various tests of pseudo-binary alloys were tested.

The second metal may be selected from silver, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum, vanadium, this list is not exhaustive.

Experimentation shows that the choice of silver is most particularly favorable to the intermetallic solubility at high temperature, and to obtaining a long FCC phase, because the miscibility silver-gold is complete, and because the silver can also dissolve aluminum.

In an innovative way, the invention has sought to create the phase diagram of the pseudo-binary alloy Au-Ag-Al, as visible on the figure 1 . This diagram represents, in a conventional way, the different phases, as a function, on the one hand, of the abscissa of the aluminum concentration, that is to say on the ratio between the aluminum mass and the total mass of the alloy, and on the other hand on the ordinate of the temperature, here represented in degrees Celsius. The diagram of the figure 1 represents the preferred case of a gold mass concentration of 75%, the preferred case of an 18-carat alloy.

It is a partially miscible diagram, and there are solubility limits, substantially vertical in the diagram, which separate phases each of defined composition and different from the neighbor. In each of these phases, the atoms reorganize locally to form precipitates, which are defined compounds of fixed composition.

To obtain the precipitated Al ₂ In ₅ hoped, and he alone, it should be restricted to a first field called A on figure 1 , in which only the elements of the alloy in FCC form coexist on the one hand, and the precipitates of Al ₂ Au ₅ on the other hand. The concentration of aluminum, to remain in this area, must remain below 2.1%. The range of concentrations to be respected is 0.1% to 2.1% of aluminum in order to be certain to develop only Al ₂ Au ₅ .

A second domain called B on the figure 1 corresponds to a phase where the precipitates of Al ₂ Au ₅ and AlAu ₂ coexist with the elements of the alloy in FCC form.

The third domain called C on the figure 1 corresponds to a phase where only the precipitates of AlAu ₂ coexist with the elements of the alloy in FCC form.

The diagram of the figure 1 shows that, to obtain an alloy in an optimal composition in the domain A, a method of obtaining is to heat up all the elements of the alloy, then to be in the domain D of the figure 1 , corresponding to a dissolution of aluminum. A dilution heat treatment, at a temperature between the solidus and the liquidus delimiting the D domain, allows a homogeneous solution solution: the gold is in FCC structure, thanks to the chosen element or elements of addition, in particular the silver, and the FCC structure is stable. High solubility of aluminum in the FCC_A1 phase is observed at high temperature, in particular at temperatures between 400 ° C. and 700 ° C. This or these addition elements also facilitate the solubility of aluminum in gold.

The alloy is then made metastable. The rise in temperature, carried out for example between about 400 ° C. and 700 ° C. for the end portion of area A, ideally around 650 ° C., is followed by rapid cooling, such as water quenching. , or similar. Thus, aluminum atoms do not have time to reorganize. After a variable duration, but preferably close to 24 hours, the alloy is subjected to structuring income treatment, in the temperature range defined by the solvus of the A domain for the aluminum concentration considered. In any case, this structuring income does not exceed the temperature of 400 ° C. During this income, Al ₂ Au ₅ precipitates develop and grow. Preferably, the temperature of structuring income is greater than 200 ° C to facilitate this growth of precipitates, and also limit the duration of heat treatment.

The figure 2 is a Vickers hardness chart in ordinate as a function of time on the abscissa. We see, on the example of figure 2 a structuring income at 200 ° C, a hardness greater than 250 HV is obtained very quickly, after about 2 hours. This hardness will still increase if the structuring income treatment is prolonged, but asymptotically, and it is hardly useful, even if the maximum hardness is sought, to prolong the treatment beyond ten or so years. hours, where one reaches a hardness of the order of 280 HV. The figure 2 shows, for comparison, the hardness level of 150 HV obtained with an 18 K gold alloy, or 18 karat gold, conventional.

If the structuring income is made at a lower temperature, for example 100 ° C, a hardness greater than 200 HV will be obtained after 10 to 15 hours, and the treatment must be further extended to reach a level of order of 250 HV.

The precipitate Al ₂ Au ₅ obtained is harder than gold.

It is essential, according to the invention, to promote the presence of the Al ₂ Au ₅ precipitate, and preferably to restrict the formation of precipitates composed solely of gold and aluminum to this single precipitate Al ₂ Au ₅ which has the best features, to help solve the technical problem of hardening of the alloy.

Preferably, in order to allow the optimal development of Al ₂ Au ₅ precipitates, the alloy contains no other metal than gold, aluminum, and an addition metal, preferably silver, chosen to increase intermetallic solubility and to maximize phase D, in terms of range amplitude of aluminum concentrations.

In sum, the invention differs from the prior art in that it creates the conditions for the development of Al ₂ Au ₅ precipitates, in an alloy of suitable composition comprising gold, aluminum, and at least one addition metal chosen for its ability to promote a stable FCC structure on the one hand, and to increase the solubility of aluminum in gold on the other hand, this addition metal being preferably 'money.

The optimum composition by weight is from 0.1% to 2.1% of aluminum, at least 75% of gold in order to respect the legal requirement in jewelery and jewelry, and the complement constituted by the metal of addition.

The addition metal can also be copper. It is also possible to combine several metals each having the properties that this adduct must have, namely the ability to promote a stable FCC structure on the one hand, and the ability to increase the solubility of the aluminum in gold on the other hand.

Silver is the best element, and the other metallic elements listed above can be added to adjust the tint of the alloy. This list of elements has been drawn up so that the elements contained therein satisfy the condition of increasing the solubility of aluminum in the FCC structure at high temperature.

In particular, copper is less favorable than silver for fulfilling these particular functions in the presence of gold and aluminum. The use of copper is still possible for reasons of cost, but is much less favorable than silver, and should always, in case of employment, be combined with money, paying attention that the concentration of money always be greater than the concentration of copper in the alloy.

When using other addition metals than silver, for example selected from chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum, vanadium, it is advisable to use to pay attention to the fact that aluminum could form precipitates with some of these addition metals, but it is preferable to form Al ₂ Au ₅ precipitates. So, in addition to silver and aluminum, it is preferable to use these elements alone: chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum, vanadium

In addition, each new composition with different addition metals requires a complete experiment in order to define the corresponding phase diagrams, nonexistent in the literature, to analyze the precipitates and other intermetallics created within each of the phases, to verify that these compounds do not alter the mechanical properties of the gold-based alloy. These studies and experiments are long and expensive and can not be conducted at random. They are also intended to determine, on a case by case basis, the range of aluminum concentrations to be observed in order to obtain Al ₂ Au ₅ precipitates, and preferably only this one.

In summary, the invention makes it possible to obtain a gold-based alloy with improved hardness, which consists of a mixture comprising in mass at least 75% gold, from 0.1% to 2.1%. % of a second metal chosen for its ability to form precipitated with gold, and a complement consisting of at least one addition metal chosen for its ability to promote a stable face-centered cubic FCC structure on the one hand, and for its ability to increase the solubility of said second metal in the or on the other hand, this mixture also comprising at least one such precipitate of the second metal with the gold selected to obtain a hardness greater than 250 HV.

The second metal is aluminum and the precipitate selected is the Al ₂ Au ₅ aluminum and gold precipitate, which makes it possible to obtain an alloy with very good hardness characteristics, which is greater than 250 HV, and in particular in the vicinity of 280 HV. This Al ₂ Au ₅ precipitate also provides the alloy with good resistance during its transformation or its machining, because it does not make the alloy brittle.

Preferably, the addition metal is silver, which ensures good dissolution of the entire mixture.

In a particular embodiment, the addition metal is silver and is supplemented with another addition metal to adjust the color of the alloy.

• on choisit un second métal pour son aptitude à constituer des précipités avec l'or ;
• on choisit au moins un métal d'addition pour son aptitude à favoriser une structure cubique faces centrées FCC stable d'une part, et pour son aptitude à augmenter la solubilité dudit second métal dans l'or d'autre part ;
• on crée les conditions d'insertion, dans une structure cubique faces centrées FCC résultant de la mise en solution d'un mélange d'or, de ce second métal, et de ce métal d'addition ou de ces métaux d'addition, de précipités du second métal avec l'or ;
• on prépare un mélange comportant en masse au moins 75% d'or, de 0,1% à 2,1% du second métal, et un complément constitué du au moins un métal d'addition, ce second métal et ce métal d'addition étant choisis pour obtenir au moins un précipité sélectionné du second métal avec l'or d'une dureté supérieure à 250 HV ;
• on met en solution ce mélange par une mise en température entre 400°C et 700 °C;
• on effectue un refroidissement brusque après la mise en solution;
• on effectue après ce refroidissement brusque un traitement de revenu structurant à une température comprise entre 200°C et 400°C pour donner naissance à au moins un précipité sélectionné du second métal avec l'or ;
• on fait croître de façon contrôlée ce précipité sélectionné ou ces précipités sélectionnés en maintenant le revenu structurant pendant une durée suffisante pour obtenir la dureté requise ;
• on effectue un refroidissement à l'ambiante.

The process for obtaining such a gold-based alloy, with improved hardness, according to the invention comprises the steps according to which:

• a second metal is chosen for its ability to form precipitates with gold;
• at least one addition metal is chosen for its ability to promote a stable FCC face-centered cubic structure on the one hand, and for its ability to increase the solubility of said second metal in gold on the other hand;
• the insertion conditions are created in a cubic face-centered FCC structure resulting from the dissolving of a mixture of gold, of this second metal, and of this additive metal or of these addition metals, precipitated from the second metal with gold;
• a mixture comprising, by mass, at least 75% gold, from 0.1% to 2.1% of the second metal, and a complement consisting of at least one addition metal, this second metal and said metal of addition being selected to obtain at least one selected precipitate of the second metal with gold of hardness greater than 250 HV;
• this mixture is brought into solution by bringing the temperature to between 400 ° C. and 700 ° C .;
• an abrupt cooling is carried out after the dissolution;
• after this quenching, a structuring income treatment is carried out at a temperature of between 200 ° C. and 400 ° C. to give rise to at least one selected precipitate of the second metal with gold;
• the selected precipitate or selected precipitates are controlled grown by maintaining the structuring income for a time sufficient to obtain the required hardness;
• cooling is carried out at room temperature.

Preferably, the selection of selected precipitates is restricted to a single precipitate.

Advantageously, the structuring income is made at least 24 hours after the quenching.

Aluminum is preferably chosen for the second metal, and the precipitate of Al ₂ Au ₅ aluminum and gold is selected as the selected precipitate.

Advantageously, the addition metal is silver.

In an alternative embodiment, silver is used as the addition metal and another addition metal with silver-like characteristics is added to adjust the color of the alloy.

The invention also relates to a timepiece, jewelery or jewelery comprising at least one component made of such an alloy.

Claims (8)

1. Use of an aluminium and gold precipitate for hardening a 18 carat gold-based alloy, characterized in that said precipitate is an aluminium and gold precipitate Al₂Au₅ for hardening a gold alloy comprising at least 75% of gold, 0.1% to 2.1% of aluminium, and the complement to 100%, comprised between 22,4% and 24,0%, of at least one additional metal, chosen for its ability to favour a stable FCC structure on the one hand, and for its ability to increase the solubility of aluminium in gold on the other hand, said use resulting from the insertion of said aluminium and gold precipitate Al₂Au₅ into an FCC structure resulting from the solution heat treatment of said mixture comprising gold, aluminium, said at least one additional metal, and aluminium and gold precipitates, in that said insertion is realised by a method for obtaining a 18 carat gold-based alloy, with improved hardness, according to which:

   - a second metal is chosen for its ability to form precipitates with gold, said second metal being aluminium;

   - at least one additional metal is chosen for its ability to favour a stable FCC on the one hand, and for its ability to increase the high temperature solubility of said second metal in gold on the other hand;

   - the conditions are created for inserting precipitates of aluminium with gold into an FCC structure resulting from the solution heat treatment of a mixture of gold, aluminium and said at least one additional metal;

   - a mixture is prepared comprising in weight percent at least 75% of gold, 0.5% to 2.1% of said second metal, and a complement made with said at least one additional metal, aluminium and said additional metal being selected to obtain at least one said particular precipitate of aluminium with gold to form an intermetallic compound giving said alloy a hardness of more than 250HV;

   - the selection of said at least one selected precipitate is restricted to only the single precipitate Al₂Au₅ ;

   - said mixture is solution heat treated by being heated to between 650°C and 700°C;

   - after said solution heat treatment, rapid cooling is performed in the form of a hardening;

   - after said rapid cooling, a tempering structuring treatment is carried out at a temperature of between 200°C and 250°C to create said selected aluminium and gold precipitate Al₂Au₅;

   - said selected aluminium and gold precipitate Al₂Au₅ is grown in a controlled manner by maintaining said tempering structuring treatment for a sufficient period of time to obtain the desired hardness;

   - cooling at ambient temperature is carried out,
   and still characterized in that said additional metal is selected from among following elements: silver, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum, vanadium.
2. Use of a precipitate according to claim 1, characterized in that said aluminium and gold precipitate Al₂Au₅ forms an intermetallic compound giving said alloy a hardness of more than 250 HV, in order to improve the structural hardening of said alloy.
3. Use of a precipitate according to claim 1 or 2, characterized in that said additional metal is silver.
4. Use of a precipitate according to claim 1 or 2, characterized in that said additional metal is silver, and is completed by another additional metal of lower concentration than that of silver.
5. Use of a precipitate according to claim 4, characterized in that said other additional metal is copper.
6. Use of a precipitate according to any of the preceding claims, characterized in that said rapid cooling is achieved by water hardening.
7. Use of a precipitate according to any of the preceding claims, characterized in that said structuring tempering treatment is performed at least 24 hours after said rapid cooling.
8. Piece of orology or jewellery including at least one component made of a 18 carat gold-based alloy with improved hardness including a selected aluminium and gold precipitate Al₂Au₅ used according to any of the preceding claims and inserted in an FCC structure resulting from the solution heat treatment of a mixture of gold, aluminium and said at least one additional metal, and of said selected aluminium and gold precipitate Al₂Au₅, said component including at least 75% of gold, 0.1% to 2.1% of aluminium, and the complement to 100%, comprised between 22,4% and 24,0%, of at least one selected additional metal, and still characterized in that said additional metal is selected from among following elements: silver, chromium, copper, iron, hafnium, manganese, niobium, palladium, platinum, vanadium.

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