CH716441B1 - Process for manufacturing precious metal alloys and precious metal alloys thus obtained. - Google Patents

Abstract

The present invention relates to a method of making a powder of an alloy formed from a boride of a precious metal, the method comprising reacting a source of said precious metal with a source of boron in a salt or a mixture of salts in the molten state. The powder can be enriched with precious metal by sintering. The present invention also relates to an alloy formed from a boride of a precious metal, said alloy comprising crystalline nanoparticles of MxBy with M which is a precious metal, distributed in an amorphous matrix of B or in an amorphous matrix of B and M z B a. A precious metal alloy obtained is an alloy of 18 carat gold and boron of composition AuB 6. Alloys and alloy powders are used in the manufacture of parts for watchmaking or jewelry.

Description

Technical field of the invention

The present invention relates to a method of manufacturing precious metal alloys. The present invention also relates to such alloys of precious metals and to parts made with these alloys. In particular, the present invention relates to a process for manufacturing light alloys of precious metals obtained from gold, silver, platinum, palladium, ruthenium or iridium. The light alloys of precious metals in question here are titratable, that is to say that they are alloys whose ratio between the mass of the precious metal which enters into the composition of the alloy and the total mass of this alloy is determined by law.

Technological background of the invention

[0002] For the purposes of the present invention and in what follows, the term “precious metal” is understood to mean a metal chosen from the group consisting of gold, silver, platinum, palladium, ruthenium and iridium.

[0003] Precious metals such as gold are used in many fields such as jewelry and watchmaking. Gold has the disadvantage of being easily deformable with the corollary that a simple impact is enough to deform the jewel made using this precious metal. This is why we tried very early to improve the mechanical properties of gold by alloying it with other metallic elements. In this regard, silver and copper are the two main metals used to improve the rigidity of gold. The alloying of gold with other metallic elements such as silver or copper leads to metallic alloys whose hardness is greater than that of gold. However, these gold alloys have the drawback of having a high density. This is why attempts have been made to alloy gold with metallic elements of lower density.

Tests were carried out to try to alloy gold (Au) which is a heavy metal, that is to say a metal with a high density (approximately 19.3 g.cm <-3> ), with boron (B) which is a very light metal, that is to say of which the density is low (approximately 2.3 g.cm <-3>). However, attempts to date to try to ally gold and boron have failed or, at best, resulted in very low boron dissolution rates, not allowing for consider industrial production. The materials resulting from the combination of gold and boron have indeed proven to be unstable and it has been found impossible to achieve titratable massive components such as 18 karat gold using this combination. These problems are explained in particular by the fact that during a conventional process of alloying by melting the elements, it is not possible to mix the gold and the boron. Indeed, because of its high density, gold tends to sediment at the bottom of the crucible, while boron, which has a lower density, floats.

[0005] Thus, many documents such as chapter 10 of the Handbook of Solid State Chemistry, First Edition, Edited by Richard Dronskowski, Shinichi Kikkawa, and Andreas Stein, published in 2017 note the impossibility of producing rich precious metal borides boron, namely metal borides MxBy where M is a metal with a y to x ratio greater than or equal to 1.

For example, in the case of palladium, it was possible to obtain metal borides whose boron content did not exceed Pd6B to Pd2B. We have succeeded in obtaining PtB0.7 for platinum, which is at the lower limit of metal borides rich in boron. For 18-carat gold, that is to say containing 75% by mass of gold, it is necessary to have a composition close to or equal to AuB5.7, which, to the knowledge of the Applicant, does not has not yet been completed.

Summary of the invention

The object of the present invention is to provide a process for manufacturing light alloys of precious metals making it possible in particular to obtain light alloys of precious metals which are stable from a physicochemical point of view and with the aid of which it is possible to produce massive components. More precisely, the process according to the invention consists in producing an alloy of a precious metal and boron by reacting a source of said precious metal with a source of boron in a mixture of molten salts acting as a solvent. The source of boron is in the state of powder, possibly weakly aggregated, and the source of precious metal is also in the state of powder. The mixture of the source of boron, of the source of precious metal and of the salt (s) can then be subjected to gentle grinding, for example carried out by means of a mortar, this operation being carried out in a dry atmosphere. that is to say free from moisture, and preferably inert.

Preferably, the source of boron is a sodium borohydride and the source of precious metal is a chloride of said precious metal. The alloy resulting from this process is formed from precious metal boride nanoparticles MxBy where M is the precious metal distributed in a boron matrix B. Preferably, the y / x ratio of the precious metal boride nanoparticles MxBy is greater than or equal to 1 and, more preferably, greater than or equal to 2. The process according to the invention thus makes it possible to produce alloys of precious metals rich in boron.

According to one embodiment of the invention, the alloy of precious metal and boron is directly used to manufacture a part by powder metallurgy.

According to another embodiment of the invention, the alloy of precious metal and boron resulting from the process by synthesis of molten salts according to the invention is enriched in precious metal before manufacturing the part by powder metallurgy .

The present invention thus relates to the alloy of precious metal and boron directly resulting from the manufacturing process by synthesis in molten salts as well as the alloy enriched in precious metal. It also relates to parts, in particular timepieces or jewelry parts, made with the alloy of precious metal and boron directly resulting from the manufacturing process by reaction with molten salts or with this same alloy enriched in precious metal. Indeed, it is possible that the y / x ratio is too high to achieve, for example, an 18 carat gold. In this case, the boron matrix is enriched with the precious metal.

The process according to the invention makes it possible to obtain alloys of precious metal and of boron which exhibit both excellent mechanical properties and a low density. To the knowledge of the Applicant, the process according to the invention offers, for the first time, the possibility of combining, on an industrial scale, a component of very low density, in this case boron, with a precious metal. , especially but not exclusively gold, the density of which is high. Remarkably, in the process according to the invention, the precious metal selected and the boron combine intimately, without at any time a phenomenon of segregation between the two materials being observed.

Detailed description of an embodiment of the invention

The present invention relates to a method of manufacturing a boride of a precious metal, also referred to below as an alloy of precious metal and of boron, and a method of manufacturing a part made of this alloy. The alloy is produced by synthesis in molten salts, a synthesis also known by its Anglo-Saxon name Synthesis in Molten Salts or SMS. This synthesis consists in bringing together the reactive substances of the precious metal and of boron in a medium comprising salts. When the whole is heated, the salts melt, thus acting like a liquid medium. The synthesis of the alloy of precious metal and boron in molten salts uses a source of metal and a source of boron. The source of metal can be selected from the group comprising sulfates, carbonates, acetates, nitrates, acetylacetonates and halides of the precious metal. Preferably, the source of precious metal is a halide and, more precisely still, a chloride of the precious metal (MCIx). The precious metal is chosen from gold (Au), silver (Ag), platinum (Pt), palladium (Pd), ruthenium (Rh) and iridium (Ir) and, more preferably, from gold, silver, platinum and iridium. The source of boron can be chosen from the group comprising boranes (BxHy) and borohydrides (MBH4). Preferably, the source of boron is sodium borohydride (NaBH4). Thus, the reaction is preferably carried out in the presence of a chloride of the precious metal, such as AuCl3 for gold (Au), and of sodium borohydride (NaBH4). As regards the salts acting as a reaction medium, they are preferably soluble in water to allow recovery of the boride after the reaction. By way of example, it may be a mixture of one or more alkali metal salts and more precisely of halides, carbonates, sulphates or even nitrates. Preferably, it is a eutectic mixture of lithium chloride and potassium chloride in a ratio of 45-50% by weight of LiCl and 55-50% by weight of KCI which has a melting point around 355 ° C. The salt is preferably present in a molar quantity greater than that of the total molar quantity of boron in the source of boron and of the metal in the source of precious metal.

The salt is typically solid at room temperature and is melted at a temperature between 100 and 1000 ° C, preferably between 355 and 900 ° C during the reaction. Ideally, one places oneself above the melting temperature of the mixture of salts, but below the vaporization temperature of this mixture. For example, the Lil / KI mixture partially vaporizes above 850 ° C.

[0015] In addition, the reactive medium can optionally comprise one or more additives having the function of controlling the size of the particles and / or the morphology of the boride obtained. It may, by way of example, be an iodide such as potassium or sodium iodide. The amount of additive is preferably between 1 and 100 moles per mole of metal of the precious metal source.

The method can be carried out at ambient pressure or at a pressure greater than ambient pressure. The atmosphere can be controlled. Thus, the use of lithium and potassium salts requires having to work in an inert atmosphere, due to the sensitivity of these chemicals to water and / or oxygen. Therefore, the precursors are handled and mixed under an inert argon atmosphere. The actual synthesis is carried out under an atmosphere of argon and not of nitrogen, given that nitrogen is capable of reacting with certain species of boron and of leading to the formation of boron nitride.

The method is carried out by mixing the source of precious metal, the source of boron and the salt (s). The whole is heated to the desired temperature to melt the salt or the mixture of salts and maintained at this temperature for a time preferably between 30 minutes and 10 hours. After the reaction, the reactive medium is preferably allowed to cool naturally. Metal borides are obtained in the form of aggregates dispersed in a volume of fixed salts. To remove the salts, washing / centrifugation cycles are carried out in a polar solvent such as water or methanol.

The alloy resulting from this molten salt manufacturing process is in the form of a powder formed from aggregates of crystalline nanoparticles of metal boride MxBydispersed in an amorphous boron B matrix. The term “nanometric particles” is understood to mean particles whose size is between 5 and 200 nm, preferably between 10 and 100 nm. Aggregates typically have a size between 0.3 and 1 micrometer. Preferably, the stoichiometric ratio y / x of the metal boride MxBy which composes the crystalline nanoparticles is greater than or equal to 1 and, more preferably, greater than or equal to 2. Thus, for an 18 carat gold alloy, the nanoparticles must meet the requirement. AuByavec composition y close to 6.

According to a first embodiment of the invention, the alloy resulting from the manufacturing process by synthesis of molten salts is directly used to manufacture a part by powder metallurgy. The powder formed from the aggregates is used as such or is ground beforehand to obtain a powder with a d50 of less than 70 μm. In other words, 50% of the particles forming the powder have a diameter less than or equal to 70 μm.

According to a second embodiment of the invention, the alloy is enriched in precious metal before the manufacture of the part by powder metallurgy. This enrichment is carried out via the additional steps consisting of: provide a quantity of the powder of the aforementioned alloy, the powder possibly coming directly from the process of synthesis in molten salts or being ground to reach the d50 of less than 70 μm; take a quantity of precious metal powder. It may be the same precious metal as that used as the source to obtain the alloy by synthesis in molten salts. It is also possible to envisage enriching the alloy with another precious metal or even with a mixture of precious metals. This powder has a d50 of less than 70 μm; mix these two quantities of powders and sinter the resulting mixture in order to obtain, after sintering, an alloy comprising crystalline nanoparticles of metal boride MxBy with M which is the precious metal, distributed in a matrix formed of amorphous boron B and metal boride MzBa , with z and a which can be equal to or less than x and y respectively. Note that the stoichiometry of the metal boride particles dispersed in the boron matrix is usually not the same as that of the metal boride particles which form the matrix with boron. The particles of metal boride which form the matrix with boron often have a mole fraction z of the metal equal to or even slightly greater than the mole fraction a of boron.

It will be specified that elements other than precious metals such as, for example, nickel, can be added to the mixture during this step. It will also be noted that at the end of the sintering operation, the product obtained is reduced to the powder state, typically by micronization.

The part manufacturing process, whether with the alloy according to the first variant or with the alloy according to the second variant, then comprises the following steps: compact the powder by applying uniaxial pressure; subjecting said compacted powder to a spark plasma sintering treatment (Spark Plasma Sintering or flash sintering) under a pressure of between 0.5 GPa and 10 GPa, or else to a hot isostatic compression treatment (Hot Isostatic Pressing or HIP ) under a pressure of between 80 bars and 2200 bars, the treatment being carried out at a temperature of between 400 ° C and 2100 ° C in order to obtain at least one ingot of an alloy of precious metal and boron; machining the precious metal and boron alloy ingot in order to obtain the desired part, or else reducing the precious metal and boron alloy ingot to the powder state by a micronization treatment, and obtaining the desired part by treating the powder resulting from the micronization treatment.

Once the precious metal and boron alloy ingot has been micronized, a first possibility to obtain the desired solid part consists in introducing the powder resulting from the micronization treatment in a mold and in subjecting this mold at uni-axial or isostatic pressure.

Once the precious metal and boron alloy ingot has been micronized, a second possibility for obtaining the desired solid part consists in subjecting the powder resulting from the micronization treatment to a three-dimensional additive manufacturing treatment.

The three-dimensional additive manufacturing processing can be of the direct printing type. The three-dimensional additive manufacturing techniques of the direct type available are laser sintering, also known by its Anglo-Saxon name Selective Laser Melting or SLM, and sintering by electron bombardment also known by its Anglo-Saxon name E-beam melting.

The three-dimensional additive manufacturing processing can be of the indirect printing type. The indirect type three-dimensional additive manufacturing techniques available are: Inkjet printing (Inkjetting): the powder resulting from the micronization treatment of the precious metal boron alloy ingot is dispersed in the ink. The ink is printed layer after layer, each layer being cured by exposure to radiation from a light source, for example UV, before depositing the next layer. nanoparticle jetting (NPJ): this technique, developed in particular by the XJet company, is similar to liquid inkjet printing, except that the ink is composed of nanoparticles in suspension resulting from the micronization treatment. The suspension is then sprayed, then dried layer by layer. Digital Light Projecting (DLP): this technique consists in projecting by reflection on a mirror the image of the part that you want to structure on a powder bed containing the powder particles resulting from the micronization treatment dispersed in a photopolymer.

Once we have micronized the precious metal and boron alloy ingot, a third possibility to obtain the desired solid part consists in subjecting the powder resulting from the micronization treatment of the ingot to an additive manufacturing treatment three-dimensional, injection or micro-injection in the presence of a polymeric binder. Thus, the powder resulting from the micronization treatment of the precious metal and boron alloy ingot is mixed with the polymeric binder in order to obtain a feedstock. A green part is then produced, the shape of which corresponds to the profile of the part sought by subjecting the feedstock either to an injection or to a micro-injection, or to an additive manufacturing technique.

Among the techniques available for indirect additive manufacturing in the presence of a polymeric binder, mention may be made of: Solvent on Granulate jetting: this technique consists in spraying a solvent on a bed of aggregates formed by the feedstock or feedstock. The dimensions of these aggregates are of the order of 10 μm to 50 μm. The desired part is printed layer by layer, the aggregates agglomerating thanks to the binder. FFD (Fused Filament Deposition): filaments whose dimensions are in the millimeter range are made by agglomerating the feedstock or feedstock. These filaments are then heated and the material from which they are made escapes from a nozzle whose diameter is of the order of 40 μm and make it possible to print the desired part in three dimensions. micro-extrusion.

[0029] As a variant, the mixing between the binder and the powder can be carried out directly during additive manufacturing using the binder jetting technique which consists in projecting an ink jet containing a solvent and a binder on a bed of powder formed by the powder particles resulting from the micronization treatment.

As for the binder, it is chosen from the group formed by polyethylene glycol (PEG), cellulose acetate butyrate (CAB), nano-cellulose (nanometric derivative of cellulose), corn starch, sugar, polylactic acid (Polylactic Acid or PLA), polyethylene, polypropylene, synthetic or natural wax and stearic acid.

A brown part (brown body) is obtained by subjecting the green part to a step of removing the polymeric binder called debinding step during which the green part is treated chemically, then thermally in an oven to burn the binder residual polymer, this debinding step typically taking place in the gas phase in an atmosphere of nitric acid or oxalic acid and at a temperature between 100 ° C and 140 ° C.

Finally, the brown part is subjected to a sintering treatment in a protected atmosphere and at a temperature between 700 ° C and 1800 ° C in order to obtain the desired part.

It will be noted that it is possible, after the sintering treatment, to subject the part resulting from the sintering step to a post-treatment step by hot isostatic compression (Hot Isostatic Pressing or HIP) under a pressure between 500 bars and 2200 bars, and at a temperature between 600 ° C and 2100 ° C.

The part thus manufactured is made of a precious metal boride alloy. This alloy is, according to the first variant, formed of crystalline nanoparticles of MxBy where M is the precious metal distributed in an amorphous matrix of boron B. According to the second variant, the alloy enriched in precious metal comprises the crystalline nanoparticles of MxByeparticles in a amorphous matrix of boron B and precious metal boride MzBa. The precious metal M is chosen from gold (Au), silver (Ag), platinum (Pt), palladium (Pd), ruthenium (Rh) and iridium (Ir). Preferably, it is chosen from gold, silver, platinum and iridium and more preferably it is gold. Preferably, the y / x ratio of the MxBy nanoparticles is greater than or equal to 1, more preferably, it is greater than or equal to 2. As for the a / z ratio, it is typically less than or equal to 1.

The light alloys of precious metals in question here are titratable, that is to say that they are alloys of which the ratio between the mass of the precious metal which enters into the composition of the alloy and the total mass of this alloy is determined by law. A remarkable precious metal alloy obtained by the process of the invention is an 18 carat gold and boron alloy of AuB6 composition with a density of between 6.6 and 7 g / cm <3>. It will also be noted that the gold powder used in the context of the present invention is preferably a 24 carat 1⁄2 bright yellow gold powder.

The part can, in particular, be a timepiece or jewelry piece and, more precisely, a covering part such as a caseband, a back, a bezel, a pusher, a bracelet link, a dial, needle, dial index, etc.

It goes without saying that the present invention is not limited to the embodiments which have just been described and that various modifications and simple variants can be envisaged by those skilled in the art without departing from the scope of the invention. as defined by the appended claims.

Claims (23)

1. A method of manufacturing an alloy powder of a precious metal boride from a precious metal and boron, the precious metal being chosen from the group formed by gold, silver, platinum , palladium, ruthenium and iridium, the process consisting in reacting a source of said precious metal with a source of boron in one or more salts in the molten state and comprising for this purpose the following steps: - mixing the source of boron, the source of precious metal and the salt (s) in the solid state; - Heating the mixture to a temperature between 100 and 1000 ° C and, preferably, between 355 and 900 ° C, to react the source of boron and the source of precious metal in order to obtain the metal boride of the precious metal; - cool the mixture; - Separating the solidified salt (s) from the precious metal boride, said precious metal boride being in the form of a powder comprising aggregates formed of crystalline nanoparticles of precious metal boride MxBy distributed in an amorphous B matrix, the ratio y / x crystalline nanoparticles of precious metal boride MxBy being greater than or equal to 1 and, more preferably, greater than or equal to 2. 2. A method of manufacturing an alloy powder according to claim 1, characterized in that the source of said precious metal is chosen from the group formed by sulphates, carbonates, acetates, nitrates, acetylacetonates and halides. precious metal. 3. A method of manufacturing an alloy powder according to claim 2, characterized in that the source of said precious metal is a chloride of the precious metal MClx where M is the precious metal. 4. A method of manufacturing an alloy powder according to one of the preceding claims, characterized in that the boron source is chosen from the group formed by boranes and borohydrides. 5. A method of manufacturing an alloy powder according to claim 4, characterized in that the source of boron is sodium borohydride NaBH4. 6. A method of manufacturing an alloy powder according to one of the preceding claims, characterized in that it comprises the following additional steps to enrich the precious metal alloy powder of precious metal boride: - Take at least a quantity of powder of said precious metal which serves as a source of precious metal and / or of a powder of another precious metal; - Take a quantity of said precious metal boride alloy powder; - mixing the powder of said precious metal and / or of said other precious metal with the alloy powder of said precious metal boride to obtain a mixture of said precious metal boride powder with said precious metal and / or with said other precious metal; - sinter the resulting mixture, and - Reducing said precious metal boride enriched in precious metal resulting from the sintering operation to a powder state. 7. A method of manufacturing an alloy powder according to claim 6, characterized in that the powder of said precious metal and / or of said other precious metal has a d50 of less than 70 μm, and in that the powder of the boride of Precious metal has a d50 less than 70 µm. 8. A method of manufacturing a part using an alloy powder of a precious metal boride obtained by means of the method according to one of the preceding claims, characterized in that it comprises the following steps : - provision of precious metal boride alloy powder; - compacting the precious metal boride alloy powder by applying uniaxial pressure; - Submission of the compacted powder to a spark plasma sintering treatment also called Spark Plasma Sintering or flash sintering under a pressure between 0.5 GPa and 10 GPa, or else to a hot isostatic compression treatment also called Hot Isostatic Pressing or HIP under a pressure between 80 bars and 2200 bars, the treatment being carried out at a temperature between 400 ° C and 2100 ° C in order to obtain at least one ingot of an alloy of the boride of the precious metal, and - machining of said ingot in order to obtain the desired part, or else - reduction to the powder state of said ingot by a micronization treatment, and obtaining the desired part by treatment of the powder resulting from the micronization treatment. 9. A method of manufacturing a part according to claim 8, characterized in that, in order to obtain the desired part, the powder resulting from the micronization treatment is introduced into a mold and subjected to uniaxial or isostatic pressure. 10. A method of manufacturing a part according to claim 8, characterized in that, in order to produce the desired part, the powder resulting from the micronization treatment is subjected to a three-dimensional additive manufacturing treatment. 11. A method of manufacturing a part according to claim 10, characterized in that the three-dimensional additive manufacturing processing is of the direct printing type chosen from the group formed by laser sintering called Selective Laser Melting or for short SLM and sintering by electron bombardment known as E-beam melting. 12. A method of manufacturing a part according to claim 10, characterized in that the three-dimensional additive manufacturing processing is of the indirect printing type chosen from the group formed by Inkjetting, nanoparticle jetting and Digital Light Projecting. 13. A method of manufacturing a part according to claim 8, characterized in that, in order to produce the desired part, the powder resulting from the micronization treatment is subjected to a three-dimensional additive manufacturing treatment, injection or micro- injection in the presence of a polymeric binder. 14. A method of manufacturing a part according to claim 13, characterized in that it comprises the steps of: mixing of the powder resulting from the micronization treatment of the ingot with the polymeric binder in order to obtain a so-called feedstock feedstock; - Making a green part, called green body, the shape of which corresponds to the profile of the desired part, by subjecting the feedstock to an injection or micro-injection; - obtaining a brown part, called brown body, by subjecting the green part to a step of removing the polymeric binder called debinding step during which the green part is treated chemically, then thermally in an oven to burn the binder residual polymer, this debinding step being carried out in the gas phase in an atmosphere of nitric acid or oxalic acid and at a temperature between 100 ° C and 140 ° C; - Submission of the brown part to a sintering treatment in a protected atmosphere and at a temperature between 700 ° C and 1800 ° C in order to obtain the desired part. 15. A method of manufacturing a part according to claim 13, characterized in that it comprises the steps of: - production of a green part, called green body, the shape of which corresponds to the profile of the desired part by subjecting the powder resulting from the micronization treatment of the ingot to the three-dimensional additive manufacturing treatment; - Obtaining a brown part, called brown body, by subjecting the green part to a step of removing the polymeric binder called debinding step during which the green part is treated chemically and then heat in an oven to burn the polymeric binder residual, this debinding step being carried out in the gas phase in an atmosphere of nitric acid or oxalic acid and at a temperature between 100 ° C and 140 ° C; - Submission of the brown part to a sintering treatment in a protected atmosphere and at a temperature between 700 ° C and 1800 ° C in order to obtain the desired part. 16. A method of manufacturing a part according to claim 15, characterized in that the additive manufacturing technique is chosen from the group formed by Solvent on Granulate jetting, Fused Filament Deposition and micro-extrusion. 17. A method of manufacturing a part according to claim 15, characterized in that the additive manufacturing technique is Binder jetting. 18. A method of manufacturing a part according to one of claims 14 to 17, characterized in that, after the sintering treatment, the part resulting from the sintering step is subjected to a compression post-treatment step. Hot Isostatic Pressing or HIP for short, at a pressure between 500 bars and 2200 bars, and at a temperature between 600 ° C and 2100 ° C. 19. A method of manufacturing a part according to one of claims 13 to 18, characterized in that the binder is chosen from the group formed by polyethylene glycol, cellulose acetate butyrate, nano-cellulose, starch. corn, sugar, polylactic acid, polyethylene, polypropylene, synthetic or natural wax and stearic acid. 20. Alloy formed from a boride of a precious metal chosen from the group comprising gold Au, silver Ag, platinum Pt, palladium Pd, ruthenium Rh and iridium Ir, said alloy comprising nanoparticles crystalline MxBy with M which is the precious metal, the y / x ratio of the crystalline nanoparticles of precious metal boride MxBy being greater than or equal to 1 and, more preferably, greater than or equal to 2, the crystalline nanoparticles of MxBy being distributed in a matrix amorphous of boron B or in an amorphous matrix of B and precious metal boride MzBa with z and a which may be equal to or less than x and y respectively. 21. Alloy according to claim 20, characterized in that the precious metal is gold and in that it is 18 carat gold with a composition MxByoù x is equal to 1 and y is close to 6. 22. Alloy according to one of claims 20 and 21, characterized in that it contains other alloyed elements. 23. Piece for timepieces or piece for jewelry made in the alloy according to one of claims 20 to 22.