Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C32/00—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ
  • C22C32/001—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ with only oxides
  • C22C32/0015—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ with only oxides with only single oxides as main non-metallic constituents
  • C22C32/0021—Matrix based on noble metals, Cu or alloys thereof

Definitions

• the present invention relates to the field of metal alloys. It concerns, more particularly, on the one hand, a method for obtaining a piece of a silver alloy (Ag) and, on the other hand, the base alloy used to achieve this.
• the silver-based alloys are common. For example, silver is mixed with a few tenths of percent of magnesium (Mg) and nickel (Ni), the latter acting as a grain refiner. By internal oxidation of magnesium to magnesium oxide (MgO), this alloy becomes very hard and has interesting mechanical qualities. In addition and unlike purely metallic alloys, its hardness and the size of its grains are retained after high temperature treatments, such as brazing. It is also an excellent conductor. Such properties make it suitable for particular use, particularly in electrical contact springs, in certain pieces of jewelry, such as fasteners, and in cable ducts for high temperature superconductor.
• Mg magnesium
• Ni nickel
• nickel is a serious drawback. Indeed, this metal is highly allergenic, which greatly limits its use in jewelry. Furthermore, it has been found that nickel is a poison for the superconducting material and an alloy Ag-Mg-Ni may be used directly in the ducts for superconducting cables at high temperature. As described in EP-02405215.1 in the name of the applicant, it is necessary to insert a layer of pure silver between the alloy layer and the superconducting cable.
• the various constituents are, for example, melted by induction in a graphite crucible and then, the liquid is poured into a steel ingot mold or graphite.
• the ingot is then deformed cold or hot in the desired form and exposed to a current of air or oxygen, at a temperature varying from 650 to 730 ° C., which causes the oxidation of magnesium to MgO.
• This operation makes it possible to harden the alloy while retaining, thanks to the presence of nickel which plays the role of refiner, grains of size less than 20 ⁇ m.
• GB 866 082 discloses a document direct oxidation process of magnesium of an Ag-Mg alloy. However, if one works well, resulting alloy has large grains, making it brittle and unsuitable for the intended applications.
• Figure 1 shows a metallographic section of a plate of Ag-Mg alloy (0.9 atomic% of Mg) was exposed for 1 hour to a stream of oxygen at a temperature of 650 ° C. Observations and measurements carried out show that the outer layers 10 of the plate undergo oxidation and exhibit a high hardness of about 155HV, compared with the starting alloy having a hardness of 50HV. It should be noted, however, that the size of the constituent grains of the alloy is of the order of 50 .mu.m.
• the present invention aims to provide an alloy retaining the Ag-Mg-Ni properties, particularly due to its fine grains, and without the drawbacks mentioned above.
• the invention relates to a method for producing an alloy part based on silver, characterized in that it consists in building an initial alloy containing silver and at least one metal soluble in silver at contents between 0.04 and 4 atomic%, and capable of forming a stable oxide at high temperature, then successively carrying out the following operations:
• the oxygenation is carried out by exposing the pre-alloy to an oxygen stream at a temperature of about 300 ° C.
• the initial alloy is a part having the desired final shape.
• complete oxidation occurs in the continuity of the partial oxidation.
• the initial alloy is a part having an intermediate form, such as a wire, a tube or tape.
• partial oxidation is carried out by placing the oxygenated room for about an hour in an inert atmosphere or under vacuum, at a temperature between 400 and 850 ° C. The part is then put into its final form before total oxidation.
• the initial alloy is in the form of powder.
• the powder is compacted before oxygenation, so keep an open porosity throughout its thickness.
• the piece thus obtained is hot extruded, causing its partial oxidation. It is then put into its final form before total oxidation.
• the initial alloy is in powder form, but the latter is compacted after oxygenation.
• the total oxidation is carried out by exposing the workpiece to an oxidizing atmosphere at a temperature between 400 and 850 ° C.
• the invention also relates to a silver-based alloy, characterized in that it contains at least one metal, soluble in silver and capable of forming an oxide stable at high temperature and which, by internal oxidation, hardens it, while making it possible to obtain a final grain size of less than 20 ⁇ m.
• the metal alloyed with silver is selected from magnesium, aluminum, titanium, gallium, manganese and zinc or a combination of these metals.
• the content is between 0.04 and 4 atomic%.
• Figures 2 and 3 are metallographic sections of Ag-Mg alloy plates, respectively, after step d 'oxygenation and after total oxidation according to the invention.
• a first implementation mode of the invention one starts from a workpiece in a single Ag-Mg alloy having the desired final shape.
• the alloy used is generally in the cold worked condition, with a cross-section reduction rate of about 50 to 95%. Any preliminary heat treatments were carried out under an inert or reducing atmosphere at a temperature low enough to maintain a fine grain. Typically, the various operations lasted one hour at a temperature of about 500 ° C.
• the alloy has a Mg content equal to that referred to the final application. Generally, this content is between 0.04 and 4 atomic%.
• the first phase of the process consists in subjecting the workpiece oxygenation. To this end, a current of oxygen flows in contact with it, at a temperature of approximately 300 ° C., for a time necessary to obtain the desired penetration. Typically, this period is 24 hours for a penetration of 50 microns but may be reduced by increasing the oxygen partial pressure. Under these conditions, oxygen diffuses inside the room and dissolved in silver without oxidizing the magnesium significantly. The hardness of the alloy is not increased and grain remains end.
• Figure 2 shows the effect of the oxygenation on an alloy plate
• the outer layers 12 have small grains, less than 20 ⁇ m.
• the hardness measurement gives 57HV for the external layers and 51 HV for the central layer 14.
• the part is placed under a stream of air or oxygen, at a temperature of between 400 and 850 ° C., preferably around 600 ° C.
• the magnesium is then oxidized to MgO.
• the duration of this phase depends on the temperature, the partial pressure of oxygen and the thickness of the desired oxide layer.
• the oxygen dissolved in silver oxide during the step of oxygenating immediately part magnesium.
• the amount of oxygen dissolved in silver being insufficient to oxidize all the magnesium present in the alloy, the partial oxidation is then and is formed of MgO precipitate particles that fit into the matrix of the alloy and prevent the coarsening of Ag-Mg grains by blocking their joints.
• the oxygen present in the atmosphere continues the oxidation of magnesium for a while, so as to oxidize at least one outer layer.
• the alloy hardens so but, thanks to the presence of precipitated particles that act as dispersoid, the size of the grains is less than 20 .mu.m.
• the prior oxygenation has not been carried out over the entire thickness of the part, it is therefore important that the oxidation takes place at a depth equal to or less than that of the oxygenated layer, in order to '' avoid any risk of breakage.
• the alloy thus obtained has qualities similar to a conventional Ag-Mg-Ni alloy but, since it does not contain nickel, it is not allergenic and does not pollute superconductive materials at high temperature. However, the alloy formed is very hard and can therefore be difficult to shape.
• an intermediate piece for example in the form of a wire, a tube or a strip, made of an Ag-Mg alloy. This part first undergoes, as in the first embodiment, an oxygenation phase.
• the workpiece is placed for approximately one hour in vacuum or in an inert atmosphere (e.g. nitrogen or argon) at a temperature between 400 and 850 ° C.
• an inert atmosphere e.g. nitrogen or argon
• MgO magnesium oxide
• the workpiece is still malleable and its final form is then given to him, for example, by rolling, drawing, cutting, bending, stamping or drawing ..., these techniques being well known to those skilled in the art .
• the starting material is an alloy of silver and magnesium in powder form, which is then compacted, while maintaining an open porosity throughout its thickness, an intermediate form for example, a cylindrical billet of diameter 100mm and length 500mm. Then, as in the first embodiment, the workpiece undergoes oxygenation phase.
• the following operation is a hot extrusion of the compacted part. To do this, it is first preheated, to a temperature between 400 ° C and 850 ° C and in an inert atmosphere, which automatically causes the initiation of the partial oxidation phase. We then proceed to the extrusion and final shaping of the part before performing, finally, the complete oxidation of magnesium.
• the phase of oxygenation may well take place before compacting the alloy.
• the magnesium content is between 0.04 and 4 atomic%. It goes without saying, however, that the magnesium may be replaced partially or completely by any soluble metal silver to the above contents and capable of curing in forming the high temperature stable oxide. Furthermore, to provide a material with acceptable mechanical properties, these elements must, in the oxidized state, provide alloy grains of size less than 20 ⁇ m. For example, aluminum, titanium, gallium, manganese or zinc can be used, for example.
• a method which allows to obtain a silver-based alloy, made very hard by the presence of a metal oxide and retaining a grain • particularly fine.
• the resulting alloy can be particularly used for certain pieces of jewelry, without presenting any particular risk of allergy, or even in sheaths for superconductive cable at high temperature, without polluting the superconductive material.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Powder Metallurgy (AREA)
• Materials For Medical Uses (AREA)

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• 2004
  • 2004-04-02 EP EP04725262A patent/EP1613786A1/de not_active Withdrawn
  • 2004-04-02 WO PCT/CH2004/000208 patent/WO2004087972A1/fr not_active Application Discontinuation
  • 2004-04-02 US US10/551,995 patent/US20060272753A1/en not_active Abandoned

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