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Method of producing a silver alloy part and alloy used for sameRelated Patent Categories: Metal Treatment, Process Of Modifying Or Maintaining Internal Physical Structure (i.e., Microstructure) Or Chemical Properties Of Metal, Process Of Reactive Coating Of Metal And Process Of Chemical-heat Removing (e.g., Flame-cutting, Etc.) Or Burning Of Metal, Heating Or Cooling Of Solid Metal, Noble Metals (i.e., Silver(ag), Gold(au), Osmium(os), Iridium(ir), Platinum(pt), Ruthenium(ru), Rhodium(rh), Palladium(pd)) Or Alloy Base ThereofMethod of producing a silver alloy part and alloy used for same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060272753, Method of producing a silver alloy part and alloy used for same. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present invention relates to the field of metal alloys. More particularly it relates, on the one hand, to a method for obtaining a part made of an alloy based on silver (Ag) and, on the other hand, to the base alloy used to achieve this. [0002] Silver-based alloys are commonplace. For example silver is mixed with a few tenths of a percent of magnesium (Mg) and nickel (Ni), the latter metal acting as grain refiner. By internal oxidation of the magnesium into magnesium oxide (MgO), this alloy becomes very hard and has useful mechanical properties. In addition, and unlike purely metallic alloys, its hardness and its grain size are preserved after high-temperature treatments, such as brazing. It is also an excellent conductor. Such properties particularly destine it to be used for example in electrical contact springs, in certain jewelry parts, such as clasps, and in high-temperature superconducting cable jackets. [0003] However, the presence of nickel is a serious drawback since this metal is highly allergenic, which greatly limits is use in jewelry. Moreover, it has been found that nickel is a poison for superconducting materials, and an Ag--Mg--Ni alloy cannot be used directly in the jackets for high-temperature superconducting cables. As disclosed in the document EP-02405215.1 in the name of the Applicant, it is therefore necessary to insert a layer of pure silver between the alloy layer and the superconducting cable. [0004] In the current state of the art, to manufacture conventional Ag--Mg--Ni alloy parts, the various constituents are for example induction-melted in a graphite crucible and then the liquid is poured into a steel or graphite mold. The ingot is then cold-deformed or hot-deformed into the desired form and exposed to a stream of air or oxygen at a temperature that may vary from 650 to 730.degree. C., which causes the magnesium to oxidize to MgO. This operation hardens the alloy while still maintaining, thanks to the presence of nickel, which acts as a refiner, grains smaller in size than 20 .mu.m. [0005] To avoid the abovementioned problems caused by the presence of nickel, it would be conceivable simply to apply the same method to a silver/magnesium mixture. [0006] Document GB 866 082 discloses a method for the direct oxidation of magnesium in an Ag--Mg alloy. However, if such an operation is carried out, the resulting alloy has coarse grains, making it brittle and unsuitable for the intended applications. To illustrate the foregoing, FIG. 1 shows a metallographic section of an Ag--Mg (0.9 at % Mg) alloy sheet that has been exposed for 1 h to a stream of oxygen at a temperature of 650.degree. C. Observations and measurements made show that the outer layers 10 of the sheet undergo oxidation and have a high Vickers hardness of around 155, compared with the starting alloy of 50 hardness. However, it should be noted that the size of the constituent grains of the alloy is around 50 .mu.m. [0007] The object of the present invention is to provide an alloy that has the same properties as Ag--Mg--Ni, particularly due to its fine grain size, but does not have the abovementioned drawbacks. [0008] More precisely, the invention relates to a method of producing a part made of a silver-based alloy, characterized in that it consists in taking an initial alloy containing silver and at least one metal soluble in silver at contents of between 0.04 and 4 at % and capable of forming a stable oxide at high temperature, and then carrying out in succession the following operations: [0009] oxygenation of the initial alloy so as to dissolve oxygen into the silver that it contains; [0010] partial oxidation of the soluble metal so as to form precipitate particles that prevent the alloy grains from coarsening; and [0011] complete oxidation, on at least an outer layer, of the soluble metal into an oxide stable at high temperature. [0012] Advantageously, the oxygenation is carried out by exposing the initial alloy to a stream of oxygen at a temperature of about 300.degree. C. [0013] According to a first embodiment, the initial alloy is a part having the desired final form. In this case, the complete oxidation takes place straight after the partial oxidation. [0014] According to a second embodiment, the initial alloy is a part having an intermediate form, such as a wire, a tube or a strip. In this case, the partial oxidation is carried out by placing the oxygenated part for about one hour in an inert atmosphere or in a vacuum, at a temperature of between 400 and 850.degree. C. The part is then made into its final form before the complete oxidation. [0015] According to a third embodiment, the initial alloy is in the form of powder. In this case, the powder is compacted before the oxygenation, so as to preserve an open porosity over its entire thickness. The part thus obtained is extruded hot, which causes it to undergo partial oxidation. It is then made into its final form before the complete oxidation. [0016] According to a fourth embodiment, which constitutes a simple variant of the third embodiment, the initial alloy is also in powder form, but this powder is compacted after the oxygenation. [0017] In all cases, the complete oxidation is carried out by exposing the part to an oxidizing atmosphere at a temperature of between 400 and 850.degree. C. [0018] The invention also relates to a silver-based alloy, characterized in that in that it contains at least one metal which is soluble in silver and capable of forming a stable oxide at high temperature, and which, by internal oxidation, hardens it, while still providing a final grain size of less than 20 .mu.m. [0019] Advantageously, the metal alloyed with the 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 at %. [0020] Other features of the invention will become apparent from the description that follows, given with reference to the appended drawing, in which FIGS. 2 and 3 are metallographic sections of Ag--Mg alloy sheets, after the oxygenation step and after the complete oxidation according to the invention, respectively. [0021] A first method of implementing the invention starts with a part made of a simple Ag--Mg alloy in the desired final form. The alloy used is generally in the work-hardened state, with a section reduction ratio of around 50 to 95%. The optional prior heat treatments were carried out in an inert or reducing atmosphere, at temperature low enough to preserve a fine grain structure. Typically, the various operations lasted one hour at a temperature of about 500.degree. C. The alloy has a magnesium content equal to that intended for the final application. In general, this content is between 0.04 and 4 at %. [0022] The first phase of the method consists in oxygenating the part. For this purpose, a stream of oxygen flows in contact with it at a temperature of about 300.degree. C. for long enough to obtain the desired penetration. Typically, this duration is 24 hours for a penetration of 50 microns, but it may be shortened if the oxygen partial pressure is increased. Under these conditions, the oxygen diffuses into the part and dissolves in the silver without appreciably oxidizing the magnesium. The hardness of the alloy does not increase and its fine grain structure remains. [0023] FIG. 2 clearly shows the effect of this oxygenation on a sheet of Ag--Mg (0.9 at % Mg) alloy. The outer layers 12 of small, less than 20 .mu.m, grain size may be distinguished. Moreover, the Vickers hardness measurement gives a value of 57 for the outer layers and 51 for the central layer 14. Next, in a second phase, the part is placed in a stream of air or oxygen, at a temperature of between 400 and 850.degree. C., preferably about 600.degree. C. The magnesium is then oxidized to MgO. The duration of this phase depends on the temperature, the oxygen partial pressure and the thickness of the oxide layer desired. [0024] However, if the oxidation process thus carried out is examined in greater detail, it will be seen that it comprises two phases. Firstly, the oxygen dissolved in the silver during the oxygenation step immediately oxidizes some of the magnesium. Since the amount of oxygen dissolved in the silver is insufficient to oxidize all the magnesium present in the alloy, the oxidation is therefore partial and precipitate particles are formed that are inserted into the matrix of the alloy and prevent coarsening of the Ag--Mg grains by blocking their grain boundaries. Immediately after this, the oxygen present in the atmosphere continues the oxidation of the magnesium for a certain time, so as to oxidize at least an outer layer of it. The alloy therefore hardens but, thanks to the presence of the precipitate particles that act as dispersoids, the size of these grains is less than 20 .mu.m. [0025] This result is illustrated in FIG. 3. The outer layers 16 that were oxygenated during the first phase of the method have, after oxidation, fine grains with a size of less than 20 .mu.m. The central layer 18 was not oxygenated and, to underline the contribution of the oxygenation phase, it was oxidized during the second phase. Thus, the central region can be directly compared with FIG. 1. Its hardness is that of an Ag--MgO alloy, but the grains are coarse and make the material brittle. The Vickers hardness measurement gives a value of 136 for the outer layers and 147 for the center. [0026] For practical use, if the prior oxygenation is not carried out over the entire thickness of the part, it is therefore important for the oxidation to take place to a depth equal to or less than that of the oxygenated layer, so as to avoid any risk of fracture. [0027] The alloy thus obtained has properties similar to a conventional Ag--Mg--Ni alloy but, since it contains no nickel, it is not allergenic and does not contaminate high-temperature superconducting materials. However, the alloy produced is very hard and therefore cannot be easily formed. Continue reading about Method of producing a silver alloy part and alloy used for same... 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