FR2985521A1 - Producing copper alloy used in aeronautics, comprises fragmenting initial alloy to obtain elemental fragments, mechanically synthesizing fragments by grinding the fragments in ball mill with high energy and flash sintering obtained powders - Google Patents

Abstract

Producing a copper alloy with high mechanical properties, from an initial alloy, comprises fragmenting the initial alloy to obtain elemental fragments, which correspond to pieces of the initial alloy, mechanically synthesizing the fragments consisting of grinding the fragments with high energy by mechanical impact of balls on the fragments in a mill, to obtain a powder whose particles have a size of nanometric crystalline structures, and flash sintering the powders by subjecting the powder to a high current, low voltage and high frequency.

Description

The present invention relates to the field of copper-based alloys. The present invention will find its application mainly in the field of the manufacture of copper-based alloys having a very high level of mechanical characteristics. The invention more particularly relates to a method, the steps and parameters of which make it possible to obtain an alloy having a very high level of mechanical characteristics. The invention also relates to an alloy obtained by said process. Traditionally, it is known from the state of the art to make alloys comprising both copper and beryllium. These alloys are known to exhibit outstanding strength and electrical conductivity characteristics for a copper-based alloy. Such an alloy may for example consist of a CuBe2 alloy with 98% copper and 2% beryllium according to AMS 4533: C17200. However, beryllium has the disadvantage of being highly toxic. Indeed, this metal is likely to cause a disease called chronic beryllium disease, or berylliosis, which affects the lungs and can progress to severe cardiorespiratory failure. In addition, several studies have shown that exposure to beryllium can lead to the development of lung cancers. As a result, beryllium and its derivatives are classified as carcinogens. It is therefore necessary to replace the alloys comprising beryllium, while maintaining a high level of mechanical characteristics. There are also beryllium-free copper alloys having mechanical characteristics equivalent to those of the CuBe2 alloy. Such an alloy may for example comprise, in addition to copper, nickel Ni and tin Sn, such as CuNil5Sn8. The latter is an alloy known to exhibit high spinodal decomposition curing by combining heat treatments and cold-forming operations, but the cold-converting operation required to move from an elastic limit of 800 MPa to 1000 MPa. can not be implemented if the remarkable size of the product (diameter and / or thickness) exceeds 50 mm.

Methods for working metals to obtain improved powders or alloys are also known from the state of the art. In particular, we know a technique called "flash sintering" which consists of heating a powder but without leading to the merger. This technique combines the use of a uniaxial press and a pulsed current passage of high intensity and low voltage. Research involving, in particular, metal powders subjected to the flash sintering technique has been conducted by scientists (Xie et al., 2003, Song et al., 2005, Wen et al., 2010). However, the methods known from the state of the art do not make it possible to obtain a copper-based alloy having a very high level of mechanical characteristics. The invention offers the possibility of overcoming the various drawbacks of the state of the art by proposing a process which makes it possible to obtain a copper-based alloy, constituting an alternative to existing copper-beryllium alloys, and meeting a certain required for a very high level of mechanical characteristics to be achieved. To this end, the present invention relates to a process for obtaining a cuprous alloy with a very high level of mechanical characteristics from an initial alloy characterized in that: said original alloy is fragmented to obtain elementary fragments which correspond to pieces of the initial alloy, a mechano-synthesis step is carried out on said elementary fragments, the mecanosynthesis consisting of a grinding of said high energy fragments, said grinding being obtained by mechanical shocks of balls on said fragments in a mill, the mechano-synthesis to obtain powders whose particles have crystalline structures with a nanometric size. a flash sintering step is carried out consisting in consolidating the powders obtained in the previous step by subjecting them to a current of high intensity, low voltage and high frequency. By crystalline structure, or structure of a crystal, is meant the arrangement of atoms in a crystal, said atoms periodically repeating themselves in space. According to one embodiment, the fragmentation of said alloy is made from raw parts, such as ingots, bars or sheets. According to a different example, the fragmentation of said alloy can be obtained by spraying the liquid alloy. In a particularly interesting way, the process according to the present invention is carried out on an initial copper-aluminum-nickel-iron alloy, or a copper-nickelaluminium alloy, or a copper-nickel-silicon alloy. Preferably, the process may in particular be carried out on a cuprous alloy CuA111Ni5Fe5 whose mass composition comprises, in addition to copper, substantially 11% of aluminum and substantially 5% of nickel and substantially 5% of iron. In a particularly preferred manner, a grinding additive is added to the powder during the mechano-synthesis step, said additive consisting of an organic acid chosen from stearic acid and / or oleic acid. More preferably, said additive is added in a proportion of between 0.1 and 1% by weight of the powder, and preferably the proportion of additive is substantially equal to 0.5% of the weight of the powder. According to another advantageous exemplary embodiment, an agglomerated ceramic having a nanometric structure is added to the powder during the mechano-synthesis step, said ceramic consisting of aluminum oxide A1202 and / or oxide titanium TiO 2 and / or zirconium oxide ZrO 2.

More preferably still, said ceramic is added to the powder in a proportion of between 1 and 5% of the weight of said powder and preferably between 1 and 2%. According to yet another embodiment, activated or unactivated powders are introduced to the treated powder during the mechano-synthesis step to obtain a dual structure comprising a hard phase and a ductile phase, said activated powders consisting of powders which have already been ground beforehand by mechanical synthesis and said unactivated powders being powders which have not yet been crushed. The term "hard phase" is understood to mean a phase comprising a nanometric particle volume fraction while, in contrast, a ductile phase has a low nanometric volume fraction. Particularly advantageously, the activated or unactivated powders are introduced into the powder during the mechano-synthesis step in proportions of between 10 and 70% of the weight of said powder. According to a particularly preferred embodiment of the process according to the present invention, the mechano-synthesis step is carried out in a planetary mill comprising at least one rotary disc supporting grinding containers loaded with elementary fragments to be treated, said disc rotary mill rotating in one direction at a rotation speed of between 250 and 400 rpm and grinding containers rotating in the opposite direction at a rotational speed of between 100 and 200 rpm. Interestingly, the duration of the mechano-synthesis step is between 1h and 30h. According to yet another preferred embodiment, the parameters applied during the flash sintering step are the following: an initial pressure of 2kN, a consolidation pressure of between 10 and 25 kN, a consolidation temperature. between 450 ° C. and 950 ° C., preferably between 750 ° C. and 875 ° C., a temperature rise substantially equal to 100 ° C./min, a duration of maintenance of the consolidation temperature of between 1s and 900s. preferably between 120 and 180 s.

Other features and advantages of the invention will emerge from the following detailed description of non-limiting embodiments of the invention, with reference to the appended figures in which: FIG. 1 schematically illustrates the various steps of the method, the FIG. 2 represents the schematic diagram of a spectrometer of DNPA (Small-angle Neutron scattering).

The method according to the present invention relates in particular to alloys based on copper and also including, in particular but not limited to, nickel and / or aluminum and / or other constituents such as iron, tin, nickel and / or aluminum. silicon, chromium, manganese. The required level in terms of mechanical characteristics to be achieved by applying the method according to the invention, for example on an initial alloy, called NCS, comprising copper, aluminum, nickel and iron, is the following: a resistance at rupture (Rm) substantially between 800 and 1200 MPa (mega Pascals), and / or a conventional yield strength corresponding to an elongation at 0.2% (Re 0.2) substantially greater than 800 MPa, and / or a Vickers hardness (HV) substantially greater than 310, and / or an elongation (A%) substantially greater than 2%. Particularly advantageously, the method according to the present invention allows the NCS alloy to which it is applied to combine all the characteristics. mentioned above, in particular for applications in the field of aeronautics. In comparison, the same conventional NCS alloy has lower mechanical properties: a 900 MPa Rm resistance, a 680 MPa Re 0.2 conventional yield strength, a 255 Vickers HV hardness and an A elongation. % of 6%. These characteristics are insufficient for an alloy intended to manufacture a part used in aeronautics.

The parameter Rm is defined as being the limit of the tensile strength, that is to say the tensile force from which the tensile specimen reaches the rupture. The conventional elastic limit Re 0.2% corresponds to the stress from which a material stops deforming in an elastic, reversible manner, and thus begins to irreversibly deform. The Vickers hardness, or HV, of an alloy is measured by the impression made by a diamond pyramid under a given load for 15 seconds. Finally, the elongation at break or percentage elongation, denoted A%, defines the capacity of an alloy to elongate before breaking when it is stressed in tension. More particularly, the alloys to which the method according to the present invention can be applied may be standardized cuprous alloys of which one and / or the other mechanical characteristics do not meet the level required for a certain application. For example, the copper-based alloys on which the process can be used to obtain an alloy having exceptional mechanical characteristics are in particular: copper-nickel-aluminum alloys (Cu-Ni-Al): for example, the alloy called K5 which comprises about 80% copper, 14% nickel and 3% aluminum. copper-nickel-silicon alloys (Cu-Ni-Si): in particular, the alloy noted NS30 which consists of approximately 88% copper, 7% nickel, 4% silicon and also 1% chromium. copper-aluminum-nickel-iron (Cu-Al-NiFe) alloys: for example, the NCS alloy which is composed of about 70% copper, 11% aluminum, 5% nickel and 5% iron . The alloy on which the process is applied can also be CuA111Ni5Fe5 which has a Vickers hardness of 230 or 270 with a heat treatment or else CuA111Ni5Fe5 with a Vickers hardness of 370 obtained by a cold work hardening. Such an alloy has proportions of 11% by weight of aluminum, 5% by weight of nickel, 5% by weight of iron, and the balance of copper. The mechanical characteristics of a metal or an alloy are determined by the microscopic structure thereof, and in particular by the interaction between dislocation movements and their micro-structural environment. The object of the invention is therefore to improve the mechanical characteristics of an alloy by modifying the movements and the nature of the dislocations but without totally hindering them, otherwise obtaining a too fragile alloy. It is therefore necessary to introduce, through mechanical and / or thermal treatments, crystalline defects within the alloy; these may consist in particular of nanostructures and / or nanoparticles generated in situ or introduced, such as grain boundaries, nanoparticles of oxides or carbides, interphases, dislocations, atomic substitutions, precipitates etc. Thus, in a particularly advantageous manner, the process according to the present invention makes it possible to obtain an alloy in which, in particular, crystalline defects have been introduced in order to prevent the propagation of dislocations. The method according to the present invention thus advantageously comprises a first step which consists of a fragmentation of an initial alloy making it possible to obtain elementary fragments. These latter correspond to pieces of the initial alloy and typically have a size of substantially between 1 and 10 mm after the fragmentation step. The fragmentation is preferably carried out on raw parts, which are manufactured from the initial alloy whose mechanical characteristics are to be improved. In particular, such blanks may consist of raw casting ingots, bars or sheets derived in particular from the transformation of ingots.

However, such an embodiment is not limiting of the invention. For example, as an alternative to the fragmentation carried out on raw solid parts of the alloy in question, the fragments can be obtained by spraying liquid metal.

It is not necessary that the elementary fragments each have the exact composition of the alloy under consideration. In this case, it will be necessary to mix fragments of different compositions so that the mixture has the required composition. There are several fragmentation conditions, and these may or may not require a prior embrittling heat treatment. This heat treatment may for example consist of a quenching treatment. The fragmentation step according to the present invention therefore makes it possible in particular to obtain elementary fragments from solid pieces. Advantageously, it is not necessary to mix fragments of different compositions, since each fragment has the composition of the starting solid part. On the other hand, the starting piece can be made fragile before fragmentation, which favors the generation of crystalline defects in each fragment and the rupture of surface oxide layers which would be likely to hinder the subsequent steps of the process. The various types of fragmentation that can be applied in the process according to the present invention have been optimized and vary depending on the starting alloy on which the process is carried out.

Indeed, according to a particular example, the process is carried out on alloys whose compositions have been detailed above and for which preferred modes of the method according to the invention have been determined. However, the implementation of the method on this type of alloy is not limiting of the invention. Indeed, said method can also be applied to other types of alloys. The various types of fragmentation that can be carried out will be detailed in the examples below which describe preferred but non-limiting embodiments of the method according to the invention. The second step of the process consists of a step called "mechano-synthesis". This mechano-synthesis step consists in the application, in a mill, of a succession of mechanical shocks of beads on the elementary fragments obtained by the preliminary fragmentation step. In case of use of elementary fragments having different compositions, the mechano-synthesis step will advantageously make it possible to ensure the synthesis of the powders. The application of these mechanical shocks will make it possible to obtain micrometric powders from elementary fragments having a millimetric size (between 1 mm and 10 mm). The powders obtained thus have grains of powder, or particles, of micrometric size; in addition, said particles have crystalline structures having singularities with a size of the order of one nanometer. The powders obtained after the mechano-synthesis step are therefore called nanostructured micrometric powders or nanostructured powders. Advantageously, the mechano-synthesis makes it possible to avoid all risks related to the exposure of nanoparticles. The particles before mechano-synthesis are millimetric (typically 1 to 10 mm). After mechanical synthesis the powders are composed of micrometric particles which advantageously all have the same composition; in addition, said particles are nanostructured. These powders obtained by mechano-synthesis are characterized by a stored energy which leads to a mechanical activation that can facilitate a subsequent consolidation step. This step can then be carried out at lower temperatures and for a shorter time. Another advantage of the mechano-synthesis technique lies in the fact that it makes it possible to obtain materials in quantities which are expressed in kilograms, or even in tons. Advantageously, the mechano-synthesis step is carried out in a "planetary mill" apparatus. The planetary mill consists of at least one rotating disc which supports grinding containers, also called jars, in which are placed the elementary fragments from the fragmentation step and which must be ground into powder form. The jars of the planetary mill advantageously contain grinding balls which may in particular be oxide balls or balls of tungsten carbide or hard steel. The number and the diameter of said grinding balls can vary, in particular according to the initial alloy on which the method according to the present invention is implemented. This advantageously makes it possible to optimize the mechano-synthesis step as much as possible. In the process according to the invention, preferably, the balls which serve to grind the fragments and then the powder may have a diameter of 15 mm and / or 12 mm and / or 8 mm and / or 4 mm. Thus, the mechanosynthesis may be carried out either in the presence of a single type of bead or of several types of bead. Typically, each of the jars of the mill is filled with the same amount of powder that is to be milled, for example each jar contains 10 g of said powder. According to a particularly advantageous embodiment, the grinding is carried out with a speed of rotation of the rotating disc of between 250 and 400 revolutions / min and in a particular direction of rotation. More preferably still, this speed is substantially close to 350 rpm. Preferably, the grinding containers in which the grinding powder is placed rotate at a rotation speed of between 100 and 200 revolutions / min in the opposite direction of the rotation disc. During this grinding step, the fragments and then the powders will continuously be flattened, fractured and welded. Fracture and powder welding are the critical steps to obtain a homogeneous mixture of said powders. The parameters applied during the process according to the invention, and particularly during the fragmentation or mechano-synthesis step, lead to a stabilization of the nanostructure of the powders obtained. Indeed, the combination of the rotational speeds used in the present process and the diameters of the rotating disk, jars or grinding balls, will generate different trajectories and impact forces that will achieve the desired goal, namely the reduction of the size of the particles, or grains, crushed powders. Advantageously, the mechano-synthesis step is carried out for a duration that can vary between 1 h and 30 h. Such a duration makes it possible to obtain an optimal structure of the ground powders. According to a particularly advantageous embodiment, a grinding additive is added to the powder which is being grinded during the mechano-synthesis step. This grinding additive consists of an organic acid selected from oleic acid and / or stearic acid. More particularly, oleic acid is a monounsaturated fatty acid while stearic acid is a saturated fatty acid. During the mechano-synthesis step, certain alloys having a ductile structure, that is to say with an ability to deform plastically without breaking, tend to agglomerate with the media of the mill (balls). and containers) and to aggregate to form large particles, up to several millimeters in diameter. The grinding additive must therefore make it possible to avoid such aggregation, which consists of a cold welding of the particles. Thus, in the presence of additive, the powders obtained are finer. In a particularly preferred manner, these organic acids are added to the treated powder in the planetary mill containers in a proportion of between 0.1% and 1% by weight of the said treated powder. More preferably still, the proportion of organic acid added is substantially close to 0.5% of the weight of the treated powder. In a particularly advantageous manner, these organic acids will cause the formation of a film on the one hand on the surface of the grains of the powder, and on the other hand on the inner surface of the jars or grinding containers as well as on the surface. grinding balls. The formation of such a film causes a change in the chemical nature of the surfaces it covers. This will advantageously prevent the cold welding of the grains, the powder but also slow down the process of agglomeration of grains between them. Thus, the powders obtained are thinner which will contribute to the improvement of the mechanical characteristics of the treated alloy.

However, it was also very important to optimize the amount of organic acid added to the powder. Indeed, if we add an unsuitable amount of this acid, in particular too much of said organic acid, it is then likely to interfere with the grinding of the powders. This is explained by the fact that the presence of a film on the surface of the balls, powders and grinding containers modifies the friction coefficients of the surfaces and a too large amount of organic acid can hinder the smooth running of the surface. grinding powders.

Advantageously, a first milling time, or mechano-synthesis, is carried out in the absence of organic acid and then at a second milling time during which the milled powder is mixed in the milling container. acid audit. In particular, the time of introduction of the organic acid to the ground powder corresponds to a grinding time varying from 4 to 20 hours. In other words, the organic acid is added to the powder when it has already undergone grinding for a period of 4h to 20h. According to another embodiment of the process according to the present invention, it is also advantageous, with the aim of further increasing the mechanical characteristics, to add agglomerated ceramics having a nanometric structure to the powder being grinded during the process. step of mechano-synthesis. Said ceramics preferably consist of agglomerated oxides of nanometric structure and / or carbides. Indeed, the nanostructured powders may have a structural instability inherent in their structure. It is then necessary, to stabilize these nanostructured powders, to slow down or stop the growth of the grain size. For this, it is advantageous to introduce ceramics to the milled powder during the mechano-synthesis step. Preferably, these added ceramics consist of aluminum oxide Al 2 O 2 and / or titanium oxide TiO 2 and / or zirconium oxide ZrO 2. These oxides will participate in the formation of second phase inclusions which will act as stopping sites for the grain boundaries. Thus, thanks to the addition of ceramics, in particular oxides, the growth of the grain size will be considerably slowed down. According to a particular embodiment of the process according to the present invention, the ceramics are added in a preferential proportion of between 1 and 5% of the weight of the ground powder. More preferably still, this proportion is substantially between 1 and 2%. According to yet another advantageous embodiment of the present process, activated or unactivated powders may also be added to the powder which is ground during the mechano-synthesis.

The activated powders consist of powders that have already undergone a mechano-synthesis step. Advantageously, this mechano-synthesis step making it possible to obtain the activated powders is of shorter duration, typically 2 hours. On the other hand, the unactivated powders have not undergone a prior grinding step by mechano-synthesis. The contact between the milled powder and the activated or non-activated powder is preferentially done during the mechano-synthesis step. However, such an embodiment is not limiting; indeed, such contacting can also take place at another time of the process, especially during the filling of the sintering container. The introduction of these activated powders or not allows to obtain a dual structure. For the record, the dual structure of an alloy is a structure in which a distinction can be made between a hard phase, called martensite, and a ductile phase, ferrite. The presence of a dual structure within the alloy by introduction of activated powders or not further increases the mechanical characteristics of the alloy obtained.

Preferably, the activated or unactivated powders are added to the ground powder, for example in the grinding container, in a proportion of between 10 and 70% by weight of said ground powder. The mechano-synthesis step is therefore an essential step of the method according to the present invention. It advantageously makes it possible to obtain powders having a nanostructure, that is to say a certain proportion of particles having nanostructures whose size is of the order of one nanometer or even ten nanometers. These nanostructures are advantageously preserved, at least in part, during the implementation of the next step of the process, which consists of a consolidation. The third step of the present process therefore consists of the consolidation of the nanostructured powders, also called "flash sintering". This step makes it possible to densify the powder obtained after grinding by mechano-synthesis, but without conducting said powder until melting. Under the effect of energy, the grains will weld together, without going to the fusion, thus allowing the cohesion of the part and the consolidation of the powders. Flash sintering, also called SPS for Spark Plasma Sintering, is particularly advantageous compared with traditional hot sintering because it offers reduced working times and therefore better conservation of the nanostructures developed during the mechano-synthesis. The principle of the flash sintering technique is based on the use of a uni-axial press and on the combination of this one with the passage of a pulsed current of high intensity, which can be between 5000 and 15000A, and under low voltage, typically less than 50V, preferably of the order of 10 to 20V. Flash sintering is also preferably carried out at high frequency, for example of the order of 1000 Hz. The pressure applied during this flash sintering step, also called consolidation pressure, is relatively low, preferably between 10 and 25 kN and even more preferably between 16 and 23 kN. Preferably, the flash sintering is carried out under vacuum by pulsed current directly through the powders resulting from the mechano-synthesis. One of the advantages of implementing a flash sintering step is that this step is very fast; in fact, the densification of the grains of the powder is carried out on a scale of one minute. In contrast, conventional densification methods generally require approximately 1 h to allow consolidation of the alloy. Thus, in a particularly preferred manner, the flash sintering step is carried out on the powders obtained after the mechano-synthesis step for a duration of substantially between 1 and 900 seconds and even more preferably between 120 and 180. seconds. Such a holding time of the consolidation temperature makes it possible to obtain an alloy having optimum mechanical characteristics. In this regard, the consolidation temperature applied during this flash sintering step is advantageously between 450 ° C. and 950 ° C., and more advantageously still, this consolidation temperature is between 750 ° C. and 875 ° C. According to a particularly interesting embodiment of the flash sintering step, the rise in temperature is substantially 100 ° C / min. Preferably, the initial piston pressure applied during consolidation by sintering is 2 kN. According to a preferred embodiment, in order to carry out the flash sintering step, a graphite container having a diameter of 20 mm is manually filled with a ground powder mass, resulting from the mechano-synthesis step, said mass being preferentially between 5 and 10g. Advantageously, intermediate steps can be introduced during the powder consolidation process by flash sintering. For example, degassing can be performed, which corresponds to a delayed pressurization. It is also planned to be able to maintain a temperature range, especially at a temperature substantially close to 450 ° C, for a period of approximately 3 min and a pressure of 5kN. Finally, it is possible to implement a temperature rise phase, without applying pressure, and to apply only a final pressure when the maximum temperature is reached. The method according to the present invention is therefore particularly interesting because it allows, once the mechano-synthesis step is complete, obtaining nanostructured powders, that is to say comprising particles having structures. crystalline particles ranging substantially from nanometer to about ten nanometers. The presence of this nanostructure, which is advantageously preserved during the subsequent consolidation step by flash sintering, and which can be reinforced by the presence of the ceramic nanoparticles, makes it possible to obtain an alloy which has very high mechanical characteristics, very higher than those of the same alloy developed in the traditional way. The nanostructures of the undoped alloys, that is to say obtained by the process according to the invention but without the addition of ceramics, have been studied by the technique of Neutron scattering with small angles (DNPA) which makes it possible to detect particles or fine inhomogeneities at the nanometer scale. For metal alloys, the DNPA technique makes it possible to obtain quantitative statistical information, for example the mean size and size distribution, the shape, or the volume fraction, on nanoscale particles in samples of macroscopic dimensions. , of the order of centimetercube. This technique is particularly interesting for the study of materials obtained by mechano-synthesis because it makes it possible to follow the evolution of the nanostructure at each stage of the elaboration, that is to say after grinding or after consolidation treatment. flash sintering. The principle of the technique is as follows: if a monochromatic beam of thermal neutrons is sent onto a solid sample containing heterogeneities or nanoparticles, the transmitted beam is enlarged, and this broadening is roughly inversely proportional to the average particle size . The DNPA technique has been implemented in particular on the NCS and K5 alloys, the compositions of which are detailed above, after these have been subjected to the various steps of the process according to the present invention described above. More particularly, this technique made it possible to follow the evolution of the internal structure of the alloy, during the various stages of the process, and particularly when the alloy is in the raw state (before mechano-synthesis), then at the state of powder (after mechano-synthesis) and finally in the consolidated state (after the step of sintering-flash). The schematic diagram of a DNAP spectrometer is shown in appended FIG. The first series of measurements were performed on samples whose size and percentage of nanoparticles are known. This allowed to validate the technique used in copper alloys. These "control" alloys tested are a Cu-0.7% Cr-0.05% Zr alloy with structural hardening and a Cu-A1203 dispersoid alloy. The results of these analyzes have made it possible to show that, for the undoped K5 and NCS tested alloys, that is to say in which there was no addition of ceramics, the diffused signal is very important and results the distribution of sizes of different nanostructured objects. The results relating to each of the alloys are detailed in the following examples, which are intended to illustrate the preferred embodiments of the method according to the present invention and are in no way limiting of the invention. EXAMPLE 1 Application of the Process to the Undoped NCS Alloy From raw ingots obtained with the NCS-listed alloy whose composition is listed above, a pre-milling step is carried out by machining by milling. This machining leads to the formation of chips that have a shape of coils whose total length varies between 60 and 100 mm for an approximate width of 5 mm. These coils are then broken up with a knife mill (Fritsch - Labo NRG), and the size of the fragments obtained varies between 1 * lmm2 and 7 * 7mm2, typically 2 * 4mm2. The thickness of these fragments is less than 1 mm. The yield of the fragmentation step is greater than 99.5%.

A mass of 10 g of these fragments is then milled in a planetary mill type Vario Mill from Fritsch. The grinding container has a radius of 32.5mm. Steel balls are added to said grinding container. More particularly, with respect to the NCS alloy, 5 steel balls having a diameter of 15 mm are added. The grinding conditions applied are then as follows: the speed of the mill rotation plate rotates at a rotation speed of 350 rpm in a defined direction and the grinding containers rotate in the opposite direction at a speed of 200 rpm. The grinding lasts 12 hours. During the grinding stage, stearic acid is added in a proportion of 0.5% by weight of the ground powder, ie approximately 0.05 g of stearic acid. This is introduced to the milled powder 4 hours after the grinding of said powders has started. The flash sintering conditions applied are as follows: an initial pressure of the pistons of 2kN a consolidation pressure of 15 to 23kN a consolidation temperature of 700 at 800 ° C a rise in temperature substantially equal to 100 ° C / min a temperature holding time of 120 and 180 s. The samples obtained are then analyzed according to the DNPA technique. The results obtained are as follows: in the consolidated state after sintering, two size distributions having a spherical shape are obtained: the first centered between 11.8 nm and 15.8 nm and the second centered between 0.9 nm and 35 nm. 2.1 nm. The respective proportions of these particles are between 0.03% and 0.081% for the first distribution and between 0.024 and 0.11% for the second.

EXAMPLE 2 Application of the Process to the Undoped K5 Alloy From rough ingots obtained with the alloy denoted K5, a fragmentation step is carried out by performing machining by milling. Fragments having a helical shape 10 are then obtained, which have an approximate length of 10 mm for a width of about 2 to 3 mm and a thickness of less than 1 mm. The K5 alloy fragments are directly usable for crushing. A mass of 10 g of these fragments is then milled in a planetary mill of the "Vario Mill" type from Fritsch. The grinding container has a radius of 32.5mm. Steel balls are added to said grinding container. More particularly, with respect to the K5 alloy, 5 steel balls with a diameter of 15 mm and 20 balls of 4 mm are added. The grinding conditions applied are equivalent to those used in Example 2. During the grinding step, stearic acid is added in a proportion of 0.5% by weight of the ground powder, ie approximately 0.05 g of stearic acid. This is introduced to the ground powder after 4 hours or after 8 hours or after 10 hours of grinding. The flash sintering conditions applied are as follows: an initial pressure of the pistons of 2kN a consolidation pressure of 15 to 23kN a consolidation temperature of 750 to 850 ° C a rise in temperature substantially equal to 100 ° C / min a duration maintaining the temperature of 120 to 180sec. After analysis by DNPA, the results obtained are as follows: in the consolidated state, that is to say after the sintering step, a first size distribution having an ellipsoidal shape whose mean radius is obtained equivalent 5 was calculated: the radius of the particles obtained is between 12.6 nm and 14.1 nm. The proportion of these ellipsoidal particles is between 0.03% and 0.06%. The second distribution obtained concerns spherical particles whose radius is between 1.9 nm and 2.3 nm. The proportion of these spherical particles of the order of 0.04% to 0.09%. Of course, the invention is not limited to the examples illustrated and described above which may have variants and modifications without departing from the scope of the invention. 15

Claims (14)

CLAIMS1.Procédé for obtaining a copper alloy with a very high level of mechanical characteristics from an initial alloy 5 characterized in that: - said initial alloy is fragmented to obtain elementary fragments which correspond to pieces of l initial alloy, - a mechano-synthesis step is carried out on said elementary fragments, the mechanosynthesis consisting in grinding said high energy fragments, said grinding being obtained by mechanical shocks of beads on said fragments in a mill, the mechanosynthesis making it possible to obtain powders whose particles have crystalline structures with a nanometric size; a flash sintering step is carried out consisting in consolidating the powders obtained in the preceding step by subjecting them to a stream of high intensity, low voltage and high frequency. 20 2.Procédé for obtaining a cuprous alloy according to claim 1 characterized in that the fragmentation of said alloy is made from raw parts, such as ingots, bars or sheets. 3.Procédé for obtaining a cuprous alloy according to claim 1 characterized in that the fragmentation of said alloy can be obtained by spraying the liquid alloy. 4.Procédé for obtaining a cuprous alloy according to any one of claims 1 to 3 characterized in that it is implemented on an initial alloy copper-aluminum-nickel-iron, or on a copper-nickelaluminium alloy, or a copper-nickel-silicon alloy. 5.Procédé for obtaining a cuprous alloy according to claim 4 characterized in that it is implemented on a cuprous alloy CuA111Ni5Fe5 whose mass composition comprises, in addition to copper, substantially 11% of aluminum and substantially 5% nickel and substantially 5% iron. 6.Procédé for obtaining a cuprous alloy according to any one of the preceding claims characterized in that a grinding additive is added to the powder during the mechano-synthesis step, said additive consisting of organic acid selected from stearic acid and / or oleic acid. 7.Procédé for obtaining a cuprous alloy according to claim 6 characterized in that said additive is added in a proportion between 0.1 and 1% of the weight of the powder, preferably the proportion of additive is substantially equal at 0.5% of the weight of the powder. 8.Procédé for obtaining a cuprous alloy according to any one of claims 1 to 7 characterized in that an agglomerated ceramic having a nanometric structure is added to the powder during the mechano-synthesis step, said ceramic consisting of aluminum oxide Al 2 O 3 and / or titanium oxide TiO 2 and / or zirconium oxide ZrO 2. 20 9.Procédé for obtaining a cuprous alloy according to claim 8 characterized in that said ceramic is added to the powder in proportions between 1 and 5% of the weight of said powder and preferably between 1 and 2%. 10. Process for obtaining a cuprous alloy according to any one of claims 1 to 9 characterized in that activated or non-activated powders are introduced to the treated powder during the mechano-synthesis step to obtain a dual structure comprising a hard phase and a ductile phase, said activated powders consisting of powders which have already been previously milled by mechanical synthesis and said unactivated powders being powders which have not yet been ground. 11. Process for obtaining a cuprous alloy according to claim 10, characterized in that the activated or unactivated powders are introduced to the powder during the mechano-synthesis step in proportions of between 10 and 10 parts. and 70% by weight of said powder. 12. Process for obtaining a cuprous alloy according to any one of claims 1 to 11, characterized in that the mechano-synthesis step is carried out in a planetary mill comprising at least one rotary disc supporting grinding containers loaded with elementary fragments to be treated, said rotating disc rotating in one direction at a rotational speed of between 250 and 400 rpm and the grinding containers rotating in the opposite direction at a speed of rotation of between 100 and 200 rpm. 13. Process for obtaining a cuprous alloy according to any one of claims 1 to 12 characterized in that the duration of the mechano-synthesis step varies between 1h and 30h. 15 14. Process for obtaining a cuprous alloy according to any one of claims 1 to 13, characterized in that the parameters applied during the flash sintering step are the following: an initial pressure of 2kN, a consolidation pressure of between 10 and 25 kN; a consolidation temperature of between 450 ° C. and 950 ° C., preferably between 750 ° C. and 875 ° C.; a temperature rise of substantially 100 ° C./min; a maintenance time of the consolidation temperature of between 1s and 900s, preferably between 120 and 180s.