EP0587494B1 - Composite material base on a magnesium alloy containing zirconium and reinforced by carbon and process for making it - Google Patents

Description

The subject of the present invention is a composite material comprising a metal matrix based on magnesium alloy and carbon reinforcement formed of carbon or graphite fibers or filaments.

The incorporation of particles, fibers or mineral whiskers, to metallic matrices made of low density metals such as alloys based on aluminum or magnesium, provides composite materials with much higher specific mechanical properties to those of traditional alloys, in particular with regard to breaking strength, the rigidity and the conservation of these properties in fatigue and thermal cycling. Among the different reinforcements that can be incorporated into such metal matrices, carbon fibers or graphite have special interest because of their excellent intrinsic characteristics: very high breaking strength (up to 4400 MPa), very high rigidity (up to 800 GPa), coefficient slightly negative thermal expansion and low density (1.76 to 2.18).

So, composite materials including such carbonaceous reinforcements have already found applications in different industrial sectors such as automotive (piston, liner, connecting rods), aeronautics or aerospace (structural elements shuttles, reflector supports, etc.), and it’s very likely that they’re going to boom important in the coming years.

The known methods of manufacturing such composite materials call for techniques in solid phase or in liquid phase techniques.

In the case of solid phase techniques, one can realize, for example, the material by extrusion of a mixture of the constituents or by thermocompression of layers of fibers forming the reinforcement, which were previously metallized.

Manufacturing processes by techniques in liquid phase can consist of infiltrating the molten metal or alloy constituting the matrix, under more or less strong pressure, in woven or non-woven preforms, or to be mixed constituents in a pasty medium, possibly in the presence of an easily removable binder. The pressure metal infiltration techniques or alloy melted in sheets unidirectional stretched long fibers or in woven preforms currently lead materials with the characteristics highest mechanical values.

However, when using the technique infiltration with light metals such as aluminum or magnesium, we meet certain difficulties in obtaining a satisfactory Liaison between the fibers of the reinforcement and the metallic matrix.

Indeed, in the case of aluminum-based matrices, the infiltration of aluminum or aluminum alloy into sheets or preforms made up of carbonaceous fibers poses certain problems because the carbon is not chemically stable in presence of pure or alloyed aluminum. Thus, as soon as the temperature exceeds about 450 ° C, a reaction takes place between the fibers and the matrix which leads to the formation of aluminum carbide Al ₄ C ₃ , which results in pitting damage to the surface of the fibers and rapid degradation of their intrinsic mechanical characteristics.

Furthermore, despite the chemical interaction leading to the formation of aluminum carbide, aluminum wetting of fibers is very bad, which forces infiltration to be carried out under high pressure, and the adhesion between aluminum and the fibers in the composite remain weak.

To overcome these difficulties, we have developed various surface treatments for carbon or graphite fibers, these treatments consisting for example in coating the surface of fibers of a suitable compound such as carbide titanium or zirconium, as described in US-A- 4,600,661 with reference to the document Japanese 49-18891 and by Nieh and Vidoz in J. of Am. Ceram. Soc. flight. 65, n ° 5, May 1982, p. 227-230.

In US-A-4 600 661, the reaction is avoided between the fibers and the aluminum matrix by adding from titanium or zirconium to molten aluminum during infiltration, which leads to formation on the fibers of a layer of Ti carbide or Zr instead of aluminum carbide. However, this technique poses other problems to control the formation of this layer so as to maintain the mechanical properties of the fibers.

In the article by Nieh et al, a layer of Ti or Zr carbide is formed on the fibers before including them in an aluminum matrix, which makes it possible to control the formation of the layer and leads to a double TiC layer. and Ti ₄ Sn ₂ C ₂ promoting the wetting of the fibers by aluminum.

However, the need to perform a such treatment complicates the development process and slows the development of this type of material composite.

Also, we developed materials carbon-reinforced composites using a matrix metallic based on magnesium because magnesium has two important advantages over with aluminum. On the one hand, magnesium has a density much lower than that of aluminum (1.74 at instead of 2.7), which can allow obtaining still specific mechanical characteristics higher. On the other hand, unlike aluminum, magnesium has excellent chemical inertness towards carbon, provided that the latter is sufficiently graphitized, which most reinforcing fibers are marketed. Therefore, no degradation sensitive to the intrinsic characteristics of a reinforcement made of such fibers cannot result a chemical reaction with the metal of the matrix, during hot processing of composite materials using a magnesium-based matrix and a carbon reinforcement.

Document US-A-3 888 661 describes the manufacture of composite materials having a matrix based on magnesium and fiber reinforcement graphite, by hot isostatic compression with addition of titanium, possibly replaced by Cr, Si, Zr, Ni or Hf. The titanium content can go up to 4% by weight of the composite, preferably 1.3 to 1.84 by weight.

However, although carbon fibers are not degraded by chemical reaction with magnesium, two major problems remain for the manufacture of such composite materials. The first is to achieve uniform penetration liquid magnesium within the fibers of reinforcement, the second is to obtain adhesion sufficient at the metal / fiber interface.

To solve these two problems, various reinforcing fiber treatments have been offered, in particular consistent treatments coating the surface of the fibers with a metal, by example of nickel, or insertion compounds wettable by the liquid matrix as it is described in Chemical Abstract vol. 86 (2), no.7835k and FR-A- 2 259 916.

We also made coatings with titanium base on graphite fibers intended to be infiltrated by a liquid magnesium alloy as described in Chemical Abstract, vol. 106 (20), n ° 161024h and vol. 106 (8) No. 54243g. However, there are certain difficulties in obtaining with these treatments a uniform coating of each fiber of the carbon reinforcement. In addition, the need perform a preliminary treatment of the fibers, before their infiltration by the metal of the matrix, complicates the material development process composite.

We also know a production process of a composite material associating a matrix based on magnesium with carbon reinforcement, in which one uses for the matrix an alloy of magnesium comprising 2 to 8% by weight of zinc, less 2% by weight of zirconium and less than 1% by weight aluminum to prevent deterioration of the carbon reinforcement by the alloy of the matrix and thereby increasing the mechanical strength of the resulting composite material, as described in US-A-4,600,661. In this method, the addition zinc to magnesium alloy lowers the point of this alloy and improves its fluidity in the molten state, which allows infiltration to be carried out under pressure of the carbon reinforcement by the alloy magnesium in better conditions. Through elsewhere, the limitation of zirconium contents and aluminum alloy avoids reactions considered harmful between Al, Zr and the carbon of the carbon reinforcement.

According to the invention, it has been found that, contrary to this teaching, a reaction between the zirconium and the carbon of the fibers reinforcement could be beneficial and that the problems for wetting carbon fibers with magnesium and adhesion between these fibers and the matrix of magnesium could be resolved by adding magnesium-based matrix zirconium without degrading the mechanical characteristics composite material.

The subject of the present invention is a composite material according to claim 1.

Advantageously, the carbide of the layer corresponds to the formula: ZrC _x Mg _y in which x and y are such that:
0.3 <x <1, and
0.02 <y <0.12

In this material, the presence of the layer of carbide ZrC _x Mg _y on the fibers makes it possible to solve the problems of wettability of the fibers by the metal of the matrix and of adhesion between the fibers and the matrix, without reducing the characteristics. mechanical of the composite material.

The formation of such a carbide layer to solve these problems that result from the choice of a magnesium-based matrix, has never been considered so far.

Indeed, as described in US-A-4 600 661, we have already used a carbide layer of zirconium or titanium carbide, formed on carbon fibers by addition of zirconium or titanium to molten aluminum, to avoid the harmful formation of aluminum carbide on these fibers during their infiltration by aluminum. So in this case it was not a layer of zirconium carbide containing in solid solution magnesium as in the invention. Otherwise, the function of this layer was to prevent the harmful reaction between the carbon of the fibers and the metal of the matrix, reaction which does not occur not in the case of a magnesium matrix.

In addition, it is indicated in this document that the zirconium carbide layer reduces the adhesion between the carbon fibers and the metal of the matrix, which is precisely one of the problems solved in the invention thanks to the use of a carbide layer such as ZrC _x Mg _y .

According to the invention, carbon fibers or graphite used in the carbon reinforcement, have a high breaking strength, for example more than 1800MPa, and a high modulus of elasticity, for example greater than 200GPa. To get a good wetting and good adhesion to the interface between metal and fibers it is essential that the surface of the latter is mainly made of carbon. It is therefore advisable to desensify carefully the fibers before their impregnation with liquid metal. On the other hand, we can indifferently use for making the fiber material of bare carbon or graphite or fibers of carbon or graphite having on their surface a layer of carbon or graphite, for example 0.03 to 0.3 µm thick, previously deposited by known techniques such as pyrolysis of liquid organic precursors or gas, PVD .... The carbon reinforcement can be present in different forms.

Thus, it can comprise at least one cylindrical wick made up of 200 to 24,000 carbon or graphite fibers having an average diameter d of less than 50 μm.

It can also be formed from one or more unidirectional sheets each obtained by spreading and / or juxtaposition of one or more wicks of carbon or graphite fibers having an average diameter d of less than 50 μm. In the case where several unidirectional plies are used, the reinforcement may be formed by a stack of these possibly crossed plies.

The reinforcement can also be constituted by a two-dimensional or three-dimensional preform made by weaving wicks of carbon fibers or graphite having an average diameter d of less than 50 μm, using conventional techniques.

The reinforcement can also be formed by a porous nonwoven body made up of wicks possibly broken carbon or graphite fibers retained by an easily removable binder.

With such reinforcements, one can obtain composite materials in the form of a wire continuous, continuous ribbon, plate or solid of complex shape resulting from impregnation reinforcements mentioned above by the liquid alloy of the matrix.

The composite materials of the invention can be manufactured by a process consisting in infiltrating into the reinforcement formed of carbon fibers or graphite preferably having an average diameter d of less than 50 μm, a liquid magnesium alloy containing zirconium, the content of zirconium of this alloy being maintained at saturation throughout the duration of the infiltration, and the infiltration being carried out at a temperature of 650 to 850 ° C., maintaining this temperature for a duration ranging from 2 min to 1 h.

The infiltration of the liquid alloy of magnesium in carbon reinforcement can be realized by any technique, for example by immersion of the carbon reinforcement in an alloy bath molten magnesium, or by bringing to a temperature from 650 to 850 ° C a set comprising carbon reinforcement, magnesium alloy and possibly zirconium powder.

In the latter case, we can for example carry out hot compression infiltration of an alternating stack of fiber reinforcements carbon or graphite, alloy strip magnesium and zirconium powder.

Under these conditions, the zirconium carbide layer containing magnesium in solid solution, for example of formula ZrC _x Mg _y , is formed directly on the fibers at the fiber-matrix interface based on magnesium, during the infiltration of the alloy melted in the carbon reinforcement.

The formation of this layer promotes, on the one hand, good wetting of the fibers by the alloy of magnesium melted and leads, on the other hand, to obtaining a satisfactory connection between fibers and the magnesium-based matrix.

Indeed, it has been discovered that the presence of zirconium in a sufficiently concentrated solution (more than 0.3% by weight), for example from 0.3 to 1% by weight, in magnesium allows the molten magnesium to penetrate close by. close to the interior of all the open pores of the reinforcement, without it being necessary to resort to a high infiltration pressure and / or to coat each fiber of the reinforcement with an appropriate deposit beforehand. This excellent penetration power results from the fact that a continuous and adherent thin layer of carbide ZrCxMg _y , in particular ZrC _0.38 Mg _0.09 , is formed very quickly on the surface of each fiber of the reinforcement by chemical reaction with zirconium. dissolved in magnesium. This carbide, which constitutes the interaction layer, is similar to zirconium carbide ZrC with cubic symmetry, but it contains a little magnesium in solid solution and can be non-stoichiometric. It corresponds to the chemical formula ZrC _x Mg _y with x between 0.3 and 1 and including between 0.02 and 0.12.

X-ray diffraction analysis of the ZrC _x Mg _y phase obtained by reaction at 727 ° C between powders of graphite, Zr and Mg in atomic proportions 20/10/70, for 60 h showed that it it was a cubic symmetry phase belonging to the same symmetry class as the zirconium carbide ZrC, having a mesh parameter a = 4.697 (4) Å close to that of pure ZrC (a = 4.6930 Å) . This is why the carbide ZrC _x Mg _y is considered _to be a non-stoichiometric zirconium carbide containing a little magnesium in solid solution, which slightly expands its mesh. From the mesh parameter a , the density d _c of the carbide ZrC _x Mg _y was calculated on the assumption that magnesium occupies interstitial sites in the structure of ZrC and for the formula ZrC _0.38 Mg _0.09 which has been determined experimentally by analysis. A d _c value of 6.28 g / cm ³ was thus obtained, which is little different from that given for pure ZrC (6.634 g / cm ³ ).

It has also been observed that the carbide ZrC _x Mg _y is poorly crystallized even after 206 hours of heat treatment. Also, the deposit formed on the fibers with much shorter durations is almost amorphous, which is very favorable for the conservation of the mechanical properties of carbon or graphite fibers.

The growth of the layer of ZrC _x Mg _y on the fibers takes place by unidirectional diffusion of carbon in solid phase through the layer. As a result, the thickness of this layer, which is proportional to the square root of the infiltration time, grows at a speed the slower the thicker the layer. The growth process is therefore kinetically self-regulating. After complete infiltration and solidification of the magnesium-based matrix, strong chemical bonds are established via the layer of carbide ZrC _x Mg _y between the matrix and the fibers, which ensures good charge transfer in the material. composite and good conservation of its mechanical properties in fatigue. Furthermore, the interface between the matrix and the reinforcement can not evolve by chemical reaction because the growth kinetics of compound ZrC _x Mg _y becomes almost zero when the matrix is in a solid state.

The variation of the thickness (in μm) of the ZrC _x Mg _y layer formed on the surface of P 55 carbon fibers as a function of the contact time (in min) with an MgZr alloy saturated with Zr and as a function of the temperature (in K) for temperatures ranging from 930 to 1100K is given by the expression: log 10 e = (3,885-5434 / T) + 1 / 2log 10 t in which t represents time (in min) and T represents temperature (in K).

Although this expression is very approximate, it provides an indication of the order of magnitude of the thickness of the layer. Thus, after 15 min of interaction at 670 ° C, the thickness of the layer is of the order of 0.05 μm.

This expression is only valid for P 55 or P 100 fibers, the characteristics are given in the attached table, or for fibers having a deposit on the surface of pyrolytic carbon obtained for example by cracking under low methane pressure around 1400 ° C. Indeed, after 4 p.m. at 670 ° C, a thickness is observed layer on the order of 1µm on M 40B fibers PAN precursor (polyacrylonitrile) while in the case of P 55 or P 100 fibers, the thickness layer is approximately 0.6 µm in the same conditions.

These differences are probably due to variation in the degree of graphitation of the fibers and / or difference in orientation of graphitic sheets at the level of the surface in contact with the liquid. Indeed, the sheets are oriented parallel to the interface in the case of fibers formed from pitch while oriented perpendicular to the interface in the case of fibers formed from PAN.

Since the carbon contained in the carbide layer ZrC _x Mg _y formed at the fiber-matrix interface comes from the attack on the surface of the fibers by zirconium, it is important that the thickness of the interaction layer is not too important for the carbon or graphite fibers to retain their excellent intrinsic mechanical characteristics. In this regard, the use of fibers previously coated with a carbon deposit of about 0.1 μm in thickness may be advantageous. Preferably, the thickness of the carbide layer ZrC _x Mg _y must be at most equal to 5 d / 100 with d representing the diameter of the fibers. Preferably, it is equal to about d / 100. Thus, in the case of carbon fibers of 6 to 7 μm in average diameter, the thickness of the carbide layer ZrC _x Mg _y is preferably less than 0.06-0.07 μm and does not exceed 0.3 to 0 , 35µm. In the case of fibers of 10 μm of average diameter, these values are respectively of the order of 0.1 and 0.5 μm.

With such layer thicknesses, the mechanical properties of the composite material have values little changed from the values theoretical predictable by the law of mixtures. Thus, a totally unidirectional carbon reinforcement infiltrated by an Mg-Zr alloy may exhibit breaking strength at least equal to 70% and a modulus of elasticity at least equal to 90% of theoretically predictable values by the law of mixtures.

As we saw above, the thickness of the carbide layer formed at the interface between the matrix and the reinforcement depends on both the nature of the fibers which constitute the reinforcement, contact time between fibers and matrix liquid, and the temperature at which it is maintained.

For medium graphite fibers such as P 55 fibers, a layer of carbide 0.3µm thick can be obtained in 60min at a temperature of 747 ° C. Since it is preferable that the interaction layer has a much lower thickness, and that this thickness varies proportionally with the square root time, it is clear that the desired thickness will be obtained quickly and that the production conditions of composite material are perfectly compatible with conventional industrial procedures liquid phase infiltration by gravity or under medium pressure of the metal forming the matrix.

According to the invention, the amount of zirconium present in the magnesium alloy used for the infiltration of the carbon reinforcement must be sufficient to allow the growth of a thick layer adequate while maintaining the process of reactive wetting its kinetically self-regulating character. Generally, the amount of zirconium present in the magnesium alloy is 0.3 to 1% by weight.

However, like the solubility of zirconium in magnesium is relatively low (0.6 0.7% by weight) at temperatures from 650 to 850 ° C generally used for infiltration it may be advantageous to maintain the zirconium title liquid magnesium at a sufficient level high, which may be equal to the solubility limit zirconium in magnesium at temperature infiltration.

This can be achieved by techniques known to those skilled in the art, for example by partial metallization of the reinforcement with zirconium using the vapor deposition technique, by adding pieces, bars, plates or zirconium cylinders, in a magnesium bath melted, or by performing the bath fusion in a zirconium or zirconia crucible.

The minimum quantity of zirconium that must be added to the magnesium matrix to obtain a layer of ZrC _x Mg _y of desired thickness on carbon fibers of radius r _f and density d _f depends in particular on the volume fraction of fibers V _f of the composite material.

To evaluate this minimum mass of zirconium, we first calculate the mass m _Zr contained in a layer of thickness e on a filament of radius r _f and of length L. As e remains small before r _f , the volume of the layer V _c , is equal to V vs = 2π.r f .eL The mass of the layer m _c is therefore equal to: m vs = 2π.r f .eLd vs where d _c is the density of the layer which is 6,28g.cm ^-3 . If x ₀ is the mass fraction of Zr in the layer, the mass of Zr contained in it therefore has the expression: m Zr = 2π.r f .eLd vs .x 0 For the atomic composition indicated above (ZrC _0.38 Mg _0.09 ), x ₀ = 0.93.

We must now reduce to the volume fraction of fiber in the composite, V _f . For this, we express in two different ways the mass of fibers m _f in the volume unit of the composite. On the one hand, if d _f is the density of the fibers: m f = π.r f 2 .Ld f on the other hand, V _f being the volume fraction of fibers in the composite: m f = V f .d f By combining (4) and (5) we have: L = V f /π.r f 2 By replacing L in (3) by its expression given by (6), we obtain for the unit of volume of the composite: m Zr = 2π.r f .eV f .d vs .x 0 /π.r f 2 and by simplifying: m Zr = 2.d vs .x 0 V f .e / r f If we now express the ratio m _Zr / m _Mg , still for the unit of volume of the composite, it comes: m Zr / m Mg = 2.d vs .x 0 .eV f / r f .d Mg . (1-V f )

We thus obtain the minimum value that must initially have the ratio m _Zr / m _Mg in the matrix so that after exhaustion of Zr, we obtain a layer of thickness e on fibers of radius r _f in a composite where the volume fraction of these fibers is equal to V _f . We notice that this ratio, increasing function of V _f , is proportional to e and inversely proportional to r _f . If we replace d _c , x ₀ and d _Mg with their value, we arrive at: m Zr / m Mg = 6.713.eV f / r f . (1-V f )

Thus, for composite materials whose volume fraction of fibers varies from 5 to 55%, the ratio m _Zr / m _Mg varies from 0.0035 to 0.082 if it is desired to obtain a layer thickness of 0.1 μm on fibers 10µm in diameter.

To satisfy this condition, we use usually magnesium alloys containing 0.3 to 1% by weight of zirconium.

The zirconium which remains in the magnesium matrix after infiltration and formation of the layer of ZrC _x Mg _y does not affect the characteristics of the matrix. On the contrary, the addition of zirconium to magnesium improves the mechanical properties of magnesium and several formulas of industrial alloys based on magnesium integrate this element as the main additive.

In the magnesium matrix, others elements may also be present with the zirconium, in particular zinc, thorium, silver, and rare earths. The magnesium matrix must however do not contain aluminum because it combines with zirconium to give unwanted refractory intermetallic compounds; in addition, aluminum reacts with carbon reinforcement causing it to be damaged by bites like we saw it above.

Thus, one can use in the invention commercial magnesium alloys containing zirconium and other additives except aluminum. These addition elements can be for example Zn and rare earths.

The composite materials of the invention, which comprise a magnesium matrix associated with a carbonaceous reinforcement consisting of carbon fibers, exhibit high mechanical properties, for example a breaking strength and a specific modulus of elasticity as high as 0.6.10 ⁶ and 1.6.10 ⁸ N. m.kg ^-1 respectively in a direction parallel to the fibers, when using fibers of type FT 700 with a volume fraction of fibers of 50%. In addition, when carbon fibers with high elastic modulus are used, such as fibers of type FT 700 or P 100 which have a sufficiently negative coefficient of thermal expansion, the composite materials exhibit excellent dimensional stability over a wide range. temperature range (-180 to + 150 ° C). The aforementioned characteristics associated with good thermal conductivity and good chemical stability make the composite materials of the invention particularly suitable for the production of structural elements or apparatus supports usable in a space environment, the corrosion problems inherent in the use of a magnesium-based matrix is not critical in this case.

Other features and benefits of the invention will appear better on reading following examples given of course as illustrative and not limiting.

The characteristics of carbon fibers used in these examples are given in the attached table.

Example 1.

In this example, we use fibers of carbon type M40B desensitized in wicks of 3000 fibers of 6.5µm average diameter and we placed in a graphite crucible around a Zirconium-saturated Mg-Zr alloy cylinder (500mg alloy + 150mg of zirconium). We heat the whole thing at 677 ° C and this temperature is maintained for 4 p.m.

Under these conditions, we observe the ascent capillary of the molten alloy along the wick of fibers over a height of 2mm. We also note that the alloy has completely infiltrated inside of the wick and that an interaction layer of a thickness of around 1µm has formed around each fiber.

When operating at 677 ° C for 15 min, we always observe the capillary rise of the magnesium alloy along the strand of fibers on a height of 1 to 2mm as well as infiltration full of it but the reaction layer which is nevertheless present on the fibers, has a much smaller thickness, around 0.1 µm.

Example 2.

In this example, we prepare a material composite using a preform as reinforcement three-dimensional carbon and realizing infiltration of the magnesium matrix into reinforcement from a magnesium alloy bath containing zirconium.

The carbon preform is a preform 1x1x3cm parallelepiped which has been made by three-dimensional weaving of carbon wicks T 300 each formed of 6000 carbon fibers each having an average diameter of the order of 6 μm. The open porosity of the preform is around 60%.

After desensimizing the preform, we immerses it to a depth of 10cm in a container graphite containing a liquid alloy bath commercial magnesium zirconium comprising 0.4% at 1% by weight of zirconium, the volume of the bath being 50 times the apparent volume of the preform.

To passivate the graphite container containing the alloy bath, we put it beforehand in contact with a similar bath for 24 hours at 750 ° C. After 6min of immersion of the preform in the bath maintained at a temperature of 657 ° C., we remove the preform and bring it back to temperature ambient.

It can thus be seen that practically all of the fibers of the reinforcement have been covered with solidified metal, a consequence of the excellent reactive wetting which has developed during immersion. A continuous layer of ZrMg _x C _y with a thickness of 0.04 to 0.06 µm was formed at the interface between the metal and each of the fibers.

The composite material thus obtained can be used as a local insert for a room more complex in shape produced by foundry.

Example 3 .

In this example, we make a part of composite material in the form of a ribbon from a wick of carbon fibers and a magnesium-zirconium alloy. The reinforcement is consisting of an untwisted wick formed of 3000 carbon fibers of 7µm in diameter, each fiber with carbon coating on the surface pyrolytic about 0.1 µm thick beforehand deposited by Joule heating the fibers around 1300 ° C under low methane pressure.

To make the composite tape, we continuously scrolls the wick of carbon fibers in the bath of magnesium-zirconium alloy of which the zirconium title is kept at saturation by contact with an excess of zirconium. this is produced by means of an axle return pulley horizontal which drives vertically from top to down the wick of carbon fibers in the bath of liquid alloy, the radius of the pulley being of 3cm and its axis being maintained above the bath surface, while the wick of fibers marching through his throat is submerged to a depth 2cm below the surface of the bath. After passing on the pulley and crossing of the alloy bath, the wick of fibers is driven on a small axis roller horizontal outside the bath but at a distance close enough to its surface for solidification metal infiltrated into the drill bit on the roller. The pulley and all parts in contact with the liquid bath are produced graphite and passivated as in Example 2 by prior contact with an analog bath for 24 hours at 750 ° C.

To achieve infiltration, we maintain the bath temperature at 670 ° C and its title in saturated zirconium (0.6% by weight of zirconium at this temperature) by placing the bottom of the bath pieces of metallic zirconium.

After scrolling for 2 min in the liquid alloy bath, a composite ribbon is obtained consisting of 3000 carbon fibers infiltrated to the core by the magnesium - zirconium alloy, the average volume fraction of these fibers being of the order of 45% and each of them being coated with a layer of ZrC _x Mg _y of 0.03 to 0.04 μm thick.

These tapes can be used for manufacture by hot compression of bars, profiles or thin pieces of revolution.

Example 4.

In this example we prepare a part composite material from webs unidirectional carbon fibers, strips of magnesium - zirconium alloys and small particle size zirconium powder (less than 25µm).

For this purpose, we have alternating layers unidirectional layers of wicks P 100 carbon fibers on which we have dispersed zirconium powder and strips in alloy RZ5 which contains 3.5 to 5% by weight of Zn, 0.4 to 1% by weight of Zr and 0.75 to 1.75% of earth rare, the rest being magnesium, so have 53% by weight of wicks, 1.5% by weight of zirconium powder and 45.5% by weight of strip of RZ5 alloy.

Place the stack thus formed in a sealed enclosure, and then it is subjected to uniaxial compression under 50MPa at a temperature from 670 ° C for 15min.

A composite plate is thus obtained containing approximately 50% by volume of P 100 wicks coated in a metal matrix having the composition of the starting alloy RZ 5, each fiber being coated with a layer ZrC _x Mg _y of 0.04 to 0.06µm thick.

EXAMPLE 5.

In this example, we make a material composite from carbon fiber webs which have been coated with RZ5 alloy and zirconium metallic by sputtering.

For this purpose, we spread carbon fibers FT 700, then it is deposited on them by spraying cathodic in a magnetron field a first layer of alloy RZ 5, then a second layer of metallic zirconium. We thus obtain a semi-finished product in which the weight proportion of fibers is 58%, the weight proportion of alloy is 40.4% and the proportion by weight of zirconium 1.6%.

After draping these semi-finished products around of a cylindrical mandrel, they are subjected to a isostatic compression under 15MPa at 650 ° C for 20min.

A composite material is thus obtained consisting of 55% by volume of FT 700 fibers coated with a layer ZrC _x Mg _y of 0.04 to 0.06 μm thick and coated in a matrix having the composition of the alloy RZ 5 .

This embodiment is more particularly suitable for making tubular parts.

Comparative example .

In this example, we prepare a material carbon fiber composite in a magnesium matrix but add magnesium titanium instead of zirconium, and it is heated to 727 ° C for 4h or 64h.

After 4 hours, we observe the formation of a layer of titanium carbide but you don't get the excellent wetting that develops in the case of Mg-Zr alloys. After 64h, this translates by the fact that no contact exists in certain points of the metal-fibers interface while a interaction zone more than a micrometer thick is visible in other places.

Thus, the development of a good anchorage reactive between carbon fibers and alloys of magnesium is a specific characteristic with the zirconium additive.

• la vitesse de dégradation de la couche d'oxyde MgO susceptible de faire écran à l'interface métal/fibres,
• la vitesse de formation et de croissance de la couche ZrC_xMg_y,
• la valeur de l'angle de contact alliage liquide-carbure.

The following factors can be expected to influence the reactive wetting process:

• the rate of degradation of the MgO oxide layer capable of shielding the metal / fibers interface,
• the rate of formation and growth of the ZrC _x Mg _y layer,
• the value of the liquid-carbide alloy contact angle.

Thus, it appears that zirconium allows to achieve a favorable compromise between these factors, which is not the case for titanium.

Although in the examples described above, we only illustrated the techniques of infiltration by immersion in a liquid bath or by hot compression of solid products, it is clear that any other infiltration technique known to those skilled in the art can be used in the process of the invention.

Thus, it is possible to carry out a soaking impregnation, a gravity infiltration or an infiltration under medium pressure to produce a composite material according to the invention, as long as the temperature and the time during which the carbonaceous reinforcement is in contact with the alloy, are compatible with the formation, by chemical reaction at the metal - reinforcement interface, of an interaction layer of zirconium carbide containing magnesium in solid solution, such as ZrC _x Mg _y , of a thickness suitable, preferably less than 1/100 the diameter of the reinforcing fibers.

Claims (12)

1. Composite material comprising a magnesium alloy matrix containing 0.3 to 1% zirconium, and a reinforcement constituted by carbon or graphite fibres having a diameter d dispersed in said matrix, said fibres being coated with a continuous zirconium carbide layer containing magnesium in solid solution, said carbide layer having a thickness of at least 0.004 d and the volume fraction of fibres in the composite material is 45 to 55%.
2. Material according to claim 1, characterized in that the carbide of the layer complies with the formula: ZrC_xMg_y in which x and y are such that
   0.3 < x <1, and
   0.02 < y < 0.12.
3. Composite material according to claim 2, characterized in that the carbide of the layer complies with the formula ZrC_0.38Mg_0.09.
4. Material according to any one of the claims 1 to 3, characterized in that the carbon or graphite fibres constituting the reinforcement have on the surface a 0.03 to 0.3 µm thick carbon deposit.
5. Material according to any one of the claims 1 to 4, characterized in that the reinforcement is constituted by at least one cylindrical wick formed from 200 to 24000 carbon or graphite fibres having an average diameter d below 50 µm.
6. Material according to any one of the claims 1 to 4, characterized in that the reinforcement comprises at least one unidirectional layer obtained by spreading and/or juxtaposing one or more carbon or graphite fibre wicks having an average diameter below 50 µm.
7. Material according to any one of the claims 1 to 4, characterized in that the reinforcement is a two or three-dimensional preform produced by weaving carbon or graphite fibre wicks having an average diameter d below 50 µm.
8. Composite material according to claim 1, characterized in that the carbide layer thickness is 0.03 to 0.06 µm.
9. Process for the production of a composite material according to any one of the claims 1 to 8, characterized in that it consists of infiltrating into a reinforcement formed from carbon or graphite fibres a liquid magnesium alloy containing zirconium, the zirconium content of said alloy being maintained at saturation throughout infiltration and infiltration takes place at a temperature of 650 to 850°C and whilst maintaining this temperature for between 2 and 60 min.
10. Process according to claim 9, characterized in that infiltration takes place by immersing a carbon and graphite fibre reinforcement in a zirconiumsaturated, molten magnesium alloy bath.
11. Process according to claim 10, characterized in that infiltration takes place by the hot compression of an alternating stack of carbon or graphite fibre reinforcements, magnesium alloy strips and zirconium powder.
12. Process according to any one of the claims 9 to 11, characterized in that the magnesium alloy does not contain aluminium.