WO2000008219A1

WO2000008219A1 - Preparation of metal-matrix composite materials using ceramic particles with modified surfaces

Info

Publication number: WO2000008219A1
Application number: PCT/CA1999/000722
Authority: WO
Inventors: Don Allen Doutre; Gary Hay; Peter Wales
Original assignee: Alcan International Limited
Priority date: 1998-08-07
Filing date: 1999-08-06
Publication date: 2000-02-17
Also published as: AU5144799A; GB2356636B; CA2339803A1; DE19983449T1; JP2002522636A; GB0102848D0; GB2356636A

Abstract

A composite material is prepared by furnishing a mass of ceramic particles, such as aluminum oxide or spinel particles, and contacting a source of magnesium or other passivating metal with the mass of ceramic particles. Magnesium is deposited onto the surfaces of the particles in the absence of added oxygen and nitrogen. A source of aluminum or other matrix metal is infiltrated into the mass of ceramic particles to form an aluminum (or other metal)-containing composite material. This composite material may be further dispersed into a second source of aluminum or other matrix metal to form a composite material of a lower volume fraction of particles.

Description

PREPARATION OF METAL-MATRIX COMPOSITE MATERIALS USING CERAMIC PARTICLES WITH MODIFIED SURFACES

TECHNICAL FIELD This invention relates to the preparation of metal-matrix composite materials and, more specifically, to a method for rapid preparation of such materials and to the modification of the surfaces of ceramic particles used in such composite materials, prior to formation of the composite materials.

BACKGROUND ART In one form of a metal -matrix composite material, a reinforcement phase is embedded in a metal matrix. The reinforcement is typically equiaxed or elongated particles of a ceramic phase such as aluminum oxide, and the matrix is a pure metal or alloy such as aluminum. The particle phase and the matrix metal phase each retains its physical and chemical identity in the composite material, and each phase contributes to the properties of the final composite material.

In order to achieve good mechanical properties, the metal matrix should wet the surfaces of the ceramic particles. The wetted interface ensures good mechanical load transfer between the phases, and also minimizes the possibility of internal failure modes such as cavitation failure at the matrix/particle interfaces during deformation. The achievement of a wetted interface may be the result of a stirring process or an infiltration process. These processes are limited, in the first case by the maximum solid loading (i.e., volume fraction of particles in the composite material) that may be achieved, and in the second case, by the time taken to infiltrate a particle bed completely. It is therefore desirable to find a rapid means of obtaining a wetted interface that is effective even for high solids loading.

However, the close interfacial contact achieved as a result of good wetting may in some cases lead to degradation of the properties of the composite material through chemical interdiffusion and the formation of brittle and/or unstable phases at the interface between the particles and the matrix. For example, the alloying elements typically found in aluminum casting alloys used as the matrix of the composite material may chemically react with aluminum oxide or silicon carbide particles, forming brittle phases that reduce the fracture resistance of the composite material.

A number of techniques have been developed to achieve particle/matrix wetting while alleviating the problems associated with chemical interdiffusion and interaction between the matrix and the particles. The particles may be coated with a surface layer that resists chemical interdiffusion. Coating of the particles is usually expensive, and some coating techniques may not achieve total coating over the entire surfaces of the particles. In another approach, uncoated particles may be used to reduce the cost, and a wetting layer such as aluminum nitride formed during the composite fabrication. The chemical reactions associated with this approach are usually slow, resulting in long processing times. In another alternative, the composition of the metallic matrix may be carefully selected so as to minimize chemical interactions. While operable, this technique limits the types of matrix alloys that may be used. There is a need for an approach that achieves a wetted matrix/particle interface at a rapid rate, and which also minimizes the degradation of the composite material due to chemical interaction. The present invention fulfills this need, and further provides related advantages.

DISCLOSURE OF THE INVENTION The present invention provides an approach for modifying the surfaces of the particles to be used in a metal-matrix composite material, and preparing the composite material using the modified particles. The composite material has a wetted matrix/particle interface, and may include a passivation barrier that reduces the incidence of deleterious diffusional reactions during fabrication and service. The approach is applicable to a wide variety of combinations of particles and matrix alloys, but is most preferably applied to the commercially important composite material containing aluminum oxide particles in an aluminum-alloy matrix. The technique of the invention is readily utilized, and may be accomplished much more rapidly than available alternative procedures. Inexpensive uncoated particles may be used as the starting material, and the preferred passivation treatment of the particles is accomplished as part of the composite fabrication process. It is therefore more economical than the alternatives.

According to one aspect of the invention, there is provide a method for preparing a composite material, in which a mass of ceramic particles is furnished and a source of matrix metal is contacted with the mass to form a metal-containing composite material; wherein, prior to the contact of the matrix metal with the mass, a source of a magnesium is contacted with the mass in the absence of added oxygen or nitrogen. According to another aspect of the invention, there is provided a method of preparing a composite material, in which a mass of ceramic particles is furnished and a source of matrix metal is contacted with the mass to form a metal-containing composite material; wherein, prior to the contact of the matrix metal with the mass, the ceramic particles are contacted with a source of metal that is reactive with the ceramic particles to form a substantially continuous, non-porous layer of chemical reaction product compound thereon.

Expressed in other terms, the invention, in at least preferred forms, provides a method for preparing a composite material, in which a mass of ceramic particles is furnished and a source of matrix metal is contacted with the mass to form a metal- containing composite material; wherein, prior to the contact of the matrix metal with the mass, a source of a passivating compound is contacted with the mass such that a passivating layer is formed on the ceramic particles.

In accordance with a preferred aspect of the invention, a method for preparing a composite material comprises the steps of furnishing a mass of ceramic particles, contacting a source of magnesium to the mass of ceramic particles, such that magnesium is deposited onto at least a portion of the surfaces of the particles in the absence of added oxygen and nitrogen, and contacting a source of aluminum to the surfaces of the magnesium-treated particles to form an aluminum-containing composite material.

In one preferred embodiment, a method for preparing a composite material comprises the steps of furnishing a bed of ceramic particles in a container, the bed of ceramic particles being at a temperature greater than the melting point of aluminum, and placing a layer of molten magnesium on an upper surface of the bed of ceramic particles, whereupon the layer of molten magnesium flows into and through the bed of ceramic particles, and forms a magnesium layer on the particles. The method further includes thereafter placing a layer of molten aluminum on an upper surface of the bed of ceramic particles through which the magnesium layer is flowing, and creating a pressure differential between the layer of molten aluminum and an interior of the bed of ceramic particles, whereupon molten aluminum flows into the bed of ceramic particles to form an aluminum-containing composite material. In accordance with another preferred embodiment of the invention, a method of preparing a composite material comprises the steps of furnishing a mass of ceramic particles, and passivating the surfaces of the ceramic particles by forming a substantially continuous, non-porous layer of a reaction product compound thereon. The passivating approach includes contacting a reactive metal to the ceramic particles to form the reaction product compound as a chemical reaction product of the reactive metal and the ceramic particles. A first composite material is formed of the passivated ceramic particles and a first matrix metal. A second composite material having a lower volume fraction of particles may be formed by furnishing a source of a second matrix metal, wherein the second matrix metal is molten, and dispersing the first composite material in the second matrix metal.

The approach of the invention may be practiced with a wide variety of particles, first matrix metals, and second matrix metals. In a preferred application, the particles are an oxide-based ceramic such as aluminum oxide or spinel, and/or silicon carbide ceramic. The reactive metal is preferably magnesium (i.e., either pure magnesium or magnesium alloys), and the first matrix metal and second matrix metal are aluminum (i.e., either pure aluminum or aluminum alloys).

Thus, a preferred method for preparing a composite material comprises the steps of furnishing a mass of aluminum oxide particles, and contacting a source of magnesium to the mass of ceramic particles, such that magnesium is deposited onto the surfaces of the particles in the absence of added oxygen and nitrogen. The magnesium, which may be in the form of pure magnesium or an operable source of a magnesium alloy, reacts with the ceramic particles to produce a continuous, non-porous layer of reaction product thereon. The reaction layer is a dense spinel composition on the surfaces of the aluminum oxide particles. A source of aluminum (first matrix metal) is contacted to the surfaces of the magnesium-treated particles to form an aluminum- containing composite material. The resulting composite material may be dispersed into a second source of aluminum (second matrix metal) to dilute the composite to a lower volume fraction of particles. The present approach may be distinguished from prior approaches. The particles are not furnished in a coated form, but instead are protected by a reaction layer or coating layer formed during processing. The reaction layer is distinct from a deposited layer. In some prior techniques, a magnesium nitride layer is deposited upon uncoated particles. This wetting-enhancement layer is formed by the reaction of magnesium with atmospheric nitrogen, a slow process. The present invention produces its passivation layer by the reaction of a reactive metal with the particles themselves, which occurs much more rapidly than the nitrogen gas reaction of the prior approach.

The technique of the invention provides an approach for producing a metal- matrix/ceramic particle composite material wherein the ceramic reinforcement particles have passivated surfaces to inhibit subsequent deleterious reactions during subsequent processing, or coated surfaces to enhance subsequent wetting of the matrix to the particles. The composite material is readily manufactured in a rapid, economic fashion.

It is to be noted that the term particles is intended to mean generally equiaxed or slightly elongated regular or irregular particles, or substantially elongated particles such as fibers or whiskers. In the present invention, the use of regular, equiaxed particles is preferred.

Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 A- IB are idealized depictions of the microstructures of composite materials made according to the invention, wherein Figure 1 A illustrates a composite material having a passivation layer and Figure IB illustrates a composite material having a magnesium surface layer that enhances wettability;

Figures 2A-2E are schematic views illustrating stages during the production of a composite material by one preferred approach of the invention, wherein Figure 2 A depicts an initial stage with a layer of molten magnesium overlying a bed of ceramic particles, Figure 2B depicts the magnesium infiltrated into the bed of ceramic particles, Figure 2C depicts a layer of molten aluminum overlying the bed of magnesium-treated ceramic particles, Figure 2D depicts the bed of magnesium-treated ceramic particles infiltrated with aluminum, and Figure 2E depicts dispersing the resulting composite material into additional matrix alloy;

Figure 3 is a block flow diagram of a preferred approach for practicing the invention;

Figure 4 is a schematic view of a second embodiment of an apparatus for practicing the invention; and Figure 5 is a schematic view of a third embodiment of an apparatus for practicing the invention. BEST MODES FOR CARRYING OUT THE INVENTION

Figures 1A and IB illustrate composite materials 20 made according to the present invention. In each case, the composite material 20 has ceramic particles 22 embedded in a metal matrix 24. In the embodiment of Figure 1 A, a passivating/wetting layer 26 is present at the surfaces of the particles 22. The passivating/wetting layer 26 is typically thin, in the order of less than about 1 micrometer in thickness. Its thickness is exaggerated in Figure 1 A so that it may be depicted.

In the embodiment of Figure IB, a wetting layer 28 of magnesium is present at the surfaces of the particles 22. The wetting layer 28 is also typically thin, in the order of less than about 1 micrometer in thickness. Its thickness is exaggerated in Figure IB so that it may be depicted. According to the fabrication approaches discussed subsequently, the magnesium layer 28 is deposited on the surface of the particles, and the matrix alloy is provided thereafter to wet the surfaces of the magnesium-coated particles. Because magnesium readily dissolves in common matrix metals and alloys, such as aluminum, all or some of the wetting layer 28 may be dissolved away as the aluminum is wetted to the particles, and for this reason the wetting layer 28 is depicted by dashed lines. However, by the time that dissolution occurs, the wetting function of the magnesium layer 28 has been performed, and its dissolution is acceptable. Thus, Figure IB is intended to depict a time after the wetting layer 28 has been deposited and the matrix alloy has been introduced, but prior to partial or total dissolution of the wetting layer. It does not necessarily, but may, depict the final product.

The particles 22 may be substantially equiaxed or elongated, and may be of any operable material. Preferred particles in the Figure 1 A embodiment include aluminum oxide particles. Preferred particles in the Figure IB embodiment include silicon carbide and/or spinel (magnesium aluminum oxide) particles.

The matrix 24 for both embodiments may be any operable material. The preferred matrix material is aluminum, which term herein includes both substantially pure aluminum and aluminum alloys. The volume fraction of particles in the matrix may be varied over a wide range, according to the fabrication processes to be described subsequently. In the preferred infiltration approach, the particles may be present in relatively large fractions of from about 35 to about 70 volume percent of the total of particles and metal in the as-infiltrated material, which volume fraction may be reduced to a value of less than about 30 volume percent by subsequent dilution. In the Figure 1 A embodiment, the passivating/wetting layer 26 is selected according to the composition of the particles and the composition of the matrix material. The passivating/wetting layer 26 is the reaction product of a reactive metal having a composition different from that of the matrix material 24 with at least one chemical constituent of the ceramic of the particles 22. The reactive metal must be amenable to the selected fabrication technique. In the case of an aluminum oxide particle and an aluminum matrix, the reactive metal is preferably magnesium. The term "magnesium" as used herein includes both substantially pure magnesium and alloys of magnesium having sufficient magnesium to permit the surface reaction with the ceramic particles. For the cases of most interest, operable alloys of the magnesium reactive metal must have at least about 10 weight percent magnesium. Alloys of magnesium are preferred to pure magnesium for several practical reasons. Molten pure magnesium has a tendency to rapid chemical reaction with oxygen, leading to the possibility of a magnesium fire during commercial operations. Alloys have a lower melting point and allow lower working temperatures to ensure complete infiltration of the bed of particles by the magnesium. The density of pure magnesium is fixed, but the density of a magnesium alloy may be selected so as to be closer to that of the aluminum that follows the magnesium into the bed, thereby reducing the likelihood of gravity- induced mixing of the magnesium infiltrating layer and the aluminum that follows it. Figures 2A-2E illustrate, in diagrammatic form, one preferred process for practicing the invention to produce either the structure of Figure 1 A or the structure of Figure IB, depending upon the nature of the particles used, and Figure 3 illustrates the process in block diagram form. A bed 30 of ceramic particles is furnished in a container 32 (numeral 50 in Figure 3). The ceramic particles are, as discussed previously, typically aluminum oxide particles for the Figure 1 A embodiment and silicon carbide and/or spinel particles for the Figure IB embodiment. The bed 30 of ceramic particles is heated to a temperature greater than the melting point of the magnesium-source material used to infiltrate the bed in the next step, and also greater than the melting point of the aluminum-source material used in a subsequent step. A layer 34 of molten magnesium is placed on the top or upper surface of the bed 30 of ceramic particles in Figure 2A, numeral 52 in Figure 3. The molten magnesium may be pure magnesium or, more preferably, a magnesium alloy such as Mg-9 weight percent Al-1 weight percent zinc (AZ 91 alloy). If a magnesium alloy is used for the Figure 1A embodiment, the magnesium alloy must have at least about 10 percent by weight magnesium so that the magnesium chemically reacts with the particles such as aluminum oxide in the subsequent steps. The amount of magnesium is sufficient to produce a molten pool on top of the bed 30, and to produce, in the final composite after infiltration with both the magnesium and subsequent aluminum alloy, a desired magnesium composition in the matrix alloy of the composite. This depth is about 25 millimeters in a preferred embodiment.

As shown in Figure 2B, the material of the magnesium layer 34 spontaneously wicks into the bed 30 of ceramic particles having an upper surface 90 as a discrete band of material 92 in the upper portion of the bed, infiltrating the bed 30 to an initial infiltration front 94, numeral 54 of Figure 3. As shown in Figure 2C, after a sufficient time for this process to commence (typically a few minutes or less) but not necessarily be completed, a layer 36 of molten aluminum (which may be substantially pure aluminum or an aluminum alloy) is placed on top of the bed 30, numeral 56 of Figure 3. Aluminum will not usually spontaneously wick into the bed 30 but, as shown in Figure 2D, the presence of the magnesium containing band 92 in the top of the bed 30 assists in such infiltration, and in addition a pressure differential is created between the aluminum layer 36 and the bed 30 to force the aluminum layer 36 to flow into the bed 30 behind the magnesium band 92, numeral 58 of Figure 3. The pressure differential, which need be no more than a few tenths of an atmosphere, is created by any operable approach. It may be created by ensuring that the aluminum layer 36 completely covers the exposed surface of the bed 30 of ceramic particles and is created immediately after the magnesium band 92 is infiltrated into the bed 30. The reaction between the magnesium and any oxygen and nitrogen trapped inside the bed 30 essentially getters the oxygen and nitrogen, and the resulting partial vacuum, which the aluminum layer 36 seals, is sufficient to draw the aluminum into the bed. In an alternative embodiment, illustrated in Figure 4, a vacuum may be applied to the interior of the bed 30 to draw the aluminum layer 36 into the bed 30.

The aluminum layer 36 forms a substantially continuous infiltration layer with the magnesium band 92 initially infiltrated into the top of the bed 30, and through interdiffusion of the aluminum from the aluminum layer 36 into the magnesium-rich band 92 and vice versa, the magnesium concentration at the infiltration front is gradually reduced towards the bulk concentration, which is the concentration resulting after full infiltration and diffusional equilibration. However, as the diffusion process is slower than the magnesium- and vacuum-enhanced infiltration rate, the infiltration front remains rich in magnesium and therefore the infiltrating magnesium metal continues to perform the functions of the initially infiltrated magnesium band 92.

The initially infiltrated magnesium band 92, and the magnesium-rich infiltration front 94 thereafter provide enhanced wetting through the high magnesium content and/or provides, in the Figure 1 A embodiment, a reaction with the aluminum oxide to form the layer 26 of spinel (magnesium aluminum oxide) on the surface of the particles 22. This layer 26 forms as a continuous layer around the entire periphery and surface of the particles and renders the particles non-reactive to further attack by the aluminum. The layer 26 is typically less than about 1 micrometer thick, but may be thicker. The layer 26 is dense (i.e., not porous) and therefore, once formed, serves as a protective passivating barrier against chemical reaction with the aluminum infiltration layer which does not contact the particles until after this passivating reaction has occurred. By contrast, if a spinel layer is formed at the surfaces of the particles by reaction of the particle material with the magnesium in a relatively dilute magnesium-containing aluminum alloy in either the initially infiltrated layer or at the infiltration front, the layer is less dense and more fuzzy in appearance when viewed microscopically, and offers little resistance to subsequent, continuing degradation by the following aluminum infiltration layer.

For particles which are non-reactive to form a passivating layer (such as SiC or spinel particles), the above process is operative to provide enhanced wetting and fast infiltration, even though no chemical reaction occurs with the particle surface to form a passivating layer.

In an alternative approach, as in the Figure IB embodiment, the magnesium may be delivered to the bed 30 by other operable means. For example, the bed 30 could be fluidized by the introduction of an inert fluidizing gas at the bottom of the bed. The magnesium in the form of a magnesium compound may be introduced into the bottom of the bed 30 in the fluidizing gas stream so as to deposit a magnesium layer on the surface of the particles throughout the bed. Similarly, magnesium vapor in an inert carrier gas may be used to permeate the bed and coat the bed particles with a layer of magnesium. A layer of aluminum alloy is provided as in the previous approach and a vacuum applied using the approaches previously described. The deposited magnesium may react to form a passivating layer (for example, if the particles are alumina, a dense spinel layer may be formed) about 1 micrometer in thickness, or in other cases may form a layer 28 of magnesium which is continuous over the surface of the particles and preferably less than about 1 micrometer in thickness. The aluminum layer readily wets the passivating/wetting layer formed on alumina particles, or the wetting layer 28 (Figure IB embodiment) previously formed on the particles, resulting in a well-wetted matrix/particle interface. In either approach, there may be a small amount of oxygen, nitrogen, or other reactive gas present initially, which is quickly gettered by the magnesium. No further oxygen or nitrogen is permitted to contact and react with the magnesium inside the bed 30. Thus, there is an "absence of added oxygen and nitrogen", as that term is used herein. There is therefore very little, if any, aluminum oxide, magnesium oxide, aluminum nitride, or magnesium nitride formed. This approach is to be contrasted with a prior approach wherein excess nitrogen gas is intentionally provided in the bed to react with infiltrated magnesium to produce magnesium nitride, which deposits upon the particles. In the present approach, by contrast, the magnesium reacts with the alumina particles, not the gas, at the particle surface. The present technique produces a more adherent and denser layer to passivate the surfaces of the particles, as compared with this prior approach.

The infiltrated material in Figure 2D typically has a relatively high volume fraction of particles in the matrix, on the order of 40 volume percent or higher as determined by the relative amount of particles and matrix alloy used in the infiltration procedure. For some applications, a lower volume fraction of particles is desired. To produce such a lower volume fraction, the composite material mixture of Figure 2D is diluted, numeral 60 of Figure 3, such as by mixing the composite material with a second matrix material 38. In one approach, as illustrated in Figure 2E, the composite material of Figure 2D may be poured into a mass of molten second matrix material 38 in a second container 40 and mixed together. Equivalently for the present purposes, the second matrix material may be poured into the container 32 and mixed with the infiltrated composite material. In either case, the relative amounts of the infiltrated composite material prepared in steps 50-58 and the second matrix material 38 are selected to yield a final composite material having a preselected volume fraction of particles. The second matrix material 38 is typically a metallic alloy which may be of the same composition as the first matrix alloy material of the layer 36, or it may be of a different composition selected to yield a preselected net composition of the final matrix. Another implementation of the invention is illustrated in Figure 5. A mixture of operable particles (e.g., silicon carbide or spinel particles) and about 1 percent by weight of a source of magnesium is introduced into a mixing container 70 through an input line 72. The preferred source of magnesium is an alloy having the composition of the magnesium-aluminum eutectic at about 69 weight percent magnesium, balance aluminum. The eutectic composition is preferred in order to minimize the required operating temperature of the mixing container 72. The mixing container 70 is heated to a temperature above the melting point of the source of magnesium, and is provided with paddles 74 that stir the mixture of particles and the source of magnesium to ensure complete coating of the particles. The magnesium-coated particles flow from the mixing container 70 through an outlet line 76. The magnesium-coated particles flow into a composite mixing container 78.

The matrix alloy is added to the composite mixing container 78 through an input line 80. The matrix alloy and the magnesium-coated particles are mixed in the composite mixing container 78 to produce a final composite composition. The composite material, with the matrix in molten form, is removed through an outlet line 82 to be cast or otherwise processed. The composite mixing container 78 may be operated in either a batch or a continuous-flow mode of operation.

EXAMPLES

The present invention has been practiced using the approach discussed in relation to Figures 2A-2E and Figure 3. In one test, 2.8 kilograms of 320 mesh silicon carbide powder was added to a 3 inch diameter by 18 inch long tube container 32 to form the 12-inch deep bed 30. The container and bed were heated to 700°C. About 150 grams of molten magnesium was poured onto the upper surface of the bed 30. The magnesium had wicked into the bed after about 3 minutes, and no magnesium layer was visible on the surface. Approximately 1.6 kilograms of aluminum alloy 359 was then poured onto the upper surface of the magnesium-infiltrated bed to form a layer 6 inches deep. The upper surface of the aluminum alloy layer fell 3.75 inches in 10 minutes, and thereafter remained at a stable height. This amount of metal was sufficient to completely infiltrate the bed at 60 percent packing density. The resulting composite material was resuspended with additional aluminum alloy 359. In a second test, 5.0 kilograms of Placor 20 aluminum oxide particles having an average size of 20 micrometers was heated in a five inch diameter steel crucible to 700°C. About 866 grams of alloy AZ91 (Mg-9 weight percent aluminum- 1 weight percent zinc), preheated to 750°C, was added to the surface of the particle bed. After about 15 seconds the magnesium alloy had wicked into the bed, and an additional 6.0 kilograms of commercial purity aluminum, preheated to 750°C, was added to the surface of the bed. The aluminum completely infiltrated the bed in about 1 minute. The composite was re-suspended by stirring to produce a 32 volume percent aluminum oxide composite, which was thereafter diluted by the addition of further commercial purity aluminum to produce a 20 volume percent aluminum oxide composite material. The composite materials produced in the first and second tests were studied metallographically. In each case, the interfaces between the particles and the matrix alloy were clean. The absence of a visible reaction product at the aluminum oxide/metal matrix interface in the second test suggests that a thin passivating layer of spinal had formed on the aluminum oxide particles.

Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.

Claims

CLAIMS:

1. A method for preparing a composite material, in which a mass of ceramic particles is furnished and a source of matrix metal is contacted with the mass to form a metal-containing composite material; characterized in that, prior to said contact of said matrix metal with the mass, a source of a magnesium is contacted with the mass in the absence of added oxygen or nitrogen.

2. A method according to claim 1 , characterized in that said contact of said source of magnesium with the mass results in at least some of the particles being coated with a layer of magnesium.

3. A method according to claim 1 or claim 2, characterized in that the step of contacting the mass with a source of matrix metal involves contacting the mass with a source of aluminum or an alloy containing aluminum.

4. A method according to claim 1, claim 2 or claim 3, characterized in that the step of contacting the mass with a source of magnesium involves contacting the mass with a source of pure magnesium or an alloy containing magnesium.

5. A method according to any preceding claim, characterized in that the step of contacting the mass with a source of matrix metal involves infiltrating the matrix metal into the mass.

6. A method according to any preceding claim, characterized in that, after the step of contacting the mass with a source of matrix metal, a second source of the matrix metal is furnished, wherein the matrix metal is molten, and the metal- containing composite material is dispersed in the second source of matrix metal.

7. A method according to any preceding claim, characterized in that the step of contacting a source of magnesium with the mass involves placing a layer of molten magnesium on an upper surface of the mass, whereupon molten magnesium flows into the mass and forms a layer of magnesium on the particles.

8. A method according to any one of claims 1 to 6, characterized in that the step of contacting a source of matrix metal with the mass involves mixing said particles with the matrix metal in molten form.

9. A method according to claim 1, characterized in that said mass of ceramic particles is furnished in a container to form a bed, the bed being held at a temperature greater than the melting point of the matrix metal, a layer of molten magnesium is placed on an upper surface of the bed, whereupon molten magnesium flows into the bed of ceramic particles and forms a magnesium layer on said particles, a layer of molten aluminum as said source of matrix metal is placed on the upper surface of the bed, and a pressure differential is created between the layer of molten aluminum and the interior of the bed, whereupon molten aluminum flows into the bed to form an aluminum- containing composite material.

10. A method according to claim 9, characterized in that the step of placing said layer of magnesium on the upper surface of the bed involves placing a layer of pure magnesium or an alloy containing magnesium.

11. A method according to claim 9 or claim 10, characterized in that the step of placing a layer of molten aluminum on the upper surface of the bed involves placing a layer of pure aluminum or an alloy containing aluminum.

12. A method according to claim 9, claim 10 or claim 11, characterized in that, after the molten aluminum has flowed into the bed, a second source of aluminum in molten form is furnished, and the aluminum-containing composite material is dispersed in the second source of aluminum.

13. A method according to claim 1 , characterized in that the step of furnishing a mass of ceramic particles involves furnishing a mass of particles that are reactive with magnesium, whereupon contact of said source of magnesium with said mass forms a substantially continuous, non-porous layer of a chemical reaction product compound on the surfaces of the ceramic particles.

14. A method according to claim 13, characterized in that the step of furnishing a mass of ceramic particles involves furnishing a mass of particles of aluminum oxide.

15. A method according to claim 13 or claim 14, characterized in that the step of contacting a source of magnesium involves contacting a source of pure magnesium or an alloy containing magnesium.

16. A method according to claim 13, claim 14 or claim 15, characterized in that step of contacting a source of matrix metal with the ceramic particles involves contacting as source of pure aluminum or an alloy containing aluminum.

17. A method according to any one of claims 13 to 16, characterized in that, after the step of contacting the mass with the source of matrix metal, a second source of matrix metal in molten form is furnished and said metal-containing composite material is dispersed in the second source of matrix metal.

18. A method according to claim 1 , characterized in that said mass of ceramic particles is furnished in a container to form a bed held at a temperature greater than the melting point of aluminum, said contacting of said source of magnesium involves placing a layer of molten magnesium on an upper surface of the bed, whereupon molten magnesium flows into an upper portion of the bed and forms a magnesium layer on the particles within the upper portion of the bed, said step of contacting a source of matrix metal involves placing a source of aluminum on the upper surface of the bed to form a substantially continuous infiltration layer with the magnesium layer, and a pressure differential is created between the layer of molten aluminum and the interior of the bed, whereupon molten aluminum and magnesium flow into the bed to form said metal- containing composite material.

19. A method according to claim 1, characterized in that said step of contacting said source of magnesium with the mass of particles involves furnishing a source of magnesium and mixing together the ceramic particles of the mass and said source of magnesium at a temperature greater than the melting point of the source of magnesium to coat magnesium onto surfaces of the particles to produce magnesium-coated particles.

20. A method according to claim 19, characterized in that the step of contacting a source of matrix metal with the mass involves mixing the magnesium-coated particles with a source of aluminum in molten form.

21. A method according to claim 19 or claim 20, characterized in that the step of furnishing the mass of particles involves furnishing a mass of particles of spinel, aluminum oxide or silicon carbide.

22. A method of preparing a composite material, in which a mass of ceramic particles is furnished and a source of matrix metal is contacted with the mass to form a metal-containing composite material; characterized in that, prior to said contact of said matrix metal with the mass, said ceramic particles are contacted with a source of metal that is reactive with the ceramic particles to form a substantially continuous, non-porous layer of chemical reaction product compound thereon.

23. A method according to claim 22, characterized in that the step of furnishing a mass of ceramic particles comprises furnishing a mass of particles of aluminum oxide.

24. A method according to claim 22 or claim 23, characterized in that the source of reactive metal is a source of magnesium.

25. A method according to claim 24, characterized in that the source of magnesium is a source of pure magnesium or a source of an alloy containing magnesium.

26. A method according to any one of claims 22 to 25, characterized in that step of contacting the ceramic particles with a source of matrix metal comprises contacting the ceramic particles with a source of aluminum.

27. A method according to claim 26, characterized in that the source of aluminum is a source of pure aluminum or a source of an alloy containing aluminum.

28. A method according to any one of claims 22 to 27, characterized in that, after the step of contacting the mass with a source of matrix metal, a source of a second matrix metal is furnished and the composite material is dispersed in the second matrix metal.