A method of forming metal matrix composite bodies by utilizing a crushed polycrystalline oxidation reaction product as a filler, and products produced thereby

Abstract: The present invention relates to a novel method for forming metal matrix composite bodies and novel metal matrix composite bodies produced thereby. Particularly, a polycry­ stalline oxidation reaction product of a parent metal and an oxidant is first formed. The polycrystalline oxidation reaction product is thereafter comminuted into an appro­ priately sized filler material (2) which can be placed into a suitable container (4) or formed into a preform. The fil­ ler material or preform of comminuted polycrystalline oxi­ dation reaction product (2) is thereafter placed into con­ tact with a matrix metal alloy (1) in the presence of an infiltration enhancer, and/or an infiltration enhancer pre­ cursor and/or an infiltrating atmosphere, at least at some point during the process, whereupon the matrix metal alloy (1) spontaneously infiltrates the filler material or pre­ form. As a result of utilizing comminuted or crushed poly­ crystalline oxidation reaction product, enhanced infiltra­ tion (e.g., enhanced rate or amount) is achieved. Moreover, novel metal matrix composite bodies are produced.

Description: Field of the Invention The present invention relates to a novel method for forming metal matrix composite bodies and novel metal matrix composite bodies produced thereby. Particularly, a polycrystalline oxidation reaction product of a parent metal and an oxidant is first formed. The polycrystalline oxidation reaction product is thereafter comminuted into an appropriately sized filler material which can be placed into a suitable container or formed into a preform. The filler material or preform of comminuted polycrystalline oxidation reaction product is thereafter placed into contact with a matrix metal alloy in the presence of an infiltration enhancer, and/or an infiltration enhancer precursor as well as an infiltrating atmosphere, at least at some point during the process, whereupon the matrix metal alloy spontaneously infiltrates the filler material or preform. As a result of utilizing comminuted or crushed polycrystalline oxidation reaction product, enhanced infiltration (e.g., enhanced rate or amount) is achieved. Moreover, novel metal matrix composite bodies are produced. Background of the Invention Composite products comprising a metal matrix and a strengthening or reinforcing phase such as ceramic particulates, whiskers, fibers or the like, show great promise for a variety of applications because they combine some of the stiffness and wear resistance of the reinforcing phase with the ductility and toughness of the metal matrix. Generally, a metal matrix composite will show an improvement in such properties as strength, stiffness, contact wear resistance, and elevated temperature strength retention relative to the matrix metal in monolithic form, but the degree to which any given property may be improved depends largely on the specific constituents, their volume or weight fraction, and how they are processed in forming the composite. In some instances, the composite also may be lighter in weight than the matrix metal per se. Aluminum matrix composites reinforced with ceramics such as silicon carbide in particulate, platelet, or whisker form, for example, are of interest because of their higher stiffness, wear resistance and high temperature strength relative to aluminum. Various metallurgical processes have been described for the fabrication of aluminum matrix composites, including methods based on powder metallurgy techniques and liquid-metal infiltration techniques which make use of pressure casting, vacuum casting, stirring, and wetting agents. With powder metallurgy techniques, the metal in the form of a powder and the reinforcing material in the form of a powder, whiskers, chopped fibers, etc., are admixed and then either cold-pressed and sintered, or hot-pressed. The maximum ceramic volume fraction in silicon carbide reinforced aluminum matrix composites produced by this method has been reported to be about 25 volume percent in the case of whiskers, and about 40 volume percent in the case of particulates. The production of metal matrix composites by powder metallurgy techniques utilizing conventional processes imposes certain limitations with respect to the characteristics of the products attainable. The volume fraction of the ceramic phase in the composite is limited typically, in the case of particulates, to about 40 percent. Also, the pressing operation poses a limit on the practical size attainable. Only relatively simple product shapes are possible without subsequent processing (e.g., forming or machining) or without resorting to complex presses. Also, nonuniform shrinkage during sintering can occur, as well as nonuniformity of microstructure due to segregation in the compacts and grain growth. U.S. Patent No. 3,970,136, granted July 20, 1976, to J. C. Cannell et al., describes a process for forming a metal matrix composite incorporating a fibrous reinforcement, e.g. silicon carbide or alumina whiskers, having a predetermined pattern of fiber orientation. The composite is made by placing parallel mats or felts of coplanar fibers in a mold with a reservoir of molten matrix metal, e.g., aluminum, between at least some of the mats, and applying pressure to force molten metal to penetrate the mats and surround the oriented fibers. Molten metal may be poured onto the stack of mats while being forced under pressure to flow between the mats. Loadings of up to about 50% by volume of reinforcing fibers in the composite have been reported. The above-described infiltration process, in view of its dependence on outside pressure to force the molten matrix metal through the stack of fibrous mats, is subject to the vagaries of pressure-induced flow processes, i.e., possible non-uniformity of matrix formation, porosity, etc. Non-uniformity of properties is possible even though molten metal may be introduced at a multiplicity of sites within the fibrous array. Consequently, complicated mat/ reservoir arrays and flow pathways need to be provided to achieve adequate and uniform penetration of the stack of fiber mats. Also, the aforesaid pressure-infiltration method allows for only a relatively low reinforcement to matrix volume fraction to be achieved because of the difficulty inherent in infiltrating a large mat volume. Still further, molds are required to contain the molten metal under pressure, which adds to the expense of the process. Finally, the aforesaid process, limited to infiltrating aligned particles or fibers, is not directed to formation of aluminum metal matrix composites reinforced with materials in the form of randomly oriented particles, whiskers or fibers. In the fabrication of aluminum matrix-alumina filled composites, aluminum does not readily wet alumina, thereby making it difficult to form a coherent product. Various solutions to this problem have been suggested. One such approach is to coat the alumina with a metal (e.g., nickel or tungsten), which is then hot-pressed along with the aluminum. In another technique, the aluminum is alloyed with lithium, and the alumina may be coated with silica. However, these composites exhibit variations in properties, or the coatings can degrade the filler, or the matrix contains lithium which can affect the matrix properties. U.S. Patent No. 4,232,091 to R. W. Grimshaw et al., overcomes certain difficulties in the art which are encountered in the production of aluminum matrix-alumina composites. This patent describes applying pressures of 75-375 kg/cm² to force molten aluminum (or molten aluminum alloy) into a fibrous or whisker mat of alumina which has been preheated to 700 to 1050°C. The maximum volume ratio of alumina to metal in the resulting solid casting was 0.25/1. Because of its dependency on outside force to accomplish infiltration, this process is subject to many of the same deficiencies as that of Cannell et al. European Patent Application Publication No. 115,742 describes making aluminum-alumina composites, especially useful as electrolytic cell components, by filling the voids of a preformed alumina matrix with molten aluminum. The application emphasizes the non-wettability of alumina by aluminum, and therefore various techniques are employed to wet the alumina throughout the preform. For example, the alumina is coated with a wetting agent of a diboride of titanium, zirconium, hafnium, or niobium, or with a metal, i.e., lithium, magnesium, calcium, titanium, chromium, iron, cobalt, nickel, zirconium, or hafnium. Inert atmospheres, such as argon, are employed to facilitate wetting. This reference also shows applying pressure to cause molten aluminum to penetrate an uncoated matrix. In this aspect, infiltration is accomplished by evacuating the pores and then applying pressure to the molten aluminum in an inert atmosphere, e.g., argon. Alternatively, the preform can be infiltrated by vapor-phase aluminum deposition to wet the surface prior to filling the voids by infiltration with molten aluminum. To assure retention of the aluminum in the pores of the preform, heat treatment, e.g., at 1400 to 1800°C, in either a vacuum or in argon is required. Otherwise, either exposure of the pressure infiltrated material to gas or removal of the infiltration pressure will cause loss of aluminum from the body. The use of wetting agents to effect infiltration of an alumina component in an electrolytic cell with molten metal is also shown in European Patent Application Publication No. 94353. This publication describes production of aluminum by electrowinning with a cell having a cathodic current feeder as a cell liner or substrate. In order to protect this substrate from molten cryolite, a thin coating of a mixture of a wetting agent and solubility suppressor is applied to the alumina substrate prior to start-up of the cell or while immersed in the molten aluminum produced by the electrolytic process. Wetting agents disclosed are titanium, zirconium, hafnium, silicon, magnesium, vanadium, chromium, niobium, or calcium, and titanium is stated as the preferred agent. Compounds of boron, carbon and nitrogen are described as being useful in suppressing the solubility of the wetting agents in molten aluminum. The reference, however, does not suggest the production of metal matrix composites, nor does it suggest the formation of such a composite in, for example, a nitrogen atmosphere. In addition to application of pressure and wetting agents, it has been disclosed that an applied vacuum will aid the penetration of molten aluminum into a porous ceramic compact. For example, U.S. Patent No. 3,718,441, granted February 27, 1973, to R. L. Landingham, reports infiltration of a ceramic compact (e.g., boron carbide, alumina and beryllia) with either molten aluminum, beryllium, magnesium, titanium, vanadium, nickel or chromium under a vacuum of less than 10⁻⁶ torr. A vacuum of 10⁻² to 10⁻⁶ torr resulted in poor wetting of the ceramic by the molten metal to the extent that the metal did not flow freely into the ceramic void spaces. However, wetting was said to have improved when the vacuum was reduced to less than 10⁻⁶ torr. U.S. Patent No. 3,864,154, granted February 4, 1975, to G. E. Gazza et al., also shows the use of vacuum to achieve infiltration. This patent describes loading a cold-pressed compact of AlB₁₂ powder onto a bed of cold-pressed aluminum powder. Additional aluminum was then positioned on top of the AlB₁₂ powder compact. The crucible, loaded with the AlB₁₂ compact "sandwiched" between the layers of aluminum powder, was placed in a vacuum furnace. The furnace was evacuated to approximately 10⁻⁵ torr to permit outgassing. The temperature was subsequently raised to 1100°C and maintained for a period of 3 hours. At these conditions, the molten aluminum penetrated the porous AlB₁₂ compact. U.S. Patent No. 3,364,976, granted January 23, 1968 to John N. Reding et al., discloses the concept of creating a self-generated vacuum in a body to enhance penetration of a molten metal into the body. Specifically, it is disclosed that a body, e.g., a graphite mold, a steel mold, or a porous refractory material, is entirely submerged in a molten metal. In the case of a mold, the mold cavity, which is filled with a gas reactive with the metal, communicates with the externally located molten metal through at least one orifice in the mold. When the mold is immersed into the melt, filling of the cavity occurs as the self-generated vacuum is produced from the reaction between the gas in the cavity and the molten metal. Particularly, the vacuum is a result of the formation of a solid oxidized form of the metal. Thus, Reding et al. disclose that it is essential to induce a reaction between gas in the cavity and the molten metal. However, utilizing a mold to create a vacuum may be undesirable because of the inherent limitations associated with use of a mold. Molds must first be machined into a particular shape; then finished, machined to produce an acceptable casting surface on the mold; then assembled prior to their use; then disassembled after their use to remove the cast piece therefrom; and thereafter reclaim the mold, which most likely would include refinishing surfaces of the mold or discarding the mold if it is no longer acceptable for use. Machining of a mold into a complex shape can be very costly and time-consuming. Moreover, removal of a formed piece from a complex-shaped mold can also be difficult (i.e., cast pieces having a complex shape could be broken when removed from the mold). Still further, while there is a suggestion that a porous refractory material can be immersed directly in a molten metal without the need for a mold, the refractory material would have to be an integral piece because there is no provision for infiltrating a loose or separated porous material absent the use of a container mold (i.e., it is generally believed that the particulate material would typically disassociate or float apart when placed in a molten metal). Still further, if it was desired to infiltrate a particulate material or loosely formed preform, precautions should be taken so that the infiltrating metal does not displace at least portions of the particulate or preform resulting in a non-homogeneous microstructure. Accordingly, there has been a long felt need for a simple and reliable process to produce shaped metal matrix composites which does not rely upon the use of applied pressure or vacuum (whether externally applied or internally created), or damaging wetting agents to create a metal matrix embedding another material such as a ceramic material. Moreover, there has been a long felt need to minimize the amount of final machining operations needed to produce a metal matrix composite body. The present invention satisfies these needs by providing a spontaneous infiltration mechanism for infiltrating a material (e.g., a ceramic material), which can be formed into a preform, with molten matrix metal (e.g., aluminum) in the presence of an infiltrating atmosphere (e.g., nitrogen) under normal atmospheric pressures so long as an infiltration enhancer is present at least at some point during the process. Description of Commonly Owned Patent Applications The subject matter of this application is related to that of several other copending and co-owned patent applications. Particularly, these other copending patent applications describe novel methods for making metal matrix composite materials (hereinafter sometimes referred to as "Commonly Owned Metal Matrix Patent Applications"). A novel method of making a metal matrix composite material is disclosed in Commonly Owned EP-A-291441. According to the method of said invention, a metal matrix composite is produced by infiltrating a permeable mass of filler material (e.g., a ceramic or a ceramic-coated material) with molten aluminum containing at least about 1 percent by weight magnesium, and preferably at least about 3 percent by weight magnesium. Infiltration occurs spontaneously without the application of external pressure or vacuum. A supply of the molten metal alloy is contacted with the mass of filler material at a temperature of at least about 675°C in the presence of a gas comprising from about 10 to 100 percent, and preferably at least about 50 percent, nitrogen by volume, and a remainder of the gas, if any, being a nonoxidizing gas, e.g., argon. Under these conditions, the molten aluminum alloy infiltrates the ceramic mass under normal atmospheric pressures to form an aluminum (or aluminum alloy) matrix composite. When the desired amount of filler material has been infiltrated with the molten aluminum alloy, the temperature is lowered to solidify the alloy, thereby forming a solid metal matrix structure that embeds the reinforcing filler material. Usually, and preferably, the supply of molten alloy delivered will be sufficient to permit the infiltration to proceed essentially to the boundaries of the mass of filler material. The amount of filler material in the aluminum matrix composites produced according to the White et al. invention may be exceedingly high. In this respect, filler to alloy volumetric ratios of greater than 1:1 may be achieved. Under the process conditions in the aforesaid White et al. invention, aluminum nitride can form as a discontinuous phase dispersed throughout the aluminum matrix. The amount of nitride in the aluminum matrix may vary depending on such factors as temperature, alloy composition, gas composition and filler material. Thus, by controlling one or more such factors in the system, it is possible to tailor certain properties of the composite. For some end use applications, however, it may be desirable that the composite contain little or substantially no aluminum nitride. It has been observed that higher temperatures favor infiltration but render the process more conducive to nitride formation. Said invention allows the choice of a balance between infiltration kinetics and nitride formation. An example of suitable barrier means for use with metal matrix composite formation is described in Commonly Owned EP-A-323945. According to the method of this invention, a barrier means (e.g., particulate titanium diboride or a graphite material such as a flexible graphite tape product sold by Union Carbide under the tradename Grafoil®) is disposed on a defined surface boundary of a filler material and matrix alloy infiltrates up to the boundary defined by the barrier means. The barrier means is used to inhibit, prevent, or terminate infiltration of the molten alloy, thereby providing net, or near net, shapes in the resultant metal matrix composite. Accordingly, the formed metal matrix composite bodies have an outer shape which substantially corresponds to the inner shape of the barrier means. The method of Patent Application EP-A-291441 was improved upon by Commonly Owned EP-A-333629. In accordance with the methods disclosed in this Patent Application, a matrix metal alloy is present as a first source of metal and as a reservoir of matrix metal alloy which communicates with the first source of molten metal due to, for example, gravity flow. Particularly, under the conditions described in this patent application, the first source of molten matrix alloy begins to infiltrate the mass of filler material under normal atmospheric pressures and thus begins the formation of a metal matrix composite. The first source of molten matrix metal alloy is consumed during its infiltration into the mass of filler material and, if desired, can be replenished, preferably by a continuous means, from the reservoir of molten matrix metal as the spontaneous infiltration continues. When a desired amount of permeable filler has been spontaneously infiltrated by the molten matrix alloy, the temperature is lowered to solidify the alloy, thereby forming a solid metal matrix structure that embeds the reinforcing filler material. It should be understood that the use of a reservoir of metal is simply one embodiment of the invention described in this patent application and it is not necessary to combine the reservoir embodiment with each of the alternate embodiments of the invention disclosed therein, some of which could also be beneficial to use in combination with the present invention. The reservoir of metal can be present in an amount such that it provides for a sufficient amount of metal to infiltrate the permeable mass of filler material to a predetermined extent. Alternatively, an optional barrier means can contact the permeable mass of filler on at least one side thereof to define a surface boundary. Moreover, while the supply of molten matrix alloy delivered should be at least sufficient to permit spontaneous infiltration to proceed essentially to the boundaries (e.g., barriers) of the permeable mass of filler material, the amount of alloy present in the reservoir could exceed such sufficient amount so that not only will there be a sufficient amount of alloy for complete infiltration, but excess molten metal alloy could remain and be attached to the metal matrix composite body. Thus, when excess molten alloy is present, the resulting body will be a complex composite body (e.g., macrocomposite), wherein an infiltrated ceramic body having a metal matrix therein will be directly bonded to excess metal remaining in the reservoir. Each of the above-discussed Commonly Owned Metal Matrix Patent Applications describes methods for the production of metal matrix composite bodies and novel metal matrix composite bodies which are produced therefrom. The entire disclosures of all of the foregoing Commonly Owned Metal Matrix Patent Applications are expressly incorporated herein by reference. Moreover, several copending patent applications, and one issued Patent, which are also commonly owned (hereinafter sometimes referred to as "Commonly Owned Ceramic Matrix Patent Applications"), describe novel methods for reliably producing ceramic materials and ceramic composite materials. The method is disclosed generically in Commonly Owned EP-A-155831. This patent application discloses a method of producing self-supporting ceramic bodies grown as the oxidation reaction product of a molten parent precursor metal which is reacted with a vapor-phase oxidant to form an oxidation reaction product. Molten metal migrates through the formed oxidation reaction product to react with the oxidant thereby continuously developing a ceramic polycrystalline body which can, if desired, include an interconnected metallic component. The process may be enhanced or in certain cases enabled by the use of one or more dopants alloyed with the parent metal. For example, in the case of oxidizing aluminum in air, it is desirable to alloy magnesium and silicon with the aluminum to produce alpha-alumina ceramic structures. The method of EP-A-155831 was improved upon by the application of dopant materials to the surface of the parent metal, as described in Commonly Owned EP-A-169067. A similar oxidation phenomenon was utilized in producing ceramic composite bodies as described in Commonly Owned EP-A-193292. This application discloses novel methods for producing a self-supporting ceramic composite body by growing an oxidation reaction product from a parent metal precursor into a permeable mass of filler, (e.g., a silicon carbide particulate filler or an alumina particulate filler) thereby infiltrating or embedding the filler with a ceramic matrix. The resulting composite, however, has no defined or predetermined geometry, shape, or configuration. A method for producing ceramic composite bodies having a predetermined geometry or shape is disclosed in EP-A-245192. In accordance with the method in this Patent Application, the developing oxidation reaction product infiltrates a permeable self-supporting preform of filler material (e.g., an alumina or a silicon carbide preform material) in a direction towards a defined surface boundary to result in predetermined geometric or shaped composite bodies. Each of the above-discussed commonly owned ceramic matrix patent applications describes methods for the production of ceramic matrix composite bodies and novel ceramic matrix composite bodies which are produced therefrom. The entire disclosures of all the foregoing commonly owned metal matrix patent applications are expressly incorporated herein by reference. As discussed in these Commonly Owned Ceramic Matrix Patent Applications and Patent, novel polycrystalline ceramic materials or polycrystalline ceramic composite materials are produced by the oxidation reaction between a parent metal and an oxidant (e.g., a solid, liquid and/or a gas). In accordance with the generic process disclosed in these Commonly Owned Ceramic Matrix Patent Applications, a parent metal (e.g., aluminum) is heated to an elevated temperature above its melting point but below the melting point of the oxidation reaction product to form a body of molten parent metal which reacts upon contact with an oxidant to form the oxidation reaction product. At this temperature, the oxidation reaction product, or at least a portion thereof, is in contact with and extends between the body of molten parent metal and the oxidant, and molten metal is drawn or transported through the formed oxidation reaction product and towards the oxidant. The transported molten metal forms additional fresh oxidation reaction product upon contact with the oxidant, at the surface of previously formed oxidation reaction product. As the process continues, additional metal is transported through this formation of polycrystalline oxidation reaction product thereby continually "growing" a ceramic structure of interconnected crystallites. The resulting ceramic body may contain metallic constituents, such as non-oxidized constituents of the parent metal, and/or voids. Oxidation is used in its broad sense in all of the Commonly Owned Ceramic Matrix Patent Applications in this application, and refers to the loss or sharing of electrons by a metal to an oxidant which may be one or more elements and/or compounds. Accordingly, elements other than oxygen may serve as an oxidant. In certain cases, the parent metal may require the presence of one or more dopants in order to influence favorably or to facilitate growth of the oxidation reaction product. Such dopants may at least partially alloy with the parent metal at some point during or prior to growth of the oxidation reaction product. For example, in the case of aluminum as the parent metal and air as the oxidant, dopants such as magnesium and silicon, to name but two of a larger class of dopant materials, can be alloyed with aluminum and the created growth alloy is utilized as the parent metal. The resulting oxidation reaction product of such a growth alloy comprises alumina, typically alpha-alumina. Novel ceramic composite structures and methods of making the same are also disclosed and claimed in certain of the aforesaid Commonly Owned Ceramic Matrix Patent applications which utilize the oxidation reaction to produce ceramic composite structures comprising a substantially inert filler (note: in some cases it may be desirable to use a reactive filler, e.g., a filler which is at least partially reactive with the advancing oxidation reaction product and/or parent metal) infiltrated by the polycrystalline ceramic matrix. A parent metal is positioned adjacent to a mass of permeable filler (or a preform) which can be shaped and treated to be self-supporting, and is then heated to form a body of molten parent metal which is reacted with an oxidant, as described above, to form an oxidation reaction product. As the oxidation reaction product grows and infiltrates the adjacent filler material, molten parent metal is drawn through previously formed oxidation reaction product within the mass of filler and reacts with the oxidant to form additional fresh oxidation reaction product at the surface of the previously formed oxidation reaction product, as described above. The resulting growth of oxidation reaction product infiltrates or embeds the filler and results in the formation of a ceramic composite structure of a polycrystalline ceramic matrix embedding the filler. As also discussed above, the filler (or preform) may utilize a barrier means to establish a boundary or surface for the ceramic composite structure. Summary of the Invention This invention relates to an improved method for forming a metal matrix composite body by infiltrating a permeable mass of filler material or a preform which comprises a comminuted polycrystalline oxidation reaction product which is grown by an oxidation reaction between a molten parent metal and an oxidant in accordance with the teachings of the aforementioned Commonly Owned Ceramic Matrix Patent Applications. It has been unexpectedly discovered that the comminuted form of the polycrystalline oxidation reaction product provides for enhanced kinetics of infiltration of a matrix metal into a permeable mass of filler material or preform, and/or lower process temperatures, and/or a reduced likelihood of metal/particle reactions and/or lower costs. Moreover, the present invention may achieve increased volume fractions of filler material. Once a comminuted polycrystalline oxidation reaction product is obtained and formed into a filler material or a preform, a metal matrix composite body is then produced by infiltrating the permeable mass of filler material or preform. Specifically, an infiltration enhancer and/or an infiltration enhancer precursor as well as an infiltrating atmosphere are in communication with the filler material or a preform, at least at some point during the process, which permits molten matrix metal to spontaneously infiltrate the filler material or preform. Moreover, rather than supplying an infiltration enhancer precursor, an infiltration enhancer may be supplied directly to at least one of the preform, mass of filler material, and/or matrix metal. Ultimately, at least during the spontaneous infiltration, the infiltration enhancer should be located in at least a portion of the filler material or preform. For example, a matrix metal (e.g., an aluminum alloy) is positioned such that it is in communication with a surface of a permeable mass of filler material or a preform (e.g., ceramic particles, whiskers and/or fibers) so that when the matrix metal is in the molten stage, it can spontaneously infiltrate the permeable mass of filler material or preform. Moreover, if an infiltration enhancer or an infiltration enhancer precursor is not inherently supplied by the comminuted polycrystalline oxidation reaction product, the same can be added to at least one of the matrix metal and comminuted oxidation reaction product (whether as a filler material or preform). The combination of comminuted polycrystalline oxidation reaction product, matrix metal, supply of infiltration enhancer precursor and/or infiltration enhancer, and infiltrating atmosphere causes the matrix metal to spontaneously infiltrate the filler material or preform. It is noted that this application discusses primarily aluminum matrix metals which, at some point during the formation of the metal matrix composite body, are contacted with magnesium, which functions as the infiltration enhancer precursor, in the presence of nitrogen, which functions as the infiltrating atmosphere. Thus, the matrix metal/infiltration enhancer precursor/infiltrating atmosphere system of aluminum/magnesium/nitrogen exhibits spontaneous infiltration. However, other matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems may also behave in a manner similar to the aluminum/magnesium/nitrogen system. For example, similar spontaneous infiltration behavior has been observed in the aluminum/strontium/nitrogen system; the aluminum/zinc/oxygen system; and the aluminum/calcium/nitrogen system. Accordingly, even though the aluminum/magnesium/nitrogen system is discussed primarily herein, it should be understood that other matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems may behave in a similar manner and are intended to be encompassed by the invention. When the matrix metal comprises an aluminum alloy, and the comminuted polycrystalline oxidation reaction product comprises a comminuted alumina polycrystalline oxidation reaction product, the aluminum alloy is contacted with the preform or filler material in the presence of, for example, magnesium and/or may be exposed to magnesium at some point during the process. The aluminum alloy and filler material or preform are contained in a nitrogen atmosphere for at least some portion of the process. Under these conditions, the preform or filler material will be spontaneously infiltrated and the extent or rate of spontaneous infiltration and formation of metal matrix composite body will vary with the given set of processing conditions including, for example, the concentration of infiltration enhancer precursor (e.g., magnesium) and/or infiltration enhancer provided to the system (e.g., in the aluminum alloy and/or in the preform), the size and/or composition of the filler material or preform, the concentration of nitrogen in the infiltrating atmosphere, time permitted for infiltration, and/or the temperature at which infiltration occurs. Spontaneous infiltration typically occurs to an extent sufficient to embed substantially completely the preform or filler material. Definitions " Aluminum ", as used herein, in conjunction with both ceramic matrix composite bodies and metal matrix composite bodies, means and includes essentially pure metal (e.g., a relatively pure, commercially available unalloyed aluminum) or other grades of metal and metal alloys such as the commercially available metals having impurities and/or alloying constituents such as iron, silicon, copper, magnesium, manganese, chromium, zinc, etc., therein. An aluminum alloy for purposes of this definition is an alloy or intermetallic compound in which aluminum is the major constituent. " Balance Non-Oxidizing Gas ", as used herein, in conjunction with metal matrix composite bodies means that any gas present in addition to the primary gas comprising the infiltrating atmosphere is either an inert gas or a reducing gas which is substantially non-reactive with the matrix metal under the process conditions. Any oxidizing gas which may be present as an impurity in the gas(es) used should be insufficient to oxidize the matrix metal to any substantial extent under the process conditions. " Barrier " or " barrier means ", as used herein, in conjunction with ceramic matrix composite bodies means any material, compound, element, composition, or the like, which, under the process conditions, maintains some integrity, is not substantially volatile (i.e., the barrier material does not volatilize to such an extent that it is rendered non-functional as a barrier) and is preferably permeable to a vapor-phase oxidant (if utilized) while being capable of locally inhibiting, poisoning, stopping, interfering with, preventing, or the like, continued growth of the oxidation reaction product. " Barrier " or " barrier means ", as used herein, in conjunction with metal matrix composite bodies means any suitable means which interferes, inhibits, prevents or terminates the migration, movement, or the like, of molten matrix metal beyond a surface boundary of a permeable mass of filler material or preform, where such surface boundary is defined by said barrier means. Suitable barrier means may be any such material, compound, element, composition, or the like, which, under the process conditions, maintains some integrity and is not substantially volatile (i.e., the barrier material does not volatilize to such an extent that it is rendered non-functional as a barrier). Further, suitable "barrier means" includes materials which are substantially non-wettable by the migrating molten matrix metal under the process conditions employed. A barrier of this type appears to exhibit substantially little or no affinity for the molten matrix metal, and movement beyond the defined surface boundary of the mass of filler material or preform is prevented or inhibited by the barrier means. The barrier reduces any final machining or grinding that may be required and defines at least a portion of the surface of the resulting metal matrix composite product. The barrier may in certain cases be permeable or porous, or rendered permeable by, for example, drilling holes or puncturing the barrier, to permit gas to contact the molten matrix metal. " Carcass " or " Parent Metal Carcass " or " Matrix Metal Carcass ", as used herein, refers to any of the original body of parent metal or matrix metal remaining which has not been consumed during formation of the ceramic body, ceramic composite body or the metal matrix composite body, and typically, which remains in at least partial contact with the formed body. It should be understood that the carcass may also typically include some oxidized constituents of the parent metal or matrix metal and/or a second or foreign metal therein. " Ceramic ", as used herein, should not be unduly construed as being limited to a ceramic body in the classical sense, that is, in the sense that it consists entirely of non-metallic and inorganic materials, but rather refers to a body which is predominantly ceramic with respect to either composition or dominant properties, although the body may contain minor or substantial amounts of one or more metallic constituents (isolated and/or interconnected, depending on the processing conditions used to form the body) derived from the parent metal, or reduced from the oxidant or a dopant, most typically within a range of from about 1-40 percent by volume, but may include still more metal. " Dopants ", as used herein, in conjunction with ceramic matrix composite bodies means materials (alloy constituents or constituents combined with and/or included in and/or in or on a filler) which, when used in combination with the parent metal, favorably influence or promote the oxidation reaction process and/or modify the growth process to alter the microstructure and/or properties of the product. While not wishing to be bound by any particular theory or explanation of the function of dopants, it appears that some dopants are useful in promoting oxidation reaction product formation in cases where appropriate surface energy relationships between the parent metal and its oxidation reaction product do not intrinsically exist so as to promote such formation. Dopants may: create favorable surface energy relationships which enhance or induce the wetting of the oxidation reaction product by the molten parent metal; form a "precursor layer" at the growth surface by reaction with alloy, oxidant, and/or filler, that (a) minimizes formation of a protective and coherent oxidation reaction product layer(s), (b) may enhance oxidant solubility (and thus permeability) in molten metal, and/or (c) allows for transport of oxidant from the oxidizing atmosphere through any precursor oxide layer to combine subsequently with the molten metal to form another oxidation reaction product; cause microstructural modifications of the oxidation reaction product as it is formed or subsequently, alter the metallic constituent composition and properties of such oxidation reaction product; and/or enhance growth nucleation and uniformity of growth of oxidation reaction product. " Filler ", as used herein, in conjunction with both metal matrix and ceramic matrix composite bodies is intended to mean either single constituents or mixtures of constituents which are substantially non-reactive with and/or of limited solubility in the metal (e.g., parent metal) and/or oxidation reaction product and may be single or multi-phase. Fillers may be provided in a wide variety of forms, such as powders, flakes, platelets, microspheres, whiskers, bubbles, etc., and may be either dense or porous. "Filler" may also include ceramic fillers, such as alumina or silicon carbide as fibers, chopped fibers, particulates, whiskers, bubbles, spheres, fiber mats, or the like, and coated fillers such as carbon fibers coated with alumina or silicon carbide to protect the carbon from attack, for example, by a molten aluminum parent metal. Fillers may also include metals. " Growth Alloy ", as used herein, in conjunction with ceramic or ceramic composite bodies means any alloy containing initially or at some point during processing obtaining a sufficient amount of requisite constituents to result in growth of oxidation reaction product therefrom. " Infiltrating Atmosphere ", as used herein, in conjunction with metal matrix composite bodies means that atmosphere which is present which interacts with the matrix metal and/or preform (or filler material) and/or infiltration enhancer precursor and/or infiltration enhancer and permits or enhances spontaneous infiltration of the matrix metal to occur. " Infiltration Enhancer ", as used herein, in conjunction with metal matrix composite bodies means a material which promotes or assists in the spontaneous infiltration of a matrix metal into a filler material or preform. An infiltration enhancer may be formed from, for example, (1) a reaction of an infiltration enhancer precursor and the infiltrating atmosphere to form a gaseous species and/or (2) a reaction product of the infiltration enhancer precursor and the infiltrating atmosphere and/or (3) a reaction product of the infiltration enhancer precursor and the filler material or preform. Moreover, the infiltration enhancer may be supplied directly to at least one of the filler material or preform, and/or matrix metal, and/or infiltrating atmosphere and function in a substantially similar manner to an infiltration enhancer which has formed as a reaction between an infiltration enhancer precursor and another species. Ultimately, at least during the spontaneous infiltration, the infiltration enhancer should be located in at least a portion of the filler material or preform to achieve spontaneous infiltration. " Infiltration Enhancer Precursor " or " Precursor to the Infiltration Enhancer ", as used herein, in conjunction with metal matrix composite bodies means a material which when used in combination with (1) the matrix metal, (2) the preform or filler material and/or (3) an infiltrating atmosphere forms an infiltration enhancer which induces or assists the matrix metal to spontaneously infiltrate the filler material or preform. Without wishing to be bound by any particular theory or explanation, it appears as though it may be necessary for the precursor to the infiltration enhancer to be capable of being positioned, located or transportable to a location which permits the infiltration enhancer precursor to interact with the infiltrating atmosphere and/or the preform or filler material and/or metal. For example, in some matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems, it is desirable for the infiltration enhancer precursor to volatilize at, near, or in some cases, even somewhat above the temperature at which the matrix metal becomes molten. Such volatilization may lead to: (1) a reaction of the infiltration enhancer precursor with the infiltrating atmosphere to form a gaseous species which enhances wetting of the filler material or preform by the matrix metal; and/or (2) a reaction of the infiltration enhancer precursor with the infiltrating atmosphere to form a solid, liquid or gaseous infiltration enhancer in at least a portion of the filler material or preform which enhances wetting; and/or (3) a reaction of the infiltration enhancer precursor within the filler material or preform which forms a solid, liquid or gaseous infiltration enhancer in at least a portion of the filler material or preform which enhances wetting. " Liquid-Phase Oxidant " or " Liquid Oxidant ", as used herein, in conjunction with ceramic matrix composite bodies means an oxidant in which the identified liquid is the sole, predominant or at least a significant oxidizer of the parent or precursor metal under the conditions of the process. Reference to a liquid oxidant means one which is a liquid under the oxidation reaction conditions. Accordingly, a liquid oxidant may have a solid precursor, such as a salt, which is molten at the oxidation reaction conditions. Alternatively, the liquid oxidant may have a liquid precursor (e.g., a solution of a material) which is used to impregnate part or all of the filler and which is melted or decomposed at the oxidation reaction conditions to provide a suitable oxidant moiety. Examples of liquid oxidants as herein defined include low melting glasses. If a liquid oxidant is employed in conjunction with the parent metal and a filler, typically, the entire bed of filler, or that portion comprising the desired ceramic body, is impregnated with the oxidant (e.g., by coating or immersion in the oxidant). " Matrix Metal " or " Matrix Metal Alloy ", as used herein in conjunction with metal matrix composite bodies, means that metal which is utilized to form a metal matrix composite (e.g., before infiltration) and/or that metal which is intermingled with a filler material to form a metal matrix composite body (e.g., after infiltration). When a specified metal is mentioned as the matrix metal, it should be understood that such matrix metal includes that metal as an essentially pure metal, a commercially available metal having impurities and/or alloying constituents therein, an intermetallic compound or an alloy in which that metal is the major or predominant constituent. " Matrix Metal/Infiltration Enhancer Precursor/Infiltrating Atmosphere System " or " Spontaneous System ", as used herein in conjunction with metal matrix composite bodies, refers to that combination of materials which exhibit spontaneous infiltration into a preform or filler material. It should be understood that whenever a "/" appears between an exemplary matrix metal, infiltration enhancer precursor and infiltrating atmosphere, the "/" is used to designate a system or combination of materials which, when combined in a particular manner, exhibits spontaneous infiltration into a preform or filler material. " Metal Matrix Composite " or " MMC ", as used herein in conjunction with metal matrix composite bodies, means a material comprising a two- or three-dimensionally interconnected alloy or matrix metal which has embedded a preform or filler material. The matrix metal may include various alloying elements to provide specifically desired mechanical and physical properties in the resulting composite. A Metal "Different" , as used in conjunction with ceramic matrix composite bodies and/or metal matrix composite bodies, means a metal which does not contain, as a primary constituent, the same metal as the matrix metal or parent metal (e.g., if the primary constituent of the matrix metal or parent metal is aluminum, the "different" metal could have a primary constituent of, for example, nickel). " Nitrogen-Containing Gas Oxidant ", as used herein in conjunction with ceramic matrix composite bodies, is a particular gas or vapor in which nitrogen is the sole, predominant or at least a significant oxidizer of the parent or precursor metal under the conditions existing in the oxidizing environment utilized. " Oxidant ", as used herein in conjunction with ceramic matrix composite bodies, means one or more suitable electron acceptors or electron sharers and may be a solid, a liquid or a gas or some combination of these (e.g., a solid and a gas) at the oxidation reaction conditions. Typical oxidants include, without limitation, oxygen, nitrogen, a halogen, sulphur, phosphorus, arsenic, carbon, boron, selenium, tellurium, and or compounds and combinations thereof, for example, silica or silicates (as a source of oxygen), methane, ethane, propane, acetylene, ethylene, propylene (the hydrocarbon as a source of carbon), and mixtures such as air, H₂/H₂O and CO/CO₂ (source of oxygen), the latter two (i.e., H₂/H₂O and CO/CO₂) being useful in reducing the oxygen activity of the environment. " Oxidation Reaction Product ", as used herein in conjunction with ceramic matrix composite bodies, means one or more metals in any oxidized state wherein the metal(s) has given up electrons to or shared electrons with another element, compound, or combination thereof. Accordingly, an "oxidation reaction product" under this definition includes the product of reaction of one or more metals with one or more oxidants. " Oxygen-Containing Gas Oxidant ", as used herein in conjunction with ceramic matrix composite bodies, is a particular gas or vapor in which oxygen is the sole, predominant or at least a significant oxidizer or the parent or precursor metal under the conditions existing in the oxidizing environment utilized. " Parent Metal ", as used herein in conjunction with ceramic matrix composite bodies, means that metal(s) (e.g., aluminum, silicon, titanium, tin and/or zirconium) which is the precursor of a polycrystalline oxidation reaction product and includes that metal(s) as an essentially pure metal, a commercially available metal having impurities and/or alloying constituents therein, or an alloy in which that metal precursor is the major constituent. When a specified metal is mentioned as the parent or precursor metal (e.g., aluminum, etc.), the metal identified should be read with this definition in mind unless indicated otherwise by the context. " Preform " or " Permeable Preform ", as used herein conjunction with ceramic matrix composite bodies and metal matrix composite bodies, means a porous mass of filler or filler material which is manufactured with at least one surface boundary which essentially defines a boundary for infiltrating matrix metal, such mass retaining sufficient shape integrity and green strength to provide dimensional fidelity prior to being infiltrated by the matrix metal. The mass should be sufficiently porous to accommodate spontaneous infiltration of the matrix metal thereinto. A preform typically comprises a bonded array or arrangement of filler, either homogeneous or heterogeneous, and may be comprised of any suitable material (e.g., ceramic and/or metal particulates, powders, fibers, whiskers, etc., and any combination thereof). A preform may exist either singularly or as an assemblage. " Reservoir ", as used herein, means a separate body of parent metal or matrix metal positioned relative to a mass of filler or a preform so that, when the metal is molten, it may flow to replenish, or in some cases to initially provide and subsequently replenish, that portion, segment or source of parent metal or matrix metal which is in contact with the filler or preform and infiltrating or reacting to form the oxidation reaction product. The reservoir may also be used to provide a metal which is different from the matrix metal. " Second or Foreign Metal ", as used herein in conjunction with ceramic or metal matrix composite bodies, means any suitable metal, combination of metals, alloys, intermetallic compounds, or sources of either, which is, or is desired to be, incorporated into the metallic component of a formed ceramic or metal matrix composite body in lieu of, in addition to, or in combination with unoxidized constituents of the parent metal. This definition includes intermetallic compounds, alloys, solid solutions or the like formed between the parent metal and a second metal. " Solid-Phase Oxidant " or " Solid Oxidant ", as used herein in conjunction with ceramic matrix composite bodies, means an oxidant in which the identified solid is the sole, predominant or at least a significant oxidizer of the parent or precursor metal under the conditions of the process. When a solid oxidant is employed in conjunction with the parent metal and a filler, it is usually dispersed throughout the entire bed of filler or that portion of the bed into which the oxidation reaction product will grow, the solid oxidant being, for example, particulates admixed with the filler or coatings on the filler particles. Any suitable solid oxidant may be thus employed including elements, such as boron or carbon, or reducible compounds, such as silicon dioxide or certain borides of lower thermodynamic stability than the boride reaction product of the parent metal. For example, when boron or a reducible boride is used as a solid oxidant for an aluminum parent metal, the resulting oxidation reaction product comprises aluminum boride. In some instances, the oxidation reaction of the parent metal may proceed so rapidly with a solid oxidant that the oxidation reaction product tends to fuse due to the exothermic nature of the process. This occurrence can degrade the microstructural uniformity of the ceramic body. This rapid exothermic reaction can be ameliorated by mixing into the composition relatively inert fillers which absorb the excess heat. An example of such a suitable inert filler is one which is identical, or substantially identical, to the intended oxidation reaction product. " Spontaneous Infiltration ", as used herein in conjunction with metal matrix composite bodies, means the infiltration of matrix metal into the permeable mass of filler or preform occurs without requirement for the application of pressure or vacuum (whether externally applied or internally created). " Vapor-Phase Oxidant ", as used herein in conjunction with ceramic matrix composite bodies, identifies the oxidant as containing or comprising a particular gas or vapor and means an oxidant in which the identified gas or vapor is the sole, predominant or at least a significant oxidizer of the parent or precursor metal under the conditions obtained in the oxidizing environment utilized. For example, although the major constituent of air is nitrogen, the oxygen content of air is the sole oxidizer for the parent metal because oxygen is a significantly stronger oxidant than nitrogen. Air therefore falls within the definition of an "Oxygen-Containing Gas Oxidant" but not within the definition of a "Nitrogen-Containing Gas Oxidant" (an example of a "nitrogen-containing gas" oxidant is forming gas, which typically contains about 96 volume percent nitrogen and about 4 volume percent hydrogen) as those terms are used herein and in the claims. Brief Description of the Figures The following Figures are provided to assist in understanding the invention, but are not intended to limit the scope of the invention. Similar reference numerals have been used wherever possible in each of the Figures to denote like components, wherein: Figure 1 is a schematic cross-section of an assemblage of materials utilized to produce a ceramic composite body according to Example 1. Figure 2 is a schematic cross-section of an assemblage of the materials utilized to produce a metal matrix composite body in accordance with Example 1. Figure 3 is a photomicrograph at 400x of a section of the metal matrix composite body formed according to Example 1. Detailed Description of the Invention and Preferred Embodiments To form a ceramic or ceramic composite body which is to be comminuted in accordance with the method of the present invention (i.e., to form a filler material or preform for use in the formation of metal matrix composite bodies), a parent metal (i.e., the growth alloy), which may be doped as explained below in greater detail, is formed into an ingot, billet, rod, plate or the like and is placed into or contained within an inert bed, crucible or other refractory container. The parent metal may comprise one or more pieces, ingots or the like and may be suitably shaped by any appropriate means. The parent metal may be oxidized in conjunction with a dopant material (described below in greater detail). A permeable mass of filler material, or, in a preferred embodiment, a permeable, shaped preform (described below in greater detail) is manufactured so as to have at least one defined surface boundary and to be permeable to a vapor-phase oxidant, when such a vapor-phase oxidant is used alone or in combination with another oxidant, and to be permeable to the infiltrating oxidation reaction product when a permeable mass is utilized, the parent metal can be placed on top of said permeable mass. Alternatively, the preform is placed adjacent to, and preferably in contact with, at least one surface of, or a portion of a surface of, the parent metal such that at least a portion of the defined surface boundary of the preform is generally positioned distantly, or outwardly spaced apart, from the surface of the parent metal. The preform preferably is in contact with a surface of the parent metal; but when desired, the preform may be partially immersed, but not totally immersed, in the molten metal. Total immersion would cut-off or block access of the vapor-phase oxidant into the preform and thus inhibit proper development of the oxidation reaction product which embeds the preform. However, when a vapor-phase oxidant is not used (i.e., the only oxidant used at the process conditions is a solid oxidant or a liquid oxidant), then total immersion of the preform in a molten parent metal becomes a viable alternative. Formation of the oxidation reaction product will occur in a direction towards the defined surface boundary. The set-up of parent metal and permeable mass or preform are placed in a suitable container such as a boat formed of alumina or a castable refractory and inserted into a furnace. The atmosphere in the furnace may contain an oxidant to permit vapor-phase oxidation of molten parent metal to occur. The furnace is then heated up to process conditions. Additionally, electric heating is typically used to achieve the temperature utilized by the invention. However, any heating means which can cause the oxidation reaction growth and the matrix metal to become molten and does not adversely affect either is acceptable for use with the invention. A preform useful in the manufacture of the composite body, when at least one oxidant is a vapor-phase oxidant, is one that is sufficiently porous or permeable to permit the vapor-phase oxidant to permeate into the preform so as to contact the parent metal. The preform also should be self-supporting and sufficiently permeable to accommodate the development or growth of the oxidation reaction product as a matrix within the preform without substantially disturbing, upsetting, or otherwise altering the configuration or geometry of the preform. A solid, liquid, or vapor-phase oxidant, or a combination of such oxidants, may be employed. For example, typical oxidants include, without limitation, oxygen, nitrogen, a halogen, sulphur, phosphorus, arsenic, carbon, boron, selenium, tellurium, and/or compounds and combinations thereof, for example, silica (as a source of oxygen), methane, ethane, propane, acetylene, ethylene, and propylene (as sources of carbon), and mixtures such as air, H₂/H₂O and CO/CO₂ the latter two (i.e., H₂/H₂O and CO/CO₂) being useful in reducing the oxygen activity of the environment. Accordingly, the ceramic structure of the invention may comprise an oxidation reaction product comprising one or more of oxides, nitrides, carbides, borides and oxynitrides. More specifically, the oxidation reaction product may, for example, be one or more of aluminum oxide, aluminum nitride, silicon carbides, silicon boride, aluminum boride, titanium nitride, zirconium nitride, titanium boride, zirconium boride, titanium carbide, zirconium carbide, silicon nitride, hafnium boride and tin oxide. Although the oxidation reaction is usually described as employing a vapor-phase oxidant, either alone or in conjunction with an oxidant which is a solid or liquid under the process conditions, it should be understood that the utilization of a vapor-phase oxidant is not necessary to produce the ceramic matrix composite body. When a vapor-phase oxidant is not employed and an oxidant which is a solid or liquid under the process conditions is used, the preform need not be permeable to the surrounding atmosphere. However, the preform should still be sufficiently permeable to accommodate the development or growth of the oxidation reaction product as a matrix within the preform without substantially disturbing, upsetting, or otherwise altering the configuration or geometry of the preform. The use of solid or liquid oxidants may create an environment within the preform more favorable to the oxidation kinetics of the parent metal than the environment outside the preform. This enhanced environment is beneficial in promoting matrix development within the preform to the boundary and minimizing overgrowth. When a solid oxidant is employed, it may be dispersed through the entire preform or through a portion of the preform adjacent to the parent metal, such as in particulate form and admixed with the preform, or it may be utilized as coatings on the particles comprising the preform. Suitable solid oxidants may include suitable elements, such as boron or carbon, or suitable reducible compounds, such as silicon dioxide (as a source of oxygen) or certain borides of lower thermodynamic stability than the boride reaction product of the parent metal. If a liquid oxidant is employed, the liquid oxidant may be dispersed throughout the entire preform or a portion thereof adjacent to the molten parent metal. Reference to a liquid oxidant means one which is a liquid under the oxidation reaction conditions, and so a liquid oxidant may have a solid precursor, such as a salt, which is molten or liquid at the oxidation reaction conditions. Alternatively, the liquid oxidant may be a liquid precursor, e.g. a solution of a material, which is used to coat part or all of the porous surfaces of the preform and which is melted or decomposed at the process conditions to provide a suitable oxidant moiety. Examples of liquid oxidants as herein defined include low melting glasses. As explained in the Commonly Owned Patent Applications and Patent, the addition of dopant materials, in conjunction with, for example, aluminum parent metal, can favorably influence the oxidation reaction process. The function or functions of the dopant material can depend upon a number of factors other than the dopant material itself. These factors include, for example, the end product desired, the particular combination of dopants when two or more dopants are used, the use of externally applied dopants in combination with an alloyed dopant, the concentration of the dopant(s), the oxidizing environment, and the process conditions. The dopant or dopants used in conjunction with the parent metal (1) may be provided as alloying constituents of the parent metal, (2) may be applied to at least a portion of the surface of the parent metal such as by spray coating or painting, (3) may be added to the filler material, or any combination of techniques (1), (2) and (3) may be employed. For example, an alloyed dopant may be used in combination with an externally applied dopant. A source of the dopant may be provided by placing either a dopant powder or a rigid body of dopant in contact with at least a portion of the parent metal surface. For example, a thin sheet of silicon-containing glass can be placed upon a surface of an aluminum parent metal. When the aluminum parent metal (which may be internally doped with Mg) overlaid with the silicon-containing material is heated in an oxidizing environment (e.g., in the case of aluminum in air, between about 850°C to about 1450°C, preferably about 900°C to about 1350°C), growth of the polycrystalline ceramic material occurs. In the case where the dopant is externally applied to at least a portion of the surface of the aluminum parent metal, the polycrystalline aluminum oxide structure generally grows substantially beyond the dopant layer (i.e., to beyond the depth of the applied dopant layer). In any case, one or more of the dopants may be externally applied to the parent metal surface. Additionally, any concentration deficiencies of the dopants alloyed within the parent metal may be augmented by additional concentration of the respective dopant(s) applied external to the parent metal. Useful dopants for an aluminum parent metal, particularly with air as the oxidant, include, for example, magnesium, zinc and silicon, in combination with each other or in combination with other dopants described below. These metals, or a suitable source of the metals, may be alloyed into the aluminum-based parent metal at concentrations for each of between about 0.1-10% by weight based on the total weight of resulting doped metal. Concentrations within this range appear to initiate the ceramic growth, enhance metal transport and favorably influence the growth morphology of the resulting oxidation reaction product. The concentration range for any one dopant will depend on such factors as the combination of dopants and the process temperature. Other dopants which are effective in promoting alumina polycrystalline oxidation reaction product growth, from aluminum parent metal systems are, for example, germanium, tin and lead, especially when used in combination with magnesium. One or more of these other dopants, or a suitable source of them, is alloyed into the aluminum parent metal system at concentrations for each of from about 0.5 to about 15% by weight of the total alloy; however, more desirable growth kinetics and growth morphology are obtained with dopant concentrations in the range of from about 1-10% by weight of the total parent metal alloy. Lead as a dopant is generally alloyed into the aluminum-based parent metal at a temperature of at least 1000°C so as to make allowances for its low solubility in aluminum; however, the addition of other alloying components, such as tin, will generally increase the solubility of lead and allow the alloying materials to be added at a lower temperature. In the case of an aluminum parent metal and with air as the oxidant, particularly useful combinations of dopants include (a) magnesium and silicon or (b) magnesium, zinc and silicon. In such examples, a preferred magnesium concentration falls within the range of from about 0.1 to about 3% by weight, for zinc in the range of from about 1 to about 6% by weight, and for silicon in the range of from about 1 to about 10% by weight. Additional examples of dopant materials, useful with an aluminum parent metal, include sodium and lithium, which may be used individually or in combination with one or more other dopants depending on the process conditions. Sodium and lithium may be used in very small amounts (in the parts per million range, typically about 100-200 parts per million) and each may be used alone or together, or in combination with other dopant(s). Calcium, boron, phosphorus, yttrium, and rare earth elements such as cerium, lanthanum, praseodymium, neodymium and samarium are also useful dopants, and herein again especially when used in combination with other dopants. The dopant materials, when used externally, are usually applied to a portion of a surface of the parent metal as a uniform coating thereon. The quantity of dopant is effective over a wide range relative to the amount of parent metal to which it is applied and, in the case of aluminum, experiments have failed to identify either upper or lower operable limits. For example, when utilizing silicon in the form of silicon dioxide externally applied as the dopant for an aluminum based parent metal using air or oxygen as the oxidant, quantities as low as 0.00003 gram of silicon per gram of parent metal, or about 0.0001 gram of silicon per square centimeter of exposed parent metal surface, together with a second dopant source of magnesium, have been used to produce the polycrystalline ceramic growth phenomenon. It also has been found that a ceramic structure is achievable from an aluminum-silicon alloy parent metal using air or oxygen as the oxidant by using MgO as the dopant in an amount greater than about 0.0008 gram of Mg per gram of parent metal to be oxidized and greater than 0.003 gram of Mg per square centimeter of parent metal surface upon which the MgO is applied. Where the parent metal is aluminum internally doped with magnesium and the oxidizing medium is air or oxygen, it has been observed that magnesium is at least partially oxidized out of the alloy at temperatures of from about 820 to 950°C. In such instances of magnesium-doped systems, the magnesium forms a magnesium oxide and/or magnesium aluminate spinel phase at the surface of the molten aluminum alloy and during the growth process such magnesium compounds remain primarily at the initial oxide surface of the parent metal alloy (e.g., the "initiation surface") in the grown ceramic structure. Thus, in such magnesium-doped systems, an aluminum oxide-based structure is produced apart from the relatively thin layer of magnesium aluminate spinel at the initiation surface. Where desired, this initiation surface can be readily removed as by grinding, machining, polishing or grit blasting prior to using the polycrystalline ceramic product. In an alternative embodiment of the invention, during growth of the polycrystalline oxidation reaction product, a different vapor-phase oxidant can be introduced. In this context, "different" should be understood as meaning having a composition which is chemically different from the composition of an initial vapor (or solid) phase oxidant. Thus, the second oxidation reaction product formed with the "different" vapor-phase oxidant will result in the formation of two ceramic bodies or phases integrally attached to each other having graded properties (e.g., a layer can be formed on a first formed ceramic composite body). In another embodiment, a ceramic composite body is first completely formed, and thereafter the completely formed ceramic composite body is exposed to an oxidant, preferably a "different" oxidant than that which was used to form the oxidation reaction product which serves as a matrix for the embedded filler material in the ceramic composite body. In this alternative embodiment, residual interconnected parent metal in the ceramic composite body is drawn towards at least one surface of the ceramic composite body and is caused to react with the "different" oxidant, thus forming a different oxidation reaction product on a substrate of a first formed oxidation reaction product. In yet another embodiment of the invention, the metallic constituent in the ceramic composite body can be tailored by changing the composition thereof. Specifically, for example, a second metal can be alloyed with or diffused into the parent metal during, for example, growth of oxidation reaction product to change favorably the composition, and thus, mechanical, electrical and/or chemical properties of the parent metal. To assist in forming a shaped ceramic composite body, a barrier means can be used in conjunction with a filler material or a preform. Specifically, a barrier means which is suitable for use with this invention may be any suitable means which interferes, inhibits, or terminates growth or development of the oxidation reaction product. Suitable barrier means may be any material, compound, element, composition, or the like, which, under the process conditions of this invention, maintains some integrity, is not volatile and preferably is permeable to a vapor-phase oxidant, if a vapor-phase oxidant is used, while being capable of locally inhibiting, poisoning, stopping, interfering with, preventing, or the like, continued growth of the oxidation reaction product. It appears that one category of barrier means is that class of materials which may be substantially non-wettable by the transported molten parent metal. A barrier of this type appears to exhibit substantially little or no affinity for the molten metal, and growth is terminated or inhibited by the barrier means. Other barriers tend to react with the transported molten parent metal to inhibit further growth either by dissolving into and diluting the transported metal excessively or by forming solid reaction products (e.g., intermetallics, which obstruct the molten metal transport process). A barrier of this type may be a metal or metal alloy, including any suitable precursor thereto such as an oxide or a reducible metal compound, or a dense ceramic material. Because of the nature of the growth inhibition or obstruction process with this type of barrier, growth may extend into or somewhat beyond the barrier before growth is terminated. Nevertheless, the barrier reduces any final machining or grinding that may be required of the formed oxidation reaction product. As stated above, the barrier should preferably be permeable or porous, and therefore, when a solid, impermeable wall is used, the barrier should be opened in at least one zone or at one or both ends to permit the vapor-phase oxidant to contact the molten parent metal. Suitable barriers particularly useful in this invention in the case of using aluminum parent metals and employing air as oxidant are calcium sulfate, calcium silicate, and tricalcium phosphate. These barriers appear to react locally with developing oxidation reaction product to form an impermeable calcium aluminate layer which locally terminates further growth of oxidation reaction product. Such barriers typically may be applied as a slurry or paste to the surfaces of a filler bed which preferably is preshaped as a preform. The barrier means also may include a suitable combustible or volatile material that is eliminated on heating, or a material which decomposes on heating, in order to increase the porosity and permeability of the barrier means. Still further, the barrier means may include a suitable refractory particulate to reduce any possible shrinkage or cracking which otherwise may occur during the process. Such a particulate having substantially the same coefficient of expansion as that of the filler bed is especially desirable. For example, if the preform comprises alumina and the resulting ceramic comprises alumina, the barrier may be admixed with alumina particulate, desirably having a mesh size of about 20-1000. The alumina particulate may be mixed with the calcium sulfate, for example, in a ratio ranging from about 10:1 to 1:10, with the preferred ratio being about 1:1. In one embodiment of the invention, the barrier means includes an admixture of calcium sulfate (i.e. Plaster of Pairs and portland cement. The portland cement may be mixed with the Plaster of Paris is a ratio of 10:1 to 1:10, with the preferred ratio of portland cement to Plaster of Paris being about 1:3. Where desired, portland cement may also be used alone as the barrier material. Another embodiment, when using an aluminum parent metal and air as the oxidant comprises using as a barrier Plaster of Paris admixed with silica in a stoichiometric amount, but there can be an excess of Plaster of Paris. During processing, the Plaster of Paris and silica react to form calcium silicate, which results in a particularly beneficial barrier in that it is substantially free of fissures. In still another embodiment, the Plaster of Paris is admixed with about 25-40 weight percent calcium carbonate. On heating, the calcium carbonate decomposes emitting carbon dioxide, thereby enhancing the porosity of the barrier means. Other particularly useful barriers for aluminum-based parent metal systems include ferrous materials (e.g., a stainless steel container), chromia and other refractory oxides, which may be employed as a superimposed wall or container to the filler bed, or as a layer to the surface of a filler bed. Additional barriers include dense, sintered or fused ceramics such as alumina. These barriers are usually impermeable, and therefore are either specially fabricated to allow for porosity or require an open section such as an open end. The barrier means may form a friable product under the reaction conditions and can be removed as by abrading to recover the ceramic body. The barrier means may be manufactured or produced in any suitable form, size, and shape, and preferably is permeable to the vapor-phase oxidant. The barrier means may be applied or utilized as a film, paste, slurry, pervious or impervious sheet or plate, or a reticulated or foraminous web such as a metal or ceramic screen or cloth, or a combination thereof. The barrier means also may comprise some filler and/or binder. The size and shape of the barrier means depends on the desired shape for the ceramic product. By way of example only, if the barrier means is placed or situated at a predetermined distance from the parent metal, growth of the ceramic matrix would be locally terminated or inhibited where it encounters the barrier means. Generally, the shape of the ceramic product is the inverse of the shape of the barrier means. For example, if a concave barrier is at least partially spaced from a parent metal, the polycrystalline growth occurs within the volumetric space defined by the boundary of the concave barrier and the surface area of the parent metal. Growth terminates substantially at the concave barrier. After the barrier means is removed, a ceramic body remains having at least a convex portion defined by the concavity of the barrier means. It should be noted that with respect to a barrier means having porosity, there may be some polycrystalline material overgrowth through the interstices, although such overgrowth is severely limited or eliminated by the more effective barrier materials. In such a case, after the barrier means is removed from the grown polycrystalline ceramic body, any polycrystalline overgrowth may be removed from the ceramic body by grinding, grit blasting or the like, to produce the desired ceramic part with no remaining overgrowth of polycrystalline material. By way of a further illustration, a barrier means spaced from a parent metal, and having a cylindrical protuberance in the direction of the metal, will produce a ceramic body with a cylindrical recess inversely replicating the same diameter and depth of the cylindrical protuberance. In order to achieve minimal or no polycrystalline material overgrowth in the formation of ceramic composites, the barrier means may be placed one, or positioned in close proximity to, the defined surface boundary of any filler bed or preform. Disposal of the barrier means on the defined surface boundary of the bed or preform may be performed by any suitable means, such as by layering the defined surface boundary with the barrier means. Such layer of barrier means may be applied by painting, dipping, silk screening, evaporating, or otherwise applying the barrier means in liquid, slurry, or paste form, or by sputtering a vaporizable barrier means, or by simply depositing a layer of a solid particulate barrier means, or by applying a solid thin sheet or film of barrier means onto the defined surface boundary. With the barrier means in place, growth of the polycrystalline oxidation reaction product terminates upon reaching the defined surface boundary of the preform and contacting the barrier means. In a preferred embodiment for manufacturing a ceramic matrix composite body, a permeable shaped preform (described below in greater detail) is formed having at least one defined surface boundary with at least a portion of the defined surface boundary having or superimposed with the barrier means. It should be understood that the term "preform" may include an assembly of separate preforms ultimately bonded into an integral composite. The preform is placed adjacent to and in contact with one or more parent metal surfaces or a portion of a surface of the parent metal such that at least a portion of the defined surface boundary having or superimposed with the barrier means is generally positioned distantly or outwardly from the metal surface, and formation of the oxidation reaction product will occur into the preform and in a direction towards the defined surface boundary with the barrier means. The permeable preform is part of the lay-up, and upon heating in a furnace, the parent metal and the preform are exposed to or enveloped by the vapor-phase oxidant, which may be used in combination with a solid or liquid oxidant. The metal and oxidant react, and the reaction process is continued until the oxidation reaction product has infiltrated the preform and comes in contact with the defined surface boundary having or superimposed with the barrier means. Most typically, the boundaries of the preform, and of the polycrystalline matrix, substantially coincide; but individual constituents at the surfaces of the preform may be exposed or may protrude from the matrix, and therefore infiltration and embeddment may not be complete in terms of completely surrounding or encapsulating the preform by the matrix. The barrier means prevents, inhibits or terminates growth upon contact with the barrier means, and substantially no overgrowth of the polycrystalline material occurs. The resulting ceramic composite product includes a preform infiltrated or embedded to its boundaries by a ceramic matrix comprising a polycrystalline material consisting essentially of the oxidation reaction product of the parent metal with the oxidant and, optionally, one or more metallic constituents such as non-oxidized constituents of the parent metal or reduced constituents of an oxidant. Generally, the oxidation reaction is continued for a time sufficient to deplete the source of parent metal. The carcass is removed such as by striking with a hammer to provide a ceramic or ceramic composite body. Once the ceramic or ceramic composite body has been formed, it must then be comminuted prior to using it as a filler material for formation of a metal matrix composite body. Particularly, in the practice of the present invention, the polycrystalline oxidation reaction product is ground, pulverized or the like and formed into a mass of filler material, or preferably, the filler material is shaped into a preform. The ceramic or ceramic composite body can be comminuted by techniques such as jaw crushing, impact milling, roller milling, gyratory crushing, or other conventional techniques depending largely upon the desired particle size for use in the metal matrix composite body. The ground or milled ceramic material is sized by seiving and recovered for use as a filler material or preform. It may be desirable to first crush the ceramic body into large pieces of about 0.6 to about 1.3 cm (about 1/4 inch to about 1/2 inch) with, for example, a jaw crusher, hammer mill, etc. Thereafter, the large pieces could be ground into finer particles of, for example, 297 µm (50 mesh) or finer, by means such as ball milling, impact milling, etc. The particulate can then be screened to obtain size fractions of a desirable size. Suitable filler materials may range in size from about <74 µm (-200 mesh) to about 25 µm (500 mesh), or finer, depending upon the ceramic composite which was made and the metal matrix composite which is to be made (e.g., the intended use for the formed metal matrix composite body). Once the comminuted oxidation reaction product has been formed into a desirable particle size as a filler material , or formed into a preform, it is then necessary to infiltrate the filler material or preform spontaneously with matrix metal. In order to effect spontaneous infiltration of the matrix metal into the preform, an infiltration enhancer should be provided to the spontaneous system. An infiltration enhancer could be formed from an infiltration enhancer precursor which could be provided (1) in the matrix metal; and/or (2) in the preform; and/or (3) from an external source into the spontaneous system. Moreover, rather than supplying an infiltration enhancer precursor, an infiltration enhancer may be supplied directly to at least one of the preform, and/or matrix metal, and/or infiltrating atmosphere. Ultimately, at least during the spontaneous infiltration, the infiltration enhancer should be located in at least a portion of the filler material or preform. In a preferred embodiment it is possible that the infiltration enhancer precursor can be at least partially reacted with the infiltrating atmosphere such that infiltration enhancer can be formed in at least a portion of the preform prior to or substantially simultaneously with contacting the preform with the matrix metal e.g., magnesium as the infiltration enhancer precursor and nitrogen was the infiltrating atmosphere. An example of a matrix metal/infiltration enhancer precursor/infiltrating atmosphere system is the aluminum/mangesium/nitrogen system. Specifically, an aluminum matrix metal can be contained within a suitable refractory vessel such as an alumina boat which, under the process conditions, does not react with the aluminum matrix metal and/or the filler material or preform when the aluminum is made molten. A preform material can be contacted with the molten aluminum matrix metal. Moreover, rather than supplying an infiltration enhancer precursor, an infiltration enhancer may be supplied directly to at least one of the preform, and/or matrix metal, and/or infiltrating atmosphere. Particularly, the infiltration enhancer can be residual magnesium in the comminuted oxidation reaction product filler. Ultimately, at least during the spontaneous infiltration, the infiltration enhancer should be located in at least a portion of the filler material or preform. Under the conditions employed in the method of the present invention, in the case of an aluminum/magnesium/nitrogen spontaneous infiltration system, the preform should be sufficiently permeable to permit the nitrogen-containing gas to penetrate or permeate the preform and contact the molten matrix metal. Moreover, the permeable preform can accommodate infiltration of the molten matrix metal, thereby causing the nitrogen-permeated preform to be infiltrated spontaneously with molten matrix metal to form a metal matrix composite body. The extent of spontaneous infiltration and formation of the metal matrix composite will vary with a given set of process conditions, including magnesium content of the aluminum alloy, magnesium content of the preform, amount of magnesium nitride in the preform, the presence of additional alloying elements (e.g., silicon, iron, copper, manganese, chromium, zinc, and the like), average size of the filler material (e.g., particle diameter) comprising the preform, surface condition and type of filler material, nitrogen concentration of the infiltrating atmosphere, time permitted for infiltration and temperature at which infiltration occurs. For example, for infiltration of the molten aluminum matrix metal to occur spontaneously, the aluminum can be alloyed with at least about 1% by weight, and preferably at least about 3% by weight, magnesium (which functions as the infiltration enhancer precursor), based on alloy weight. Auxiliary alloying elements, as discussed above, may also be included in the matrix metal to tailor specific properties thereof. (Additionally, the auxiliary alloying elements may affect the minimum amount of magnesium required in the matrix aluminum metal to result in spontaneous infiltration of the filler material or preform). Loss of magnesium from the spontaneous system due to, for example, volatilization should not occur to such an extent that no magnesium was present to form infiltration enhancer. Thus, it is desirable to utilize a sufficient amount of initial alloying elements to assure that spontaneous infiltration will not be adversely affected by volatilization. Still further, the presence of magnesium in both of the preform and matrix metal or the preform alone may result in a reduction in the required amount of magnesium to achieve spontaneous infiltration (discussed in greater detail later herein). The volume percent of nitrogen in the nitrogen atmosphere also affects formation rates of the metal matrix composite body. Specifically, if less than about 10 volume percent of nitrogen is present in the infiltrating atmosphere, very slow or little spontaneous infiltration will occur. It has been discovered that it is preferable for at least about 50 volume percent of nitrogen to be present in the atmosphere, thereby resulting in, for example, shorter infiltration times due to a much more rapid rate of infiltration. The infiltrating atmosphere (e.g., a nitrogen-containing gas) can be supplied directly to the filler material or preform and/or matrix metal, or it may be produced or result from a decomposition of a material. The minimum magnesium content required for molten matrix metal to infiltrate a filler material or preform depends on one or more variables such as the processing temperature, time, the presence of auxiliary alloying elements such as silicon or zinc, the nature of the filler material, the location of the magnesium in one or more components of the spontaneous system, the nitrogen content of the atmosphere, and the rate at which the nitrogen atmosphere flows. Lower temperatures or shorter heating times can be used to obtain complete infiltration as the magnesium content of the alloy and/or preform is increased. Also, for a given magnesium content, the addition of certain auxiliary alloying elements such as zinc permits the use of lower temperatures. For example, a magnesium content of the matrix metal at the lower end of the operable range, e.g., from about 1 to 3 weight percent, may be used in conjunction with at least one of the following: an above-minimum processing temperature, a high nitrogen concentration, or one or more auxiliary alloying elements. When no magnesium is added to the preform, alloys containing from about 3 to 5 weight percent magnesium are preferred on the basis of their general utility over a wide variety of process conditions, with at least about 5 percent being preferred when lower temperatures and shorter times are employed. Magnesium contents in excess of about 10 percent by weight of the aluminum alloy may be employed to moderate the temperature conditions required for infiltration. The magnesium content may be reduced when used in conjunction with an auxiliary alloying element, but these elements serve an auxiliary function only and are used together with at least the above-specified minimum amount of magnesium. For example, there was substantially no infiltration of nominally pure aluminum alloyed only with 10 percent silicon at 1000°C into a bedding of 25 µm (500 mesh), 39 Crystolon (99 percent pure silicon carbide from Norton Co.). However, in the presence of magnesium, silicon has been found to promote the infiltration process. As a further example, the amount of magnesium varies if it is supplied exclusively to the preform or filler material. It has been discovered that spontaneous infiltration will occur with a lesser weight percent of magnesium supplied to the spontaneous system when at least some of the total amount of magnesium supplied is placed in the preform or filler material or a higher temperature of infiltration is used. It may be desirable for a lesser amount of magnesium to be provided in order to prevent the formation of undesirable intermetallics in the metal matrix composite body. In the case of a silicon carbide preform, it has been discovered that when the preform is contacted with an aluminum matrix metal, the preform containing at least about 1% by weight magnesium and being in the presence of a substantially pure nitrogen atmosphere, the matrix metal spontaneously infiltrates the preform. In the case of an alumina preform, the amount of magnesium required to achieve acceptable spontaneous infiltration is slightly higher. Specifically, it has been found that when an alumina preform, when contacted with a similar aluminum matrix metal, at about the same temperature as the aluminum that infiltrated into the silicon carbide preform, and in the presence of the same nitrogen atmosphere, at least about 3% by weight magnesium may be required to achieve similar spontaneous infiltration to that achieved in the silicon carbide preform discussed immediately above. It is also noted that it is possible to supply to the spontaneous system infiltration enhancer precursor and/or infiltration enhancer on a surface of the alloy and/or on a surface of the preform or filler material and/or within the preform or filler material prior to infiltrating the matrix metal into the filler material or preform (i.e., it may not be necessary for the supplied infiltration enhancer or infiltration enhancer precursor to be alloyed with the matrix metal, but rather, simply supplied to the spontaneous system). If the magnesium was applied to a surface of the matrix metal it may be preferred that said surface should be the surface which is closest to, or preferably in contact with, the permeable mass of filler material or vice versa; or such magnesium could be mixed into at least a portion of the preform or filler material. Still further, it is possible that some combination of surface application, alloying and placement of magnesium into at least a portion of the preform could be used. Such combination of applying infiltration enhancer(s) and/or infiltration enhancer precursor(s) could result in a decrease in the total weight percent of magnesium needed to promote infiltration of the matrix aluminum metal into the preform, as well as achieving lower temperatures at which infiltration can occur. Moreover, the amount of undesirable intermetallics formed due to the presence of magnesium could also be minimized. The use of one or more auxiliary alloying elements and the concentration of nitrogen in the surrounding gas also affects the extent of nitriding of the matrix metal at a given temperature. For example, auxiliary allowing elements such as zinc or iron included in the alloy, or placed on a surface of the alloy, may be used to reduce the infiltration temperature and thereby decrease the amount of nitride formation, whereas increasing the concentration of nitrogen in the gas may be used to promote nitride formation. The concentration of magnesium in the alloy, and/or placed onto a surface of the alloy, and/or combined in the filler or preform material, also tends to affect the rate and extent of infiltration at a given temperature. Consequently, in some cases where little or no magnesium is contacted directly with the preform or filler material, it may be preferred that at least about three weight percent magnesium be included in the alloy. Alloy contents of less than this amount, such as one weight percent magnesium, may require higher process temperatures or an auxiliary alloying element for infiltration. The temperature required to effect the spontaneous infiltration process of this invention may be lower: (1) when the magnesium content of the alloy alone is increased, e.g. to at least about 5 weight percent; and/or (2) when alloying constituents are mixed with or part of the permeable mass of filler material or preform; and/or (3) when another element such as zinc or iron is present in the aluminum alloy. The temperature also may vary with different filler materials. In general, spontaneous and progressive infiltration will occur at a process temperature of at least about 675°C, and preferably a process temperature of at least about 750°C-850°C. Temperatures generally in excess of 1200°C do not appear to benefit the process, and a particularly useful temperature range has been found to be from about 675°C to about 1200°C. However, as a general rule, the spontaneous infiltration temperature is a temperature which is above the melting point of the matrix metal but below the volatilization temperature of the matrix metal. Moreover, the spontaneous infiltration temperature should be below the melting point of the filler material. Still further, as temperature is increased, the tendency to form a reaction product between the matrix metal and infiltrating atmosphere increases (e.g., in the case of aluminum matrix metal and a nitrogen infiltrating atmosphere, aluminum nitride may be formed). Such reaction product may be desirable or undesirable based upon the intended application of the metal matrix composite body. Additionally, electric resistance heating is typically used to achieve the infiltrating temperatures. However, any heating means which can cause the matrix metal to become molten and does not adversely affect spontaneous infiltration, is acceptable for use with the invention. In the present method, for example, a permeable preform comes into contact with molten aluminum in the presence of a nitrogen-containing gas (e.g., forming gas which is 96% N₂ and 4% H₂) maintained for the entire time required to achieve infiltration. This is accomplished by maintaining a continuous flow of gas into contact with the preform and molten aluminum matrix metal. Although the flow rate of the nitrogen-containing gas is not critical, it is preferred that the flow rate be sufficient to compensate for any nitrogen lost from the atmosphere due to nitride formation in the alloy matrix, and also to prevent or inhibit the incursion of air which can have an oxidizing effect on the molten metal. The method of forming a metal matrix composite is applicable to a wide variety of filler materials, and the choice of filler materials will depend on such factors as the matrix alloy, the process conditions, the reactivity of the molten matrix alloy with the filler material, the ability of the filler material to conform to the infiltrating matrix metal, and the properties sought for the final composite product. For example, when aluminum is the matrix metal, suitable filler materials include (a) oxides, e.g. alumina; (b) carbides, e.g. silicon carbide; (c) borides, e.g. aluminum dodecaboride, and (d) nitrides, e.g. aluminum nitride. In a preferred embodiment, crushed oxidation reaction product is utilized as a filler material. Further, the crushed oxidation reaction product can be used either alone or in combination with other filler materials to provide the permeable mass or preform for infiltration. If there is a tendency for the filler material to react with the molten aluminum matrix metal, this might be accommodated by minimizing the infiltration time and temperature or by providing a non-reactive coating on the filler. The filler material may comprise a substrate, such as carbon or other non-ceramic material, bearing a coaling to protect the substrate from attack or degradation. Suitable coatings include ceramic oxides, carbides, borides and nitrides. Ceramics which can be utilized in the present method include alumina and silicon carbide in the form of particles, platelets, whiskers and fibers. The fibers can be discontinuous (in chopped form) or in the form of continuous filament, such as multifilament tows. Further, the ceramic mass or preform may be homogeneous or heterogeneous. The size and shape of the filler material utilized to form the ceramic oxidation reaction product, or that filler material which is mixed with the ceramic oxidation reaction product once crushed, can be any suitable material that may be required to achieve the properties desired in the composite. Thus, the material may be in the form of particles, whiskers, platelets or fibers since infiltration is not restricted by the shape of the filler material. Other shapes such as spheres, tubules, pellets, refractory fiber cloth, and the like may be employed. In addition, the size of the material does not limit infiltration, although a higher temperature or longer time period may be needed for complete infiltration of a mass of smaller particles than for larger particles. Further, the mass of filler material (shaped into a preform) to be infiltrated should be permeable (i.e., permeable to molten matrix metal and to the infiltrating atmosphere). The method of forming metal matrix composites according to the present invention, not being dependent on the use of pressure to force or squeeze molten metal matrix into a preform or a mass of filler material. The invention permits the production of substantially uniform metal matrix composites having a high volume fraction of filler material and low porosity. Higher volume fractions of filler material on the order of at least about 50 percent may be achieved by using a lower porosity initial mass of filler material and/or particles of varying sizes to increase the packing efficiency. Higher volume fractions also may be achieved if the mass of filler is compacted or otherwise densified provided that the mass is not converted into either a compact with close cell porosity or into a fully dense structure that would prevent infiltration by the molten alloy. It has been observed that for aluminum infiltration and matrix formation around a ceramic filler, wetting of the ceramic filler by the aluminum matrix metal may be an important part of the infiltration mechanism. Moreover, at low processing temperatures, a negligible or minimal amount of metal nitriding occurs resulting in a minimal discontinuous phase of aluminum nitride dispersed in the metal matrix. However, as the upper end of the temperature range is approached, nitridation of the metal is more likely to occur. Thus, the amount of the nitride phase in the metal matrix can be controlled by varying the processing temperature at which infiltration occurs. The specific process temperature at which nitride formation becomes more pronounced also varies with such factors as the matrix aluminum alloy used and its quantity relative to the volume of filler material, the filler material to be infiltrated, and the nitrogen concentration of the infiltrating atmosphere. For example, the extent of aluminum nitride formation at a given process temperature is believed to increase as the ability of the alloy to wet the filler decreases and as the nitrogen concentration of the atmosphere increases. It is therefore possible to tailor the constituency of the metal matrix during formation of the composite to impart certain characteristics to the resulting product. For a given system, the process conditions can be selected to control the nitride formation. A composite product containing an aluminum nitride phase will exhibit certain properties which can be favorable to, or improve the performance of, the product. Further, the temperature range for spontaneous infiltration with an aluminum alloy may vary with the ceramic material used. In the case of alumina as the filler material, the temperature for infiltration should preferably not exceed about 1000°C if it is desired that the ductility of the matrix be not reduced by the significant formation of nitride. However, temperatures exceeding 1000°C may be employed if it is desired to produce a composite with a less ductile and stiffer matrix. To infiltrate silicon carbide, higher temperatures of about 1200°C may be employed since the aluminum alloy nitrides to a lesser extent, relative to the use of alumina as filler, when silicon carbide is employed as a filler material. More importantly, when using crushed or comminuted oxidation reaction growth product as the filler, temperatures from about 750-850°C can be used. Particularly, the polycrystalline material formed by the directed oxidation process may contain metallic components such as nonoxidized parent metal. The amount of metal can vary over a wide range of 1 to 40 percent by volume, and sometimes higher, depending largely upon the degree of exhaustion (conversion) of parent metal in the production of ceramic or ceramic composite bodies. It may be desirable to separate at least some of the residual metal or carcass of parent metal from the oxidation reaction product before using the material as a filler. This separation can be accomplished before and/or after the polycrystalline material has been crushed or ground. The oxidation reaction product in some cases may be more easily fractured than the metal, and therefore, it may be possible in some cases to partially separate the oxidation reaction product from metal by comminuting and screening. However, in accordance with the present invention, the comminuted oxidation reaction product which is utilized, either alone or in combination with another filler material, exhibits an affinity for the molten alloy, apparently attributable to an affinity between like substances under the process conditions and/or due to the presence of one or more auxiliary alloying elements. Because of this affinity, it has been observed that enhanced infiltration kinetics, and consequently infiltration occurs at a somewhat faster rate relative to substantially the same process using a commercially available ceramic filler, that is, a filler not produced by the directed oxidation process. However, if another filler material is to be mixed with a comminuted oxidation reaction product, the amount of comminuted oxidation reaction product should be supplied in a quantity which is sufficient to achieve enhanced infiltration kinetics (e.g., at least about 10-25 percent by volume of the filler material should comprise comminuted oxidation reaction product). In addition, when comminuted oxidation reaction product is utilized as the filler material, it has been observed that the process can be conducted at lower temperatures, which is advantageous from a cost and handling standpoint. Also, at lower temperatures, the molten metal is less susceptible to react with the filler and formation of an undesirable reaction product which may have a deleterious effect on the mechanical properties of the metal matrix composite. One factor which appears to contribute to the enhanced infiltration of the present invention is the presence of an auxiliary alloying element and/or aluminum parent metal intimately associated with the filler. For example, when alumina as the oxidation reaction product is formed upon the oxidation reaction of aluminum in air, a dopant material typically is used in association with or in combination with the aluminum parent metal, as explained in the Commonly Owned Patent Applications. The parent metal or the dopant, or a portion thereof, may not be exhausted from the reaction system, and therefore may become dispersed throughout part or substantially all of the polycrystalline ceramic material. In such a case, the parent metal or the dopant material may be concentrated at or on a surface of the comminuted oxidation reaction product or the parent metal or dopant may be bonded within the oxidation reaction product. Without wishing to be bound by any particular theory or explanation, it is believed that when the polycrystalline material is comminuted for use as a filler, the matrix metal used to spontaneously infiltrate the comminuted oxidation reaction product may exhibit an affinity for this filler due to the parent metal and/or dopant material included in the filler. Specifically, residual parent metal and/or dopants may enhance the infiltration process by serving as useful auxiliary alloying elements in the production of the final composite product; and/or may function as an infiltration enhancer; and/or may function as an infiltration enhancer precursor. Accordingly, a comminuted oxidation reaction product may inherently provide at least a portion of the requisite infiltration enhancer and/or infiltration enhancer precursor needed to achieve spontaneous infiltration of a matrix metal into a filler material or preform. Moreover, it is possible to use a reservoir of matrix metal to assure complete infiltration of the filler material and/or to supply a second metal which has a different composition from the first source of matrix metal. Specifically, in some cases it may be desirable to utilize a matrix metal in the reservoir which differs in composition from the first source of matrix metal. For example, if an aluminum alloy is used as the first source of matrix metal, then virtually any other metal or metal alloy which was molten at the processing temperature could be used as the reservoir metal. Molten metals frequently are very miscible with each other which would result in the reservoir metal mixing with the first source of matrix metal so long as an adequate amount of time is given for the mixing to occur. Thus, by using a reservoir metal which is different in composition than the first source of matrix metal, it is possible to tailor the properties of the metal matrix to meet various operating requirements and thus tailor the properties of the metal matrix composite. A barrier means may also be utilized in combination with the present invention. Specifically, the barrier means for use with this invention may be any suitable means which interferes, inhibits, prevents or terminates the migration, movement, or the like, of molten matrix alloy (e.g., an aluminum alloy) beyond the defined surface boundary of the filler material. Suitable barrier means may be any material, compound, element, composition, or the like, which, under the process conditions of this invention, maintains some integrity, is not volatile and preferably is permeable to the gas used with the process as well as being capable of locally inhibiting, stopping, interfering with, preventing, or the like, continued infiltration or any other kind of movement beyond the defined surface boundary of the filler material. Suitable barrier means includes materials which are substantially non-wettable by the migrating molten matrix alloy under the process conditions employed. A barrier of this type appears to exhibit little or no affinity for the molten matrix alloy, and movement beyond the defined surface boundary of the filler material or preform is prevented or inhibited by the barrier means. The barrier reduces any final machining or grinding that may be required of the metal matrix composite product. As stated above, the barrier preferably should be permeable or porous, or rendered permeable by puncturing, to permit the gas to contact the molten matrix alloy. Suitable barriers particularly useful for aluminum matrix alloys are those containing carbon, especially the crystalline allotropic form of carbon known as graphite. Graphite is essentially non-wettable by the molten aluminum alloy under the described process conditions. A particular preferred graphite is a graphite tape product that is sold under the trademark Grafoil®, registered to Union Carbide. This graphite tape exhibits sealing characteristics that prevent the migration of molten aluminum alloy beyond the defined surface boundary of the filler material. This graphite tape is also resistant to heat and is chemically inert. Grafoil® graphite material is flexible, compatible, conformable and resilient. It can be made into a variety of shapes to fit any barrier application. However, graphite barrier means may be employed as a slurry or paste or even as a paint film around and on the boundary of the filler material or preform. Grafoil® is particularly preferred because it is in the form of a flexible graphite sheet. In use, this paper-like graphite is simply formed around the filler material or preform. Other preferred barrier(s) for aluminum metal matrix alloys in nitrogen are the transition metal borides (e.g., titanium diboride (TiB₂)) which are generally non-wettable by the molten aluminum metal alloy under certain of the process conditions employed using this material. With a barrier of this type, the process temperature should not exceed about 875°C, for otherwise the barrier material becomes less efficacious and, in fact, with increased temperature infiltration into the barrier will occur. The transition metal borides are typically in a particulate form (1-30 µm). The barrier materials may be applied as a slurry or paste to the boundaries of the permeable mass of ceramic filler material which preferably is preshaped as a preform. Other useful barriers for aluminum metal matrix alloys in nitrogen include low-volatile organic compounds applied as a film or layer onto the external surface of the filler material or preform. Upon firing in nitrogen, especially at the process conditions of this invention, the organic compound decomposes leaving a carbon soot film. The organic compound may be applied by conventional means such as painting, spraying, dipping, etc. Moreover, finely ground particulate materials can function as a barrier so long as infiltration of the particulate material would occur at a rate which is slower than the rate of infiltration of the filler material. Thus, the barrier means may be applied by any suitable means, such as by covering the defined surface boundary with a layer of the barrier means. Such a layer of barrier means may be applied by painting, dipping, silk screening, evaporating, or otherwise applying the barrier means in liquid, slurry, or paste form, or by sputtering a vaporizable barrier means, or by simply depositing a layer of a solid particulate barrier means, or by applying a solid thin sheet or film of barrier means onto the defined surface boundary. With the barrier means in place, spontaneous infiltration substantially terminates when metal reaches the defined surface boundary and contacts the barrier means. Various demonstrations of the present invention are included in the Examples immediately following. However, these Examples should be considered as being illustrative and should not be construed as limiting the scope of the invention as defined in the appended claims. Example 1 Figure 1 shows an assembly in cross-section, which can be used to grow an oxidation reaction product. Particularly, a parent metal bar (1) measuring 3.8 x 10.2 x 22.8 cm (1-1/2 x 4 x 9 inches) and comprised of a slightly modified 380.1 aluminum alloy from Belmont Metals was placed upon a bedding (2) of 216 µm (90 grit) El Alundum®, supplied by Norton Co., both of which were contained in a high-purity alumina refractory boat (4). The alumina boat was obtained from Bolt Technical Ceramics and had a purity of 99.7 percent. The parent metal bar (1) was placed within the El Alundum® bedding (2) such that a surface of the bar (1) was approximately flush with the bedding (2). The aluminum alloy (1) comprised about 2.5-3.5% Zn, 3.0-4.0% Cu, 7.5-9.5% Si, 0.8-1.5% Fe, 0.2-0.3% Mg, 0-0.5% Mn, 0-0.001% Be and 0-0.35% Sn. The aluminum alloy bar was externally doped by applying approximately 5 grams of 150 µm (140 grit) silica particles (3) substantially only on a top surface of the aluminum alloy bar (1) such that a ceramic body would grow only from a surface of the alloy (1) toward the atmosphere (e.g., away from the bedding (2)). The boat (4) containing the bedding (2), aluminum alloy (1) and dopant (3) was placed into an electric resistance furnace which was heated to a temperature of about 1100°C at a rate of about 200°C per hour and held there for a period of time sufficient to permit molten aluminum alloy to react with oxygen in the air environment to produce oxidation reaction product. During the heating, air was allowed to circulate into the furnace in order to provide oxidant. The oxidation reaction product which grew formed a "loaf" above the aluminum alloy (1). The boat (4), and its contents, was then allowed to cool. The final oxidation reaction product (i.e., the loaf) was removed from the boat and parent metal carcass was removed by striking it with a hammer. The oxidation reaction product was then placed into a jaw crusher and was crushed into golf ball or pea size chunks. The chunks of oxidation reaction product were placed into a porcelain jar along with aluminum oxide grinding media and water. Ball milling reduced the size of the chunks to smaller particles. Moreover, because the oxidation reaction product may contain unoxidized residual parent metal from the parent aluminum alloy, it was necessary to control the pH of the solution during ball milling, thereby reducing any reaction between the aluminum and the water. The ball milling was continued for about 36 hours. After ball milling, the contents of the porcelain jar were dried and sifted using conventional techniques. And chunks remaining after ball milling which were greater than 840 µm (20 mesh) were placed back into the ball mill and ground again at a later time. The particles of crushed oxidation reaction which were smaller than 149 µm (100 mesh) and greater than 74 µm (-200 mesh) were collected. Figure 2 shows an assembly in cross-section, which can be used to infiltrate a matrix metal into a comminuted oxidation reaction product. Particularly, the comminuted oxidation reaction product (12) was placed in a high purity alumina boat (14) similar to the one used above to form the oxidation reaction product. An ingot of matrix metal (10) to be infiltrated was placed on top of the crushed oxidation reaction product (12) such that said matrix metal (10) extended above the surface of the comminuted filler (12). The aluminum alloy (10) which was used to spontaneously infiltrate the crushed oxidation reaction product (12) was a bar or ingot of matrix metal measuring about 2.5 x 5.1 x 1.3 cm (1 inch by 2 inches by 1/2 inch). The matrix metal aluminum alloy had a composition which contained about 5 percent silicon by weight and 5 percent magnesium by weight. The alumina boat (14) containing this assemblage of materials was placed into an electric resistance heated muffle furnace. The muffle furnace was sealed such that substantially only the infiltrating gas was present. In this case, forming gas was used for the infiltrating atmosphere (i.e. 96 volume percent nitrogen and 4 volume percent hydrogen). The forming gas was passed through the muffle furnace at a rate of about 350 cm³/minute. The muffle furnace was heated over a period of about 10 hours until a temperature of about 800°C was reached. The furnace was maintained at this temperature for about 5 hours. Then the furnace was cooled down for a period of 5 hours. The assemblage was then removed from the furnace and it was observed that the matrix metal (10) had substantially completely embedded the filler material (12). Figure 3 shows a photomicrograph taken at 400X of the resultant metal matrix composite body produced in accordance with Example 1. The darker regions (20) correspond to the crushed oxidation reaction product filler and the lighter regions (21) correspond to the matix metal. Example 2 This Example is a comparative example. In this Example, commercially available 216 µm (90 grit) 38 Alundum, which is a fused aluminum oxide grain obtained from Norton Co., was placed into an alumina boat. The same matrix metal utilized in Example 1 was placed thereon. The materials were placed into the same arrangement as discussed in Example 1 and shown in Figure 2. The assemblage was placed into a muffle furnace and heated in accordance to Example 1. After cooling, the boat was removed and inspected. No significant infiltration of the aluminum alloy matrix metal had occurred. Example 3 This Example is a comparative example. In order to establish that the crushed oxidation reaction product of the invention permits a lower temperature for spontaneous infiltration to occur, the following experiment was conducted. Specifically, the procedure of Example 2 was repeated except that a higher infiltrating temperature was utilized. Specifically, a boat containing the assemblage of materials according to Example 2 was placed into a muffle furnace and heated in accordance to Example 1 at the higher temperature of about 900°C. The furnace was cooled and the boat was removed. After inspection, it was discovered that substantially complete infiltration of the matrix metal had been achieved. The above Example demonstrates the desirability of utilizing a crushed oxidation reaction product as a filler material. Particularly, it has been discovered that enhanced infiltration kinetics are achieved when a crushed oxidation reaction product is utilized as a filler material. While the preceding Examples have been described with particularity, various modifications to these Examples may occur to an artisan of ordinary skill, and all such modifications should be considered to be within the scope of the claims appended hereto.

Claims: A method for producing a metal matrix composite comprising a ceramic filler embedded in the matrix of a matrix metal, said method comprising providing (1) a filler in the form of a comminuted ceramic or ceramic composite material obtained by the oxidation reaction of a molten parent metal with at least one of a vapor-phase oxidant, a liquid oxidant and a solid phase oxidant, optionally in the presence of a second filler material, to grow as oxidation reaction product a ceramic or ceramic composite material, and (2) a matrix metal, and at temperatures above the melting point of the matrix metal and in the presence of (3) at least one of an infiltration enhancer precursor and an infiltration enhancer, and (4) an infiltrating atmosphere permitting or enhancing spontaneous infiltration of the matrix metal and being in communication with at least one of the filler and the matrix metal for at least a portion of the period of infiltration, causing the matrix metal to spontaneously infiltrate at least a portion of the filler. The method of claim 1, further comprising the step of supplying said at least one of an infiltration enhancer precursor and an infiltration enhancer to at least one of the matrix metal, the filler and the infiltrating atmosphere. The method of claim 1 or 2, wherein said comminuted oxidation reaction product inherently comprises at least one of an infiltration enhancer and an infiltration enhancer precursor. The method of any of the preceding claims, further comprising the step of defining a surface boundary of the filler with a barrier, wherein the matrix metal spontaneously infiltrates the filler up to the barrier. The method of any of the preceding claims, wherein the temperature during spontaneous infiltration is greater than the melting point of the matrix metal, but lower than the volatilization temperature of the matrix metal and the melting point of the filler. The method of any of the preceding claims, wherein said oxidation reaction product comprises at least one material selected from the group consisting of oxides, nitrides, carbides, borides, and oxynitrides. The method of claim 6, wherein said oxidation reaction product comprises at least one material selected from the group consisting of aluminum oxide, aluminum nitride, silicon carbide, silicon boride, aluminum boride, titanium nitride, zirconium nitride, titanium boride, zirconium boride, titanium carbide, hafnium boride and tin oxide. The method of any of the preceding claims, wherein said oxidation reaction product is comminuted to a size ranging from about 74 µm (200 mesh) to about 25 µm (500 mesh).

Inventors: AGHAJANIAN MICHAEL KEVORK, NEWKIRK MARC STEVENS, AGHAJANIAN, MICHAEL KEVORK, NEWKIRK, MARC STEVENS
Applicants: LANXIDE TECHNOLOGY CO LTD, LANXIDE TECHNOLOGY COMPANY, LP.
Application year: 1989
Main classification: C22C1
IPC codes: C22C21, B22D19, C22C47, C22C1