BARRIER MATERIALS FOR MAKING METAL MATRIX COMPOSITES

Abstract: The present invention relates to a novel process for forming metal matrix composite bodies by using a barrier material. Particularly, an infiltration enhancer and/or an infiltration enhancer precursor and/or an infiltrating atmosphere are in communication with a 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 up to the barrier material. Such spontaneous infiltration occurs without the requirement for the application of any pressure or vaccum. Accordingly, shaped metal matrix composite bodies can be produced having superior surface finish.

Description: DESCRIPTION BARRIER MATERIALS FOR MAKING METAL MATRIX COMPOSITES Technical Field The present invention relates to a novel process for forming metal matrix composite bodies by using a barrier material. Particularly, an infiltration enhancer and/or an infiltration enhancer precursor and/or an infiltrating atmosphere are in communication with a 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 up to the barrier material. Such spontaneous infiltration occurs without the requirement for the application of any pressure or vacuum. Accordingly, shaped metal matrix composite bodies can be produced having superior surface finish. Background Art 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 matrix metal. Generally, a metal matrix composite will show an improvement in such properties as strength, stiffness, contact wear resistance, coefficient of thermal expansion (C.T.E.), density, thermal conductivity 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 specific stiffness (e.g., elastic modulus over density), wear resistance, thermal conductivity, low coefficient of thermal expansion (C.T.E.) and high temperature strength and/or specific strength (e.g., strength over density) 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 m<?trix-alumina composites. This patent describes applying pressures of 75-375 kg/cm 2 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 β 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. 0094353. 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. Landingha , 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 AIB^ powder onto a bed of cold-pressed aluminum powder. Additional aluminum was then positioned on top of the AlB j 2 powder compact. The crucible, loaded with the AIB^ 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 HOOT and maintained for a period of 3 hours. At these conditions, the molten aluminum penetrated the porous A1B 12 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), up to an applied barrier material in the presence of an infiltrating atmosphere (e.g., nitrogen) under normal atmospheric pressures so long as an infiltration enhancer precursor and/or infiltration enhancer is present at least at some point during the process. Description of Commonly Owned U.S. Patents and Patent Applications This application is a Continuation-in-Part Application of U.S. Patent Application Serial No. 07/520,915, filed on May 9, 1990, in the names of Michael K. Aghajanian et al., and entitled "Method of Making Metal Matrix Composite Bodies With Use of A Barrier". The subject matter of this application is related to that of several other copending and commonly owned patent applications and issued Patents. Particularly, these other copending and commonly owned patent applications and issued Patents describe novel methods for making metal matrix composite materials (hereinafter sometimes referred to as "Commonly Owned Metal Matrix Patents and Patent Applications"). A novel method of making a metal matrix composite material is disclosed in Commonly Owned U.S. Patent No. 4,828,008, which issued on May 9, 1989, from U.S. Patent Application Serial No. 049,171, filed May 13, 1987, in the names of White et al . , and entitled "Metal Matrix Composites", which published in the EPO on November 17, 1988, as Publication No. 0291441. According to the method of this White et al . 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 β 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, howeveζ, 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. The White et al . 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 U.S. Patent No. 4,935,055, which issued on June 19, 1990, from U.S. Patent Application Serial No. 141,642, filed January 7, 1988, in the names of Michael K. Aghajanian et al., and entitled "Method of Making Metal Matrix Composite with the Use of a Barrier", which published in the EPO on July 12, 1989, as Publication No. 0323945. According to the method of this Aghajanian et al . invention, a barrier means (e.g., particulate titanium diboride or a graphite material such as a flexible graphite foil product sold by Union Carbide under the trade name 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 U.S. Patent No. 4,828,008 was improved upon by Commonly Owned and Copending U.S. Patent Application Serial No. 517,541, filed on April 24, 1990, which was a continuation of U.S. Patent Application Serial No. 168,284, filed March 15, 1988 (and now abandoned), in the names of Michael K. Aghajanian and Marc S. Newkirk and entitled "Metal Matrix Composites and Techniques for Making the Same", and which published in the EPO on September 20, 1989, as Publication No. 0333629. In accordance with the methods disclosed in these U.S. Patent Applications, 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., a macrocomposite), wherein an infiltrated ceramic body having a metal matrix therein will be directly bonded to excess metal remaining in the reservoir. Further improvements in metal matrix technology can be found in commonly owned and copending U.S. Patent Application Serial No. 521,043, filed May 9, 1990, which was a continuation-in-part of U.S. Patent Application Serial No. 484,753, filed February 23, 1990, which was a continuation-in-part of U.S. Patent Application Serial No. 432,661, which was filed on November 7, 1989 (and now abandoned), which was a continuation-in-part of U.S. Patent Application Serial No. 416,327, filed October 6, 1989 (and now abandoned), which was a continuation-in-part application of U.S. Patent Application Serial No. 349,590, filed May 9, 1989 (and now abandoned), which in turn was a continuation-in-part application of U.S. Patent Application Serial No. 269,311, filed November 10, 1988 (and now abandoned), all of which were filed in the names of Michael K. Aghajanian et al . and all of which are entitled "A Method of Forming Metal Matrix Composite Bodies By A Spontaneous Infiltration Process, and Products Produced Therefrom." According to these Aghajanian et al . applications, spontaneous infiltration of a matrix metal into a permeable mass of filler material or preform is achieved by use of an infiltration enhancer and/or an infiltration enhancer precursor and/or an infiltrating atmosphere which are in communication with the filler material or preform, at least at some point during the process, which permits molten matrix metal to spontaneously infiltrate the filler material or preform. Aghajanian et al . disclose a number of matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems which exhibit spontaneous infiltration. Specifically, Aghajanian et al . disclose that spontaneous infiltration behavior has been observed in the aluminum/magnesium/nitrogen system; the aluminum/strontium/nitrogen system; the aluminum/zinc/oxygen system; and the aluminum/calcium/nitrogen system. However, it is clear from the disclosure set forth in the Aghajanian et al . applications that the spontaneous infiltration behavior should occur in other matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems. Each of the above-discussed Commonly Owned Metal Matrix Patents and 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 Patents and Patent Applications are expressly incorporated herein by reference. Summary of the Invention A metal matrix composite body is produced by spontaneously infiltrating a molten matrix metal into a permeable mass of filler material or a preform having at least one surface boundary established or defined by a barrier means. Specifically, an infiltration enhancer and/or an infiltration enhancer precursor and/or an infiltrating atmosphere are in communication with the filler material or preform, at least at some point during the process, which permits molten matrix metal to spontaneously infiltrate the filler material or preform up to the barrier material. A barrier material, typically, inhibits the transport of molten matrix metal beyond itself, thereby permitting the formation of shaped metal matrix composite bodies. The barrier material of the instant invention may be a physical barrier, a reactive barrier, or any combination of the two. The barrier material should prevent the molten matrix metal from infiltrating beyond the desired boundaries of the filler material or preform and, preferably, provide a smooth surface finish to the final metal matrix composite body. Further, the barrier should not react or dissolve into the molten matrix metal or the filler material, unless such behavior is desired, e.g., when a reactive barrier is utilized. Any material or combination of materials which satisfy the above-described criteria for a particular matrix metal/infiltration enhancer and/or infiltration enhancer precursor and/or infiltrating atmosphere/filler material system may be utilized as a barrier material in that system. To obtain spontaneous infiltration, a precursor to an infiltration enhancer may be supplied to at least one of, a filler material or preform, and/or a matrix metal and/or an infiltrating atmosphere. The supplied infiltration enhancer precursor may thereafter react with at least one of the filler material or preform and/or the matrix metal and/or the infiltrating atmosphere to produce infiltration enhancer in at least a portion of, or on, the filler material or preform. Ultimately, at least during the spontaneous infiltration, infiltration enhancer should be in contact with at least a portion of the filler material or preform. In another preferred embodiment of the invention, 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 in contact with at least a portion of the filler material or preform. This application discusses various examples of matrix metals, which at some point during the formation of a metal matrix composite, are contacted with an infiltration enhancer precursor, in the presence of an infiltrating atmosphere. Thus, various references will be made to particular matrix metal/infiltration enhancer precursor/infiltratiηg atmosphere systems which exhibit spontaneous infiltration. However, it is conceivable that many other matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems other than those discussed in this application may behave in a manner similar to the systems discussed above herein. Specifically, spontaneous infiltration behavior has been observed in the aluminum/magnesium/nitrogen system; the aluminum/strontium/nitrogen system; the aluminum/zinc/oxygen system; and the aluminum/calcium/nitrogen system. Accordingly, even though this application discusses only those systems referred to above herein (with particular emphasis being placed upon the aluminum/magnesium/nitrogen system), it should be understood that other matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems may behave in a similar manner. In a preferred embodiment for achieving spontaneous infiltration into a permeable mass of filler material or a preform, molten matrix metal is contacted with the preform or filler material. The preform or filler material may have admixed therewith, and/or at some point during the process, be exposed to, an infiltration enhancer precursor. Moreover, in a preferred embodiment, the molten matrix metal and/or preform or filler material communicate with an infiltrating atmosphere for at least a portion of the process. In another preferred embodiment, the matrix metal and/or preform or filler material communicate with an infiltrating atmosphere for substantially all of the process. The preform or filler material will be spontaneously infiltrated by molten matrix metal, and the extent or rate of spontaneous infiltration and formation of metal matrix composite will vary with a given set of processing conditions including, for example, the concentration of infiltration enhancer precursor provided to the system (e.g., in the molten matrix alloy and/or in the filler material or preform and/or in the infiltrating atmosphere), the size and/or composition of the filler material, the size and/or composition of particles in the preform, the available porosity for infiltration into the preform or filler material, the time permitted for infiltration to occur, 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. Moreover, by varying the composition of the matrix metal and/or the processing conditions, the physical and mechanical properties of tfte formed metal matrix composite bodies may be engineered to any particular application or need. Further, by subjecting a formed metal matrix composite body to a post treatment process (e.g., directional solidification, heat treatment, etc.) the mechanical and/or physical properties may be further engineered to meet any particular application or need. Still further, by controlling the processing conditions during the formation of a metal matrix composite the nitrogen content of the formed metal matrix composite may be tailored to meet a wide range of industrial applications. Moreover, by controlling the composition and/or size (e.g., particle diameter) and/or geometry of the filler material or the material comprising the preform, the physical and/or mechanical properties of the formed metal matrix composite can be controlled or engineered to meet any number of industrial needs. For example, it has been discovered that wear resistance of the metal matrix composite can be increased by increasing the size of the filler material (e.g., increasing the average diameter of the filler material particles), given that the wear resistance of filler material is higher than that of the matrix metal. However, strength and/or toughness may tend to increase with decreasing filler size. Further, the thermal expansion coefficient of the metal matrix composite may decrease with increasing filler loading, given that the coefficient of thermal expansion of the filler is lower than the coefficient of thermal expansion of the matrix metal. Still further, the mechanical and/or physical properties (e.g., density, elastic and/or specific modulus, strength and/or specific strength, etc.) of a formed metal matrix composite body may be tailored depending on the loading of the filler material in the loose mass or in the preform. For example, by providing a loose mass or preform comprising a mixture of filler particles of varying sizes and/or shapes, wherein the density of the filler is greater than that of the matrix metal, a higher filler loading, due to enhanced packing of the filler material, may be achieved, thereby resulting in a metal matrix composite body with an increased density. By utilizing the teachings of the present invention, the volume percent of filler material or preform which can be infiltrated can vary over a wide range. The lower volume percent of filler that can be infiltrated is limited primarily by the ability to form a porous filler material or preform, (e.g., about 10 volume percent); whereas the higher volume percent of filler or preform that can be infiltrated is limited primarily by the ability to form a dense filler material or preform with at least some interconnected porosity (e.g., about 95 volume percent). Accordingly, by practicing any of the above teachings, alone or in combination, a metal matrix composite can be engineered to contain a desired combination of properties. Definitions "Aluminum", as used herein, 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, 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, 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 orgrinding 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, etc. "Carcass" or "Carcass of Matrix Metal", as used herein, refers to any of the original body of matrix metal remaining which has not been consumed during formation of the metal matrix composite body, and typically, if allowed to cool, remains in at least partial contact with the metal matrix composite body which has been formed. It should be understood that the carcass may also include a second or foreign metal therein. "Filler", as used herein, is intended to include either single constituents or mixtures of constituents which are substantially non- reactive with and/or of limited solubility in the matrix metal and may be single or multi-phase. Fillers may be provided in a wide variety of forms and sizes, such as powders, flakes, platelets, microspheres, whiskers, bubbles, fibers, particulates, fiber mats, chopped fibers, spheres, pellets, tubules, refractory cloths, 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 ceramic-coated fillers such as carbon fibers coated with alumina or silicon carbide to protect the carbon from attack, for example, by a molten aluminum matrix metal. Fillers may also include metals. "Hot-Topping", as used herein, refers to the placement of a substance on one end (the "topping" end) of an at least partially formed metal matrix composite which reacts exothermically with at least one of the matrix metal and/or filler material and/or with another material supplied to the topping end. This exothermic reaction should provide sufficient heat to maintain the matrix metal at the topping end in a molten state while the balance of the matrix metal in the composite cools to solidification temperature. "Infiltrating Atmosphere", as used herein, 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 . "Infiltration Enhancer", as used herein, 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 with an 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 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, and the infiltration enhancer may be at least partially reducible by the matrix metal. "Infiltration Enhancer Precursor" or "Precursor to the Infiltration Enhancer", as used herein, 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 the matrix 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 t e 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. "Matrix Metal" or "Matrix Metal Alloy", as used herein, means that metal which is utilized to form a metal matrix composite body (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, 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 that 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, 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" from the Matrix Metal means a metal which does not contain, as a primary constituent, the same metal as the matrix metal (e.g., if the primary constituent of the matrix metal is aluminum, the "different" metal could have a primary constituent of, for example, nickel). "Nonreactive Vessel for Housing Matrix Metal" means any vessel which can house or contain a filler material (or preform) and/or molten matrix metal under the process conditions and..not react with the matrix me,tal and/or the infiltrating atmosphere and/or infiltration enhancer precursor and/or the filler material or preform in a manner which would be significantly detrimental to the spontaneous infiltration mechanism. The nonreactive vessel may be disposable and removable after the spontaneous infiltration of the molten matrix metal has been completed. "Physical Barrier", as used herein, is any barrier which is not a reactive barrier. "Preform" or "Permeable Preform", as used herein, 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. "Reactive Barrier", as used herein, means a barrier which reacts with at least one of the molten matrix metal, the filler material, the infiltration enhancer, the infiltration enhancer precursor, or the infiltrating atmosphere to either provide or remove a substance from the spontaneous infiltration system thereby interferring, inhibiting, preventing, or terminating, 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 reactive barrier. For example, the reactive barrier could react with the molten matrix metal upon contact with the molten matrix metal to form a compound which interferes, inhibits, prevents, or terminates the migration, movement, or the like, of the molten matrix metal; or alternatively, the reactive barrier could locally react with the infiltration enhancer or infiltration enhancer precursor resulting in the removal of the infiltration enhancer or infiltration enhancer precursor from the immediate vicinity of the reactive barrier which in turn interferes, inhibits, prevents, or terminates the migration, movement, or the like, of molten matrix metal beyond the area from which the infiltration enhancer precursor or infiltration enhancer has been removed. The reactive barrier should react with at least one of the molten matrix metal, infiltration enhancer precursor, infiltration enhancer, filler material or infiltrating atmosphere, at a rate which is sufficient to prevent the molten matrix metal from migrating or infiltrating to a significant (or undesirable) extent beyond the desired boundaries of the filler material or preform. Therefore, a material will qualify as a reactive barrier only if the kinetics of reaction between the material and at least one of the molten matrix metal, infiltration enhancer, infiltration enhancer precursor, filler material, or infiltrating atmosphere is sufficiently rapid to prevent any significant (or undesirable) infiltration or migration of the molten matrix metal beyond the desired boundaries of the filler material or preform. A material which functions as a barrier material and reacts with at least one of the molten parent metal, infiltration enhancer, infiltration enhancer precursor, filler material, or infiltrating atmosphere, to produce or remove a substance from the spontaneous infiltration system wnich results in the interference, inhibition, prevention, or termination of the migration, movement, or the like, of molten matrix metal, is not necessarily a reactive barrier. For example, the barrier may be functioning as a physical barrier because the kinetics of reaction between the material and at least one of the molten matrix metal, infiltration enhancer, infiltration enhancer precursor, filler material, or infiltrating atmosphere are too slow to effectively interfere, inhibit, prevent, or terminate the migration, movement, or the like, of molten matrix metal beyond a surface boundary of a permeable mass of filler material or preform. Thus, the primary method by which a material functions as a barrier material determines whether the material will be classified as a physical barrier or a reactive barrier. "Reservoir", as used herein, means a separate body of 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 matrix metal which is in contact with the filler or preform. "Spontaneous Infiltration", as used herein, means that 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). 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. Figure 1 is a cross-sectional schematic of a typical lay-up used to form a shaped metal matrix composite body as described below; Figure 2 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 1; Figure 3 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 2; Figures 4a-4e are schematic cross-sectional views which depict the significant processing steps used to fabricate the shaped metal matrix composite body of Example 3; Figure 5 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 4; Figure 6 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 5; Figure 7 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 6; Figure 8 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 7; Figure 9 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 8; Figure 10 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 9; Figure 11 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 10; Figure 12 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 11; Figure 13 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 12; Figure 14 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 13; and Figure 15 is a cross-sectional schematic view of a lay-up used to form a shaped metal matrix composite body in accordance with Example 14. Detailed Description of the Invention and Preferred Embodiments The present invention relates to forming a shaped metal matrix composite by spontaneously infiltrating a molten matrix metal into a filler material or preform up to a barrier material. Particularly, an infiltration enhancer and/or an infiltration enhancer precursor and/or an infiltrating atmosphere are in communication with the filler material or preform, at least at some point during the process, which permits molten matrix metal to spontaneously infiltrate the filler material or preform up to the barrier material. With reference to Figure 1, a simple lay-up 10 for forming a spontaneously infiltrated metal matrix composite is illustrated. Specifically, a filler or preform 1, which may be of any suitable material, as discussed in detail below, is placed in a vessel 2. The interior surfaces of the vessel 2 are coated with a barrier material 4. A matrix metal 3 is placed on or adjacent to the filler or preform 1 to form a setup. The setup is then placed within a mass of support material 5 contained within a refractory boat 6. The lay-up, comprising the refractory boat 6 and its contents, is thereafter placed in a furnace and heated to a temperature above the melting point of the matrix metal to initiate spontaneous infiltration. As set forth above, an infiltration enhancer and/or an infiltration enhancer precursor and/or an infiltrating atmosphere are in communication with the filler material or preform, at least at some point during the process, which permits the molten matrix metal to spontaneously infiltrate the filler material or preform. Without wishing to be bound by any particular theory or explanation, when an infiltration enhancer precursor is utilized in combination with at least one of the matrix metal, and/or filler material or preform and/or infiltrating atmosphere, the infiltration enhancer precursor may react to form an infiltration enhancer that induces or assists molten matrix metal to spontaneously infiltrate a filler material or preform. Moreover, 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 at least one of the infiltrating atmosphere, and/or the preform or filler material, and/or molten matrix 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. Thus, for example, if an infiltration enhancer precursor was included or combined with, at least at some point during the process, molten matrix metal, it is possible that the infiltration enhancer precursor could volatilize from the molten matrix metal and react with at least one of the filler material or preform and/or the infiltrating atmosphere. Such reaction could result in the formation of an infiltration enhancer which may be a solid species, if such solid species was stable at the infiltration temperature, said solid species being capable of being deposited on at least a portion of the filler material or preform as, for example, a coating. Moreover, it is conceivable that such solid species could be present as a discernable solid within at least a portion of the preform or filler material. If such a solid species was formed, molten matrix metal may have a tendency to react with the solid species (e.g., the molten matrix metal may reduce the formed solid species) such that infiltration enhancer precursor may become associated with (e.g., dissolved in or alloyed with) the molten matrix metal. Accordingly, additional infiltration enhancer precursor may then be available to volatilize and react with another species (e.g., the filler material or preform and/or infiltrating atmosphere) and again form a similar solid species. It is conceivable that a continuous process of conversion of infiltration enhancer precursor to infiltration enhancer followed by a reduction reaction of the infiltration enhancer with,molten matrix metal to again form additional infiltration enhancer precursor, and so on, could occur, until the result achieved is a spontaneously infiltrated metal matrix composite. In order to effect spontaneous infiltration of the matrix metal into the filler material or 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 filler material or preform; and/or (3) from the infiltrating atmosphere; and/or (4) 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 filler material or 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 of the invention, it is possible that the infiltration enhancer precursor can be at least partially reacted with the infiltrating atmosphere such that the infiltration enhancer can be formed in at least a portion of the filler material or preform prior to or substantially contiguous with contacting the filler material or preform with the matrix metal (e.g., if magnesium was the infiltration enhancer precursor and nitrogen was the infiltrating atmosphere, the infiltration enhancer could be magnesium nitride which would be located in at least a portion of the preform or filler material). An example of a matrix metal/infiltration enhancer precursor/infiltrating atmosphere system is the aluminum/magnesium/nitrogen system. Specifically, an aluminum matrix metal can be contained within a suitable refractory vessel which, under the process conditions, does not adversely react with the aluminum matrix metal and/or the filler material when the aluminum is made molten. A filler material or preform can thereafter be contacted with molten aluminum matrix metal and spontaneously infiltrated. Moreover, rather than supplying an infiltration enhancer precursor, an infiltration enhancer may be supplied directly to at least one of the preform or filler material, 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. Under the conditions employed in the method of the present invention, in the case of an aluminum/magnesium/nitrogen spontaneous infiltration system, the preform or filler material should be sufficiently permeable to permit the nitrogen-containing gas to penetrate or permeate the filler material or preform at some point during the process and/or contact the molten matrix metal. Moreover, the permeable filler material or 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 and/or cause the nitrogen to react with an infiltration enhancer precursor to form infiltration enhancer in the filler material or preform and thereby result in spontaneous infiltration. 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 or filler material, amount of magnesium nitride in the preform or filler material, 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 or the filler material, surface condition and type of filler material or preform, 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 percent by weight, and preferably at least about 3 percent 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 (or filler material) and matrix metal or the preform (or filler material) alone may result in a reduction in required amount of magnesium to achieve spontaneous infiltration (discussed in greater detail later herein). The volume percent of nitrogen in the infiltrating 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 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 the 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 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. 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 is 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). For example, in the aluminum/magnesium/nitrogen system, if the magnesium was applied to a surface of the matrix metal it may be preferred that the 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 alloying 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 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 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 , in the aluminum/magnesium/nitrogen system spontaneous and progressive infiltration will occur at a process temperature of at least about 675 β C, and preferably a process temperature of at least about 750 β C- 800"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 β C to about 1000 β 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 filler material or preform comes into contact with molten aluminum in the presence of, at least sometime during the process, a nitrogen-containing gas. The nitrogen-containing gas may be supplied by maintaining a continuous flow of gas into contact with at least one of the filler material or preform and/or 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 any nitride formation, 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, 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, magnesia, zirconia; (b) carbides, e.g. silicQn carbide; (c) borides, e.g. aluminum dodecaboride, titanium diboride, and (d) nitrides, e.g. aluminum nitride, and (e) mixtures thereof. 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 ceramic coating to protect the substrate from attack or degradation. Suitable ceramic coatings include oxides, carbides, borides and nitrides. Ceramics which are preferred for use 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 filler material or preform may be homogeneous or heterogeneous. It also has been discovered that certain filler materials exhibit enhanced infiltration relative to filler materials having a similar chemical composition. For example, crushed alumina bodies made by the method disclosed in commonly owned U.S. Patent No. 4,713,360, entitled "Novel Ceramic Materials and Methods For Making Same", which issued on December 15, 1987, in the names of Marc S. Newkirk et al . , exhibit desirable infiltration properties relative to commercially available alumina products. Moreover, crushed alumina bodies made by the method disclosed in Commonly Owned U.S. Patent No. 4,851,375, which issued on July 25, 1989, in the names of Marc S. Newkirk et al., and entitled "Methods of Making Composite Ceramic Articles Having Embedded Filler", also exhibit desirable infiltration properties relative to commercially available alumina products. The subject matter of each of these issued Patents is herein expressly incorporated by reference. Thus, it has been discovered that complete infiltration of a permeable mass of ceramic material can occur at lower infiltration temperatures and/or lower infiltration times by utilizing a crushed or comminuted body produced by the method of the aforementioned U.S. Patents. The size, shape, chemistry and volume percent of the filler material (or preform) can be any that may be required to achieve the properties desired in the composite. Thus, the filler 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 filler 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 or vice-versa depending on the particular reaction conditions. Average particle diameters as small as a micron or less to about 1100 microns or more can be successfully utilized in the present invention, with a range of about 2 microns through about 1000 microns being preferred for a vast majority of commercial applications. Further, the mass of filler material (or preform) to be infiltrated should be permeable (i.e., contain at least some interconnected porosity to render it permeable to molten matrix metal and/or to the infiltrating atmosphere). Moreover, by controlling the size (e.g., particle diameter) and/or geometry and/or composition of the filler material or the material comprising the preform, the physical and mechanical properties of the formed metal matrix composite can be controlled or engineered to meet any number of industrial needs. For example, wear resistance of the metal matrix composite can be increased by increasing the size of the filler material (e.g., increasing the average diameter of the filler material particles) given that the filler material has a higher wear resistance than the matrix metal. However, strength and/or toughness may tend to increase with decreasing filler size. Further, the thermal expansion coefficient of the metal matrix composite may decrease with increasing filler loading, given that the coefficient of thermal expansion of the filler is lower than the coefficient of thermal expansion of the matrix metal. Still further, the mechanical and/or physical properties (e.g., density, coefficient of thermal expansion, elastic and/or specific modulus, strength and/or specific strength, etc.) of a formed metal matrix composite body may be tailored depending on the loading of the filler material in the loose mass or in the preform. For example, by providing a loose mass or preform comprising a mixture of filler particles of varying sizes and/or shapes, wherein the density of the filler is greater than that of the matrix metal, a higher filler loading, due to enhanced packing of the filler materials, may be achieved, thereby resulting in a metal matrix composite body with an increased density. By utilizing the teachings of the present invention, the volume percent of filler material or preform which can be infiltrated can vary over a wide range. The lower volu e percent of filler that can be infiltrated is limited primarily by the ability to form a porous filler material or preform, (e.g., about 10 volume percent); whereas the higher volume percent of filler or preform that can be infiltrated is limited primarily by the ability to form a dense filler material or preform with at least some interconnected porosity (e.g., about 95 volume percent). Accordingly, by practicing any of the above teachings, alone or in combination, a metal matrix composite can be engineered to contain a desired combination of properties. 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 matrix metal into a preform or a mass of filler material, 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 may be achieved by using a lower porosity initial mass of filler material. 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 closed cell porosity or into a fully dense structure that would prevent infiltration by the molten alloy. Specifically, volume fractions on the order of about 60 to 80 volume percent can be achieved by methods such as vibrational packing, controlling particle size distribution, etc. However, alternative techniques can be utilized to achieve even higher volume fractions of filler. Volume fractions of filler on the order of 40 to 50 percent are preferred for thermo-forming in accordance with the present invention. At such volume fractions, the infiltrated composite maintains or substantially maintains its shape, thereby facilitating secondary processing. Higher or lower particle loadings or volume fractions could be used, however, depending on the desired final composite loading after thermo-forming. Moreover, methods for reducing particle loadings can be employed in connection with the thermo-forming processes of the ^resent invention to achieve lower particle loadings. ... has sen 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. Further, the wetting of the filler by molten matrix metal may permit a uniform dispersion of the filler throughout the formed metal matrix composite and improve the bondiag of the filler to the matrjx metal . 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 or preform, 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 not be 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 β 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. Further, the constituency of the matrix metal within the metal matrix composite and defects, for example, porosity, may be modified by controlling the cooling rate of the metal matrix composite. For example, the metal matrix composite may be directionally solidified by any number of techniques including: placing the container holding the metal matrix composite upon a chill plate; and/or selectively placing insulating materials about the container and/or pjacing "hot-topping" materials in contact with the container. Further, the constituency of the metal matrix may be modified after formation of the metal matrix composite. For example, exposure of the formed metal matrix composite to a heat treatment may improve the tensile strength (the standard test for tensile strength is ASTM-D3552-77, reapproved 1982) of the metal matrix composite. For example, a desirable heat treatment for a metal matrix composite containing a 520.0 aluminum alloy as the matrix metal may comprise heating the metal matrix composite to an elevated temperature, for example, to about 430'C, which is maintained for an extended period (e.g., 18-20 hours). The metal matrix may then be quenched in boiling water at about 100 β C for about 20 seconds (i.e., a T-4 heat treatment) which can temper or improve the ability of the composite to withstand tensile stresses. 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 from 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 resulting metal matrix composite. A barrier means may also be utilized in combination with the spontaneous infiltration of molten matrix metal into a filler material or preform. 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 ceramic filler. Barrier means may be used during spontaneous infiltration or in any molds or other fixtures utilized in connection with thermo-forming of the spontaneously infiltrated metal matrix composite, as discussed in greater detail below. There are many different types of materials which are useful as barrier means for the spontaneous infiltration process. Generally, these materials fall into two broad categories, reactive barriers and physical barriers. A reactive barrier is a material which reacts with at least one of the molten matrix metal, infiltration enhancer, infiltration enhancer precursor, filler material, and infiltrating atmosphere to either produce or remove a substance from the spontaneous infiltration system resulting in the interference, inhibition, prevention, or termination, of the migration, movement or the like, of molten matrix alloy beyond the defined surface boundary of the filler material or preform. For example, a reactive barrier may react with a molten matrix metal to form a compound which acts as a barrier to further infiltration of the molten matrix metal. Alternatively, a reactive barrier may locally consume or modify at least one of the infiltration enhancer, infiltration enhancer precursor, or infiltrating atmosphere, in such a way as to prevent further infiltration of the molten matrix metal into the affected region. Examples of reactive barriers for use with an aluminum alloy matrix metal and a nitrogen atmosphere are CaC0 3 , AIPO^, and colloidal Siθ2- Physical barriers can be defined as those barriers which are not reactive barriers. Physical barriers include those materials which are effective barriers and could be reactive barriers but do not function as reactive barriers because the reaction kinetics between the material and the molten matrix metal and/or infiltration enhancer and/or infiltration enhancer precursor and/or filler material and/or infiltrating atmosphere, are too slow to allow the material to function effectively as a reactive barrier. In addition, physical barriers also include those materials which are simply nonreactive with any of the spontaneous infiltration system components, or nonwettable by the matrix metal. Examples of physical barriers for an aluminum alloy matrix metal with a nitrogen atmosphere are colloidal graphite and certain glass forming materials. The glass forming materials can be mixed with a particulate material and are sometimes effective as physical barriers even when mixed with particulate material which would not normally act as a barrier material. Certain types of material may act partially as a reactive barrier and partially as a physical barrier. In those situations, the material would be classified as a physical barrier or a reactive barrier depending on which aspect was more effective at preventing the molten matrix metal from infiltrating beyond the desired boundaries of a mass of filler material or a preform. An example of a material which may function as both a physical barrier and a reactive barrier is very fine alumina particulate. For example, Grade A-17 alumina having an average particle size of about 3.5 microns (produced by Alcoa Industrial Products, Bauxite AR) is believed to function as both a reactive barrier and a physical barrier. However, it is further believed that such fine particulate alumina acts more as a reactive barrier than as a physical barrier. Therefore, the fine particulate alumina would be classified as a reactive barrier. Additional barrier discussion, including discussion of various specific types of barrier materials, is included in the following paragraphs. 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 foil product that is sold under the trademark Grafoil ® , registered to Union Carbide. This g aphite foil exhibits sealing characteristics that prevent the migration of molten aluminum alloy beyond the defined surface boundary of the filler material. This graphite foil is also resistant to heat and is chemically inert. Grafoil ® graphite foil is flexible, compatible, conformable and resilient. It can be made into a variety of shapes to fit any barrier application. Grafoil ® is particularly preferred in the method of the instant invention 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. Moreover, other 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. Certain types of colloidal graphite are particularly preferred in this embodiment of the instant invention. Additional examples of barrier materials which are useful in the method of the present invention include calcium containing compounds such as calcium sulfate or calcium sulfide, either alone or in combination with other materials such as graphite or colloidal silica. Moreover, certain boron-containing compounds have also been found to be useful as barrier materials, especially with aluminum alloys. For example, transition metal borides, such TiB 2 , have been found to be effective barriers as well as boron-containing compounds such as boron nitride. As discussed above, barrier means may be used during spontaneous infiltration or in any molds or other fixtures utilized in connection with thermo-forming of the spontaneously infiltrated metal matrix composite. The previous paragraph discusses the use of carbon, and especially the crystalline allotropic form of carbon known as graphite, as a barrier material which may be used during spontaneous infiltration. In addition, carbon, especially in the form of graphite, can be used as a barrier material for thermo-forming the spontaneously infiltrated metal matrix composite. For example, if a formed metal matrix composite body is subjected to temperatures above the melting point of the matrix metal, some of the matrix metal may bleed out of the surface of the formed metal matrix composite body. This phenomenon can be controlled by utilizing a barrier material which contacts or coats the surfaces of the metal matrix composite body which would be affected by the above-described phenomenon. Thus, a metal matrix composite body formed by spontaneous infiltration could be heat treated at a temperature which is above the melting point of the matrix metal without any bleeding.of matrix metal which might undesirably affect the properties of the metal matrix composite body. Such heat treating could be used to affect or enhance one or more properties of the final metal matrix composite body. In addition, the metal matrix composite body could be deformed after the spontaneous infiltration step by exerting pressure on the metal matrix composite body at a temperature above the melting point of the matrix metal. In certain situations, it may be necessary to utilize a barrier material to contain or shape the deformed metal matrix composite body. The above description of alternate uses for barrier materials relating to spontaneously infiltrated metal matrix composite bodies is not intended to be exhaustive. Barriers will be useful in a wide variety of situations where a formed metal matrix composite body is to be utilized at or above the temperature of the melting point of the matrix metal. Other preferred barrier(s) for aluminum metal matrix alloys in nitrogen are the transition metal borides (e.g., titanium diboride (ΗB2)) 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, with increased temperature, infiltration into the barrier will occur. Moreover, the particle size of the barrier material may affect the ability of the material to inhibit spontaneous infiltration. The transition metal borides are typically in a particulate form (1-30 microns). 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 or by encasing a filler material or preform within a barrier structure. 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 the infiltrating matrix 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 This Example demonstrates that thin metal matrix composite bodies having excellent surface finish can be produced by first coating a graphite mold housing and barrier means with a colloidal graphite prior to assembling the lay-up for the infiltration. The setup used to fabricate these thin metal matrix composites having excellent surface finish is shown schematically in Figure 2. A rectangular shaped mold 12 having a wall thickness of about 0.25 inches and an inner cavity measuring about 2.3 inches (58 mm) in length, about 0.44 inches (11 mm) in width and about 1.0 inch (25 mm) in height was machined from a solid piece of graphite. A graphite insert 14 was fabricated such that when the insert was placed into the bottom of the graphite mold a 0.10 inch (2.5 mm) deep and a 0.03 inch (0.76 mm) wide channel was created between the perimeter of the insert and the inside wall of the rectangular mold. The interior surfaces of the mold and the exterior surfaces of the insert were then spray coated with a mixture comprising by volume equal parts of DAG® 154 colloidal graphite (Acheson Colloids Company, Port Huron, MI) and ethyl alcohol. After drying the coating 16 in air at ambient temperature for about 1/2 hour, the coating and drying process was repeated two more times to produce a total of three colloidal graphite coatings 16 on the graphite mold 12 and graphite insert 14. The coated graphite mold and insert were then placed separately into a resistance heated air atmosphere furnace at substantially room temperature. The furnace temperature was then increased to a temperature 5 of about 380 β C at a rate of about 400 β C per hour. After maintaining a temperature of about 380 β C for about 2 hours, power to the furnace was turned off and the furnace was allowed to cool to substantially room temperature. A metal gasket 18 about 0.030 inch (0.76 mm) thick and sized to fit 10 in the above-described channel between the graphite mold 12 and the graphite insert 14 and comprising by weight about 12 percent silicon and the balance aluminum was positioned in said channel. A slurry 20 comprising by weight about 2 percent -325 mesh (particle diameter less than about 45 μm) magnesium powder (Reade Manufacturing Company, 15 Lakehurst, NJ), about 48 percent ethyl alcohol, and the balance a -325 mesh (particle diameter less than about 45 μm) metal alloy powder (Alcan Chemicals Division of Aluminum Company of Canada, Ltd., Montreal, PQ, Canada) comprising by weight about 7.5 to 9.5 percent silicon, about 0.7- 1.0 percent iron, about 3.0-4.0 percent copper, <0.50 percent manganese, 20 about 0.20-0.30 magnesium, <0.50 percent nickel, about 2.7-3.5 percent zinc, <0.35 percent tin, and the balance aluminum, was disposed on the top of the metal gasket 18 and was permitted to flow freely within the above- described channel, thus covering any areas which were not already covered by the metal gasket 18. The mold 12 and its contents were then placed 25 onto a vibration table and vibrated for about 60 seconds. The vibration was ceased and the slurry 20 was allowed to dry for about an additional two minutes. The metal powder on top of the metal gasket was removed with a small paint brush until the top of the gasket was again visible. Residual loose powder located on the surface of the gasket was blown out 30 of the mold using compressed air. This procedure was continued until the only remaining loose powder was that covering areas on the bottom of the channel which were not covered by the metal gasket 18. A filler material mixture 22 was made by roll mixing for approximately 1/2 hour, in a two liter porcelain jar containing about four -35 1/2 inch (13 mm) diameter BURUNDUM® ball milling stones (U.S. Stoneware, Mahwah, NJ), approximately 412 grams of a mixture comprising by weight about 19 percent 500 grit (average particle diameter of about 17 μm) 39 CRYST0L0N ® silicon carbide (Norton Company, Worcester, MA), about 78 percent 220 grit (average particle diameter of about 16 μm) 39 CRYST0L0N ® silicon carbide and about 3 percent -325 mesh (particle diameter less than about 45 μm) magnesium powder (Reade Manufacturing Company, Lakehurst, NJ). The ball milling stones were removed from the jar and the filler material admixture 22 was poured into a stainless steel tray. The stainless steel tray and its contents were then placed into a vacuum oven. The oven chamber was evacuated to about 30 inches (762 mm) of mercury vacuum and the oven temperature was raised to about 150 β C. The filler material 22 was subjected to this environment for at least about 12 hours prior to its incorporation in the lay-up. The fabrication of the filler material mixture 22 was completed by removing the stainless steel tray and its contents from the vacuum oven, pouring the filler material admixture 22 into a plastic jar, capping the jar, and shaking the jar by hand for about 5 minutes. About 14 grams of the filler material mixture 22 were poured into the graphite mold 12, a quantity sufficient to fill the channel between the mold 12 and the graphite insert 14 and to cover the top of the insert 14 to a depth of about 0.18 inch (4.6 mm). The filler material mixture 22 was thereafter leveled within the mold 12. The surface of the filler material mixture 22 was then dusted with -50 mesh (average particle diameter of about 300 μm) ALFA® magnesium particulate 24 (Alfa Products, Division of Johnson Matthey Company, Ward Hill, MA) until a concentration of about 6.2 milligrams per square centimeter was achieved. Matrix metal ingots 26 comprising by weight about 12 percent silicon, about 2.5 percent nickel, about 1 percent copper, and the balance aluminum, and weighing a total of about 25 grams were scored with a coarse file to remove as much oxide skin on the ingots as possible. The ingots 26 were then placed into and supported by two copper foil slings 28 positioned above the filler bed, each sling measuring about 1 inch (25 mm) long and about 1/8 inch (3 mm) wide. The mold 12 and its contents were then placed into a stainless steel container 30 measuring about 7 inches (178 mm) long, about 3.25 inches (83 mm) wide and about 5 inches (127 mm) high on top of an approximately 1/4 inch (6 mm) thick layer of Grade A aluminum nitride particulate 32 (average particle diameter 4.0 μm) (Starck B, Hermann C. Starck, Inc., New York, NY). The stainless steel container 30 and its contents were placed into a resistance„heated controlled atmosphere furnace. The furnace chamber was evacuated to about 30 inches (762 mm) of mercury vacuum. The furnace temperature was then increased from substantially room temperature to about 200 β C at a rate of about 400 β C per hour. After holding the stainless steel container 30 and its contents in the furnace under vacuum and at a temperature of about 200°C for about 2 hours, the furnace chamber was backfilled with nitrogen to substantially atmospheric pressure. A nitrogen gas flow rate of about 5 liters per minute was established. The furnace temperature was increased from about 200°C to about 800 β C at a rate of about 90 β C per hour. After maintaining a temperature of about 800 β C for about 3 hours, the temperature was decreased to about 750 β C at a rate of about 400 β C per hour. At a temperature of about 750 β C, the stainless steel container 30 and its contents were removed from the furnace and the graphite mold 12 and its contents were placed upon a water cooled copper chill plate to directionally solidify the molten metal 26 contained within the graphite mold 12. The top and sides of the mold 12 were covered with CARBORUNDUM ® FIBERFRAX ® ceramic fiber insulation (Standard Oil Engineered Materials Company, Niagara Falls, NY) of at least one inch (25 mm) thickness to assist in the directional solidification process. Thus, by maintaining the high temperature at the top of the graphite mold 12 and providing a heat sink at the bottom of the graphite mold 12, directional solidification of the molten matrix metal 26 contained within the graphite mold 12 was effected. After cooling to substantially room temperature, the graphite mold 12 was disassembled to reveal that the matrix metal 26 had infiltrated the filler material mixture 22 to produce a metal matrix composite body having substantially the same shape as the original graphite mold cavity, i.e., the configuration of an open-ended box. Furthermore, the metal gasket 18 first placed into the graphite mold cavity had fused and bonded to the metal matrix composite material to yield an unreinforced metal lip along the top edge of the metal matrix composite body. Thus, the final body produced in the instant Example comprised a acrocomposite body and both the metal matrix composite portion and the metal lip portion of the macrocomposite had an excellent surface finish. Exampl e 2 This Example demonstrates that a metal matrix composite barrier comprising colloidal graphite can be placed directly onto a preform to be infiltrated with a matrix metal. A cross-sectional schematic view of the setup is shown in Figure 3. About 25 grams of -325 mesh (particle diameter less than about 45 μm) AESAR ® magnesium powder (AESAR Group of Johnson Matthey Company, Seabrook, NH) was combined with about 475 grams of Grade A-200 aluminum nitride powder (Advanced Refractory Technologies, Inc., Buffalo, NY). This mixture was placed into a 1 liter plastic jar which contained about 250 grams of BURUNDUM ® alumina milling media (U.S. Stoneware, Mahwah, NJ) measuring about 1/2 inch (13 mm) in diameter by about 1/2 inch (13 mm) tall. The contents of the jar were roll mixed for about 1 hour. The roll mixed powders were then screened to separate the powder from the milling media. About 250 grams of screened powder was then poured into a pressing die. The powder was uniaxially pressed under an applied load of about 45,000 pounds to form a tile shaped preform 32. The pressed tile preform 32 measured about 3 inches (76 mm) square and about 7/8 inch (22 mm) tall. The pressed tile preform 32 was coated on all sides except one about 3 inches (76 mm) square face with an AER0DAG® G colloidal graphite 34 aerosal spray (Acheson Colloids Company, Port Huron, MI). After drying the coating in air at room temperature for about 1/2 hour, a second spray coating of colloidal graphite 34 was applied to the same surfaces of the preform 32. After drying the coated preform 32 again in air at about room temperature for about 1/2 hour, a coating of DAG® 154 colloidal graphite 36 (Acheson Colloids Company, Port Huron, MI) was painted on the surfaces that had been previously spray coated. After drying this colloidal graphite layer 36 in substantially the same manner as was used to dry the spray coated colloidal graphite layers 34, three additional coatings of DAG ® 154 colloidal graphite 36 were painted on the previously coated preform 32 surfaces with 1/2 hour drying periods between each coating step. After the last coating of colloidal graphite 36 had been applied to the preform 32, the coated preform 32, 34, 36 was allowed to air dry at about room temperature for about 3 to 5 hours. A graphite boat 38 measuring about 6 inches (152 mm) square and about 3 inches (76 mm) tall was coated on its interior surfaces with DAG ® 154 colloidal graphite 37 (described above) and allowed to air dry.at about room temperature for about 3 to 5 hours. Loose particulate El ALUNDUM ® alumina 40 (Norton Company) was poured into the coated graphite boat 36,38 to a depth of about 3/4 inch (19 mm). The coated preform 32, 34,36 was placed into the coated graphite boat 36,38 on top of the alumina bedding 40, with the uncoated face up. Additional El ALUNDUM ® alumina particulate material 40 was then poured around the sides of the preform 32,34,36 to a height substantially flush with the top of the preform. About 2 grams of -50 mesh (particle diameter less than about 300 μm) magnesium powder 42 (Hart Corporation, Tamaqua, PA) was spread evenly over the exposed face of the preform 32. An ingot of matrix metal 44 measuring about 5 inches (127 mm) square and about 1 inch (25 mm) thick and comprising by weight about 12 percent silicon and the balance substantially pure aluminum was placed into the graphite boat 38 on top of the magnesium powder coated preform 32,42 so that one 5 inch (127 mm) square surface of the matrix metal ingot 44 contacted the 3 inch (76 mm) square magnesium coated surface 42 of the preform 32. The graphite boat 38 and its contents were placed into a resistance heated controlled atmosphere furnace. The furnace chamber was evacuated to about 30 inches (762 mm) of mercury vacuum and then backfilled with nitrogen gas. A nitrogen gas flow rate of about 3 liters per minute was established and maintained throughout the heating cycle. The furnace temperature was then increased from substantially room temperature to about 550°C at a rate of about 200°C per hour. After maintaining the temperature of about 550 β C for about 2 hours, the furnace temperature was increased to about 775 β C at a rate of about 200°C per hour. After maintaining a temperature of about 775°C for about 15 hours, the furnace temperature was then decreased to substantially room temperature at a rate of about 200 β C per hour. Once the furnace chamber had reached substantially room temperature, the graphite boat 38 and its contents were removed from the furnace. The preform 32 and residual matrix metal 44 were removed from the graphite boat 38 to reveal that the matrix metal 44 had completely infiltrated the preform 32 to produce a metal matrix composite. The matrix metal infiltration, however, did not proceed past the boundaries of the preform 32. Further inspection revealed that a metal matrix composite of near net shape and having excellent surface finish was produced. Example 3 This example illustrates the use of an investment shell as a reactive barrier to define the exterior surfaces of a metal matrix composite body. A flange 50, as illustrated in Figure 4a, measuring about 3 inches (76 mm) in diameter by about 2 inches (51 mm) in height was spray coated with Grade MS-122 fluorocarbon release agent dry lubricant 52 (Miller- Stephenson Company, Inc., Danbury, CT). GI-1000 rubber molding compound 54 (Plastic Tooling Supply Company, Exton, PA) was cast around the spray coated flange to form a rubber mold 54 inversely replicating the shape of the flange 50 exterior. After curing the rubber molding compound 54 in air for about 12 hours, the spray coated flange 50, 52 was separated from the mold 54. An exact wax model 56 of the original flange 50 was then made by casting Grade 5550-K. GRN. FLK. molten wax 58 (Yates Manufacturing Company, Chicago, IL) at a temperature of about 110 β C into the rubber mold cavity 60 left after the removal of the flange 50, as depicted in Figure 4b. The wax 58 was then allowed to cool to room temperature. Before solidification, however, a mandrel 62 was inserted into the wax 58, and the wax 58 subsequently solidified around the mandrel 62, locking it in place. After the wax 58 had cooled to substantially room temperature, the wax model 56 and the mandrel 62 were separated as a unit from the rubber mold 54. An investment shell 64 was then built up on the surface of the wax model 56, as depicted in Figure 4c. Specifically, the wax model 56 was dipped into a slip 66 or slurry comprising by weight about 67 percent pure calcium carbonate (Standard Ceramic Supply Company, Division of Chem-Clay Corp., Pittsburgh, PA), about 16.5 percent NYAC0L® 2040 colloidal 40 weight percent silica (Nyacol Products, Inc., an affiliate of PQ Corporation, Ashland, MA), and about 16.5 percent water. The slip-coated wax model 55,66 was then dusted or stuccoed with dry 90 grit (average particle diameter of about 216 μm) 37 CRYST0L0N ® silicon carbide 68 (Norton Co., Worcester, MA). The wax model 56 and the developing investment shell 66,68 were then dried for about 1/2 hour at about room temperature in air. This dip-dust-dry sequence was then repeated. The coated wax model was then dipped into a slurry 70 comprising by weight about 67 percent 500 grit (average particle diameter about 17 μm) 37 CRYSTOLON ® silicon carbide (Norton Co.), about 16.5 percent NYACOL ® 2040 colloidal 40 weight percent silica, and about 16.5 percent water. The coated wax model was then stuccoed with 24 grit (average particle diameter of about 1035 μm) 37 CRYSTOLON® silicon carbide 72 (Norton Co.), after which it was again dried for about 1/2 hour in air at about room temperature. This second dip-stucco-dry sequence was then repeated an additional three times to form the sequence of coatings depicted in Figure 4d. The coated wax model 56,64 was then fired in a gas heated air atmosphere furnace at a temperature of about 700 β C for about 10 minutes to remove the wax model 56 from the investment shell 64. The investment shell 64 was then transferred from the about 700 β C gas fired furnace to a resistance heated air atmosphere furnace at a temperature of about 750°C for about 2 to 3 hours to drive off the chemically bound water and to outgas and rigidize the shell 64. After this second firing, the investment shell 64 was withdrawn from the approximately 750 β C furnace and placed on an approximately room temperature refractory plate to cool. 1000 grit (average particle diameter of about 5 μm) E67 ALUNDUM ® alumina powder (Norton Company) was vacuum dried for about five hours at about 150T under about 30 inches (762 mm) of mercury vacuum. A -325 mesh (particle diameter less than about 45 μm) AESAR ® magnesium powder (described above) was added to the alumina powder to produce an admixture 74 comprising by weight about 10 percent magnesium. About 160 grams of the admixture 74 was placed into a dry 1.1 liter alumina jar containing about 325 grams of dry one-half inch (13 mm) diameter BURUNDUM ® alumina stones (U.S. Stoneware, Mahwah, NJ) and milled for about two hours. The milled admixture 74 was removed from the alumina jar and vacuum dried a second time for at least about four hours at about 150 β C under about 30 inches (762 mm) of mercury vacuum. The milled and dried admixture 74 was poured into the investment shell 64, which was then placed onto a vibration table and vibrated for about 30 seconds. The investment shell 64 was lightly hand tapped during the vibration period to collapse any void spaces in the admixture 74. After the vibration period, the admixture 74 was leveled and -50 mesh (particle diameter less than about 30 μm) AESAR® magnesium particulate 76 (AESAR Group of Johnson Matthey Company, Seabrook, NH), was sprinkled evenly over the surface of the admixture 74 contained within the investment shell 64 until a concentration of about 23 milligrams of magnesium per square centimeter of admixture surface was achieved. The admixture-filled investment shell 67,74 was then placed in a rectangular stainless steel can 78 (AISI type 316 stainless steel) measuring about 10 inches (254 mm) by about 12 inches (305 mm) by about 10 inches (254 mm) tall (see Figure 4e). An about 270 gram ingot 80 of standard 520 aluminium alloy having a nominally identified composition by weight of about <0.25% Si, <0.30% Fe, <0.25% Cu, <0.15 Mn, 9.5-10.6% Mg, <0.15% Zn, <0.25 Ti and the balance aluminum, was placed in the investment shell 64 in contact with the magnesium coated surface 76 of the admixture 74. About 5 grams of TI-LOY 97 titanium sponge 82 (Chemalloy Co., Bryn Mawr, PA) was placed in the stainless steel can 78 outside of the investment shell 64. The function of the titanium sponge 82 was to act as an oxygen-getter. A copper sheet 84 measuring about 14 inches (356 mm) by about 16 inches (406 mm) by about 6 mils (0.15 mm) thick was placed over the opening of the can 78. Those portions of the copper sheet 84 extending over the sides of the can 78 were folded down against the sides of the can 78 to form an isolated chamber. A nitrogen gas feed tube 86 extended through one side of the stainless steel can 78. The stainless steel can 78 and its contents were placed into a resistance heated air atmosphere furnace. The furnace was closed, and a nitrogen gas flow rate of about 2.5 liters per minute was provided to the interior of the can through the feed tube. The furnace was then heated to about 800 β C over a period of about 1.5 hours and maintained at about 800°C for about 1 hour. After the above-described heating period, the investment shell 64 and its contents were removed from the furnace at about 800"C and placed onto a bed of sand at room temperature. After the investment shell 64 and its contents had cooled to substantially room temperature, the investment shell 64 was removed to reveal that the matrix metal 80 had infiltrated the admixture 74 to form a metal matrix composite in the shape of the flange 50. Example 4 This Example demonstrates that a slip cast alumina body can serve as a barrier material for metal matrix composite formation. Grade A-17 alumina powder (Alcoa Industrial Chemicals Division, Bauxite, AR) was stirred into a quantity of water containing DARVAfJ ® 821A dispersant (R. T. Vanderbilt Company, Inc., Norwalk, CT) such that the composition of the final slurry comprised by weight about 15 percent water, about 0.1 percent dispersant and the balance alumina. About 500 milliliters of this slurry was poured into a one liter plastic jar and roll mixed for about 16 hours. The slip was then cast into a two piece plaster of paris mold having an internal cavity measuring about 1.5 inches (38 mm) square and about 7 inches (178 mm) tall. When the casting on the mold wall reached a thickness of about 1/4 inch (6 mm), the excess slip was drained from the mold and the casting was allowed to dry for several hours. The slip cast shell 90 was then removed from the two piece mold and allowed to dry in air for an additional 24 hours. After drying, the slip cast shell 90 was placed onto a layer of 90 grit (average particle diameter of about 216 μm) 38 ALUNDUM® alumina (Norton Co.) which was supported by a refractory plate measuring about 6 inches (152 mm) by about 11 inches (279 mm) by about 1 inch (25 mm) thick (see Figure 5). The refractory plate bearing the slip cast shell 90 was then placed into a resistance heated air atmosphere furnace and heated to about 1050°C over a period of about 24 hours. The furnace temperature was maintained at about 1050°C for about 2 hours after which time the furnace was allowed to cool naturally. Once the furnace temperature had substantially returned to room temperature, the fired slip cast alumina shell 90 was removed from the furnace. About 2 percent by weight -325 mesh (particle diameter less than about 45 μm) magnesium powder (Hart Corporation, Tamaqua, PA) was added to a filler material comprising 220 grit (average particle diameter of about 66 μm) 39 CRYST0L0N ® green silicon carbide (Norton, Co.). About 500 milliliters of this particulate mixture 92 was placed into a one liter plastic jar and roll mixed for about an hour. About 80 grams of this mixture 92 was then poured into the fired slip cast alumina shell 90. About 0.5 grams of -50 mesh (particle size less than about 300 μm) magnesium powder 94 (Hart Corporation, Tamaqua, PA) was then evenly distributed on top of the silicon carbide filler mixture 92. A matrix metal ingot 96 measuring about 3/4 inch (19 mm) square and about 4 inches (102 mm) long and comprising by weight about 11.0-13.0%Si , <2.0%Fe, <1.0%Cu, <0.35%Mn, <0.1%Mg, <0.5%Ni, <0.5%Zn, <0.15%Sn, and the balance aluminum, was placed into the fired slip cast alumina shell 90 on top of the magnesium powder 94 to form a lay-up. The lay-up was then placed onto an about 1 inch (25 mm) thick layer of loose 90 grit (average particle diameter of about 216 μm) El ALUNDUM® (Norton Co.) alumina powder 98 contained within a stainless steel can 100 measuring about 6 inches (153 mm) square and about 6 inches (153 mm) tall. An additional 3 inches (76 mm) of 90 grit El ALUNDUM® powder 98 was poured into the stainless steel can 100 around the lay-up. The stainless steel can 100 and its contents were then placed into a resistance heated controlled atmosphere furnace. The furnace chamber was evacuated to about 30 inches (762 mm) of mercury vacuum and then backfilled with nitrogen gas. A nitrogen gas flow rate of about 4 liters per minute was established. The furnace was then heated from about room temperature to about 775 β C at a rate of about 200 β C per hour. The furnace temperature was maintained at about 775"C for about 11 hours before it was reduced to about 750 β C at a rate of about 150 β C per hour. At about 750 β C, the stainless steel can 100 was removed from the furnace and placed onto a graphite chill plate. The top of the residual pool of matrix metal 96 contained within the lay-up was then covered with a particulate mixture of a FEED0L ® No. 9 hot topping agent (Foseco, Inc., Cleveland, OH) to create an exothermic reaction on the top of the lay-up. By providing a heat source on the top of the lay-up and a heat sink on the bottom of the lay-up, directional solidification of the molten matrix metal 96 was effected. After the stainless steel can 100 and its contents had cooled to substantially room temperature, the fired slip cast alumina shell 90 and its contents were removed from the can 100. The fired slip cast alumina shell 90 was then removed by sandblasting to reveal that the matrix metal 96 had infiltrated the silicon carbide filler material 92 to produce a metal matrix composite body. Further inspection revealed that the metal matrix composite body had closely replicated the interior shape of the slip cast alumina shell 90. In addition, little or no infiltration had occurred into the slip cast alumina shell 90. Example 5 This Example demonstrates that fine grained alumina material is not only useful as a barrier material in the form of a shell or container for a loose mass of filler material, but also can be applied directly to a preform as a coating. About 25 grams of -325 mesh (particle diameter less than about 45 μm) AESAR ® magnesium powder (AESAR Group of Johnson Matthey Company, Seabrook, NH) were added to about 475 grams of Grade A-200 LX aluminum nitride powder (Advanced Refractory Technologies, Inc., Buffalo, NY). This admixture was placed into a one liter plastic jar which contained about 250 grams of 1/2 inch (13 mm) diameter by 1/2 (13 mm) inch tall BURUNDUM ® alumina milling media. The contents of the jar were roll mixed for about one hour. The roll mixed powders were then screened to separate the powder from the milling media. About 150 grams of screened powder were then poured into a pressing die. The powder was uniaxially pressed under an applied load of about 45,000 lbs to form a tile-shaped preform 102. The preform 102 measured about 3 inches (76 mm) square and about 1/2 inch (13 mm) tall . The preform 102 was then coated on all sides, except one 3 inch (76 mm) square face, with a slurry comprising by weight about 33 percent Grade A-1000 alumina 104 (Alcoa Industrial Chemical Divisions, Bauxite, AR) of sub icron particle size and the balance ethyl alcohol (see Figure 6). After drying the coating in air at room temperature for about 1/2 hour, two additional coatings of submicron alumina 104 were spray coated onto the preform tile over the first coating with a 1/2 hour drying period between the second and third coat. After the last coating of submicron alumina 104 had been applied to the preform 102, the coated preform 102, 104 was placed into a forced air drying oven at a temperature of about 70"C and allowed to dry for about 1 hour. A graphite boat 106 measuring about 6 inches (152 mm) square and about 3 inches (76 mm) tall was filled to a uniform depth of about 3/4 inch (19 mm) with 90 grit (average particle diameter of about 216 μm) 38 ALUNDUM ® alumina particulate 108 (Norton, Co.). The coated preform was then placed into the graphite boat and centered on top of the alumina bedding, uncoated face up. Additional alumina particulate bedding material 108 was then poured around the sides of the preform 102 to a height substantially flush with the top of the preform 102. About 2 grams of -50 mesh (particle diameter less than about 300 μm) magnesium particulate 110 (Hart Corporation, Tamaqua, PA) were then spread evenly over the exposed face of the preform 102. An ingot of matrix metal 112 measuring about 5 inches (127 mm) square and about 1 inch (25 mm) thick and comprising by weight abojit 11.0 to 13.0 percent silicon, < 2.0 percent iron, < 1.0 percent copper, < 0.35 manganese, < 0.10 percent magnesium, < 0.50 percent nickel, < 0.50 percent zinc, < 0.15 percent tin and the balance aluminum, was placed on top of the magnesium particulate coating 110 on the exposed face of the preform 102 to form a lay-up. The graphite boat 106 and its contents were placed into a resistance heated controlled atmosphere furnace. The furnace chamber was evacuated to about 30 inches (762 mm) of mercury vacuum and then backfilled with nitrogen gas. A gas flow rate of about 3 liters per minute was established. The furnace temperature was increased from substantially room temperature to a temperature of about 525°C at a rate of about 200 β C per hour. After maintaining a temperature of about 525 β C for about 1 hour, the furnace temperature was then increased to about 775 β C at a rate of about 200°C per hour. After maintaining a temperature of about 775 β C for about 10 hours, the furnace temperature was decreased to about 675°C at a rate of about 200 β C per hour. The flow rate of nitrogen gas was maintained at about 3 liters per minute throughout the heating cycle. At a temperature of about 675 β C, the graphite boat 106 and its contents were removed from the furnace and placed on top of a water cooled aluminum chill plate. FEED0L ® No. 9 particulate hot topping material (Foseco, Inc., Cleveland, OH) was poured on top of the residual molten matrix metal 112 contained within the graphite boat 106 to cause an exothermic reaction which supplied heat to the surface of the residual molten matrix metal 112. An about 2 inch (51 mm) thick layer of CERABLANKET ® ceramic fiber insulation (Manville Refractory Products, Denver, CO) was placed over the top of the graphite boat 106 to help maintain the high temperature at the top of the graphite boat 106. Thus, by providing a heat source at the top of the graphite boat 106 and a heat sink at the bottom of the graphite boat 106, directional solidification of the molten metal 112 contained within the graphite boat 106 was effected. After the graphite boat 106 and its contents had cooled to substantially room temperature, the lay-up was removed from the graphite boat 106. The submicron alumina 104 barrier coating was then removed by sandblasting. Inspection of the resulting body revealed that the matrix metal 112 had infiltrated the preform 102 tile to produce a metal matrix composite body having substantially the same shape as the original preform 102. Further inspection revealed that-.the coating of fine participate alumina 104 on the preform 102 was effective in preventing fine infiltration of the matrix metal 112 into the 90 grit alumina bedding material 108 around the preform 102. Thus, this Example demonstrates that a fine particulate alumina material can also serve as a barrier material when it is applied directly (as a spray coating) to the surfaces of a preform of filler material. Example 6 This Example demonstrates that the surface finish of a metal matrix composite body made using a slip cast barrier shell can be improved by coating the barrier shell with colloidal graphite prior to use. Grade A-17 alumina powder (Alcoa Industrial Chemicals Div., Bauxite, AR) was stirred into a quantity of water containing DARVAN® 821A dispersant (R. T. Vanderbilt Company, Inc., Norwalk, CT) to produce a slurry comprising by weight about 15 percent water, about 0.1 percent dispersant, and the balance particulate alumina. About 500 milliliters of the slurry was poured into a one liter plastic jar and roll mixed for about 16 hours to form a slip. The slip was then cast into a two piece plaster of paris mold having an internal cavity measuring about 9 inches (229 mm) in height and having a diameter of about 2 inches (51 mm) for about the first 6 inches (152 mm) and a diameter of about 3 inches (76 mm) for about the top 3 inches (76 mm). When the casting on the mold wall reached a thickness of about 1/4 inch (6 mm), the excess slip was drained from the two piece mold and the casting was allowed to dry in air for several hours at room temperature. The casting, a slip cast alumina barrier shell 90, shown in Figure 7, was then removed from the two piece mold by separating the two mold halves. The slip cast barrier shell 90 was then allowed to dry in air for an additional 24 hours. After drying, the slip cast barrier shell 90 was placed onto a layer of 90 grit (average particle diameter of about 216 μm) 38 ALUNDUM® alumina (Norton Co.) which was supported by a cordierite refractory plate measuring about 6 inches (152 mm) by about 11 inches (279 mm) by about 1 inch (25 mm) thick. The refractory plate bearing the slip cast barrier shell 90 was placed into a resistance heated air atmosphere furnace and heated to about 1050°C over a period of about 24 hours. The furnace temperature was maintained at about 1050°C for about 2 hours after which time the furnace was allowed to cool naturally. Once the furnace temperature had substantially returned to room temperature, the fired slip cast barrier shell 90 was removed from the furnace. The interior of the alumina barrier shell 90 was then aerosol spray coated with AERODAG® G colloidal graphite 34 (Acheson Colloids Company, Port Huron, MI) and allowed to dry in air at ambient temperature for about 1/2 hour. Two additional colloidal graphite coatings 34 were applied in a similar fashion, with 1/2 hour drying periods between coating applications. After application of the final colloidal graphite coating 34, the coated alumina barrier shell 90 was allowed to dry in air at ambient temperature for about 3 to 5 hours. About 10,000 grams of alumina milling media measuring about 0.94 inch (24 mm) in diameter and about 0.94 inch (24 mm) in length were placed into an 8 liter porcelain ball mill. About 5000 grams of -325 mesh (particle diameter less than about 45 μm) Grade T-64 particulate alumina filler material (Alcoa Industrial Chemicals Division, Bauxite, AR.) filler material was added to the mill. After dry ball milling the alumina particulate for about 6 hours, the milling media was removed and about 200 grams of -325 mesh (particle diameter less than about 45 μm) magnesium particulate (Hart Corporation, Tamaqua, PA) was added. The admixture 120 of ball milled alumina particulate and magnesium particulate was then roll mixed for about 2 hours. About 1 gram of -50 mesh (particle diameter less than 300 μm) magnesium particulate 94 (Hart Corporation) was sprinkled onto the bottom of the graphite coated 34 alumina barrier shell 90. About 125 grams of the roll mixed filler admixture 120 was then poured into the graphite coated alumina 34 barrier shell 90 on top of the layer of magnesium particulate 94. The surface of the filler admixture was then leveled and about 1 gram of the -50 mesh magnesium particulate material 94 was sprinkled evenly over the surface of the filler admixture 120. An approximately 144 gram ingot of a matrix metal 122 comprising by weight < 0.25 percent silicon, < 0.3 percent iron, < 0.25 percent copper, < 0.15 percent manganese, about 9.5-10.6 percent magnesium, < 0.15 percent zinc, < 0.25 percent titanium, and the balance aluminum, was placed into the coated alumina barrier shell 90 on top of the magnesium particulate 94 dusted surface of the filler admixture 120 to form a lay-up. The lay-up was then placed into a stainless steel container 100 measuring about 10 inches (254 mm) tall and about 4 inches (102 mm) in diameter. A bedding material 124 comprising 220 grit (average particle diameter of abo±it 66 μm) 38 ALUNDUM ® alumina (Norton Co.) was poured into the stainless steel container 100 around the coated alumina barrier shell 90 to a level about 1/2 to 1 inch (13 mm - 25 mm) from the top of the barrier shell 90, to provide physical support for the shell and its contents. The stainless steel container 100 and its contents were placed into a resistance heated controlled atmosphere furnace. The furnace chamber was evacuated to about 20 inches (308 mm) of mercury vacuum and then backfilled with nitrogen gas to substantially atmospheric pressure. A nitrogen gas flow rate of about 4 liters per minute was established. The furnace temperature was increased from substantially room temperature to about 550°C at a rate of about 200*C per hour. After maintaining a temperature of about 550 β C for about 1 hour, the temperature was then increased to about 775 β C at a rate of about 150 β C per hour. After maintaining a temperature of about 775 β C for about 12 hours, the temperature was decreased to about 760 β C at a rate of about 150 β C per hour. The flow rate of nitrogen gas was maintained at about 4 liters per minute during the entire heating cycle. At a temperature of about 760 β C, the stainless steel container 100 and its contents were removed from the furnace and placed upon a water cooled aluminum chill plate. FEED0L ® No. 9 particulate hot topping material was poured onto the top of the residual molten matrix metal 122 contained within the lay-up to cause an exothermic reaction which provided heat to the surface of the residual molten matrix metal. The top of the stainless steel container 100 was then covered with an approximately 2 inch (51 mm) thick layer of a CERABLANKET® ceramic fiber blanket material to help maintain the high temperature at the top of the lay-up. As described earlier, the combination of the heat source at the top of the lay-up and the heat sink at the bottom of the lay-up permitted the directional solidification of the molten matrix metal 122 contained within the lay-up. After the stainless steel container 100 and its contents had cooled to substantially room temperature, the lay-up was removed from the stainless steel container 100. The graphite coated 34 alumina barrier shell 90 was removed with light hammer blows to reveal that the matrix metal 122 had infiltrated the filler admixture 120 to produce a metal matrix composite body. Further inspection revealed that the presence of the colloidal graphite coating 34 on the interior of the slip cast, alumina barrier shell 90 resulted in less infiltration of the barrier shell 90 by the molten matrix metal 122, which in turn resulted in the formation of a metal matrix composite body which possessed improved surface finish and was closer to net shape than a similar metal matrix composite body produced using an uncoated slip cast alumina barrier shell. Example 7 This Example illustrates that a shaped dense body can be used as a physical barrier during the formation of a metal matrix composite body to define a surface of the final metal matrix composite body. A slip cast alumina barrier shell 90, shown schematically in Figure 8, was fabricated by substantially the same technique outlined in Example 6. The interior of the shell 90 was then aerosol spray coated with colloidal graphite 34, also by the same technique illustrated in Example 6. About 0.5 grams of -50 mesh (particle diameter less than 300 μm) magnesium particulate 94 (Hart Corporation, Tamaqua, PA) was sprinkled evenly over the bottom of the coated slip cast alumina barrier shell 34, 90. A hollow graphite tube 130 measuring about 13/8 inches (35 mm) in outside diameter and about 4 inches (102 mm) tall was placed into the coated alumina barrier shell 34,90 and centered on top of the magnesium particulate layer 94. The interior of the graphite tube 130 was filled with 500 grit (average particle diameter of about 17 μm) 38 ALUNDUM ® alumina 132 (Norton Co.) containing no infiltration enhancer precursor in an effort to prevent metal matrix composite formation inside of the graphite tube. Approximately 100 grams of a particulate filler admixture 120 comprising by weight about 4 percent -325 mesh (particle diameter less than 45 μm) magnesium powder (Hart Corporation) and the balance -325 mesh Grade T-64 tabular alumina (Alcoa Industrial Chemicals Division, Bauxite, AR) was processed by substantially the same technique as demonstrated in Example 6 and was then placed into the coated alumina barrier shell 90 around the graphite tube 130 and covering the top of the tube 130 to a depth of about 1 inch (25 mm). After the surface of the filler admixture 120 had been leveled, about 0.5 grams of the -50 mesh magnesium particulate 94 (Hart Corporation) was sprinkled evenly over the surface of the filler admixture 120. An approximately 225 gram ingot of a matrix metal 122 comprising by weight about 9-5-10.6 percent magnesium, <, 0.25 percent silicon, < 0.30 percent iron, < 0.25 percent copper, < 0.15 percent manganese, < 0.15 percent zinc, < 0.25 percent titanium, and the balance aluminum, and measuring about 2 inches (51 mm) in diameter and about 1 5/8 inches (41 mm) tall was placed into the coated alumina barrier shell 34,90 and centered over the magnesium particulate 94 dusted surface of the filler admixture 120 to form a lay-up. The lay-up, comprising the coated alumina barrier shell 90 and its contents, was then placed into a steel box 134 measuring about 4 inches (102 mm) square and about 7 inches (178 mm) tall. The space between the steel box 134 and the barrier shell 90 was then filled with 220 grit (average diameter of about 66 μm) 38 ALUNDUM ® alumina 124 (Norton Co.) which acted as support material for the barrier shell 90. The steel box 134 and its contents were placed into a resistance heated controlled atmosphere furnace. The furnace chamber was evacuated to about 20 inches (508 mm) of mercury vacuum and then backfilled with nitrogen gas to substantially atmospheric pressure. A nitrogen gas flow rate of about 4 liters per minute was established. The furnace temperature was increased from substantially room temperature to a temperature of about 550 β C at a rate of about 200 β C per hour. After maintaining a temperature of about 550°C for about 1 hour, the temperature was then increased to about 775°C at a rate of about 150T per hour. After maintaining a temperature of about 775 β C for about 15 hours, the temperature was decreased to about 760 β C at a rate of about 150 β C per hour. The nitrogen gas flow rate of about 4 liters per minute was maintained throughout the heating cycle. At a temperature of about 760°C, the steel box 134 and its contents were removed from the furnace and placed on top of a water cooled aluminum chill plate. FEED0L ® No. 9 hot topping particulate mixture was poured on top of the residual molten matrix metal 122 contained within the lay-up to cause an exothermic reaction which supplied heat to the surface of the residual molten matrix metal 122. An about 2 inch (51 mm) thick layer of CERABLANKET ® ceramic fiber insulation was placed over the top of the lay-up to help maintain the high temperature at the top of the lay-up and thereby assist in the directional solidification of the molten metal 122 contained within the lay-up. After the steel box 134 and its contents had cooled to about room temperature, the coated alumina barrier shell 34,90 and its contents were removed from the steel box 134. The barrier shell 90 was removed from the lay-up to reveal that at least some of the matrix metal 122 had infiltrated the filler particulate admixture 120 to form a metal matrix composite body in the shape of a tube. Some matrix metal 122 had also infiltrated into some of the alumina powder 132 contained within the graphite tube 130. The outside diameter of the metal matrix composite tube was defined by the interior surface of the coated alumina barrier shell 34,90 and the interior diameter of the metal matrix composite tube was defined by the exterior surface of the graphite tube 130. Therefore, this Example demonstrates that a solid body of graphite can be used as a physical barrier during the formation of a metal matrix composite to define a surface of the final metal matrix composite body. Example 8 This Example demonstrates the use of an investment shell containment means containing a glass forming additive which strengthens the investment shell and improves the surface finish of the formed metal matrix composite body. The experimental setup used to carry out the metal matrix composite fabrication process is shown schematically in Figure 9. A metal matrix composite body in the shape of a slotted pin measuring about 2 inches (51 mm) long and about 5/8 inch (16 mm) in diameter was made by substantially the same techniques as the flange in Example 3, but with certain differences noted below. Specifically, an investment shell 140 was built up upon a wax model of the slotted pin. The primary investment shell layer comprised by weight about 28.7 percent NYAC0L ® 1430AT colloidal silica (Nyacol Products, Inc. an affiliate of PQ Corporation, Ashland, MA), about 63.2 percent -325 mesh (particle diameter less than about 45 μm) HUBERCARB ® Q 325 calcium carbonate (J. M Huber Corporation, Calcium Carbonate Div., Quincy, IL), about 4.3 percent 500 grit (average particle diameter of about 17 μm) 39 ALUNDUM ® green silicon carbide (Norton Co.), about 2.9 percent 500 grit TETRAB0R ® boron carbide (Exolon-ESK Corporation, Tonawanda, NY), about 0.6 percent VICT0WET ® 12 wetting agent (Ransom and Randolph Inc., Maumee, OH), and about 0.3 percent DCH Antifoam defoamer (Ransom and Randolph Inc., Maumee, OH). As in Example 3, two primary investment shell layers were applied. The secondary investment shell composition comprised by weight about one part REDIP™ indicator (Ransom and Randolph Company) about two parts VICT0WET ® 12 wetting agent, about 56 parts distilled water, about 274 parts NYACOL ® 830 colloidal silica and about 700 parts RANCO SIL® No. 2 silica powder (Ransom and Randolph Company) to yield a slurry viscosity corresponding to about 15 seconds in a Zahn No. 4 cup. The slurry coated investment shell was then stuccoed or dipped in a fluidized bed of approximately 30 grit (average particle diameter of about 930 μm) RANCO® SIL B silica sand (Ransom and Randolph, Inc.). The stuccoed investment shell was again dried at a temperature of about 18"C for about 1/2 hour or until the REDIP ® indicator in the shell changed in color from yellow-green to deep orange. This second dip-stucco-dry sequence was then repeated an additional five times. No prewetting of the investment shell with colloidal silica between dippings in the secondary investment shell slurry was required. The coated wax mold was then given two firings, the first to remove the wax pattern and the second to outgas and rigidize the investment shell 140. The first firing comprised placing the coated wax pattern into a gas heated furnace at a temperature of about 800 β C. After about 15 minutes at a temperature of about 800 β C, virtually all of the Grade 5550-K. GRN. FLK. wax had either volatilized or been burned out of the shell. The shell 140 was then removed from the furnace at a temperature of about 800 β C and directly placed into a resistance heated air atmosphere furnace at a temperature of about 850 β C for the second firing. After firing the shell 140 at a temperature of about 850 β C for about 4 hours, the fired shell 140 was removed from the resistance heated furnace and set onto a refractory plate to cool. It was observed that the investment shell 140 had changed in color from light gray to nearly white as a result of at least some boron carbide in the shell oxidizing to glassy boric oxide during the second firing. Grade T-64 tabular alumina powder (Alcoa Industrial Chemical Division, Bauxite, AR) was vacuumed dried for about 12 hours at a temperature of about 150"C under about 30 inches (762 mm) of mercury vacuum. The alumina was then removed from the vacuum drying oven and combined with -325 mesh (particle diameter less than about 45 μm) AESAR ® magnesium powder to produce an admixture 142 comprising by weight about 5 percent magnesium. About 160 grams of this admixture 142 was placed into a dry 1.1 liter porcelain jar containing about 325 grams of dry about 1/2 inch (13 mm) diameter BURUNDUM® alumina stones and milled for about 2 hours. The milled admixture 142 was then removed from the porcelain jar and vacuum dried for at least four hours at about 150 * C under about 30 inches (762 mm) of mercury vacuum. About 68 grams of the milled and dried admixture 142 was then poured into the investment shell 140. The investment shell 140 was then gently tapped two or three times on a hard surface to remove any void spaces in the admixture 142. A thin layer of - 50 mesh (particle diameter less than about 300 μm) magnesium particulate 144 (Reade Manufacturing Company, Lakehurst, NJ) was then sprinkled evenly over the top surface of the filler admixture 142 until a concentration of about 12 milligrams per square centimeter was achieved. The magnesium particulate 144 acted as an infiltration enhancer precursor. The investment shell 140 and its contents were then placed into a stainless steel can 100 measuring about 10 inches (254 mm) wide by about 12 inches (305 mm) long by about 10 inches (254 mm) tall. An ingot of matrix metal 122, weighing about 167 grams and having substantially the same composition as the matrix metal employed in Example 6, was wedged into the investment shell 140, above but not in contact with, the magnesium particulate layer 144 on the surface of the filler material admixture 142. About 5 grams of Grade RMC-3 magnesium turnings 146 (Reade Manufacturing Company, Lakehurst, NJ) and about 7 grams of TI-L0Y 97 titanium sponge 82 were placed into the stainless can 100 outside of the investment shell 140. The magnesium turnings 146 and the titanium sponge 82 acted as oxygen-getters. A PERMA FOIL graphite foil sheet 148 (TTAmerica, Portland, OR) measuring about 4 inches (102 mm) square was placed over the opening in the investment shell 140. A copper sheet 84 measuring about 14 inches (356 mm) by about 16 inches (406 mm) by about 6 mils (0.15 mm) thick was placed over the opening of the stainless steel can 100. Those portions of the copper sheet 84 extending over the sides of the can 100 were folded down tightly against the sides of the can 100 to form an isolated chamber. A nitrogen gas feed tube 86 extended through one side of the can 100. The stainless steel can 100 and its contents were placed into a resistance heated air atmosphere furnace. A nitrogen gas flow rate of about 14 liters per minute was provided to the interior of the stainless steel can 100 through the feed tube 86. The furnace was then heated from substantially room temperature to a temperature of about 200 β C at a rate of about 600 β C per hour. After maintaining a temperature of about 200 β C for about 13 1/2 hours, the furnace temperature was increased to a te perature of about 760 * C at a rate of about 600 β C per hour. After maintaining a temperature of about 760 β C for about 1 hour, the nitrogen gas feed tube, which had been supplying nitrogen gas at a flow rate of about 14 liters per minute throughout the heating cycle, was disconnected from the stainless steel can 100. The can and its contents were then removed from the furnace and placed onto a water cooled copper chill plate. The copper 84 and graphite foil 148 sheets were removed from the stainless steel can 100 and the investment shell 140, respectively, and a FEEDOL ® No. 9 hot topping particulate mixture was poured onto the top of the residual molten matrix metal 122 contained within the investment shell 140 to cause an exothermic reaction which supplied heat to the surface of the residual molten matrix metal 122, thereby assisting in the directional solidification of the molten matrix metal contained within the investment shell 140. The investment shell 140 and its contents were removed from the stainless steel can 100. After the stainless steel can 100 and its contents had cooled to substantially room temperature, the investment shell 140 was then removed with light hammer blows to reveal that at least some of the matrix metal 122 had infiltrated the filler material admixture 142 to form a metal matrix composite. Further inspection revealed that the formed metal matrix composite body was of substantially the same size and shape as the mold cavity defined by the investment shell 140. In addition, the surface finish of the formed metal matrix composite body was superior to the surface finish of metal matrix composite bodies produced with investment shells which did not contain the glass forming additive utilized in this Example. Example 9 The following Example demonstrates that a loose bed of alumina particulate and glass frit can be utilized as a barrier material which will prevent molten matrix metal from infiltrating beyond the boundaries of a sediment cast preform. An aqueous solution of BLUONIC® A colloidal alumina (Buntrock Industries, Inc., Lively, VA) totalling about 261.4 grams was diluted with additional water amounting to about 522.8 grams and placed into an about 2 liter plastic jar. About 1280.9 grams of 220 grit (average particle diameter of about 66 μm) 39 CRYST0L0N® .x green silicon carbide particulate (Norton Co.) and about 548.9 grams of 500 grit (average particle diameter of about 17 μm) 39 CRYST0L0N® green silicon carbide particulate was added to the jar to prepare a slurry for sediment casting. The total slurry weight was about 2615 grams and comprised by weight about 49% 220 grit silicon carbide, about 21% 500 grit silicon carbide, about 2% colloidal alumina (dry basis), and about 28% water. After roll mixing for about 45 minutes, the slurry was poured into a silicone rubber mold with an internal cavity measuring about 7 inches (178 mm) square and about 1 1/2 inches (38 mm) deep. The mold was then vibrated to assist the sedimentation process. After vibrating for about 1/2 hour, the excess water on the surface of the forming preform was removed with a paper towel. After vibrating for about an additional 1 to 1 1/2 hours, the remaining surface water was removed and the silicone rubber mold was taken off the vibration table and placed into a freezer. After the residual water in the preform had frozen, the silicone rubber mold and the preform were removed from the freezer and the frozen sediment cast preform was withdrawn from the mold. The preform was placed on a bed of 90 grit (average particle diameter of about 216 μm) 38 ALUNDUM® alumina particulate material and allowed to dry in air at room temperature for about 16 hours. After drying, the sediment cast preform 150 was transferred to a new bedding of 90 grit alumina supported by a refractory plate and placed into a resistance heated air atmosphere furnace for firing. The furnace temperature was increased from substantially room temperature to a temperature of about 1050 β C in a period of about 10 hours. After maintaining a temperature of about 1050 β C for about 2 hours, the temperature was decreased to substantially room temperature in a period of about 10 hours. A graphite foil box 152 measuring about 8.5 inches (216 mm) square and about 4 inches (102 mm) tall and shown in Figure 10 was prepared from a sheet of GRAFOIL ® graphite foil (Union Carbide Company, Danbury, CT) and placed into a graphite boat 154 having an interior cavity measuring about 9 inches (229 mm) square and about 4 inches (102 mm) tall. The fired sediment cast preform 150 was then placed into the bottom of the graphite foil box 152. A bedding material 156 comprising by weight about 15% Grade F-69 borosilicate glass frit (Fusion Ceramics, Inc., Carrollton, OH) and the balance equal proportions of 90, 220, and 500 grit El ALUNDUM ® alumina particulate (Norton Co.) was placed into the graphite foil box 152 around the fired sediment cast preform 150 to a level substantially flush with the top of the preform 150. A thin surface layer of -100 mesh (particle diameter less than about 150 μm) magnesium powder 158 (Hart Corporation, Tamaqua, PA) was sprinkled over the top surface of the preform 150. A gating means comprising a graphite ring 160, with an inside diameter measuring about 2 1/2 inches (64 mm) and a height of about 1/2 inch (13 mm), was centered over an about 2 1/2 inch (64 mm) diameter hole in an about 7 inch (178 mm) square by about 14 mil (0.36 mm) thick sheet of GRAFOIL ® graphite foil 162. The graphite ring 160 was then adhered to the graphite foil 162 with a thin layer of an adhesive comprising by volume about 25 to 50% RIGIDLOCK® graphite cement (Polycarbon Corporation, Valencia, CA) and the balance ethyl alcohol. The joined graphite components were allowed to dry in air at room temperature for about 3 to 5 hours. The graphite foil 162 and ring 160 assembly were then placed into the graphite foil box 152 on top of the layer of -100 mesh magnesium powder 158 with the graphite ring 160 facing up. The inside of the graphite ring 160 was then filled with a dry particulate mixture 164 comprising by weight about 1% -100 mesh magnesium powder, about 1% -325 mesh magnesium powder, about 29% 90 grit 39 CRYST0L0N ® green silicon carbide and the balance 54 grit (average particle diameter of about 430 μm) green silicon carbide. Additional bedding material 156 (the above- described particulate mixture of alumina and glass frit) was then poured into the graphite box 152 around the graphite foil 162 and ring 160 assembly to a height substantially flush with the top of the graphite ring 160 but somewhat higher out towards the walls of the graphite box 152. An ingot of matrix metal 166 weighing about 1736 grams and measuring about 5 inches (127 mm) square and about 1 1/2 inches (38 mm) thick and comprising by weight about 12% silicon, about 6% magnesium, and the balance aluminum, was placed into the graphite foil box 152 and centered over the graphite ring 160 to form a lay-up. The graphite boat 154 and its contents were placed into a resistance heated controlled atmosphere furnace at room temperature. The furnace chamber was evacuated to about 30 inches (762 mm) of mercury vacuum and then backfilled with nitrogen gas. A gas flow rate of about 3 liters per minute was established. The furnace temperature was increased to about 825 β C at a rate of about 150 * C per hour. After maintaining a temperature of about 825 β C for about 20 hours, the temperature was decreased to about 700 β C at a rate of about 200 β C per hour. The flow rate of nitrogen gas was maintained at about 3 liters per minute throughout the heating cycle. At a temperature of about 700 β C, the graphite boat 154 and its contents were removed from the furnace and placed onto a water cooled aluminum chill plate. FEEDOL® No. 9 particulate hot topping material was poured onto the top of the residual molten matrix metal 166 contained within the lay-up to cause an exothermic reaction which supplied heat to the surface of the residual molten matrix metal 166. An about 2 inch (51 mm) thick layer of CERABLANKET ® ceramic fiber insulation (Manville Refractory Products, Denver, CO) was placed on top of the lay-up and across the graphite boat 154 to further assist in directional solidification process. After the graphite boat 154 and its contents had cooled to substantially room temperature, the lay-up was removed from the graphite boat 154. The bedding of alumina and glass frit material 156 was removed from around the lay-up to reveal that at least some of the matrix metal 166 had infiltrated the sediment cast preform 150 to produce a metal matrix composite tile of substantially the same size and shape as that of the preform 150. Further inspection revealed that there was little or no infiltration of the molten matrix metal 166 into the alumina and glass frit bedding material 156. Example 10 This Example demonstrates that the investment shell barrier concept works not only with wax investment patterns but also when the pattern comprises a cellulose based material such as basswood. A cross-sectional view of such an investment shell and its contents is shown schematically in Figure 11. A hollow body of triangular cross-section having one open-end and containing additional structural members was assembled by gluing together about 1/16 inch (1.6 mm) thick sheets of basswood. The lengths of the triangular sides were about 4, 4, and 3 inches (102, 102 and 76 mm), respectively. The depth of the body was about 3 inches (76 mm). The interior of the triangular body was braced with three additional sheets of basswood each about 3 inches (76 mm) long and oriented such that two were parallel to the base of the triangle aad the third was substantialjy perpendicular to the base and adjacent to the base. The basswood sheets were joined to one another with ELMER'S® wood glue (Professional Carpenters Wood Glue, Borden Company, Columbus, OH). Upon curing the glue in air at substantially room temperature for about 2 to 3 hours, the basswood fugitive investment pattern was then sealed with a protective coating of RED DEVIL ® HI-GLOSS 70 polyurethane (Red Devil Paints and Chemicals, Division of Insilco Company, Mount Vernon, NY). After drying the brushed-on polyurethane coating for about 1/2 hour, two additional coatings were applied, each with an approximately 1/2 hour drying period after the coating application. The finished basswood pattern was then joined to a Grade 5550-K. GRN. FLK. wax pattern in the shape of the desired matrix metal reservoir measuring about 3 inches (76 mm) in height, about 3 inches (76 mm) in diameter at the top and about 2 inches (51 mm) in diameter at its base where it contacted the basswood investment pattern. The basswood investment pattern was joined to the wax reservoir pattern with molten wax. The basswood and wax pattern assembly was then coated with investment shell layers in substantially the same manner as those described in Example 8. In the present Example, however, three primary investment shell coatings were applied instead of two. Furthermore, after the third secondary investment shell coating was applied, the investment shell was wrapped with a length of wire for added strength. Two additional layers of the secondary investment shell coating composition were then applied on top of the wire wrapped investment shell. Several holes were drilled in the investment shell 170 in strategic locations to assist in venting gases from the basswood during the subsequent flash firing. The coated basswood and wax reservoir investment pattern was then flash fired by placing said pattern into a gas heated air atmosphere furnace at a temperature of about 890 β C and holding at that temperature for about 15 minutes to burn out the basswood and the wax. The remaining investment shell 170 was then removed from the 890 β C gas furnace and immediately placed into a resistance heated air atmosphere furnace at a temperature of about 850 β C. After firing the investment shell 170 for about 6 hours at a temperature of about 850 β C to remove the chemically bound water and other vapors and to rigidize the shell, the investment shell 170 was removed from the furnace at a temperature of about 850 β C and placed onto a room temperature refractory plate and allowed to cool . A blend of 39 CRYSTOLON ® green silicon carbide particulates comprising about 772 grams of 220 grit (average particle diameter of about 66 μm) and about 193 grams of 500 grit (average particle diameter of about 17 μm) particles were placed into a dry 1.1 liter alumina ball mill containing about 2000 grams of dry about 1/2 inch (13 mm) diameter BURUNDUM ® alumina stones. The alumina ball mill, the ball mill lid (removed from mill) and its contents were vacuum dried for about 4 hours at a temperature of about 150 β C under about 30 inches (762 mm) of mercury vacuum. The ball mill and its contents were removed from the vacuum drier and -325 mesh (particle diameter less than 45 μm) magnesium powder (Reade Manufacturing Company, Lakehurst, NJ) was added to the silicon carbide particulate in the ball mill to produce a filler material admixture 172. The lid to the ball mill was then secured and the filler material admixture 172 was milled for about 2 hours. The ball mill lid was then removed from the alumina ball mill and the ball mill and its contents were vacuum dried a second time for at least about 4 hours at a temperature of about 150 β C under about 30 inches (762 mm) of mercury vacuum. The vent holes in the investment shell 170 were filled with a CARBORUNDUM ® FIBERFRAX ® ceramic fiber blanket 174. About 138 grams of the milled and dried filler material admixture was then poured into the investment shell 170 while the shell was shaken back and forth to allow the admixture 172 to fill in as much space as possible in the narrow walls. When all of the admixture 172 had been poured into the investment shell 170, the shell was then tapped about 5 times on a hard surface to complete the packing of the admixture 172. Magnesium particulate material 176 (-50 mesh, Reade Manufacturing Company) was sprinkled evenly over the surface of the filler material admixture 172 until a concentration of about 6 milligrams per square centimeter of filler material surface was achieved. An ingot of matrix metal 178 weighing about 901 grams, and having substantially the same composition as the matrix metal ingot used in Example 4, was then placed onto a sling 180 comprising several copper foil strips suspended over the magnesium particulate 176 dusted surface of the filler material admixture 172. The ends of the copper strips were wrapped over the walls of the investment shell. The copper foil sling 180 served to prevent the matrix metal 178 from contacting the filler material admixture 172 until the matrix metal 178 was in a molten state. A PERMA FOIL graphite foil sheet 182 measuring about 4 inches (102 mm) square was placed over the opening at the top of the investment shell 170. The investment shell 170 and its contents were then placed into a stainless steel can 184 measuring about 10 inches (254 mm) wide by about 10 inches (254 mm) long by about 10 inches (254 mm) tall. About 7 grams of Grade RMC-3 magnesium turnings 186 and about 12 grams of TI-L0Y 97 titanium sponge 188 were placed into the stainless steel can 184 outside of the investment shell 170. The magnesium turnings 186 and the titanium sponge 188 were utilized as oxygen-getters. Two sheets of copper foil 190 each measuring about 14 inches (356 mm) by about 16 inches (406 mm) by about 6 mils (0.15 mm) thick were placed over the opening of the stainless steel can 184. Those portions of the sheets 190 extending over the sides of the can 184 were folded down tightly against the sides of the can 184 to form an isolated chamber. The stainless steel can 184 and its contents were placed into a resistance heated air atmosphere furnace. A nitrogen gas flow rate of about 15 liters per minute was provided to the interior of the can 184 through a feed tube 192 which extended through one side of the can 184. The furnace was then heated from substantially room temperature to a temperature of about 790 β C at a rate of about 400 β C per hour. After maintaining a temperature of about 790 β C for about 3 1/2 hours, the nitrogen gas feed tube 192 was disconnected from the stainless steel can 184 and the can and its contents were removed from the furnace and placed onto a water cooled copper chill plate. The copper 190 and graphite foil 182 sheets were removed from the stainless steel can 184 and the investment shell 170, respectively, and FEEDOL® No. 9 hot topping particulate mixture was pυured onto the top of the residual molten matrix metal 178 contained within the investment shell 170 to cause an exothermic reaction which provided heat to the surface of the residual molten matrix metal 178 and thereby assisted in the directional solidification of the molten matrix metal 178 contained within the investment shell 170. After the stainless steel can 184 and its contents had cooled to room temperature, the investment shell 170 and its contents were removed from the stainless steel can 184. The investment shell 170 was removed to reveal that the matrix metal 178 had infiltrated the filler material admixture 172 to form a metal matrix composite. Further inspection revealed that the formed metal matrix composite body was of substantially the same size and shape as the mold cavity defined by the investment shell, which in turn was defined by the shape of the basswood and wax patterns, respectively. Example 11 This Example demonstrates that a particulate mixture comprising a glass frit may be formed into a rigid self-supporting body which may in turn be used as a mold or other shape-defining means for supporting, confining and shaping a loose permeable mass of a filler material which is to be infiltrated by a molten matrix metal in a spontaneous infiltration process to form a shaped metal matrix composite body. More importantly, this Example also demonstrates the use of a thin layer of colloidal graphite as a physical barrier material in the formation of an aluminum metal matrix composite. Figure 12 is a cross-sectional schematic view of a lay-up employed in fabricating the shaped metal matrix composite body by the above technique. The interior of a steel box measuring about 7.5 inches (191 mm) square by about 4 inches (152 mm) high was lined with one layer of GRAFOIL ® graphite foil (Union Carbide Company, Carbon Products Division, Cleveland, OH) measuring about 15 mils (0.38 mm) thick. A particulate mixture comprising by weight about 13 percent Grade F-69 glass frit (-325 mesh, Fusion Ceramics, Inc., Carrollton, OH) having substantially all particles smaller than about 45 μm, about 26 percent 90 grit El ALUNDUM ® alumina (Norton Co., Worcester, MA, average particle size of about 216 μm) and the balance 36 grit 38 ALUNDUM ® alumina (Norton Co., average particle size of about 710 μm) was poured into the graphite foil lined steel can to a depth of about 2 inches (51 mm) and leveled. A stainless steel bowl was then placed on top of the particulate mixture and additional particulate mixture was poured around the bowl to create a hemispherical depression measuring about 5 1/2 inches (140 mm) in diameter. The stainless steel bowl was left in the particulate mixture contained within the GRAFOIL ® coated steel box to form an assembly for firing. The assembly, comprising the steel can and its contents, was then placed into an air atmosphere furnace and heated to a temperature of about 850°C at a rate of about 200 β C per hour. After maintaining a temperature of about 850 β C for about 5 hours, the furnace and its contents werβ cooled back to about room temperature at a rate of about 200 β C per hour. Upon cooling to about room temperature, the assembly was removed from the furnace and disassembled to reveal that the alumina/glass frit particulate mixture had rigidized into a self-supporting shape defined by the interior of the GRAFOIL ® coated steel box and the exterior of the stainless steel bowl. This self-supporting alumina/glass frit shape comprised a shell mold which was used to contain filler material and matrix metal in the spontaneous infiltration process described below. Referring to Figure 12, the self-supporting alumina/glass frit shell mold 200 was brush coated with DAG® 154 colloidal graphite (Acheson Colloids Company, Port Huron, MI) on its interior surfaces to form a thin layer 202 of colloidal graphite. The colloidal graphite coated self- supporting shell mold 200 was then allowed to dry in air at ambient temperature until substantially all of the solvent carrier had volatilized from the colloidal graphite coating layer 202. After three additional colloidal graphite coatings were applied to the coated self-supporting shell mold in a similar fashion, the coated shell mold was placed into a resistance heated air atmosphere furnace at about room temperature. The furnace temperature was then increased to a temperature of about 400 β C at a rate of about 400 β C per hour. After maintaining a temperature of about 400 β C for about 10 hours, power to the heating elements in the furnace was interrupted and the furnace and its contents were allowed to cool to about room temperature. Upon cooling the furnace and its contents to about room temperature, the coated shell mold was recovered and the hemispherical cavity in the shell mold was buffed with a paper towel. A fifth coating of colloidal graphite was then applied to the coated shell mold in substantially the same fashion as were the previous four coatings and this fifth coating was cured in substantially the same manner as was the previous four coatings except that the dwell at temperature comprised an about 4-hour soak at a temperature of about 380 β C. The coated self-supporting shell mold 200 was then placed into a graphite boat 204 which measured about 10 inches (254 mm) square by about 4 inches (152 mm) high with the hemispherical cavity facing toward the open side of the graphite boat. About 55 pounds (25 kg) of 39 CRYSTOLON ® silicon carbide particulates (Norton Co.) comprising 430 μm and 216 μm average particle size, respectively, in about a 70:30 weight ratio, respectively, was placed into an approximately 6.6 gallon (25 liter) porcelain jar mill containing about 35 pounds (16 kg) of about 0.5 inch (13 mm) in diameter by about 0.5 inch (13 mm) in height cylindrical BURUNDUM® ball milling stones (U.S. Stoneware Corp.) The jar was then sealed and placed onto a rotating mill rack for about 24 hours to ball mill the silicon carbide particulates. To the ball milled silicon carbide particulate was added approximately 2 weight percent magnesium particulate (-100 mesh, Hart Corporation, Tamaqua, PA) having substantially all particles smaller than about 149 μm, and this admixture was placed into an equivalently sized plastic jar. The jar was then sealed and the contents were roll mixed on a rotating mill rack for about 2 hours. About 400 grams of the roll mixed filler material admixture 206 was then poured into the hemispherical chamber in the coated alumina/glass frit shell mold 200 and leveled. About 4 grams of magnesium particulate 208 (-50 mesh, Hart Corporation), having substantially all particle diameters smaller than about 300 μm, was then sprinkled evenly over the surface of the ball milled filler material admixture 206. An ingot of matrix metal 210 weighing about 660 grams and comprising by weight about 15% silicon, about 2% magnesium and the balance aluminum was placed into the graphite boat 204 on top of the magnesium particulate 208 to complete the lay-up. The lay-up, comprising the graphite boat 204 and its contents, was then placed into a retort and the retort chamber was sealed from the external environment. The retort was then evacuated to about 30 inches (762 mm) of mercury vacuum and backfilled with commercially pure nitrogen gas to about atmospheric pressure. A nitrogen gas flow rate through the retort of about 4 liters per minute was thereafter established. The retort temperature was then increased from about room temperature to a temperature of about 200"C at a rate of about 200 β C per hour. After maintaining a temperature of about 200°C for about 3 hours, the temperature was then further increased to a temperature of about 800 β C at a rate of about 200 β C per hour. After maintaining a temperature of about 800 β C for about 6 hours, the temperature was then decreased to a temperature of about 700"C at a rate of about 200°C per hour. Upon reaching a temperature of about 700 β C, the retort chamber was opened and the lay-up was removed and placed onto a graphite quench plate to directionally solidify the molten matrix metal. FEEDOL ® 9 hot topping particulate material (Foseco, Inc., Cleveland, OH) was sprinkled over the surface of the residual molten matrix metal to help maintain the thermal gradient. An approximately 2 inch (51 mm) thick layer of CERABLANKET ® ceramic fiber insulation (Manville Refractory Products, Denver, CO) was placed on top of the lay-up and across the graphite boat 204 to further assist in the directional solidification process. After the graphite boat 204 and its contents had cooled to about room temperature, the coated alumina/glass frit shell mold 200 was removed from the graphite boat and the contents contained in the hemispherical shaped cavity of the shell mold were recovered to reveal that the matrix metal had melted and infiltrated the permeable mass of the filler material admixture 206 to form a metal matrix composite body. The metal matrix composite body so produced had substantially the same shape as the hemispherical cavity in the shell mold 200. Further investigation revealed that substantially no overinfiltration of the molten matrix metal beyond the boundaries of the filler material admixture 206 occurred. Accordingly, the thin layer 202 of colloidal graphite, which lined the surface of the hemispherical chamber in the alumina/glass frit shell mold 200, was successful in preventing the molten matrix metal from infiltrating beyond the boundaries of the filler material admixture 206. This Example demonstrates that a barrier material comprising a thin layer of colloidal graphite can be applied to a shaped surface of a mold comprising a fired admixture of alumina and a glass frit so that upon infiltration of a molten aluminum matrix metal into a mass of filler material contained within the shaped portion of the graphite coated mold, the molten matrix metal is prevented from infiltrating beyond the surfaces of the filler mass which are defined by the graphite barrier, thus producing a metal matrix composite having at least one surface which is defined by the graphite coated surface of the mold. Example 12 This Example demonstrates the use of an alumina/glass frit barrier material as a disposable insert used as a mold or mandrel, for a colloidal graphite coating which functions as a barrier material to define a cavity within a metal matrix composite electronic package. Figure 13 is a cross- sectional schematic view of the lay-up used to fabricate the package. A rectangular graphite mold 220 having a wall thickness of about 0.25 inch (6 mm) and an inner cavity measuring about 2 inches (51 mm) long, about 1 inch (25 mm) wide and about 1 inch (25 mm) deep was machined from a solid piece of Grade ATJ graphite (Union Carbide Corp., Carbon Products Division, Cleveland, OH). An insert for the graphite mold was then fabricated from a particulate mixture comprising alumina and glass frit. As described below, the insert was used to create the cavity or space inside of the subsequently formed electronic package and the fabrication of the alumina/glass frit insert was based on the replication of a graphite insert as a model. Referring to Figure 4a, graphite insert 50 measuring about 1.875 inches (48 mm) long, about 0.874 inch (22 mm) wide and about 0.312 inch (7.9 mm) high was spray coated with Grade MS-122 fluorocarbon release agent dry lubricant 52 (Miller-Stevenson Company, Inc., Danbury, CT) . GI- 1000 rubber molding compound 54 (Plastic Tooling Supply Company, Exton, PA) was cast around the spray coated graphite insert 50 to form a rubber mold 54 inversely replicating the shape of the graphite insert exterior. After curing the rubber molding compound 54 in air for about 12 hours, the spray coated graphite insert 50 was separated from the mold 54. Referring to Figure 4b, an exact model of the original graphite insert was then made by casting into the cavity 60 in the rubber mold 54 a slurry 226 comprising by weight about 11.5 percent Grade F-69 glass frit (Fusion Ceramics, Inc., Carrollton, OH), about 23 weight percent LUD0X® SM colloidal silica (E. I. Du Pont de Nemours and Co., Inc., Wilmington, DE) and the balance equal weight fractions of El ALUNDUM® alumina (Norton Co., Worcester, MA), the alumina having average particle sizes of about 216, 66, and 17 microns, respectively. The rubber mold 54 and its contents were then vibrated on a vibration table to assist in sedimentation of the particulates in the slurry 226. After vibrating for about 1/2 hour, the excess water on the surface of the formed sediment cast body was removed with a paper towel. After vibrating for about an additional 1-1 1/2 hours, the remaining surface water was removed and the rubber mold 54 was placed into a freezer. After the residual water of the sediment cast body had thoroughly frozen, the rubber mold and its frozen sediment cast insert body were removed from the freezer. The frozen sediment cast insert body was then withdrawn from the mold, placed onto a bed of 90 grit 38 ALUNDUM ® alumina particulate (Norton Co., average particle size of about 216 μm) and allowed to dry in air at room temperature for about 16 hours. After drying, the sediment cast insert body was transferred to a new bedding of 90 grit 38 ALUNDUM ® alumina supported by a refractory plate measuring about 12 inches (305 mm) long by about 6 inches (154 mm) wide by about 1 inch (25 mm) thick and placed into a resistance heated air atmosphere furnace for firing. The furnace temperature was then increased from about room temperature to a temperature of about 850 β C in a period of about 8 hours. After maintaining a temperature of about 850 β C for about 2 hours, the temperature was decreased to about room temperature in a period of about 8 hours. Referring to Figure 13, the recovered alumina/glass frit insert body 222 was then painted with four coats of DAG® 154 colloidal graphite (Acheson Colloids Co., Port Huron, MI) to form a thin layer 224 of colloidal graphite. Each coating was allowed to dry before the subsequent coating was applied. The colloidal graphite coated insert body 222 was then placed into an air atmosphere drying oven at a temperature of about 380°C for about 12 hours to cure the graphite coating 224. After the coated insert body 222 was retrieved from the drying oven and had cooled to about room temperature, the coated insert body 222 was then placed into the graphite mold 220. A filler material admixture was made by mixing in a plastic jar on a ball mill for about 5 hours a mixture comprising by weight about 19 percent 39 CRYST0L0N ® 500 grit (average particle diameter of about 17 μm) silicon carbide particulate (Norton Co., Worcester, MA), about 4.8 percent Mg 2 Si (Aldrich Chemical Co., -20 mesh) having substantially all particles smaller than about 1340 μm, and the balance 220 grit 39 CRYST0L0N ® (average particle diameter of about 66 μm) silicon carbide particulate. About 16 grams of the filler material admixture 228 was poured into the bottom of the graphite mold 220 to cover the coated alumina/glass frit insert body 222. The filler material admixture 228 was thereafter leveled by generally shaking the graphite mold 220 and its contents in a side-to- side motion about five times. The level surface of the filler material admixture 228 was about 0.38 inch (9.5 mm) above the upper surface of the coated alumina/glass frit insert body 222. About 0.2 grams of magnesium particulate 230 having substantially all particles smaller than about 300 μm in diameter (-50 mesh, Hart Corporation, Tamaqua, PA) was then sprinkled evenly on the level surface of the filler material admixture 228. An ingot of matrix metal 232 weighing about 31 grams and measuring about 1.75 inches (44 mm) by about 0.75 inch (19 mm) by about 0.5 inch (13 mm) and nominally comprising by weight about 12 percent silicon, about 1.0 percent copper, about 0.2 magnesium, about 2.5 percent nickel and the balance aluminum was placed on top of the magnesium particulate layer 230 overlaying the filler material admixture 228. The graphite mold 220 and its contents were then placed into a graphite boat 234 measuring about 14 inches (356 mm) long by about 10.25 inches (260 mm) wide by about 1.5 inches (38 mm) deep on top of a bedding of Grade A-200 aluminum nitride particulate 236 (Advanced Refractory Technologies, Inc., Buffalo, NY) to complete the lay-up. The lay-up comprising the graphite boat 234 and its contents was then placed into a retort. The retort chamber was then sealed to the external environment and evacuated to about 30 inches (762 mm) of mercury vacuum. Once the retort chamber was evacuated, it was backfilled with nitrogen gas to about atmospheric pressure. A nitrogen gas flow rate through the retort of about 5 liters per minute was thereafter established. The temperature in the retort was then increased from about room temperature to a temperature of about 200 β C at a rate of about 200°C per hour. After maintaining a temperature of about 200°C for about 10 hours, the temperature was then increased to a temperature of about 800°C at a rate of about 800 β C per hour. After maintaining a temperature of about 800 β C for about 5 hours, the temperature was then decreased to a temperature of about 700"C at a rate of about 800 β C per hour. After maintaining a temperature of about 700"C for about 12 hours, the retort chamber was opened and the graphite mold 220 and its contents were removed from the lay-up and placed onto a water-cooled aluminum chill plate which was at approximately room temperature and the top of the graphite mold was covered with an approximately 2 inch (51 mm) thick layer of CERABLANKET ® ceramic fiber insulation (Manville Refractory Products, Denver, CO) to further assist in the directional solidification of the molten matrix metal. After the graphite boat 220 and its contents had cooled to substantially room temperature, the graphite boat was separated from its contents to reveal that the matrix metal had infiltrated the filler material admixture 228 to form a metal matrix composite electronic package containing the alumina/glass frit insert body 222 and having the colloidal graphite coating 224 in contact with the metal matrix composite electronic package. The colloidal graphite coated alumina/glass frit insert body was then easily removed from the surrounding metal matrix composite electronic package through a grit blasting operation. The resulting metal matrix composite electronic package showed full infiltration of the matrix metal into the filler material admixture up to the boundary defined by the colloidal graphite coating layer 224 on the exterior of the alumina/glass frit insert body 222. Moreover, the cavity left behind in the metal matrix composite electronic package by the removal of the coated insert displayed a good surface finish. Accordingly, this Example demonstrates that an insert material comprising an alumina/glass frit particulate admixture can be used as a shaped substrate or mandrel for a colloidal graphite coating which functions as a barrier material to a molten matrix metal and that the shaped barrier coated insert can be used to define the interior cavity of a metal matrix composite electronic package fabricated by a spontaneous infiltration process. Example 13 This Example demonstrates the use of boron nitride as a physical barrier material for arresting the spontaneous infiltration of a matrix metal into a permeable material. The lay-up used for carrying out the spontaneous infiltration process was similar to that shown in Figure 9, but is shown with particularity in Figure 14. An investment shell was created for the purpose of containing the filler material and matrix metal to be used in carrying out the spontaneous infiltration process and for defining the shape of the metal matrix composite body to be formed. Two approximately 400 ml styrofoam cups were glued together to produce an investment pattern which was open on both ends. A coating 240 comprising COMBAT® boron nitride (Carborundum Co., Niagara Falls, NY) was then applied by dip-coating the investment pattern into a slurry comprising boron nitride. An investment shell was then built up on the surface of the boron nitride coated styrofoam investment pattern. Specifically, the investment pattern comprising the coated styrofoam cups was dipped into a slurry comprising about 29^6% by weight NYACOL ® 1430AT colloidal silica (Nyacol Products, Inc., an affiliate of PQ Corporation, Ashland, MA), about 65.1% HUBERCARB ® Q 325 calcium carbonate (-325 mesh, JM Huber Corporation, Calcium Carbonate Division, Quincy, IL) having substantially all particles smaller than about 45 microns in diameter, about 4.3% 500 grit (average particle diameter of about 17 microns) 39 CRYST0L0N® green silicon carbide particulate (Norton Company, Worcester, MA), about 0.6% VICTOWET ® 12 wetting agent (Ransom and Randolph, Inc., Maumee, OH), and about 0.3% DCH ANTIFOAM ® defoamer (Ransom and Randolph, Inc., Maumee, OH). The coated investment pattern was then dusted or stuccoed with dry 90 grit (average particle diameter of about 216 μm) RANCO® SIL No. 4 silica sand (Ransom and Randolph, Inc.). The styrofoam investment pattern and its developing investment shell were then dried for about 1/2 hour at a temperature of about 65 β F. The dried investment shell was then dipped for about 2 seconds into a bath of NYACOL® 1430AT colloidal silica (Nyacol Products, Inc.). This dip-dust-dry-wet sequence was then immediately repeated two more times. The coated investment pattern was then immediately dipped into a secondary slurry comprising by weight about 1 part REDIP ® indicator (Ransom and Randolph, Inc.), about 2 parts VICTOWET® 12 wetting agent (Ransom and Randolph, Inc.), about 56 parts distilled water, and about 274 parts NYACOL ® 830 colloidal silica (Nyacol Products, Inc.), and about 700 parts RANCO ® SIL No. 2 silica powder (Ransom and Randolph, Inc.) which had a slurry viscosity corresponding to about 15 seconds in a Zahn No. 4 cup. The slurry investment shell was then stuccoed or dipped into a fluidized bed of approximately 30 grit (average particle diameter of about 930 μm) RANCO ® SIL B silica sand (Ransom and Randolph, Inc.). The stuccoed investment shell was again dried at a temperature of about 65 β F for about 1/2 hour or until the REDIP ® indicator in the shell changed in color from yellow/green to deep orange. This second dip-stucco-dry sequence was then repeated an additional five (5) times. Wetting of the investment shell with colloidal silica between dippings in the secondary investment shell slurry was required. The coated styrofoam investment pattern and its investment shell were then flash fired by placing the pattern and the shell into a gas heated air atmosphere furnace at a temperature of about 950°C for a duration of about 15 minutes to volatilize the styrofoam. The remaining investment shell was then removed from the approximately 950 β C gas furnace and immediately placed into a resistance heated air atmosphere furnace at a temperature of about 850 β C. After firing the investment shell for about 4 hours at a temperature of about 850 β C to remove the chemically-bound water and other impurity vapors and to rigidize the shell, the investment shell was furnace cooled to a temperature of about 200 β C, and then removed from the furnace and placed onto a room temperature refractory plate and allowed to cool to about room temperature. About 400 grams of 220 and 500 grit 39 CRYST0L0N ® green silicon carbide particulates (Norton Company, Worcester, MA, average particle diameters of about 66 and 17 μm, respectively) were placed into an approximately 1.9 liter porcelain ball mill in a weight ratio of about 80:20 along with about 12 grams of magnesium particulate (-325 mesh, Atlantic Equipment Engineers, Bergenfield, NJ), having substantially all particles smaller than about 45 μm, and about 200 grams of approximately 0.5 inch (13 mm) in diameter by about 0.5 inch (13 mm) in height BURUNDUM ® cylindrical grinding media (U.S. Stoneware Corp., Mahwah, NJ). The porcelain ball mill was sealed and the contents were roll milled on a rotating mill rack for about 2 hours. Referring to Figure 14, About 50 grams of the roll milled filler material admixture 246 was then poured into the investment shell 242 while the shell was shaken back and forth to allow the filler material admixture to fill in as much space as possible in the approximately 0.125 inch (3 mm) wide walls. When all of the filler material admixture 246 had been poured into the investment shell 242, the shell was then tapped about five times on a hard surface to complete the packing of the filler material admixture. Magnesium particulate material 263 (-50 mesh, Reade Manufacturing Company, Lakehurst, NJ) having substantially all particles between about 430 μm and about 1035 μm, was then sprinkled evenly over the surface of the filler material admixture 246 until a concentration of about 6 mg of the magnesium particulate per square centimeter of filler material surface was achieved. Two ingots of matrix metal 250 each measuring about 2 inches (51 mm) long by about 1 inch (25 mm) in both width and thickness and comprising by weight about 11.0-13.0 percent silicon, < 1.2 percent iron, about 0.5-1.5 percent copper, < 0.35 percent manganese, about 0.7-1.3 percent magnesium, about 2.0-3.0 percent nickel, < 0.35 percent zinc, < 0.25 percent titanium and the balance aluminum (no inally Aluminum Association Alloy No. 336) were placed into the investment shell 242 atop the layer of magnesium particulate material 263. The investment shell 242 and its contents were then placed into a stainless steel can 252 measuring about 10 inches (254 mm) wide by about 10 inches (254 mm) long by about 10 inches (254 mm) high which was lined on its interior with GRAFOIL® graphite foil (Union Carbide Corp., Carbon Products Div., Cleveland, OH) having a nominal thickness of about 15 mils (0.38 mm). About 7 grams of Grade RMC-3 magnesium turnings 146 (Reade Manufacturing Co.) and about 12 grams of TI-LOY 97 titanium sponge 82 (Chemalloy Company, Bryn Mawr, PA) were placed into the stainless steel can 252 outside of the investment shell. The magnesium turnings and the titanium sponge were utilized as oxygen-getters. A sheet of copper foil 256 measuring about 14 inches (356 mm) by about 16 inches (406 mm) by about 6 mils (0.15 mm) thick was placed over the opening of the stainless steel can 252. Those portions of the sheet 256 extending over the sides of the can 252 were folded down tightly against the sides of the can to form an isolated chamber. The stainless steel can 252 and its contents were placed into a resistance heated air atmosphere furnace. A nitrogen gas flow rate of about 40 cubic feet per hour (19 liters per minute) was provided to the interior of the can 252 through a purge tube 258 which extended through one side of the can 252. The nitrogen gas exhausted through the space between the copper foil 256 and the exterior walls of the stainless steel can. The furnace was then heated from about room temperature to a temperature of about 100°C at a rate of about 800 β C per hour. Upon reaching a temperature of about 100 β C, the temperature was then further increased to a temperature of about 440 β C at a rate of about 170°C per hour. Upon reaching a temperature of about 440 β C, the temperature was then further increased to a temperature of about 550 β C at a rate of about 12°C per hour. Upon reaching a temperature of about 550 β C, the temperature was then further increased to a temperature of about 750°C at a rate of about 100'C per hour. After maintaining a temperature of about 750 β C for about 1 hour, the nitrogen gas purge tube 258 was disconnected from the stainless steel can 252 and the can and its contents were removed from the furnace and placed onto a water-cooled copper chill plate to effect the directional solidification of the molten matrix metal. An about 2 inch (51 mm) thick FIBERFRAX® ceramic fiber blanket (Carborundum Co.) was placed over the top of the can to assist in the directional solidification. After the stainless steel can 252 and its contents had cooled to about room temperature, the investment shell 242 and its contents were removed from the stainless steel can 252. The investment shell 242 was removed from its contents using light hammer blows to reveal that the matrix metal had infiltrated the filler material admixture to form a metal matrix composite closely replicating the shape of one of the boron nitride coated styrofoam cups. Therefore, this Example demonstrates that the boron nitride coating 240 was effective as a barrier material which limited the extent of infiltration of the matrix metal to the filler material admixture 246 and specifically prevented the infiltration of the matrix metal into the adjacent investment shell 242. Example 14 This Example demonstrates a particular investment shell coating which shows utility as a barrier material for making metal matrix composite bodies having good surface finishes. The lay-up used for carrying out the spontaneous infiltration process was similar to that illustrated in Figure 4e, but is shown with particularity in Figure 15. Referring to Figure 15, an investment shell 64 was fabricated around a wax pattern using substantially the same method as was described in Example 3, including the flash firing in the gas heated air atmosphere furnace and the 2 to 3 hour firing in the resistance heated air atmosphere furnace to drive off chemically bound water and other impurity gases and to rigidize the investment shell. A slurry comprising by weight about 5% plaster of Paris (#1 Potters' plaster, U.S. Gypsum Corp.), about 15% Grade KS-44 graphite (Lonza, Inc.), about 8% NYACOL ® 1430AT colloidal silica (Nyacol Products, Inc., an affiliate of PQ Corporation, Ashland, MA) and the balance water was prepared and poured into the investment shell 64 substantially filling the investment shell. The investment shell 64 and its contents were then placed into a vacuum chamber and the atmosphere in the chamber was evacuated to about 30 inches (762 mm) of mercury vacuum to allow at least a portion of the investment shell to be infiltrated with the slurry material. After maintaining this vacuum for about 2 minutes, the vacuum chamber was brought back to atmospheric pressure, the investment shell 64 and its contents were removed from the vacuum chamber and the uninfiltrated slurry inside the investment shell was drained out. The investment shell 64, with the vacuum infiltrated slurry coating 260 was then allowed to dry in air at ambient temperature for about 16 hours. After the now coated investment shell 64 had substantially completely dried, the coated investment shell was placed into a retort at about room temperature and a flow rate through the retort chamber of about 10 to 12 standard liters per minute of forming gas (96 volume percent N2-4 volume percent H 2 ) was established. After flowing the forming gas through the retort chamber for about 30 minutes at about room temperature, the retort temperature was increased from about room temperature to a temperature of about 900 β C at a rate of about 150 β C per hour. After maintaining a temperature of about 900 β C for about 2 hours, power to the heating elements of the retort was interrupted and the retort chamber and its contents were allowed to cool back to about room temperature. It is believed that during the heating of the retort and its contents at about 900 β C in a flowing atmosphere of forming gas, the plaster of Paris in the infiltrated slurry coating was dehydrated to form calcium sulfate which was then at least partially reduced by the hydrogen in the forming gas to form calcium sulfide. About 1000 grams of 39 CRYST0L0N® green silicon carbide particulates (Norton Company, Worcester, MA) comprising by weight about 20 percent 500 grit (average particle diameter of about 17 μm) and the balance 220 grit (average particle diameter of about 66 μm) particles were placed into a stainless steel tray and the tray and its contents were then placed into a vacuum drying apparatus and vacuum dried for about 4 hours at a temperature of about 150 Q C under about 30 inches (762 mm) of mercury vacuum. The stainless steel tray and its contents were then removed from the vacuum drying apparatus and about 1000 grams of this admixture along with about an additional 5 weight percent magnesium metal particulate (- 325 mesh, Atlantic Equipment Engineers, Bergenfield, NJ) having substantially all particles smaller than about 45 μm in diameter, were placed into a dry 10 liter porcelain ball mill (U.S. Stoneware Corp., Mahwah, NJ) containing about 2000 grams of dry, about 0.5 inch (13 mm) in diameter by about 0.5 inch (13 mm) in height BURUNDUM ® cylindrical ball milling stones (U.S. Stoneware Corp.) and ball milled for about 2 hours on a rotating mill rack. The ball milled filler material admixture was then removed from the porcelain ball mill and vacuum dried for at least about 4 hours at a temperature of about 150"C under about 30 inches (762 mm) of mercury vacuum. Referring to Figure 15, about 75 grams of the milled and dried filler material admixture 262 was then poured into the coated investment shell 64. The coated investment shell 64 was then gently tapped two or three times on a hard surface to settle the filler material admixture 262 by collapsing any unusually large voids or air spaces between the filler particles. A thin layer of magnesium particulate 263 (-24 +50 mesh particle diameters (between about 430 μm and about 1035 lm) Reade Manufacturing Company, Lakehurst, NJ) having substantially all particles smaller than about 300 lm in diameter, was then sprinkled evenly over the top surface of the filler material admixture 262 until a concentration of about 6 mg of the magnesium particulate per square centimeter of the filler material admixture surface was achieved. About 600 grams of a matrix metal in the form of ingots 264 each measuring about 2 inches (51 mm) long by about 0.5 inch (13 mm) wide by about 0.5 inch (13 mm) thick and comprising by weight about 11.0-13.0% silicon, < 1.2% iron, about 0.5-1.5% copper, < 0.35% manganese, about 0.7-1.3% magnesium, about 2.0-3.0% nickel, < 0.35% zinc, < 0.25% titanium and the balance aluminum were placed into the investment shell 64 on top of the layer of magnesium particulate 248. The investment shell 64 and its contents were then placed into a graphite boat 266 measuring about 8 inches (203 mm) wide by about 8 inches (203 mm) long by about 2.5 inches (64 mm) high. The lay-up was completed with the addition of about 7 grams of Grade RMC-3 magnesium turnings 146 (Reade Manufacturing Company) to the bottom of the stainless steel can 266 outside of the investment shell 64 to act as an oxygen getter during the spontaneous infiltration process. The lay-up, comprising the stainless steel can 266 and its contents, was placed into a retort and the retort was isolated from the external atmosphere. A commercially pure nitrogen gas flow rate of about 10-12 standard liters per minute through the retort chamber was established. After flowing nitrogen gas through the retort for about 30 minutes, the temperature in the retort was increased from about room temperature to a temperature of about 200°C at a rate of about 150°C per hour. After maintaining a temperature of about 200 β C for about 2 hours, the temperature was then further increased to a temperature of about 560°C at a rate of about 150T per hour. After maintaining a temperature of about 560 β C for about 2 hours, the temperature was then further increased to a temperature of about 800 β C at a rate of about 150 β C per hour. After maintaining a temperature of about 800 β C for about 3 hours, power to the heating elements of the retort furnace was interrupted and the retort and its contents were allowed to cool to about room temperature. After the retort and its contents had cooled to about room temperature, the retort chamber was opened and the lay-up was removed. The investment shell 64 and its contents were then removed from the stainless steel can 266. The investment shell 64 bearing the slurry coating 260 was then removed using light hammer blows to reveal that the matrix metal had infiltrated the filler material admixture 262 to produce a metal matrix composite body closely replicating the shape of the filler material admixture 262 as defined by the surrounding coated investment shell. The formed metal matrix composite body featured good surface finish with very little infiltration of the matrix metal beyond the boundary of the filler material admixture 262 into the surrounding investment shell 64. Accordingly, the present Example demonstrates that a thin layer comprising by weight about 5 parts calcium sulfate, 15 parts graphite and 8 parts silica, wherein at least a portion of the calcium sulfate has been reduced to calcium sulfide, functions effectively as a barrier material to a molten aluminum matrix metal to produce a metal matrix composite body having a good surface finish. Example 15 The method of Example 14 was substantially repeated with the following notable exceptions. The slurry 260 which was coated on the interior of the investment shell 64 comprised by weight about 20% plaster of Paris, about 8% NYAC0L ® 1430 AT colloidal silica (Nyacol Products, Inc., an affiliate of PQ Corporation, Ashland, MA) and the balance water. The firing of the investment shell comprised heating the coated and dried investment shell under about 30 inches (762 mm) of mercury vacuum from about room temperature to a temperature of about 400"C at a rate of about 75 β C per hour. After maintaining a temperature of about 400°C for about 2 hours, the temperature was then decreased to a temperature of about 100°C at a rate of about 75 β C per hour. The coated investment shell was maintained at a temperature of about lOO'C under vacuum until the coated investment shell was ready to be filled with the filler material admixture 262, the magnesium particulate layer 248 and matrix metal 264. The furnace heating schedule for conducting the spontaneous infiltration process also differed somewhat from that described in Example 14 and those differences are described with particularity herein. The retort and its contents were first evacuated to about 30 inches (762 mm) of mercury vacuum and subsequently backfilled with commercially pure nitrogen gas to about atmospheric pressure. The retort was then evacuated a second time and maintained at a vacuum of about 30 inches (762 mm) of mercury vacuum for about 0.5 hour and then backfilled a second time with nitrogen gas to substantially atmospheric pressure. A commercially pure nitrogen gas flow rate of about 10 standard liters per minute through the retort chamber was thereafter established. The temperature in the retort was then increased from about room temperature to a temperature of about 400 β C at a rate of about 150 β C per hour. After maintaining a temperature of about 400 β C for about 2 hours, the temperature was then further increased to a temperature of about 560 β C at a rate of about 150 β C per hour. The heating schedule for conducting the spontaneous infiltration process was thereafter substantially the same as that disclosed in Example 14. Inspection of the formed metal matrix composite body revealed that the investment shell coating 260 of the present Example was effective in allowing the matrix metal to spontaneously infiltrate substantially the entire mass of filler material admixture 262 contained within the investment shell 64, and prevented the infiltration of the molten matrix metal from occurring substantially beyond the boundaries of the filler material admixture 262. The surface finish of the formed metal matrix composite body was good. Accordingly, this Example demonstrates that a thin layer comprising by weight about 20 parts calcium sulfate and about 8 parts silica functions effectively as a barrier material to a molten aluminum matrix metal to produce a metal matrix composite body having a good surface finish.

Claims: Cl aims 1. A method for making a metal matrix composite, comprising: (a) providing a body of substantially non-reactive filler; (b) contacting said body of substantially non-reactive filler with a barrier means so as to define at least one surface of said body of substantially non-reactive filler; and (c) spontaneously infiltrating at least a portion of said body of filler with a molten matrix metal to contact at least a portion of said barrier means to define at least a portion of at least one surface of said metal matrix composite. 2. A method of making a metal matrix composite, comprising: contacting a body of at least one filler material with a barrier means which defines at least a portion of at least one surface of said mass of at least one filler material; infiltrating at least a portion of said at least one filler material with a molten matrix metal, at least one of the matrix metal and filler being in the presence of at least one of an infiltration enhancer precursor, and infiltration enhancer, and an infiltrating atmosphere for at least a portion of the period of infiltration; continuing said infiltration until said molten matrix metal contacts at least a portion of said barrier means, thereby terminating the infiltration of said molten matrix metal at said at least one surface boundary; and cooling said molten matrix metal to a temperature below the melting point of said matrix metal and recovering a metal matrix composite from said mass of at least one filler material, said metal matrix composite having at least a portion of at least one surface defined by said barrier means. 3. The method of any one of claims 1 or 2, wherein said barrier comprises at least one material which functions as a physical barrier or a reactive barrier. 4. The method of any one of claims 1 or 2, wherein said barrier comprises a reactive barrier which reacts with at least the matrix metal. 5. The method of claim 2, wherein said barrier comprises a reactive barrier which reacts with at least one of the matrix metal, the infiltration enhancer precursor and the infiltration enhancer. 6. The method of any one of claims 1 or 2, wherein said barrier comprises at least one material selected from the group consisting of (1) fine alumina particulate, (2) graphite, (3) titanium diboride, and (4) low-volatile organic compounds. 7. The method of claim 6, wherein said graphite comprises at least one of a graphite tape and a colloidal graphite. 8. The method of claim 7, wherein said colloidal graphite is applied by at least one process selected from the group consisting of painting, spraying, dipping and the like. 9. The method of any one of claims 1-7, wherein said matrix metal comprises aluminum. 10. A metal matrix composite body made according to any of the preceding claims.

Inventors: AGHAJANIAN MICHAEL KEVORK, BECKER KURT JOSEPH, BURKE JOHN THOMAS, DWIVEDI RATNESH KUMAR, HANNON GREGORY EUGENE, KECK STEVEN DAVID, MCCORMICK ALLYN LESLIE, TAYLOR STEVEN JOHN, WIENER ROBERT JAMES, AGHAJANIAN, MICHAEL, KEVORK, BECKER, KURT, JOSEPH, BURKE, JOHN, THOMAS, DWIVEDI, RATNESH, KUMAR, HANNON, GREGORY, EUGENE, KECK, STEVEN, DAVID, MCCORMICK, ALLYN, LESLIE, TAYLOR, STEVEN, JOHN, WIENER, ROBERT, JAMES
Applicants: LANXIDE TECHNOLOGY CO LTD, LANXIDE TECHNOLOGY COMPANY, LP
Application year: 1991
Main classification: C22C1
IPC codes: C04B35, C04B41, C22C1, C22C47