Background of the Invention

The present invention comprises a process for the preparation of metal -second phase composite material and the products of that process. In the present invention, a second phase, such as a ceramic material or an intermetallic, is formed directly in a metal, metal alloy, or intermetallic matrix, in the desired volume fraction. The second phase can comprise a ceramic, such as a boride, carbide, oxide, nitride, suicide, sulfide, oxysulfide or other compound, of one or more metals the same as or different than the matrix metal. Of special interest are the inter metallics of aluminum, such as the aluminides of titanium, zirconium, iron, cobalt, and nickel. In the present invention, the second phase is dispersed in a matrix metal, metal alloy, or intermetallic, by the direct addition of second phase-forming constituents and solvent metal into a molten matrix metal. By introducing the second phase-forming constituents in this manner, reaction of the constituents with other second phase-forming constituents added to the molten matrix metal will disperse the second phase throughout the metal bath. Cooling yields a metal or intermetallic having improved properties due to the uniform dispersion of very small particulate second phase throughout the matrix, and the resultant fine grain size of the matrix. Either the introduced second phase-forming constituents or molten matrix metal, or both, may constitute an alloy of two or more metals.

For the past several years, extensive research has been devoted to the development of metal -second phase composites, such as aluminum reinforced with fibers, whiskers, or particles of carbon, boron, sil icon carbi de, sil ica, or alumina. Metal -second phase composites with good high temperature yield strengths and creep resistance have been fabricated by the dispersion of very fine (less than 0.1 micron) oxi de or carbide particles throughout the metal or alloy matrix of composites formed by util izing powder metal l urgy techni ques. However, such composi tes typically suffer from poor ductil ity and fracture toughness, for reasons whicli are expl ai ned below.

Prior art techni ques for the production of metal-second phase composites may be broadly categorized as powder metal l urgical approaches, molten metal techniques, and internal oxi dation processes. The powder metall urgical type production of dispersion-strengthened composites woul d ideally be accomplished by mechanical ly mixing metal powders of approximately 5 micron diameter or less with an oxi de or carbide powder (preferably 0.01 micron to 0.1 micron) . High speed blending techniques or conventional procedures, such as bal l milling, may be used to mix the powders. Standard powder metal lurgy techniques are then used to form the final composite. Conventionally, however, the ceramic component is l arge, i .e. , greater than 1 micron, due to a l ack of avail abil ity, and high cost, of very small particle size material s, because their production is energy intensive, time consuming and capital intensi ve. Furthermore, production of very smal l particles inevi tably leads to contamination at the particle surface, leading to contamination at the particle- to -metal interface of the composite, which in turn compromises the mechanical properties thereof. Al so, in many cases where the particul ate material s are avail able in the desired size, they are extremely hazardous due to the r pyrophoric nature.

Alternat vely, molten metal infiltration of a continuous skeleton of the second phase materi al has been used to produce composites. In some cases, elaborate particle coating techniques have been developed to protect ceramic particles from molten metal

during molten metal infiltration and to improve bonding between the metal and ceramic. Techniques such as this have been developed to produce silicon carbide-aluminum composites, frequently referred to as SiC/Al or SiC aluminum. This approach is suitable for large particulate ceramics (for example, greater than 1 micron) and whiskers. The ceramic material, such as silicon carbide, is pressed to form a compact, and liquid metal is forced into the packed bed to fill the inter sticies. Such a technique is illustrated in U.S. Patent No. 4,444,603 to Yamatsuta et al., hereby incorporated by reference. Because this technique necessitates molten metal handling and the use of high pressure equipment, molten metal infiltration has not been a practical process for making metal-second phase composites, especially for making composites incorporating submicron ceramic particles where press size and pressure needs would be excessive and unreal stic.

The presence of oxygen in ball -mi lied powders used in prior art powder metallurgy techniques, or in molten metal infiltration, can result in a deleterious layer, coating, or contamination such as oxide at the interface of second phase and metal. The existence of such layers will inhibit inter facial bonding between the second phase and the metal matrix, adversely affecting ductility of the composite. Such weakened interfacial contact may also result in reduced strength, loss of elongation, and facilitated crack propagation. Internal oxidation of a metal containing a more reactive component has also been used to produce dispersion strengthened metals, such as copper containing internally oxidized aluminum. For example, when a copper alloy containing about 3 percent aluminum is placed in an oxidizing atmosphere, oxygen may diffuse through the copper matrix to react with the aluminum, precipitating alumina. Although this technique is limited to relatively few systems, because the two metals must have a wide difference in chemical reactivity, it has offered a possible method for dispersion hardening. However, the highest possible concentration of dispersoids formed in the resultant dispersion strengthened metal is generally insufficient to impart significant changes in properties such as modulus, hardness and the like.

In U.S. Patent No. 2,852,366 to Jenkins, hereby incorporated by reference, it is taught that up to 10 percent by weight of a metal complex can be incorporated into a base metal or alloy. The patent ^" teaches blending, pressing, and sintering a mixture of a base metal, a compound of the base metal and a non-metallic complexing element, and an alloy of the base metal and the complexing metal. Thus, for example, the reference teaches mixing powders of nickel, a nickel-boron alloy, and a nickel -titanium alloy, pressing, and sintering the mixed powders to form a coherent body in which a stabilizing unprecipitated "complex" of titanium and boron is dispersed in a nickel matrix. Precipitation of the complex phase is specifically avoided.

In U.S. Patent No. 3jl94,656, hereby incorporated by reference, Vordahl teaches the formation of a ceramic phase, such as TiBg crystallites, by melting a mixture of eutectic or near eutectic alloys. It is essential to the process of Vordahl that at least one starting ingredient has a melting point substantially lower than that of the matrix metal of the desired final alloy. There is no disclosure of the. initiation of an exothermic second phase-forming reaction at or near the melting point of the matrix metal. Bredzs et al, in U.S. Patent Nos. 3,415,697; 3,547,673; 3,666,436; 3,672,849; 3,690,849; 3,690,875; and 3,705,791, hereby incorporated by reference, teach the preparation of cermet coatings, coated substrates, and alloy ingots, wherein an exothermic reaction mechanism forms an in-situ precipitate dispersed in a metal matrix. Bredzs et al rely on the use of alloys having a depressed melting temperature, preferably eutectic alloys, and thus do not initiate a second phase-forming exothermic reaction at or near the melting temperature of the matrix metal. DeAngelis, in U.S. Patent No.4,514,268, hereby incorporated by reference, teaches reaction sintered cermets having very fine grain size. The method taught involves the dual effect of reaction between and sintering together of admixed particulate reactants that are shaped and heated at temperatures causing an exothermic reaction to occur and be substantially completed. The reaction products are then sintered by holding the reaction mass at the elevated

temperature attained as a result of the reaction. Thus, this reference relates to a sintered product, suitable for use in contact with a molten metal.

Backerud, in U.S. Patent No. 3,785,807, hereby incorporated be reference, teaches the concept of preparing a master alloy containing titanium diboride for grain refining aluminum. The patentee dissolves and reacts titanium and boron in molten aluminum at a high temperature, but requires that titanium aluminide be crystallized at a lower temperature around the titanium diboride formed. Thus, the patent teaches formation of a complex dispersoid. In recent years, numerous ceramics have been formed using a process termed "self-propagating high-temperature synthesis" (SHS). It involves an exothermic, self-sustaining reaction which propagates through a mixture of compressed powders. The SHS process involves mixing and compacting powders of the constituent elements and igniting a portion of a green compact with a suitable heat source. The source can be electrical impulse, laser, thermite, spark, etc. On ignition, sufficient heat is released to support a self-sustaining reaction, which permits the use of sudden, low power initiation of high temperatures, rather than bulk heating over long periods at lower temperatures. The SHS process occurs at higher temperatures than the present invention and is non-isothermal, yielding sintered ceramic particles having substantial variation in size. In most SHS processes, the product is a ceramic which may be relatively dense for use as a finished body, or may be crushed for use as a powder raw material. Exemplary of these techniques are the patents of Merzhanov et al , U.S. Patent Nos. 3,726,643; 4,161,512; and 4,431,448 among others, hereby incorporated by reference. In U.S. Patent No. 3,726,643, there is taught a method for producing high-melting refractory inorganic compounds by mixing at least one metal selected from Groups IV, V, and VI of the Periodic System with a non-metal, such as carbon, boron, silicon, sulfur, or liquid nitrogen, and heating the surface of the mixture to produce a local temperature adequate to initiate a combustion process. In U.S. Patent No. 4,161,512, a process is taught for preparing

titanium carbide by ignition of a mixture consisting of 80-88 percent titanium and 20-12 percent carbon, resulting in an exothermic reaction of the mixture under conditions of layer-by-layer combustion. These references deal with the preparation of ceramic materials, absent a binder.

U.S. Patent No.4,431„448 teaches preparation of a hard alloy by intermixing powders of titanium, boron, carbon, and a Group I-B binder metal or alloy, such as an alloy of copper or silver, compression of the mixture, local ignition thereof to initiate the exothermic reaction of titanium with boron and carbon, and propagation of the ignition, resulting in an alloy comprising titanium diboride, titanium carbide, and up to about 30 percent binder metal. This reference, however, is limited to the use of Group I-B metals, such as copper and silver, as binders, and requires local ignition. Products made by this method have low density, and are subjected to subsequent compression and compaction to achieve a porosity below about one percent.

U.S. Patent No.4,540,546 to Giessen et al, hereby incorporated by reference, teaches a method for rapid solidification processing of a multiphase alloy. In this process, two starting alloys react in a mixing nozzle in which a "Melt Mix Reaction" takes place between chemically reactable components in the starting alloys to form submicron particles of the resultant compound in the final alloy. The mixing and chemical reaction are performed at a temperature which is at or above the highest liquidus temperature of the starting alloys, but which is also substantially below the liquidus temperature of the final alloy, and as close to the solidus temperature of the final alloy as possible. While dispersion-strengthened alloys can be produced by this technique, there are a number of inherent difficulties. First, processing is technically complex, involving multiple furnaces. Second, efficient mixing is important if fine dispersions are to be consistently produced. Lastly, very high degrees of superheat will be required to completely dissolve the rapid solidification alloying elements, which necessarily accentuates particle growth, for example, in composites containing 10-20 percent dispersoid.

The present invention overcomes the disadvantages of the prior art noted above. More particularly, the present invention permits simplification of procedures and equipment compared to the prior art. For example, the present process obviates the need for multiple furnaces and mixing and control equipment because all of the constituents of the second phase are present in a single reaction vessel. The present invention also overcomes the need for forming complex or multiple melts of components at very high melting temperatures. Further, relatively high loading composites can be prepared without the necessity of achieving high levels of superheat in holding furnaces.

Applicants' invention also provides for a cleaner particle/metal interface compared with conventional metal -cera ic composites made by techniques using, for example, separate metal and ceramic powders, because the reinforcing particles are formed in-situ which leads to a superior product. With these facts in mind, a detailed description of the invention follows, which achieves advantages over known processes.

Summary of the Invention it is an object of the present invention to provide an inexpensive method for forming composite materials, in large quantities, consisting of finely dispersed second phase, such as a particulate ceramic, intermetallic material, or mixtures thereof, in metal, metallic alloy, or intermetallic matrices. For purposes of simplifying further description, the metal, metallic alloy, or intermetallic matrices of the final composite sought shall be referred to as the "composite matrix" or "final matrix".

It is a further object of this invention to provide a method for dispersion hardening of metals and alloys. It is a particular object of this invention to provide a method for preparation of a second phase material, such as ceramic particles, for example, titanium diboride particulates in an aluminum matrix, without the necessity for utilizing expensive submicron second phase starting materials. The second phase-forming constituents provide the desired volume fraction of submicron particulates when reacted in an

appropri ate volume of mol ten metal , molten al loy or mol ten intermetal l ic matrix. The terms "molten matrix metal " and "molten matrix metal , metal al loy, or intermetal l ic" shal l hereinafter be used i nterchangeably when referring to the molten bath of metal lic material to which the reactive second phase-forming constituents are directly added.

The present invention rel ates to a method for the production of metal -second phase composite materi al s, the method compri sing precipitating at least one second phase material in a metal matrix by contacting a preformed mixture of reactive second phase-forming constituents and solvent metal in which the second phase-forming constituents are more sol uble than the second phase material , wi th a substantially nonreactive molten matrix metal at a temperature at which sufficient diffusion of the reactive second phase-forming consti tuents into the substantial ly nonreactive solvent metal occurs to cause the second phase-forming reaction of sai d constituents to thereby precipi tate second phase particles in the molten matrix metal so as to produce a composite comprising finely divided second phase particles in the composi te matrix. The molten matrix metal itself may be of the same composition as the sol vent metal , but in cases where different metal s are used, the solvent metal must be compatible with, and preferably soluble in, the molten matrix metal , resulting in the formation of an alloy of the molten matrix metal as the composite matrix. The present invention further rel ates to a method for the production of metal -second phase composite material s, the method comprising precipitating at least one second phase material in a metal matrix by adding at least one reactive second phase-forming consti tuent to a molten mass containing at least one other reactive second phase-forming constituent and a substantial ly nonreactive solvent metal in which the second phase-forming constituents are more sol uble than the second phase material , at a temperature at which sufficient diffusion of the second phase-forming constituents into the molten mass occurs to cause a second phase-forming reaction of the constituents to thereby precipi tate second phase particles so as to produce a composite comprising finely divi ded second phase particles in a final matrix.

The present invention also relates to composite products comprising uniformly dispersed substantially unagglomerated particles of precipitated second phase material formed in-situ by adding unreacted preformed compacts of second phase-forming constituents and solvent metal to a bath of molten metal, alloy or intermetallic.

In summary, the process of the present invention, which may be referred to as the Direct Addition Process, comprises adding reactive second phase-forming constituents and solvent metal directly to a molten matrix metal and recovering a product comprising unagglomerated particles of the second phase in a final matrix.

Description of the Preferred Embodiments The present invention relates to a method for the preparation of metal -second phase composites by the direct addition of a mixture of second phase-forming constituents and a solvent metal to a molten matrix metal, resulting in the in-situ formation of finely dispersed second phase particles. The second phase-forming constituents may be added as a preform of individual powders of the reactive elements or reactive compounds and solvent metal, a compact of such reactive individual powders and solvent metal, or as alloys of such reactive elements with the solvent metal, which may be either the same as or different than the molten matrix metal. It is to be noted that a solvent metal must be present in the preform, compact or alloy to facilitate the reaction of second phase-forming constituents. The final matrix composition, or composite matrix, may thus be the same as the molten matrix metal, or an alloy of the solvent metal and the molten matrix metal. The various modes of this invention shall be discussed further hereinafter. Thus, by the method of the present invention, it is possible, in one step, to form a second phase material, in-situ, directly in a molten metal bath, which may then be cooled to form a composite.

One embodiment of the present invention for forming the subject composite materials comprises providing a substantially molten or liquid mass of a metal alloy of the solvent metal and at least one

second phase-forming consti tuent, and then adding another consti tuent of the desi red second phase material to the molten mass. Thus, the molten matrix metal al ready contains, as an al loying element, one or more second phase forming consti tuents reactive wi th the constituent or consti tuents which are added. In the presence of solvent metal , an exothermic in-situ precipitation reaction of the reactive elements is initi ated to form and disperse finely divi ded particles of second phase material in a final matrix. Thus, in one embodiment, the molten matrix metal coul d contain one of the consti tuents of the desi red second phase material and the solvent metal , for example, as preformed al loy, and one or more constituents woul d subsequently be added.

In the embodiment wherein one second phase-forming constituent is added to a molten mass containing the sol vent metal and another second phase-forming constituent, it may be necessary to agitate strongly and hold the mixture at temperature for a given time to effect the desired reaction. Examples of this embodiment incl ude the addition of titan um powder to an al uminum- boron melt or to a copper-boron melt, and the addition of boron powder to a superheated titanium-al uminum mel t. It is particul arly to be noted that the second phase-forming reaction proceeds more readily at temperatures where the melt is substantially single phase.

In the preferred embodiment, the constituent or constituents are added incremental ly to the molten matrix metal along wi th sufficient solvent metal to allow the reaction to easily proceed. With this procedure, the molten matrix metal may be different than the added sol vent metal and thus need not be a solvent for the constituents. In a particul arly preferred embodiment the compact is added to the mol ten matrix metal under an i nert atmosphere, and is quickly immersed in the melt to avoi d oxi dation or ejection of the reaction products. In industrial practice, the addition of the compact to the molten matrix metal may be made di rectly in the melting furnace or in a transfer trough of smal ler dimensions just prior to casting. The l atter procedure has a number of advantages including the avoi dance of contamination of large furnaces by second phase material s, the abil ity to induce effective stirring, for exampl e, by

- n - mechanical, inductive, or ultrasonic means, and the ability to control the atmosphere to a greater extent. Mechanical plunger or gas injection techniques may be used to introduce the compact into the molten matrix metal. Suitable gases include inert gases such as argon or helium and reactive gases such as chlorine. In the case of an aluminum molten matrix metal, chlorine, either alone or as a mixture with one of the inert gases, may be used with the added benefit of de-oxidizing the melt.

In the Direct Addition Process, it is preferable that the amount of each constituent added is such that essentially all of the constituents are consumed in the precipitation reaction, i.e., that essentially no unreacted constituent remains after the completion of the reaction. In most instances, this requirement can be met if stoichiometric quantities of the constituents are available in the compact or in the molten mass of metal. However, it may be advantageous to add one constituent above stoichiometric proportion to the compact or the molten mass of metal to essentially eliminate unwanted products which may be formed from the reaction of another constituent and the solvent metal or the matrix metal. For example, titanium aluminide which may be formed in the titanium di bo ride-aluminum composite can be removed by adding additional boron to the molten mass of aluminum, or to the compact prior to addition to the molten mass of aluminum. The boron can be in the form of elemental boron, boron alloy or boron halide. Such a boron addition also provides the benefit that any free titanium, which can adversely affect the viscosity of the melt for casting operations, is converted to titanium diboride.

The present invention is particularly directed to a novel process for the in-situ precipitation of fine particulate second phase materials, including ceramics, such as borides and carbides, and intermetallics, such as aluminides, within metal and alloy systems to produce a metal -second phase composite. However, the process described may also be used for producing larger particles of a second phase material in the matrix metal, up to the point at which such larger particles result in component embrittlement, or loss of ductility, etc. The improved properties of the novel

composites offer weight-savings in stiffness limited uses, higher operating temperature capability and associated energy efficiency improvements, and reduced wear in parts subject to erosion. A specific use of such material is in the construction of turbine engine components, such as blades.

In this context, it should be noted that the metal -second phase products of the present invention are also suitable for use as matrix materials, for example, in long-fiber reinforced composites to enhance transverse modulus, for example, compared to conventional metal matrices. Thus, for example, a particulate reinforced aluminum composite of the present invention may be used in conjunction with long SiC or carbon fibers to enhance specific directional properties. Typical fabrication routes for such materials include diffusion bonding of thin layed-up sheets, and molten metal processing.

In the present invention, a method is taught whereby second phase forming elements are caused to react in a solvent metal to form a finely-divided dispersion of the second phase material in the solvent metal, which is evenly dispersed throughout the final matrix. In accordance with the present invention, the second phase-forming constituents most easily combine at or about the melting temperature of the solvent metal, and the exothermic nature of this reaction causes a very rapid, but localized, temperature elevation or spike, which can have the effect of melting additional solvent metal (if solid solvent metal is present), simultaneously promoting the further reaction of the second phase-forming constituents.

Moreover, the present invention incorporates the novel concept that a bath of molten metallic material may be used to advantage both as a uniform heat source and as a heat sink for in-situ precipitation reactions of second phase in a solvent metal. Thus, when reactant-pl us-sol vent metal compacts are added to, and submerged under the molten matrix metal, rapid and efficient liquid-solid heat transfer into the compact is effected to initiate the second phase-forming reaction. Conversely, following reaction, the same efficient liquid-solid heat transfer serves to rapidly cool

the product to the temperature of the surrounding metal, thereby minimizing particle growth and sintering. It is surprising that a molten metal may be used effectively to both rapidly heat and rapidly cool the same reaction mass. The rapid heat up is important in the prevention of side reactions, such as intermetallic formation during slow heating, while the rapid cooling promotes fine particle size.

Another surprising aspect of the present process is that second phase particle formation may be combined with particle dispersion and composite formation into a single step operation. The prior art suggestion of preparing second phase particles in one operation and combining subsequently with metal, is therefore overcome by the present process. In addition, both the formation and dispersion processes are effected under substantially ideal conditions of fast heat and fast cool. The ensuing dispersion is then highly favorable because the transient exo therm and resultant expansion of absorbed gases causes vigorous agitation and mixing.

Exemplary of suitable second phase ceramic precipitates are the borides, carbides, oxides, nitrides, suicides, sul fides, and oxysul fides of the elements which are reactive to form ceramics, including, but not limited to, transition elements of the third to sixth groups of the Periodic Table. Particularly useful ceramic-forming or intermetallic compound-forming constituents include aluminum, titanium, silicon, boron, molybdenum, tungsten, niobium, vanadium, zirconium, chromium, hafnium, yttrium, cobalt, nickel, iron, magnesium, tantalum, thorium, scandium, lanthanum, and the rare earth elements. Particularly useful additional inter etallic-forming elements include copper, silver, gold, zinc, tin, platinum, manganese, lithium, and beryllium. Preferred second phase materials include titanium diboride, titanium carbide, zirconium diboride, zirconium carbide, zirconium disilicide, and titanium nitride.

As the solvent metal one may use any metal capable of dissolving or sparingly dissolving the constituents of the second phase, and having a lesser capability for dissolving the second phase

precipitate. Thus, the solvent metal component must act as a solvent for the specific reactants, but not for the desi red second phase precipitate. It is t be noted that the solvent metal acts primarily as a sol vent for the reactive consti tuents in the process of the present invention, and that the constituents of the second phase precipi tate have a greater affini ty for each other than either has for the sol vent metal , which does not react si gnificantly with the second phase dispersoi ds wi thin the time frame of the exothermic excursion. Suitable solvent metal s incl ude al uminum, nickel , titanium, copper, vanadium, chromium, manganese, cobalt, iron, silicon, molybdenum, beryl l ium, silver, gol d, pl atinum, tungsten, antimony, bismuth, magnesi um, lead, zinc, tin, niobi um, tantalum, hafnium, zirconium, and alloys thereof. As previously indicated, the solvent metal may be the same as or different than the molten matrix metal in which the present Direct Addi tion Process is carried out. When the solvent metal differs from the molten matrix metal , the temperature of the molten matrix metal must be above the melting point of the solvent metal , and there must be sufficient miscibi l i ty of the two l iquid metal s to ensure alloying/combination. Under these c rcumstances, it is possible, for example, to form second phase material in a particul ar sol vent metal (that may be particul arly suitable for formation of particul ates of a certain morphology, for example) and then disperse the second phase into a different matrix which may not be sui ted to particle formation. In certain instances, the molten matrix metal may comprise material other than conventional metal s, metal al loys or ntermetall ics. The molten matrix metal may, for example, be a dispersion strengthened metal such as metal containing finely dispersed erbium oxi de, thoria, al umina, etc. Similarly, it is possible to use a metal -second phase composi te, such as those prepared in accordance wi th the present invention, or in accordance with co-pending appl ication, Serial No . 662,928 and co-fi led appl ication, Serial No . , as the molten matrix metal . It is important in these cases that the preexisting dispersion be stable n the molten metal for the time /temperature required for introducing the desi red material . The advantage of util izing a material containing a second phase dispersion as the molten matrix

metal is that a bimodal distribution of second phase types, shapes, amounts, etc. may be obtained. An example would be the use of an aluminum matrix containing a dispersion of essentially equiaxed titanium diboride particles, to which the constituents necessary to form needle shaped Ti particles are added. A combination of dispersion strengthening and high temperature creep resistance is obtained. In accordance with the foregoing discussion, it must be understood that suitable molten matrix metals encompass the types of materials discussed above containing preexisting second phase dispersions;

Suitable molten matrix metals include the aforementioned aluminum, nickel, titanium, copper, vanadium, chromium, manganese, cobalt, iron, silicon, molybdenum, beryllium, silver, gold, tungsten, antimony, bismuth, platinum, magnesium, lead, zinc, tin, niobium, tantalum, hafnium, zirconium, and alloys of such metals. The molten matrix metal may be the same as the solvent metal, an alloy of the solvent metal, or a metal in which the solvent metal is soluble. Further, the molten matrix metal may be any metal that wets the second phase precipitate, in which the second phase precipitate is not soluble, and with which the second phase does not react during subsequent fabrication, and/or recasting. When alloys are utilized as the molten matrix metal, one may substantially retain the beneficial properties of the alloys, and increase, for example, the modulus of elasticity, high temperature stability, and _W ear resistance, although some loss of ductility may be encountered in certain soft alloys. For example, 7075 aluminum alloy, containing from about 5 percent to about 40 percent by weight titanium diboride, shows the same beneficial effect of age-hardening as 7075 alloy alone, but exhibits a substantial increase in modulus of elasticity, higher temperature capability, greater high temperature stability, and extremely high wear resistance. Further, the composites of the present process may be fabricated in substantially conventional fashion, by casting, forging, extruding, rolling, machining, etc. The composites of the present process may also be remelted and recast while retaining substantial uniformity in second phase particle distribution, retaining fine second phase

particle size, fine grain size, etc. , thereby maintaining associated improvements in physical properties. Aside from the obvious benefi ts in subsequent processing and fabrication, the abi l ity to remel t and recast these material s permi ts recycl ing and reuse thereof , unl ike known prior art metal -ceramic composi tes.

In accordance wi th the present invention, the Direct Addition Process enables preparation of master alloys containing a second phase concentrate, which may, in turn, be util ized to introduce the second phase into another metal in controlled fashion. It is therefore possible, by the Direct Addition Process of the present invention, to produce a composi te of rel atively hi gh second phase concentration, wherein the second phase particles formed have a desired shape, size, or morphology, and to subsequently introduce such composite into another metal to produce a composite having a desired lower concentration of second phase. For example, a master alloy may be prepared having a hi gh percentage of a second phase material , such as titanium dibori de, in a matrix metal , such as aluminum. This master al loy may then be added to a molten metal , metal alloy or intermetall ic bath, (which molten metal may be the same or different from the matrix metal of the master al loy) to achieve a composite having the desi red concentration of second phase. It is therefore possible to form composites of rel atively concentrated second phase content, wherein the second phase particles are protected from oxide or other deleterious covering l ayers which form on prior art ceramic powders, and to then introduce the composites into molten metal to redisperse the second phase, thus producing composi tes having uniform dispersions of second phase particles of desired concentrations. Additionally, with the concentrate addition procedure, it is possible to form the second phase material in a matrix metal which is conducive to the formation of particles of a desired type, size, and morphology, and thereafter i ncorporate the particles in a further molten metal in which such particles cannot be produced by the in-situ precipi tation reaction, for example, due to rel ative differences in mel ting point. Varying amounts of the second phase forming material may be incorporated into the composi te matrix, depending upon the end use

and the properties desired in the product. For instance, to produce dispersion strengthened alloys having high modulus, one may utilize a range of about 0.1 to about 30 percent by volume, and preferably from about 5 percent by volume to about 25 percent by volume, and more preferably from about 5 to about 15 percent by volume of second phase forming constituents. However, the second phase volume fraction for dispersion strengthening may be varied considerably, to produce a composite with the desired combination of properties, within the range of from about 1 percent by volume of dispersoid up to the point at which ductility is sacrificed to an unacceptable extent. The primary determining factors of the composition of the composite will be the intended use of the products. Thus, for example, for use in cutting tools the properties will be the hardness and wear resistance of the composite material produced. It is possible to effectively tailor the composition to achieve a range of desired properties by controlling the proportions of the reactant and solvent materials.

In addition to controlling the second phase amount, it is possible to effectively engineer materials by manipulating the types of matrix and second phase. Thus, for example, a reinforced matrix may be obtained by using a dispersion strengthened metal or a metal -second phase composite as the molten matrix metal. Another dispersoid type could then be added, for example as high aspect ratio needles. Depending on the final engineering requirements for the product, a wide variety of such controls may be applied to tailor the type, amount, morphology, etc. of matrix and second phase. The Direct Addition Process of the present invention may be used to further produce high purity ceramic powders of desired particle size, morphology, and composition which are expensive and in some cases impossible to produce using prior art techniques. This may be achieved by dissolving the final matrix away from the final composite, leaving the second phase dispersoids, which, due to in-situ precipitation, inherently possess superior properties over prior art ceramic powders. For example, a final composite produced by the present Direct Addition Process, containing titanium diboride dispersoids in an aluminum final matrix, may be immersed in

hydrochloric acid to dissolve the al uminum matrix, l eaving titani um diboride particles having very small size, e. g. 0.1 micron.

It is bel ieved that the prior art technique of introduction of fine second phase particles directly to a molten metal bath is technically difficult and produces metal products having l ess desirable properties upon sol idification due to a deleterious l ayer, such as an oxi de, which forms on the surface of each second phase particle at the time of or prior to introduction into the mol ten metal bath. The second phase particl es of the present invention, being formed i n-situ, do not possess this deleterious coating or l ayer. Thus, the present invention produces metal products having unexpectedly superior properties.

In the most preferred embodiment of the present invention, the reactive constituents are compacted wi th solvent metal powder so as to provi de intimate contact of the reactive consti tuents wi th the solvent metal . It must be noted that this approach may be used even when the solvent metal and the molten matrix metal are the same. As an example of the most preferred embodiment, aluminum powder may be compacted wi th titanium and boron powders to form compacts, which may then be added to a mol ten matrix metal such as copper or aluminum.

In one mode of the invention, indivi dual alloys may be reacted, one such alloy comprising an al loy of the sol vent metal with one of the consti tuents of the second phase, and the other comprising an al loy of the same solvent metal , or another metal wi th which the solvent metal readily alloys, with the other constituent of the second phase. As an example of using two alloys of a common metal , a mixture of aluminum-ti tani um alloy and aluminum-boron alloy may be added to a molten al uminum bath to form a dispersion of titanium dibori de in the molten aluminum. This al loy -al loy reaction route may, in some cases, be rel atively slower than the elemental route, yet may offer economic advantages because the alloys util ized can be cheaper than the elemental powders. In addition, when two phase alloys are util ized, it is general ly preferred that the molten matrix metal be superheated sufficiently to substanti ally complete the reaction. Further, it may be necessary to sustain the elevated temperature for substantial periods of time.

It is particularly to be noted that the prior art teaches that the combination of elemental metal or alloy powders, particularly of a coarse particulate size, would yield intermetallic compounds. In fact, conventional techniques for forming intermetallics involve, for example, reacting a mixture of titanium and aluminum, to form titanium aluminide, and a mixture of boron and aluminum to form aluminum diboride. Thus, one would expect that a mixture comprising powders of titanium, aluminum, and boron would yield an aggregate agglomeration of titanium aluminide, aluminum diboride, and possibly, titanium diboride. In contrast, the present invention provides for the production of essentially just one finely dispersed precipitate from the two reactive components in a final matrix of the third component or an alloy thereof. It is important that the second phase precipitate material not be soluble in the solvent metal, while the constituents of the second phase, individually, are at least sparingly soluble in the solvent metal. Thus, the exothermic dispersion reaction mechanism depends upon a certain amount of each second phase-forming constituent dissolving and diffusing in the solvent metal, and while in solution (either liquid or solid state), reacting exother ically to form the insoluble second phase, which precipitates rapidly as a fine particulate. The solvent metal provides a medium in which the reactive elements may diffuse and combine. Once the initial reaction has occurred, the heat released in the exothermic reaction and the expansion of absorbed gases, etc. enhance agitation, and hence dispersion of precipitated second phase in the molten matrix metal.

Regarding impurities, the molten matrix metal and the solvent metal may be alloyed in conventional manner, while in the reactive constituents, large amounts of alloying elements or impurities may cause problems in certain instances. For example, the presence of large amounts of magnesium in boron may inhibit the formation of titanium diboride in an aluminum matrix by forming a magnesium-boron complex on the surface of the boron particles, thus limiting diffusion of the boron in the solvent matrix. However, the presence of magnesium in the aluminum does not have this effect. That is, boride forming materials in the boron itself may inhibit the desired dissolution or diffusion of the boron and its subsequent reaction to

form titani um dibori de. Likewi se, thick oxi de fi lms around the starting constituent powders may al so act as barriers to diffusion and reaction. Extraneous contaminants, such as absorbed water vapor, may al so yield undesirable phases such as oxi des or hydri des, or the powders may be oxi dized to such an extent that the reactions are infl uenced.

It is al so to be noted that, in accordance wi th the present invention, the complex precipitation of a pl ural ity of systems may be caused. Thus, it is possible to precipitate complex phases, such as Ti (B _Q JC _Q 5) , or alternatively, to precipitate a mixture of titani um dibori de and zi rconi um di bori de in an aluminum matrix by direct addition of the appropri ate constituents, in accordance with the overall reaction:

Ti + Zr + 4B + Al — > TiB _£ + ZrB ₂ + AT . Substitution of titanium by zirconium, or vice versa, is al so possible, yielding bori des of the general type (Ti It is al so possible to achieve a low temperature solvent assisted reaction in a compact containi ng a metal which has a hi gh melting temperature by al loying or admixing the high melting metal with a lower melting solvent metal . For example, titanium dibori de is precipi tated at very low temperatures, such as 620°C, in cobalt, chromium, and nickel by incl uding up to 20 percent by weight al uminum. In the absence of the al loying sol vent al uminum, the reaction requires temperatures of about 900°C or greater. The low temperature reaction may be useful where it is desired to minimize the maximum temperature reached during the exothermic reaction, or to minimize the time before reaction initiation. In either case, the severity of the time /temperature treatment is reduced and there may be reduced inci dence of particle growth and si ntering. As previously discussed, in the preferred embodiment of the present invention, it is desirable to form a green compact of second phase-forming constituents and solvent metal or al loy constituents in reactant concentrations outl ined above for immersion in a molten matrix metal bath. The immersion in the molten bath effects formation of the second phase, which rapi dly disperses in the bath.

The starting powders of the prior art, such as ceramics, typically suffer from extensive oxidation due to exposure to the atmosphere, which weakens the interfacial bonding of the ceramic into the metal matrix. The method of the present invention, however, circumvents this problem because the second phase particles are formed in-situ within the solvent metal, and thus are largely protected from the formation of a deleterious oxide layer or coating.

The particle size of the second phase reaction product is dependent upon heat-up rate, reaction temperature, cool -down rate, crystal unity and composition of the starting materials.

Appropriate starting powder sizes may range from less than 5 microns to more than 200 microns. For economic reasons, one normally may utilize larger particle size powders. It has been found that the particle size of the precipitated second phase in the matrix may vary from less than about 0.01 microns to about 5 microns or larger, dependent upon such factors as discussed above.

Particle size considerations have a direct impact on the grain size of the metal -second phase composite product because the grain size is generally controlled by the inter par tide spacing of the dispersoid. The inter particle spacing varies with the volume fraction and size of the dispersoid. Thus, relatively high loadings of very fine second phase particles produce the finest grained product materials. Typically the grain size of the product of the present invention is in the vicinity of one micron for second phase volume fractions between 5 percent and 15 percent. Fine grain size is extremely important, for example, in precision casting and in applications where fatigue resistance is required. By way of illustration, it is known in the manufacture of jet engine compressor disks that fine grain size and low porosity must be achieved. To initially reduce porosity of conventional materials, the cast product is subjected to hot-isostatic pressing. However, the severity of the time/temperature treatment that can be applied is limited by the grain growth that results from long times at high temperature. Accordingly, a compromise quality is obtained between.

sufficient densifi cation while mi nimizi ng grain growth. The composi te products of the present invention may be used to advantage in appl ications like this because the stable fi nely dispersed array of precipi tates pi ns grain boundaries, thereby minimizing grain growth. Accordingly, in the above example, higher temperatures and longer times are possible in the hot-isostatic-press to reduce porosity without compromising grain size.

It has been found that some specific reactant properties have a greater impact than powder particle size on the particle size of the second phase produced. For example, the use of amorphous boron may result in the precipitation of a fi ner particle size titanium dibori de than- does the use of crystal l ine boron in an otherwise comparable mixture. The precipitation of specific particle size second phase may be selectively control led by proper control of starting composition, temperature of reaction, and cool -down rate. The cool -down period fol lowing ini tiation of the reaction and consumption of the reactive constituents is bel ieved important to achieving very small particle size, and limiting particle growth. It is known that at high temperatures, it is possible for the second phase particles to grow, for example, by dissolution-precipitation mechanisms. This shoul d al so be avoided, in most cases, because of the negative effect of l arge particle sizes on ductil ity. Rapi d temperature elevation of the molten matrix may be avoided in the Di rect Addition Process by controlling the rate of addi tion of reactive constituents. While the temperature elevation is somewhat local ized to the area in proximity to the compact added, the total molten matrix metal bath may be extensively heated when higher concentrations of second phase are produced. The cool-down or quenchi ng of the reaction is, in a sense, automatic, because once the second phase-forming constituents are completely reacted, there is no further energy released to maintain the high temperature achieved and the l arge thermal mass of metal provi des a quenching effect. However, one may control the rate of cool-down to a certain extent by control of the size and/or composi tion of the mass of material reacted, for example, by control l ing the size of the constituent and solvent metal containing compact which is added to

the molten matrix metal. That is, large thermal masses absorb more energy, and cool down more slowly, thus permitting growth of larger particles, such as may be desired for greater wear resistance, for example, for use in cutting tools. An advantage of the present invention is that, if the constituents are added to a relatively large pool of molten metal in a step-wise or incremental addition, for example, the temperature of the molten matrix metal will not change significantly during the course of the addition. Thus, potential particle growth of the second phase particles will be minimized since the elevated temperatures will only occur locally, will be quenched rapidly by the large thermal mass, and will be minimized in the bulk of the melt. Such an addition procedure is also advisable from a safety standpoint to prevent the rapid evolution of significant quantities of heat which could cause metal to be splattered, sprayed or boiled from the containment vessel. Another advantage is that the exothermic reaction of the constituents, forming the second phase material, occurring in the molten mass creates a mixing effect. This, together with the concomitant expansion of adsorbed and produced gases, aids in dispersing the second phase material throughout the mass. In addition, by having the mass molten or liquid upon addition of the constituents , the constituents are rapidly heated to reaction temperature. This promotes the formation of fine second phase particles. A further important consideration of this process is that because a molten mass of matrix metal is utilized, the matrix metal need not be formed from powdered metal, but may be formed from ingot, scrap, etc., thus resulting in a significant saving in material preparation costs.

As described previously, in selecting the constituents and the matrix metal for the composite materials produced by the Direct Addition Process, it is important that the formed second phase material have a low solubility in the molten matrix metal, for example, a maximum solubility of 5 weight percent, and preferably 1 percent or less, at the temperature of the molten mass. Otherwise, significant particle growth in the second phase material may be experienced over extended periods of time. For most uses of the

composite material s, the size of the second phase particles shoul d be as smal l as possible, and thus particle growth is undesi rable.

When the sol ubil ity of the formed second phase material in the molten matrix metal is low, the molten matrix metal with dispersed second phase particles can be mai ntained in the molten state for a consi derable period of time wi thout growth of the second phase particles. For example, a molten mass of aluminum containing dispersed titanium dibori de particles can be maintained in the molten state for three to four hours without appreci able particle growth.

Wi th certain combinations of constituents and matrix metal s, one or more of the constituents may tend to react wi th the molten matrix metal as the added constituent is heated up to the temperature at which the second phase-forming reaction occurs. This reaction product of the constituent and the molten matrix metal may be undesirable in the final composite and, in any event, reduces the amount of constituent avail able for the in-si tu second phase-forming reaction. For example, when adding ti tanium and boron to molten aluminum, titanium al umini des (e.g. Al - _j Ti ) and aluminum dibori de may be formed as the titanium and boron are heated to reaction temperature. When titanium reacts wi th aluminum, the formed ti tanium aluminide, if present as l arge pl ates, may be deleterious in the final compos te and results in a lower than desi red concentration of titanium dibori de. To help prevent the formation of such undesi rable reaction products, the added constituent or constituents may be provided with a thin barrier l ayer to retard contact of the constituents wi th the molten matrix metal until the constituents reach the reaction ini tiation temperature at which the formation of the desi red second phase material begins. The composition of such a barrier layer woul d, of course, depend upon the particul ar material s being util ized and shoul d be selected so that undesirable products are not generated. Generally, the barrier l ayer should prevent, or at least retard, wetting of the added constituents by the molten matrix metal .-

The following example teaches the preparation of a composite material by direct addition of the ceramic forming constituents to a molten mass of matrix metal.

Example 1 An aluminum 2014 alloy is loaded in a crucible and melted to a completely molten mass. A compacted but unreacted mixture containing about 41.4 weight percent titanium, about 18.6 weight percent boron, and the remainder aluminum is prepared from powders having a particle size greater than 20 microns. The compacted mixture is then added to the molten aluminum mass. A rapid reaction occurs which results in the formation of about 23 weight percent titanium diboride dispersed in the aluminum alloy. Subsequent SEM analysis of the solidified composite material determines that the particle size of the dispersed titanium diboride is less than 1 micron, considerably smaller than the particle size of the starting titanium and boron constituents.

The following example demonstrates the use of boron nitride to coat a titanium, boron, and aluminum containing compact prior to addition to molten aluminum matrix metal.

Example 2 Titanium, boron, and aluminum powders are mixed in a ball mill in the proper stoichiometric proportions to provide 80 weight percent titanium diboride forming constituents and 20 weight percent aluminum. 752 grams of this mixture are then placed in a tube and isostatically pressed to 40 ksi to form a compact. Pieces of the compact are coated with boron nitride from a spray can and then added to 2.3 kilograms of molten aluminum matrix metal at 920°C. The pieces of compact are observed to react more slowly and less violently than uncoated compact pieces under similar conditions. After all of the compact is reacted, the melt is mechanically stirred for several minutes and then poured at 950°C. The resultant composite is found to comprise approximately 20 weight percent titanium diboride second phase in an aluminum final matrix.

The fol lowing example il lustrates the use of one second phase-forming constituent above stoichiometric proportion and the abil ity to remel t and recast composites made by the process of the present invention ÷

Example 3

11 .3 kilograms of aluminum, 3.90 kilograms of copper master al loy (20 percent Cu in Al ), and 1 106 ki lograms of magnesium master alloy (25 percent Mg in Al ) are heated in an induction furnace to 850°C to form a molten matrix metal . 7.39 ki lograms of compact, prepared as in Example 2, except for the addition of 4 wei ght percent excess boron, are coated wi th boron ni tri de, slowly added to the molten matrix metal , and the melt stirred: The resul tant composite is then cast in conventional manner at 935°C to form an ingots Analysis of the composite reveal s a uniform dispersion of ti tani um dibori de second phase particl es in a 2024 aluminum al loy final matrix, with substantially no ti tanium aluminide present. The ingot is then remelted and recast. The resulting composite is analyzed and found to comprise a uniform dispersion of titanium dibori de particles, having substantial ly the same average particl e size as the original composite, in a 2024 aluminum alloy matrix.

The following example demonstrates the abil ity to disperse ti tanium diboride second phase particles throughout a 6061 aluminum alloy final matrix.

Example 4 15;3 kilograms of aluminum are melted in an induction furnace.

212 grams sil icon master alloy ( 50 percent Si in Al ) and 709 grams of magnesium master alloy (25 percent Mg in Al ) are added, and the temperature raised to 850° C_ 7.38 kilograms of compact, prepared as in Example 2, wi th the addition of 4 weight percent excess boron, are coated with boron nitri de and slowly added to the molten matrix metal . The mel t is stirred, fluxed, skimmed, stirred again, and then cast in conventional manner. Analysis of the resultant composite reveal s approximately 25 weight percent titani um dibori de particles uniformly di spersed in a 6061 aluminum alloy final matrix.

The following example demonstrates the ability to disperse titanium carbide second phase particles throughout an aluminum final matrix.

Example 5 Titanium, carbon, and aluminum powders are mixed in a ball mill in the proper stoichiometric proportions to provide 60 weight percent titanium carbide forming constituents and 40 weight percent aluminum. The mixture is placed in a tube and isostatically pressed to 40 ksi to form a compact. The compact is then added to molten aluminum matrix metal at 750°C in the proper proportion to yield approximately 10 volume percent titanium carbide second phase in an aluminum final matrix. The resultant composite is then cast in conventional manner. Optical microscopy reveals a substantially uniform dispersion of titanium carbide particles in aluminum. It is noted that the present invention has a number of advantages over methods taught by the prior art. For example, this invention circumvents the need for submicron, unagglomerated refractory metal boride starting materials, which materials are not commercially available, and are often pyrophoric. This invention also eliminates the technical problems of uniformly dispersing a second phase in a solvent metal, and avoids the problem of oxide or other deleterious layer formation at the second phase/metal interface during processing. Further, the present invention yields a metal matrix composite with a second phase precipitated therein, having superior hardness and modulus qualities over currently employed composites, such as SiC/aluminum. The metal matrix composite also has improved high temperature stability, in that the second phase is not reactive with the final matrix. Further, the metal matrix composite can be welded without degradation of material properties, and the weldments possess superior corrosion resistance, when compared to any metal matrix composites presently available. In addition, as opposed to composites presently available, the metal -second phase composite prepared by the method of the present invention can be remelted and recast while retaining fine grain size, fine particulate size, and the resultant superior physical properties associated therewith.

It is understood that the above description of the present invention is susceptible to consi derable modification, change, and adaptation by those skil led in the art, and such modifications, changes, and adaptations are intended . to be consi dered to be wi thin the scope of the present invention, which i s set forth by the appended cl aims.