CA2054018A1

CA2054018A1 - Metal matrix composite composition and method

Info

Publication number: CA2054018A1
Application number: CA002054018A
Authority: CA
Inventors: Thomas Wesley Gustafson; Charles George Fick, Iii; Dale Allen Gerard; Anil Kumar Sachdev
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1991-02-25
Filing date: 1991-10-23
Publication date: 1992-08-26
Also published as: JPH0586425A; EP0501539A2; EP0501539A3

Abstract

METAL MATRIX COMPOSITE COMPOSITION AND METHOD
Abstract of the Invention A new metal matrix composite of an aluminum based alloy and a ceramic, and a method of making it are provided.
In the preferred method, the ceramic is silicon carbide whiskers which are heated to an elevated temperature, generally in the range of between about 750°F and 2000°F. An alloy, comprising by weight about 3 to 6 percent copper, about 0.5 to 5 percent magnesium and the balance essentially aluminum, is heated to melt the alloy. The heated silicon carbide and molten alloy are mixed or intermingled. The intermingled silicon carbide and molten alloy are then cooled at a rate sufficient to sustain supersaturation of the copper and magnesium in the aluminum down to a predetermined temperature. The predetermined temperature is selected so as to permit precipitation of a strengthening copper-rich secondary metallic phase containing copper, magnesium and aluminum and consisting essentially of about 40 to 80 percent by weight copper, magnesium in an amount between about 5 and 30 percent by weight, and the balance essentially aluminum. This forms a metal matrix composite having a silicon carbide phase, an aluminum-rich primary metallic phase and a copper-rich secondary metallic phase which has the desired composition. The primary metallic phase can contain up to 10 percent of eutectic phase which is generally present as a coarse network or as isolated islands.
Preferably, cooling occurs immediately after the intermingling step and relatively rapidly to a temperature below about 550°F where precipitation of the secondary metallic phase occurs.
In a preferred embodiment, a metal matrix composite of the invention comprises a ceramic phase distributed substantially uniformly throughout the composite, and a metal phase which comprises an aluminum-rich primary metallic phase and a copper-rich and magnesium-rich secondary metallic phase distributed throughout the primary metallic phase. The secondary metallic phase contains copper, magnesium and aluminum and consists essentially of about 40 to 80 percent by weight copper, magnesium in an amount between about 5 and 30 percent by weight, and the balance essentially aluminum. Preferably, the secondary metallic phase comprises cubical-shaped structures which are about 400 angstroms on a side. Preferably, the ceramic phase comprises silicon carbide whiskers.

Description

METAL MAT~IX COMPOSITE COMPOSITION AND METHOD
Field of the Inv~n~ion This invention relates to a metal matrix composite which has a ceramic phase and an aluminum alloy phase and a method of making it.
Back~round of the Invention There is a continuing need to lighten and strenythen motor vehicles and aircraft. As a result, structural members and other parts are being constructed from lighter materials such as aluminum and magnesium. Despite the advantage of light weight possessed by these materials, they have relatively inferior mechanical characteristics, such as lower yield strength, wear resistance and tensile strength.
Further, aluminum, magnesium and alloys thereo~ are known to be relatively notch-sensitive and subject to crack propagation. These deficiencies limit the application of aluminum, magnesium and their alloys.
Therefore, it is desirable ~or such parts to be formed from composite materials having a metal phase and a reinforcement phase.
A composite structure is one which comprises heterogeneous material, that is, two or more diferent materials which are intimately combined in order to attain desired properties of the composite. For example, two different materials may be intimately combined by embedding one in a matrix of the other or impregnating the one with the other.
Metal matrix composites comprise a metal phase and a strengthening or reinforcing phase, such as ceramic particulates, whiskers and/or fibers. In a 2 ~ 8 metal matrix material, the reinforcing phase is typically dispersed, distributed and/or embedded in the metal phase. Generally, a metal matrix composite will show an improvement in such properties as strength, sti~fness, contact wear resistance, and elevated temperature strength retention as compared to the metal material alone. Metal matrix composites show great promise for a variety of applications because they combine the strength and hardness of the strengthening phase with the ductility of the metal phase.
Aluminum matrix composites reinforced with a ceramic such as alumina (Al2O33 or silicon carbide ~SiC) are of particular interest. Such metal matrix composites potentially may provide the advantages of the matrix alloy workability (i.e. aluminum is known for its weight and workability advantages) while avoiding disadvantages with regard to low strength and crack propagation. The strength of a properly ~ormulated composite material is relatively high compared to the same aluminum alloy without the reinforcing phase. Moreover, the reinforcing ceramic material should prevent the propagation of cracks through the composite. However, enhanced properties and an efficient, economical method to form the composite are needed.
It has been suggested that metal matrix composites be formed by a number of methods which intermingle the ceramic and metal alloy together so that the ceramic phase is distributed throughout the composite. In one proposed method, the alloy is first melted and then stirred while fibers are added. The : . , '~

:, stirred mixture is then permitted to cool. Another proposed method includes powder metallurgy techniques, where the metal, in the form of a powder, and the ceramic reinforcing material, in the form of particles, whiskers or fibers, are mixed and then either hot pressed or extruded. Still other proposed methods for intermingling the materials include high pressure casting or infiltration processes. In an infiltration method, the ceramic is formed into a structure and molten metal is injected under the force of pressure into voids or interstices in the structure.
It has been suggested that a bonding interface exists between the metal matrix and the ceramic reinforcement. It also has been suggested that the strength of the interface is related to the composition of the metal and good wetting of the ceramic by the metal during formation of the composite.
There is a need for an improved composition for the metal phase which provides a metal matrix composite product having desired enhanced mechanical propertiès and good bonding at the interface between the ceramic and metal phases. There is also a need for a new method ~or forming metal matrix composites which produces such an improved composition and the desired enhanced properties.
Sum~ary of the Invention A new metal matrix co~posite of an aluminum based alloy and a ceramic, and a method of making it are provided.
In the preferred method, silicon carbide ceramic is heated to an elevated temperature, generally .:.

3 ~ ~ g in the range of between about 750F and 2000F. An alloy, comprising by weight about 3 to 6 percent copper, about 0.5 to 5 percent magnesium and the balance essentially aluminum, is heated to melt the alloy. The heated ceramic and molten alloy are mixed or intermingled. The intermingled ceramic and molten alloy are then cooled at a rate sufficient to sustain supersaturation of the copper and magnesium in the aluminum down to a predetermined temperature. The predetermined temperature is selected so as to permit precipitation of a secondary metallic phase containing the three elements copper, magnesium and aluminum. The secondary ~etallic phase essentially consists of between about 40 to 80 percent by weight copper, ma~nesium in an amount between about S and 30 percent by weight, and the balance essentially aluminum. This forms a metal matrix composite having an aluminum-rich primary metallic phase and a magnesium containing copper-rich secondary metallic phase which has the desired composition. The primary metallic phase can contain up to lO percent of eutectic phase which is generally present as a coarse network or as isolated islands.
Preferably, cooling occurs immediately after the step of intermingling and proceeds sufficiently rapidly to a temperature below about 550F before precipitation occurs. Hence, essentially all of the precipitation occurs at a temperature below about 550F
and the precise composition of the precipitate will depend on the temperature(s) selected. During fabrication of components it is often not practically possible to conduct a controlled, rapid cooling or quench immediately after intermingling. Therefore, the constraints of the manufacturing process may require that the step of controlled cooling be deferred. If such a deferral causes the composite to cool at an uncontrolled rate, the composite must then be reheated to a sufficiently high temperature as to dissolve or solubilize the copper and magnesium in the aluminum including that present in the eutectic phase. This can readily be accomplished by heating the composite to between about 900F and 1000F preferably for between about 8 and 36 hours. Longer times or mechanical working could be used to insure that the eutectic phase is completely dissolved. After ~eheating, the cooling step immediately follows, where t:he composite cools at a rate sufficient to sustain supersaturation of the copper and the magnesium in the aluminum preferably to a temperature below about 550F to produce the desired precipitate below about 550F.
Preferably, the aluminum alloy melt consists of at least about 85 percent aluminum, about ~ to 5 percent copper, about 1.5 to ~.5 percent magnesium by weight, and the method includes the step of forming a self-supporting heated preform of a ceramic.
Preferably, the preform is of a silicon carbide ceramic which is intermingled with melted alloy in an infiltration-type intermingling or mixing process, by applying about 11,500 PSI of pressure to a surface of the molten aluminum alloy distal from the silicon carbide preform so as to force the alloy into the interstices of the preform (i.e. impregnate the 2 Q ~

preform).
In a preferred embodiment, a metal matrix composite product is formed in accordance with the invention, having a silicon carbide ceramic phase distributed substantially uniformly throughout the metal phase. Although silicon carbide ceramic is preferred, other ceramics, such as crystalline alumina, crystalline alumina-silica and glass, may also be selected. The metal phase comprises the aluminum-based primary metallic phase and the secondary metallic phase distributed essentially uniformly throughout the primary metallic phase wherein the secondary metallic phase contains the three elements copper, magnesium and aluminum and consists essentially of about 40 to 80 percent by weight copper, magnes.ium in an amount between about 5 and 30 percent by weight, and the balance essentially aluminum.
Desirably, the secondar~ metallic phase has cubical shaped structures which are between about 300 to 500 angstroms or 30 to 50 nm on a side, and preferably 400 angstroms (40 nm) on a side and comprises at least about 50 percent by weight copper.
Preferably, the secondary metallic phase has a volume fraction which is up to about 5 percent of the volume frac~ion of the metal phase; and the primary - metallic phase includes alpha-aluminum with a volume fraction of at least about 95 percent of the volume fraction o~ the metal phase. The primary metallic phase can contain up to 10 percent of eutectic phase which is generally present as a coarse network or as isolated islands.

Desirably, the ceramic phase comprises silicon carbide particles and an oxide. The oxide may be present in the form of an oxygen containing binder, (i.e. SiO2) admixed with silicon carbide particles.
Preferably, the ceramic phase comprises silicon carbide particles and oxides thereof, (i.e. SiO2) formed by surface oxidation of the SiC.
Objects, features and advantages of this invention are to provide a unique metal matrix composite with enhanced mechanica. properties and a process of making it, which is efficient and economical, and which facilitates the manufacture of composite components.
It is also an object to provide a unique metal matrix composite having a metal phase comprising an alloy: which enhances the mechanical properties of the composite; which provides the advantages of relativel~
high strength and low weight as compared to other materials commonly used to form articles, such as automotive, boat, airplane and other parts; which provides high strength and low weight advantages needed to increase fuel economy and recluce fuel consumption;
and which is readily adaptable to the process of casting parts.
2~ Brief Description of the Drawings These and other objects, features and advantages of this invention will be apparent from the following detailed description, appended claims and accompanying drawings in which:
Fig. 1 is a transmission electron micrograph of a metal matrix composite embodying the invention.
; 7 , .

:, Fig. 2 is an apparatus used in a method of the invention.
Fig. 3 is a diagram o~ oxide layer formation as a function of temperature.
5Fig. 4 is a diagram of atom percent of magnesium as a function of distance from an interface.
Fig. 5 is a diagram of uniaxial tensile strength as a function of weight percent magnesium.
Fig. 6 is a diagram of composite strength compared to matrix strength.
Fig. 7 is a phase diagram of a Mg-Cu-Al system.
Detailed Description of the Preferred Embodiments In a preferred embodiment a metal matrix composite of the invention 10, as shown in Fig. 1, comprises a ceramic phase 11 dist:ributed substantially uniformly throughout the composil:e 10, and a metal phase 13 which comprises an aluminum-based primary metallic phase 14 and a secondary metallic phase 15 distributed throughout the primary metallic phase 14.
The secondary metallic phase 15 contains copper, magnesium and aluminum and consists essentially of about 40 to about 80 percent by weight copper, magnesium in an amount between about 5 and 30 percent by weight and the balance essentially aluminum.
Preferably, the secondary metallic phase 15 ccmprises cubical-shaped structures which are about 400 angstroms on a side. The ceramic phase 11 comprises particles preferably in the form of fibers or single crystal whiskers having an aspect ratio (i.e. length to diameter ratio), greater than 3 to 1 and preferably 2 ~ ~3 L~

greater than 10 to 1. Preferably, the ceramic phase 11 is of a silicon carbide material, however, other ceramics, such as alumina, alumina-silicate glasses, crystalline alumina-silica and the like may be used.
The preferred method of making the metal matrix composite of the invention 10 includes the steps o~:
a) heating the ceramic to a temperature between about 750F and 2000F to preclude chilling of the melt upon contact with the ceramic and promote better wetting of the ceramic by the melt;
b) melting an alloy comprisinq, by weight, about 3 to about 6 percent copper, about 0.5 to about 5 percent magnesium and the balance essentially aluminum;
c) intermingling the heated ceramic with the melted alloy; and d) cooling the intermingled ceramic and alloy at a rate sufficient to sustain supersaturation of the copper and magnesium until a predetermined temperature is reached, the predetermined temperature being selected so as to permit precipitation of a secondary metallic phase containing copper, magnesium and aluminum and consisting essentially of about 40 to 80 percent by weight o~ copper, magnesium in an amount between about S to 30 percent by weight, and the - balance essentially aluminum, thereby forming a metal matrix composite having an aluminum-based primary metallic phase and the secondary metallic phase distributed throughout the primary metallic phase. The primary metallic phase can contain up to 10 percent of eutectic phase which is generally present as a coarse .

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network or as isolated islands.
The intermingling step is accomplished by a number of methods, such as mixing and heating powdered metal and ceramic; melting a metal and adding ceramic S while stirring; or infiltrating a ceramic preform with a melted metal. In essence, any method which achieves intermingling or dispersion of ceramic in the metal alloy may be used.
The intermingled alloy and ceramic (i.e. the composite) must be hot enough to achieve a relatively homogeneous metal alloy solution. Then the cooling step is conducted to cool the composite from an elevated temperature to a predetermined temperature at or below which the precipitation of the secondary metallic phase occurs. The cooling step is sufficiently rapid as to preclude any substantial precipitation from occurring before the predetermined temperature is reached.
Preferably, the aforesaid rapid cooling occurs immediately after the step of intermingling to achieve a temperature below about 550F thereby causing precipitation to occur below about 550F. Due to the configuration of many components it is often not practically possible to conduct a controlled, rapid cooling or quench immediately after intermingling as different portions of the component cool at different rates. Therefore, the constraints of the manufacturing prooess may require that the step of controlled cooling be deferred. If such a deferral causes the composite to cool at an uncontrolled rate, the composite must then be reheated to a temperature sufficient to . .
:, redissolve or resolubilize the copper and magnesium in the aluminum including that present in the eutectic phase. Heating to a temperature between about 900F
and about 1000F preferably for between about 8 and about 36 hours is adequate for this purpose thouqh any temperature above about 900F would be effective.
Longer times or mechanical working could be used to insure that the eutectic phase is completely dissolved.
After reheating, the cooling step immediately follows, where the composite cools at a rate sufficient to sustain supersaturation (i.e. of the copper and magnesium in the aluminum) preferably to a temperature below about 550F to produce the desired precipitate below about 550F.
The rapid cooling is desirably conducted by quenching in a liquid, preferably water, at a temperature between about 80F and 200F. Then, if desired, an aging step may follo~ the quench. The aging step desirably occurs at about 150F to about 550F for about 4 to about ~8 hours, and preferably at 300F to 400F for 4 to 8 hours. A natural aging step may be conducted at about room temperature for up to a few days. It is well known in the art that aluminum alloys generally exhibit an increase in strength over time, sometimes for years after guenching. Thus, the aging step will simply depend on the temper condition desired at the time the part is placed in use.
In order to efficiently and economically produce cast components, preferably the infiltration ~o method is used to achieve mixing or intermingling. In this preferred method, a ceramic preform is contoured , ~ .

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to the shape of the final part desired; heated;
impregnated with molten alloy metal under pressure; and then cooled at a relatively rapid, controlled rate.
The resultant composite part will be in the shape of the ~inal product desired, with little or no subsequent machinin~ being required. Importantly, no subsequent treatment such as mechanical work or heat treatment is required to impart enhanced mechanical properties, as further discussed below.
The metal matrix composite process and the physical properties of the composite will be further described by reference to the following examples.
Example 1 The preferred infiltration method was used to cast a component part from a preEorm of the preferred silicon carbide material, which was contoured to the ~inal shape desired for the part. In this method:
a) a porous ~i.e. 80% porosity) preform was ormed of a ceramic material which included silicon carbide whiskers bound together by a layer of oxides of silicon (i.e. SiO2) on the surface of the whiskers;
b) the preform was heated to a temperature between about 1450F and about 1500F;
c) an alloy comprising by weight, about 4.5 percent copper, about 0.45 percent magnesium, the balance essentially aluminum, was melted at a temperature between about 1450F and about 1500DF;
d) the heated ceramic preform was impregnated with the melted alloy by applying about 11,500 PSI of 3~ pressure to a free surface of the molten aluminum alloy distal from the preform;

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e) the pressure was maintained for about 4 minutes to enable the metal to solidify, after which the infiltrated preform and excess solidified metal was ejected;
f) the impregnated preform was naturally cooled from an ejection temperature of about 850F to room temperature at an uncontrolled rate;
g) the impregnated preform was solution treated by reheating to 977F by placing in a furnace heated to 750F and ramping up to 977F at a rate not exceeding more than 40F per hour, and holdiny ~or 16 hours followed by water quenching into 155F water at a rate of about 400F per second; and h) the solution treated impregnated preform was aged for 5 hours at 370F and air cooled. At this temperature a secondary metallic phase formed which contained the three elements, copper, magnesium and aluminum. The metallic phase essentially consisted of about 40 to about ao percent by weight copper, magnesium in an amount between about 5 and 30 percent by weight and the balance essentially aluminum. By this infiltration method, a metal matrix composite was formed having an aluminum-based primary metallic phase comprising about 95 percent by volume alpha aluminum and the secondary metallic phase distributed throughout the primary metallic phase. The primary metallic phase can contain up to 10 percent of eutectic phase which is generally present as a coarse network or as isolated islands. The resultant metal matrix composite product had a silicon carbide content of about 20 percent by volume, a yield strength of 317 megapascals lMpa)~ a . ~ :

2 ~ 8 tensile strength of 317 MPa and exhibited elongation of zero percent.
~xa~ples 2, 3, 4 and 5 In Examples 2, 3, 4 and 5 the method of Example 1 was followed except that the weight percent of magnesium was increased to 0.9 percent, 1.1 percent, 1.58 percent, and 1.9 percent, respectively. The yield strength was, respectively, 459 MPa, 476 MPa, 478 MPa and 512 MPa; the tensile strength was, respectively, 562 MPa, 546 MPa, 543 MPa and 596 MPa; and the total elongation was, respectively, 2%, 0.9%, 0.5% and 1.3%.
Table 1 is a summary of the properties obtained from the several examples.

Yield Tensile Total Example Mg Strength Strength Elonga-No. % (MPa) (MPa) (KSI) tion (~) ~
1 0.45 317 317 46 0.0 2 0.90 459 562 81 2.0 3 1.10 476 546 79 0.9 4 1.58 478 543 79 0.5 S 1.90 512 596 86 1.3 MPa - Megapascals KSI = Thousands of Pounds per Square Inch In Examples 1-5, infiltration was accomplished using a casting mold 19, (Fig. 2). The preform 20 was made of the preferred silicon carbide particles, with an oxide layer grown on the silicon carbide particles .

by heating in air at an elevated temperature. The oxide layer had a thickness of approximately 0.2 m-crons. The preform 20 was placed in a cylindrical cavity 22 of an open top die 24 having a sleeve 25 resting on a base element 26 and heated to the temperature specified. A charge of molten aluminum alloy 27 was then ladled into cavity 22 and onto the preform 20. A hydraulically driven punch 28 was advanced into cavity 22 to apply a pressure of about 11,500 PSI to a free surface 29 of the molten alloy charge 27 distal from the preform 20, to inject the alloy 27 by force of pressure into the voids of the preform 20 in about 15 to 30 seconds.
Althou~h silicon carbide particles were used in Examples 1-5, composites were also formed with either crystalline alumina-silica, or alumina-silicate glass in place of silicon carbide. The silicon carbide particles used in the examples were of the preferred whisker type, having diameters less than two microns and elongate with an aspect ratio generally greater than about 3 to 1 and pre~erably significantly greater than 10 to 1.
An oxide layer seemed to facilitate the wettiny of the ceramic by the metal alloy. An oxide layer may be present in the form of a binder added to the ceramic, or grown "in situ" by surface oxidation of the ceramic. One such binder is colloidal silica (SiO2~ or colloidal alumina (Al~03).
Preferably, the oxide layer is grown in situ by surface oxidation of the ceramic. It has been found that the thickness of the oxide layer on the silicon ,:

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carbide may be controlled. Various thicknesses of silicon oxides (i.e. SiO2) were formed by heating in air to tempe~atures in the range of 800C to 1400C.
At the lower end of the range, an oxide thickness of about 0.2 microns was achieved, at the higher end of the range, an oxide thickness of about 0.5 microns was achieved in about 10 to 16 hours. The thickness of the oxide film in microns, is shown as a function of temperature in Fig. 3.
In Examples 1-5, the alloy used was 206 aluminum available from any casting alloy supplier.
The 206 alu~inum is essentially a binary Al-4.5% Cu alloy with trace amounts of other elements. Various amounts of magnesium were added to the 206 Al-4.5~ Cu alloy to form the Al-Cu-Mg alloy melts with Mg in the range of 0.45% to 1.9% by weight.
It has been found that i3 range of about 0.5 to about 5 percent Mg and a range of about 3 to 6 percent Cu are each satisfactory, the balance being essentially aluminum. Preferably, magnesium is present in the alloy in a range of about 1.5 to 2.5 weight percent and copper is present in the alloy in a range of about 4 to 5 weight percent. Other elements typically found in alloys may also be present in low concentrations. Such typical elements include manganese (Mn), chromium (Cr), zinc (Zn), titanium (Ti), iron (Fe), vanadium (V), zirconium (Zr), nickel (Ni), bismuth (Bi), palladium (Pd), tin (Sn), beryllium (Be), silver (Ag), antimony (Sb), cobalt (Co) and silicon (Si). These typical elements may be present in concentrations up to about 3 percent but preferably are present in smaller .

quantities. Silicon, up to about 20 weight percent, may be tolerated.
It has been found that the preform should be heated, prior to impregnation, to a temperature of between about 750F and 2000F and preferably to a temperature between about 1200F and 1750F.
Preheating of the preform will facilitate impregnation and prevent the occurrence, for example, of premature solidification of the molten alloy.
The pressure at which impregnation occurs in an infiltration technique is not critical. Preferably, the pressure is on the order of 5,000 pounds per square inch or greater. Generally, the selection of the pressure is determined by the desired length of the 15 infiltration step, so long as premature cooling does not occur. ~igher pressures cause infiltration to oc~ur more rapidly and lower pressures cause infiltration to occur more slowly.
The coolin~ rate of about 400F per second after completion of solidification was found to be satisfactory. Other rates of cooling may be used so long as cooling proceeds sufficiently rapidly to a temperature below about 550F, so that essentially all of the precipitation will occur at a temperature below about 550~
The metal matrix composites of the invention comprising the preferred silicon carbide ceramic and the new second metal phase composition, as shown in Fig. 1, have the strengths as shown in Table 1. The new secondary metallic phase is in the form of cubical shaped structures, about 300 to about 500 angstroms on 2 ~

a side and preferably about 400 angstroms (40 nm) on a side. The structures are clearly visible on the transmission electron micrograph of Fig. 1. Although the secondary metallic phase consists of about 40 to 80 percent by weight copper, magnesium in an amount between about 5 and 30 percent by weight, and the balance essentially a~uminum, it has been found that the atom percent of magnesium, at the interface between the metal and the ceramic, in the as cast material, is relatively high and drops off significantly with distance from the interface (Fig. 4).
The metal matrix composite of the invention clearly exhibits improved strength compared to the unreinforced metal. As shown in Fig. 5, the reinforced matrix (i.e. the composite) has a uniaxial tensile strength (UTS) in the range of 70 to 90 thousands of pounds per square inch (KSI), as magnesium is increased from about 0.5 percent to about 2 percent. In contrast, the unreinforced matrix metal alone has a UTS
which decreases from a high of about 65 KSI down to about 15 KSI as the percentage of magnesium increases from about 0.5 percent to about 2 percent.
The metal matrix composites 10 were compared to prior art wrought composites obtained by repeated working and/or heat treating. Fig. 6 is a graph having matrix tensile strength on the X axis and composite strength on the Y axis. Sigma C represents the composite strength and Sigma M represents the matrix strength. The diagonal line is formed by points where Sigma C and Sigma M are equal. Thus, if the strength of a composite is greater than the strength of the ` 18 :, .

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matrix metal alone, such a composite would be represented on the graph by a point above the diagonal line. The composites of the invention 10, indicated on Table 1~ generally exhibit strength well in excess of 500 MPa and approachinq 600 MPa; and the composites 10 are represented by the large cross located above the diagonal line in Fig. 6.
Wrought composites, indicated by squares in Fig. 6, exhibit a wide range of strengths and in one case the strength of the wrought composite is worse than that of the matrix metal. Such wrought composites required significant additional treatment to obtain their properties, as shown in Fig. 6. In contrast, the metal matrix composites of the invention 10 in their "as cast and heat treated condition", without subsequent mechanical working treatment, have properties comparable to or better than the wrought composites.
Two comparative cast composites are indicated on Fig. 6, by small crosses. These two comparative composites were formed with either a 339 or 1275 aluminum alloy, each of which is different from the alloy of the invention. The comparative composites exhibit considerably less strength than the composite of the invention 10. The composites of the invention 10 may be subjected to subsequent working and/or heat treating to further enhance their properties over and above the improved properties shown on Fig. 6.
Although not wishing to be confined to any particular theory, it appears that the enhanced properties o the composites of the invention 10 are : ' : .:
, . : :

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achieved, at least in part, because: (1) magnesium significantly improves bonding, as is well known; (2) there is a limited amount of low melting eutectic phase in the composite; (3) magnesium, and particularly copper, provide a relatively large amount of the secondary metallic phase which is a strengthening precipitate; (4) the ceramic enhances stability o~ the secondary metallic phase; (5) the oxide layer improves strength by improving bonding at the metal-ceramic interface; and (6) the ceramic and oxide may each improve strenyth by contributing to the formation of the copper-rich second metal phase, which has not heretofore been observed in castings.
In order to produce these results the invention takes advantage of the phenomena that an alloy exists as a homogeneous solution at one temperature and decomposes into its constituents at some lower temperature. A more ~undamental description of this phenomena, which occurs als cooling takes place may be helpful. When metals dissolve in one another at an elevated temperature, desired compositions and properties may be obtained by controlling the cooling of the metal solution from the elevated temperature.
For example, the solubilities of several alloying elements in solid aluminum are much greater at elevated temperatures than at room temperature. If an aluminum alloy containing, for example, 5 percent by weight copper is heated to well over 900F, all of the copper will be in solution. If the alloy is then rapidly cooled or quenched, it becomes supersaturated, containing almost 5 percent more copper solute in solution than it can retain under equilibrium conditions, and particles of an aluminum-copper metallic phase will precipitate. The final properties will depend on the size and distribution of the precipitated particles, which in turn depend on the control of the cooling conditions. The rejection of solute to form a precipitate generally occurs in a similar manner whether the metal solution is a solid or liquid. Thus, the changes that take place when a liquid solution cools may also occur during the cooling of a solid solution. The invention takes advantage of the phenomena that an alloy exists as a homogeneous solution at one temperature and decomposes into its constituents at some lower temperature. Such lS decomposition leads to the formation of a metal phase, the structure of which is like that of the eutectic if cooling occurs from a liquid, or a eutectoid if the structure is the result of the decomposition of a solid solution. Whether the cooling occurs from a liquid homogeneous solution or a solid homogeneous solution is not critical. What is critical is preventing the rejection o~ solute until a desired (lower) temperature is reached, to form the desired precipitate.
Correspondingly, if the composition of the precipitate desired is known, the cooling conditions may be controlled so as`to selectively generate the desired precipitate.
As was described earlier, if a casting is made and controlled cooling does not immediately take place, the cast part will be permitted to cool at some random uncontrolled rate. The desired composition of the 2 ~

metal phase will, therefore, not be achieved. In this event, to take advantage of the precipitation hardening reaction, it is possible to produce a supersaturated solid solution by reheating the part, and then conducting the rapid cooling step.
Transmission electron micrographs show the existence of the cubic-shaped precipitate obtained by the method of the invention. The precipitate has no~
been observed before in castings and was tentatively determined to include one or more of the following specific compositions: Cu6Mg2A15, CuA12, CuMg~12 and/or CuMgAl.
Relative to other systems, very little is known about the Mg-Cu-Al system. Although some phases having Mg-Cu-Al have been reported, many of those which have been reported are said: (1) to be unstable and are not in e~uilibrium with aluminum; and/or (2) to require the presence of zinc, thus forming an Al-Cu-Mg-Zn phase. It has been determined that the precipitate of the invention, that is the secondary metallic phase of the invention, has a composition in the range bounded by the trapezoid shaped area sho~n on the intermetallic phase diagram of Fig. 7. This phase consists of the three elements copper, magnesium and aluminum and has over 40 percent copper and magnesium in an amount between about 5 and 30 percent by weight, with the balance essentially aluminum. Such a phase has never been reported in a cast metal matrix composite.
With regard to the phenomena of enhanced bonding, Fig. 4 shows that the atom percent of magnesium at the interace between the metal and the 2~

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2 ~ 3 ceramic is relatively high and drops off significantly with distance from the interface. It is believed that enhanced bonding is also achieved by the magnesium addition.
The enhanced mechanical properties of the composites are believed to be produced by: (1) a strong, thin bond (interface) for efficient load transfer; (2) an interface which remains stable during service; and (3) a strong, tough matrix which resists crack propagation with particle and whisker reinforcements. More specifically, the controlling of the interfacial properties and the formation of the second metàl phase which leads to the enhanced mechanical characteristics is believed to be due to a combination of the composition o~E the matrix metal phase, the oxide layer on the reinforcement and the controlled cooling and precipitation hardening of the metal matrix.
The invention provides enhanced metal matrix composite properties achieved by the specific constituents of the matrix, their volume or weight fraction, the method by which the metal matrix composite is formed ~nd the temperature conditions prevailing during formation of the metal matrix composite.
The invention also provides optimized matrix toughness, control of the matrix/reinforcement interaction and the ability to maintain the desired matrix composition during manufacture of a composite and the service life of the composite. Metal matrix composites of the invention achieve the advantages of ~.

2 ~ 8 relatively high strength and low weight as compared to other materials commonly used to form articles.
Finally, the invention provides metal matrix composites fabrica ed by a method which: (1) is cost-effective because it utilizes an intermingling impregnation process which provides a casting in the shape of the part desired; and (2) permits, alternatively, an immediate cooling step after infiltration, or reheating and cooling steps to achieve the enhanced metal matrix composite properties.
While the invention has been described primarily in terms of specific example thereof it is not intended to be limited thereto but rather only to the extent set forth hereafter in the claims which follow.

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Claims

1. A method of forming a metal matrix composite having a ceramic phase intermingled with an aluminum alloy phase comprising the steps of:
a) heating the ceramic to a temperature between about 750°F and 2000°F;
b) melting an alloy comprising, by weight, about 3 to 6 percent copper, about 0.5 to 5 percent magnesium, and the balance essentially aluminum;
c) intermingling the heated ceramic with the melted alloy; and d) cooling the intermingled ceramic and alloy at a rate sufficient to sustain supersaturation of the copper and magnesium in the aluminum until a predetermined temperature is reached, the predetermined temperature being selected so as to permit precipitation of a secondary metallic phase consisting essentially of about 40 to 80 percent by weight copper, magnesium in an amount between about 5 and 30 percent by weight, and the balance essentially aluminum, thereby forming a metal matrix composite having an aluminum-based primary metallic phase and the secondary metallic phase distributed throughout the primary metallic phase.

2. A method according to claim 1 wherein the predetermined temperature is below about 550°F.

3. A method according to claim 1 wherein the aluminum alloy comprises at least about 85 percent aluminum, about 4 to 5 percent copper and about 1.5 to 2.5 percent magnesium by weight.

4. A method according to claim 1 wherein after step (c) and immediately before step (d) the intermingled ceramic and alloy are permitted to cool and are subsequently reheated to a temperature above about 900°F for a time sufficient to dissolve the copper and magnesium in the aluminum.

5. A method of forming a metal matrix composite having a silicon carbide ceramic phase intermingled with an aluminum alloy phase comprising the steps of:
a) heating the ceramic to a temperature between about 750°F and 2000°F;
b) melting an alloy comprising, by weight, about 3 to 6 percent copper, about 0.5 to 5 percent magnesium, and the balance essentially aluminum;
c) intermingling the heated ceramic with the melted alloy; and d) cooling the intermingled ceramic and alloy at a rate sufficient to sustain supersaturation of the copper and magnesium in the aluminum until a predetermined temperature is reached, the predetermined temperature being selected so as to permit precipitation of a secondary metallic phase consisting essentially of about 40 to 80 percent by weight copper, magnesium in an amount between about 5 and 30 percent by weight, and the balance essentially aluminum, thereby forming a metal matrix composite having an aluminum-based primary metallic phase and the secondary metallic phase distributed throughout the primary metallic phase.

6. A method according to claim 5 wherein the predetermined temperature is below about 550°F.

7. A method according to claim 5 wherein the aluminum alloy comprises at least about 85 percent aluminum, about 4 to 5 percent copper and about 1.5 to 2.5 percent magnesium by weight.

8. A method according to claim 5 wherein after step (c) and immediately before step (d) the intermingled ceramic and alloy are permitted to cool and are subsequently reheated to a temperature above about 900°F for a time sufficient to dissolve the copper and magnesium in the aluminum.

9. A method according to claim 8 wherein the temperature is less than about 1000°F and the time is between about 8 and 36 hours.

10. A method according to claim 5 wherein the cooling substantially immediately succeeds the intermingling.

11. A method according to claim 5 wherein the ceramic phase comprises a plethora of silicon carbide particles and oxides thereof.

12. A method according to claim 11 wherein the particles are elongated having an aspect ratio greater than about 3 to 1.

13. A method according to claim 5 wherein the ceramic phase comprises a plethora of silicon carbide particles and an oxygen containing binder admired with the silicon carbide particles.

14. A method of forming a metal matrix composite having a silicon carbide ceramic phase intermingled with an aluminum alloy phase comprising the steps of:
a) forming a preform of the ceramic;
b) heating the preform to a temperature between about 750°F and 2000°F;
c) melting an alloy comprising by weight, about 3 to 6 percent copper, about 0.5 to 5 percent magnesium, the balance essentially aluminum;
d) impregnating the heated preform with the melted alloy; and e) cooling the impregnated preform at a rate sufficient to sustain supersaturation of the copper and magnesium in the aluminum until a predetermined temperature is reached, the predetermined temperature being selected so as to permit precipitation of a secondary metallic phase consisting essentially of about 40 to 80 percent by weight copper, magnesium in an amount between about 5 and 30 percent by weight, and the balance essentially aluminum, thereby forming a metal matrix composite having an aluminum-based primary metallic phase and the secondary metallic phase distributed throughout the primary metallic phase.

15. A method according to claim 14 wherein the preform is impregnated by applying a force of at least 5,000 PSI to a free surface of the molten alloy to force it into the preform.

16. A method according to claim 14 wherein the predetermined temperature is below about 550°F.

17. A method according to claim 14 wherein the aluminum alloy comprises at least about 85 percent aluminum, about 4 to 5 percent copper and about 1.5 to 2.5 percent magnesium by weight.

18. A method according to claim 14 wherein after step (d) and immediately before step (e) the impregnated preform is permitted to cool and subsequently reheated to a temperature above about 900°F for a time sufficient to dissolve the copper and magnesium in the aluminum.

19. A method according to claim 18 wherein the temperature is less than about 1000°F and the time is between about 8 and 36 hours.

20. A method according to claim 14 wherein the cooling substantially immediately succeeds said impregnating.

21. A method according to claim 14 wherein the ceramic phase comprises a plethora of silicon carbide particles and oxides thereof.

22. A method according to claim 21 wherein the particles are elongated having an aspect ratio greater than about 3 to 1.

23. A method according to claim 14 wherein the ceramic phase comprises a plethora of silicon carbide particles and an oxygen containing binder admixed with the silicon carbide particles.

24. A metal matrix composite comprising:
a) a silicon carbide ceramic phase distributed substantially uniformly throughout the composite; and b) a metal phase comprising an aluminum-rich primary metallic phase and a copper-rich secondary metallic phase distributed throughout the primary metallic phase, the secondary metallic phase consisting essentially of about 40 to 80 percent by weight copper, magnesium in an amount between about 5 and 30 percent by weight and the balance essentially aluminum.

25. A metal matrix composite according to claim 24 wherein the secondary metallic phase comprises cubical-shape structures which are about 300 to 500 angstroms on a side.

26. A metal matrix composite according to claim 24 wherein the secondary metallic phase comprises cubical-shape structures which are about 400 angstroms on a side.

27. A metal matrix composite according to claim 24 wherein the secondary metallic phase is distributed substantially uniformly throughout the primary metallic phase.

28. A metal matrix composite according to claim 24 wherein the secondary metallic phase has a volume fraction which is up to about 5 percent of the volume fraction of the metal phase.

29. A metal matrix composite according to claim 24 wherein the primary metallic phase includes alpha-aluminum with a volume fraction of at least about 95 percent of the volume fraction of the metal phase.

30. A metal matrix composite according to claim 24 wherein the ceramic phase comprises a plethora of silicon carbide particles and oxides thereof.

31 31. A metal matrix composite according to claim 30 wherein the particles are elongated having an aspect ratio greater than about 3 to 1.

32. A metal matrix composite according to claim 24 wherein the ceramic phase comprises a plethora of silicon carbide particles and an oxygen containing binder admixed with the silicon carbide particles.