Metal-matrix composites are either in use or prototyping for the Space Shuttle, commercial airliners, electronic substrates, bicycles, automobiles, golf clubs, and a variety of other applications. While the vast majority are aluminum matrix composites, a growing number of applications require the matrix properties of superalloys, titanium, copper, magnesium, or iron.

Like all composites, aluminum-matrix composites are not a single material but a family of materials whose stiffness, strength, density, and thermal and electrical properties can be tailored. The matrix alloy, the reinforcement material, the volume and shape of the reinforcement, the location of the reinforcement, and the fabrication method can all be varied to achieve required properties. Regardless of the variations, however, aluminum composites offer the advantage of low cost over most other MMCs. In addition, they offer excellent thermal conductivity, high shear strength, excellent abrasion resistance, high-temperature operation, nonflammability, minimal attack by fuels and solvents, and the ability to be formed and treated on conventional equipment.

Aluminum MMCs are produced by casting, powder metallurgy, in situ development of reinforcements, and foil-and-fiber pressing techniques. Consistently high-quality products are now available in large quantities, with major producers scaling up production and reducing prices. They are applied in brake rotors, pistons, and other automotive components, as well as golf clubs, bicycles, machinery components, electronic substrates, extruded angles and channels, and a wide variety of other structural and electronic applications.

Superalloy composites reinforced with tungsten alloy fibers are being developed for components in jet turbine engines that operate temperatures above 1,830 °F.

Graphite/copper composites have tailorable properties, are useful to high temperatures in air, and provide excellent mechanical characteristics, as well as high electrical and thermal conductivity. They offer easier processing as compared with titanium, and lower density compared with steel. Ductile superconductors have been fabricated with a matrix of copper and superconducting filaments of niobium-titanium. Copper reinforced with tungsten particles or aluminum oxide particles is used in heat sinks and electronic packaging.

Titanium reinforced with silicon carbide fibers is under development as skin material for the National Aerospace Plane. Stainless steels, tool steels, and Inconel are among the matrix materials reinforced with titanium carbide particles and fabricated into draw-rings and other high-temperature, corrosion-resistant components.

Compared to monolithic metals, MMCs have:

  • Higher strength-to-density ratios
  • Higher stiffness-to-density ratios
  • Better fatigue resistance
  • Better elevated temperature properties
    • -- Higher strength
    • -- Lower creep rate
  • Lower coefficients of thermal expansion
  • Better wear resistance

The advantages of MMCs over polymer matrix composites are:

  • Higher temperature capability
  • Fire resistance
  • Higher transverse stiffness and strength
  • No moisture absorption
  • Higher electrical and thermal conductivities
  • Better radiation resistance
  • No outgassing
  • Fabricability of whisker and particulate-reinforced MMCs with conventional metalworking equipment.

Some of the disadvantages of MMCs compared to monolithic metals and polymer matrix composites are:

  • Higher cost of some material systems
  • Relatively immature technology
  • Complex fabrication methods for fiber-reinforced systems (except for casting)
  • Limited service experience

Numerous combinations of matrices and reinforcements have been tried since work on MMC began in the late 1950s. However, MMC technology is still in the early stages of development, and other important systems undoubtedly will emerge.

Reinforcements: MMC reinforcements can be divided into five major categories: continuous fibers, discontinuous fibers, whiskers, particulates, and wires. With the exception of wires, which are metals, reinforcements generally are ceramics.

Key continuous fibers include boron, graphite (carbon), alumina, and silicon carbide. Boron fibers are made by chemical vapor deposition (CVD) of this material on a tungsten core. Carbon cores have also been used. These relatively thick monofilaments are available in 4.0, 5.6, and 8.0-mil diameters. To retard reactions that can take place between boron and metals at high temperature, fiber coatings of materials such as silicon carbide or boron carbide are sometimes used.

Silicon carbide monofilaments are also made by a CVD process, using a tungsten or carbon core. A Japanese multifilament yarn, designated as silicon carbide by its manufacturer, is also commercially available. This material, however, made by pyrolysis of organometallic precursor fibers, is far from pure silicon carbide and its properties differ significantly from those of monofilament silicon carbide.

Continuous alumina fibers are available from several suppliers. Chemical compositions and properties of the various fibers are significantly different. Graphite fibers are made from two precursor materials, polyacrilonitrile (PAN) and petroleum pitch. Efforts to make graphite fibers from coal-based pitch are under way. Graphite fibers with a wide range of strengths and moduli are available.

The leading discontinuous fiber reinforcements at this time are alumina and alumina-silica. Both originally were developed as insulating materials. The major whisker material is silicon carbide. The leading U.S. commercial product is made by pyrolysis of rice hulls. Silicon carbide and boron carbide, the key particulate reinforcements, are obtained from the commercial abrasives industry. Silicon carbide particulates are also produced as a by-product of the process used to make whiskers of this material.

A number of metal wires including tungsten, beryllium, titanium, and molybdenum have been used to reinforce metal matrices. Currently, the most important wire reinforcements are tungsten wire in superalloys and superconducting materials incorporating niobium-titanium and niobium-tin in a copper matrix. The reinforcements cited above are the most important at this time. Many others have been tried over the last few decades, and still others undoubtedly will be developed in the future.

Matrix materials and key composites: Numerous metals have been used as matrices. The most important have been aluminum, titanium, magnesium, and copper alloys and superalloys.

The most important MMC systems are:

  • Aluminum matrix
    • Continuous fibers: boron, silicon carbide, alumina, graphite
    • Discontinuous fibers: alumina, alumina-silica
    • Whiskers: silicon carbide
    • Particulates: silicon carbide, boron carbide
  • Magnesium matrix
    • Continuous fibers: graphite, alumina
    • Whiskers: silicon carbide
    • Particulates: silicon carbide, boron carbide
  • Titanium matrix
    • Continuous fibers: silicon carbide, coated boron
    • Particulates: titanium carbide
  • Copper matrix
    • Continuous fibers: graphite, silicon carbide
    • Wires: niobium-titanium, niobium-tin
    • Particulates: silicon carbide, boron carbide, titanium carbide.
  • Superalloy matrices
    • Wires: tungsten

Characteristics and design considerations: The superior mechanical properties of MMCs drive their use. An important characteristic of MMCs, however, and one they share with other composites, is that by appropriate selection of matrix materials, reinforcements, and layer orientations, it is possible to tailor the properties of a component to meet the needs of a specific design.

For example, within broad limits, it is possible to specify strength and stiffness in one direction, coefficient of expansion in another, and so forth. This is rarely possible with monolithic materials.

Monolithic metals tend to be isotropic, that is, to have the same properties in all directions. Some processes such as rolling, however, can impart anisotropy, so that properties vary with direction. The stress-strain behavior of monolithic metals is typically elastic-plastic. Most structural metals have considerable ductility and fracture toughness.

The wide variety of MMCs have properties that differ dramatically. Factors influencing their characteristics include:

  • Reinforcement properties, form, and geometric arrangement
  • Reinforcement volume fraction
  • Matrix properties, including effects of porosity
  • Reinforcement-matrix interface properties
  • Residual stresses arising from the thermal and mechanical history of the composite
  • Possible degradation of the reinforcement resulting from chemical reactions at high temperatures, and mechanical damage from processing, impact, etc.

Particulate-reinforced MMCs, like monolithic metals, tend to be isotropic. The presence of brittle reinforcements and perhaps of metal oxides, however, tends to reduce their ductility and fracture toughness. Continuing development may reduce some of these deficiencies.

The properties of materials reinforced with whiskers depend strongly on their orientation. Randomly oriented whiskers produce an isotropic material. Processes such as extrusion can orient whiskers, however, resulting in anisotropic properties. Whiskers also reduce ductility and fracture toughness.

MMCs reinforced with aligned fibers have anisotropic properties. They are stronger and stiffer in the direction of the fibers than perpendicular to them. The transverse strength and stiffness of unidirectional MMCs (materials having all fibers oriented parallel to one axis), however, are frequently great enough for use in components such as stiffeners and struts. This is one of the major advantages of MMCs over PMCs, which can rarely be used without transverse reinforcement.

Because the modulus and strength of metal matrices are significant with respect to those of most reinforcing fibers, their contribution to composite behavior is important. The stress-strain curves of MMCs often show significant nonlinearity resulting from yielding of the matrix.

Another factor that has a significant effect on the behavior of fiber-reinforced metals is the frequently large difference in coefficient of expansion between the two constituents. This can cause large residual stresses in composites when they are subjected to significant temperature changes. In fact, during cool down from processing temperatures, matrix thermal stresses are often severe enough to cause yielding. Large residual stresses can also be produced by mechanical loading.

Although fibrous MMCs may have stress-strain curves displaying some nonlinearity, they are essentially brittle materials, as are PMCs. In the absence of ductility to reduce stress concentrations, joint design becomes a critical design consideration. Numerous methods of joining MMCs have been developed, including metallurgical and polymeric bonding and mechanical fasteners.

Fabrication methods: Fabrication methods are an important part of the design process for all structural materials, including MMCs. Considerable work is under way in this critical area. Significant improvements in existing processes and development of new ones appear likely.

Current methods can be divided into two major categories, primary and secondary. Primary fabrication methods are used to create the MMC from its constituents. The resulting material may be in a form that is close to the desired final configuration, or it may require considerable additional processing, called secondary fabrication, such as forming, rolling, metallurgical bonding, and machining. The processes used depend on the type of reinforcement and matrix.

A critical consideration is reactions that can occur between reinforcements and matrices during primary and secondary processing at the high temperatures required to melt and form metals. These impose limitations on the kinds of constituents that can be combined by the various processes. Sometimes, barrier coatings can be successfully applied to reinforcements, allowing them to be combined with matrices that otherwise would be too reactive. For example, the application of a coating such as boron carbide permits the use of boron fibers to reinforce titanium. Potential reactions between matrices and reinforcements, even coated ones, is also an important criterion in evaluating the temperatures and corresponding lengths of time to which MMCs may be subjected in service.

Relatively large-diameter monofilament fibers, such as boron and silicon carbide, have been incorporated into metal matrices by hot pressing a layer of parallel fibers between foils to create a monolayer tape. In this operation, the metal flows around the fibers and diffusion bonding occurs. The same procedure can be used to produce diffusion-bonded laminates with layers of fibers oriented in specified directions to meet stiffness and strength requirements for a particular design. In some instances, laminates are produced by hot pressing monolayer tapes in what can be considered a secondary operation.

Monolayer tapes are also produced by spraying metal plasmas on collimated fibers, followed by hot pressing. Structural shapes can be fabricated by creep and superplastic forming of laminates in a die. An alternate process is to place fibers and unbonded foils in a die and hot press the assembly.

The boron/aluminum struts used on the space shuttle are fabricated from monolayer foils wrapped around a mandrel and hot isostatically pressed to diffusion bond the foil layers together and, at the same time, to diffusion bond the composite laminate to titanium end fittings.

Composites can be made by infiltrating liquid metal into a fabric or prearranged fibrous configuration called a preform. Frequently, ceramic or organic binder materials are used to hold the fibers in position. The latter is burned off before or during infiltration. Infiltration can be carried out under vacuum, pressure, or both. Pressure infiltration, which promotes wetting of the fibers by the matrix and reduces porosity, is often called squeeze casting.

Cast MMCs now consistently offer net or net-net shape, improved stiffness and strength, and compatibility with conventional manufacturing techniques. They are also consistently lower in cost than those produced by other methods, are available from a wide range of fabricators, and offer dimensional stability in both large and small parts.

For example, Duralcan has developed its "ice cream mixer" technology and process controls to the point where it produces up to 25 million pounds per year of aluminum composite billets. Investment casting has been modified at Cercast to cast Duralcan billets into complex, net-shape parts. Pressure casting produces net shapes with exceptional properties at Alcoa, while pressureless infiltration is used at Lanxide Corp. to fabricate net-shape components.

At the current time, the most common method used to make graphite/aluminum and graphite/magnesium composites is by infiltration. Graphite yarn is first passed through a furnace to burn off any sizing that may have been applied. Next it goes through a CVD process that applies a coating of titanium and boron which promotes wetting by the matrix. Then it immediately passes through a bath or fountain of molten metal, producing an infiltrated bundle of fibers known as a "wire." Plates and other structural shapes are produced in a secondary operation by placing the wires between foils and pressing them, as is done with monofilaments. Recent development of "air stable" coatings permits use of other infiltration processes, such as casting, eliminating the need for "wires" as an intermediate step. Other approaches are under development.

A particularly important secondary fabrication method for titanium matrix composites is superplastic forming/diffusion bonding (SPF/DB). To reduce fabrication costs, continuous processes such as pultrusion and hot roll bonding are being developed.

Three basic methods are being used to make whisker and particulate-reinforced MMCs. Two use powdered metals; the other uses a liquid-metal approach, details of which are proprietary.

The two powder-metal processes differ primarily in the way the constituents are mixed. One uses a ball mill, the other employs a liquid to aid mixing, which is subsequently removed. Mixtures are then hot pressed into billets.

Secondary processes are similar to those for monolithic metals, including rolling, extrusion, spinning, forging, creep-forming, and machining. The latter poses some difficulties because the reinforcements are very hard.