Metal-matrix composites

A metal-matrix composite (MMC) is a material in which a continuous metallic phase (the matrix) is combined with another phase (the reinforcement) that constitutes a few percent to around 50% of the total volume of the material. In the strictest sense, metal-matrix composite materials are not produced by conventional alloying. This feature differentiates most metal-matrix composites from many other multiphase metallic materials, such as pearlitic steels or hypereutectic aluminum-silicon alloys.

The particular benefits exhibited by metal-matrix composites, such as lower density, increased specific strength and stiffness, increased high-temperature performance limits, and improved wear-abrasion resistance, are dependent on the properties of the matrix alloy and of the reinforcing phase. The selection of the matrix is empirically based, using readily available alloys, and the major consideration is the nature of the reinforcing phase.

Matrices and Reinforcements

A large variety of metal-matrix composite materials exist. The reinforcing phase can be fibrous, platelike, or equiaxed (having equal dimensions in all directions), and its size can also vary widely, from about 0.1 to more than 100 |im. Matrices based on most engineering metals have been explored, including aluminum, magnesium, zinc, copper, titanium, nickel, cobalt, iron, and various aluminides. This wide variety of systems has led to an equally wide spectrum of properties for these materials and of processing methods used for their fabrication.

Reinforcements used in metal-matrix composites fall in five categories: continuous fibers, short fibers, whiskers, equiaxed particles, and interconnected networks.

Continuous Fibers

Several continuous fibers or filaments are used in metal-matrix composites. Their elastic moduli vary significantly, depending on the nature of the fiber and its fabrication process. For example, silica-alumina spinels and microcrys-talline or amorphous polycarbosilane-derived fibers possess significantly lower elastic moduli than do pure alumina or crystalline P-silicon carbide produced by chemical vapor deposition. Carbon fiber strength and modulus also vary significantly with processing, depending on the level of graphitization of the microstructure.

Short Fibers

Short fibers are less expensive, especially when they are mass-produced for other applications such as high-temperature thermal insulation. Their physical properties can be similar to those of continuous fibers; however, their reinforcing efficiency in the matrix is also far lower. Short fibers used in engineering practice include chopped carbon fibers and alumina-silica fibers.

Whiskers

Whiskers are single-crystal short fibers, produced to feature highly desirable mechanical properties due to lack of microstructural defects. Whiskers have typically been made of silicon carbide, and they are often priced far higher than short fibers. The high price and toxicity of most whiskers have prevented their application in engineering practice.

Single-crystal whiskers, because of the absence of grain boundary defects, offer much higher tensile strength than other types of discontinuous reinforcements, and thus they are preferred for certain applications of discontinu-ously reinforced metal-matrix composites. The whiskers can be aligned to a preferred orientation by conventional metallurgical processes; higher directional strengths can be achieved in finished components where fabrication is by extrusion, rolling, forging, or superplastic forming. Whiskers tend to produce anisotropic properties due to their alignment during processing, whereas particulate materials usually produce essentially isotropic properties.

Equiaxed Particles

Equiaxed particles of several ceramics, including those containing silicon carbide, aluminum oxide, boron carbide, and tungsten carbide, do not provide the possibility for preferential strengthening of the matrix along selected directions; however, their price is low and their combination with the metal is relatively easier. These reinforcements are therefore used in many metal-matrix composite systems, including mass-produced aluminum-matrix composites.

Interconnected Cellular Networks

These can be produced by several methods, such as by chemical vapor deposition of ceramic onto a pyrolizable polymer foam or by conversion of a preceramic polymer foam prior to infiltration with the molten matrix. Alternatively, some processing techniques for in-place metal-matrix composites, including directional oxidation of aluminum melts, produce interconnected reinforcing networks.

Microstructures

The microstructure of a metal-matrix composite comprises the structure of matrix and reinforcement, that is, the interface and the distribution of the reinforcement within the matrix.

Composite Properties

Composite properties depend first and foremost on the nature of the composite; however, certain detailed microstructural features of the composite can exert a significant influence on its behavior.

Physical properties of the metal, which can be significantly altered by addition of a reinforcement, are chiefly dependent on the reinforcement distribution. A good example is aluminum-silicon carbide composites, for which the presence of the ceramic increases substantially the elastic modulus of the metal without greatly affecting its density. Elastic moduli for 6061 aluminum-matrix composites reinforced with discrete silicon carbide particles or whiskers have been calculated by using the rule of mixtures for the same matrix reinforced with two types of commercial continuous silicon carbide fibers. As a result, several general facts become apparent. First, modulus improvements are significant, even with equiaxed silicon carbide particles, which are far less expensive than fibers or whiskers. However, the level of improvement depends on the shape and alignment of the silicon carbide. Also, it depends on the processing of the reinforcement: for the same reinforcement shape (continuous fibers), microcrystalline polycarbosilane-derived silicon carbide fibers yield much lower improvements than do crystalline P-silicon carbide fibers. These features, which influence reinforcement shape, orientation, and processing of modules, are quite general; they are also observed, for example, in metal-matrix composites reinforced with aluminum oxide or carbon.

Other properties, such as the strength of metal-matrix composites, depend in a much more complex manner on composite micro-structure. The strength of a fiber-reinforced composite, for example, is determined by fracture processes, themselves governed by a combination of microstructural phenomena and features. These include plastic deformation of the matrix, the presence of brittle phases in the matrix, the strength of the interface, the distribution of flaws in the reinforcement, and the distribution of the reinforcement within the composite. Consequently, predicting the strength of the composite from that of its constituent phases is generally difficult.

FIGURE M.12 Methods used to make MMCs.

Production

A variety of techniques are available for the production of continuous or discontinuous metal-matrix composites. These may be broadly classified as diffusion processes, deposition processes, and liquid processes. See Figure M.12 illustrating the methods used to make metal-matrix composites.

Fabrication

Composite processing methods combine the reinforcement with the matrix. This is accomplished while the matrix is either solid or liquid.

Typical liquid-state processes include the dispersion processes, which are casting techniques. A second set of processes involves liquid-metal impregnation; these include squeeze casting, where a preform or a bed of dispersoids is impregnated by molten alloy under hydraulic pressure. A third set comprises spray processes. In one of these, a molten metal stream is fragmented by means of a high-speed cold inert-gas jet passing through a spray gun, and dispersoid powders are simultaneously injected. A stream of molten droplets and dispersoid powders is directed toward a collector substrate where droplets recombine and solidify to form a high-density deposit.

Depending on the process, the desired microstructure, and the desired part, metal-matrix composites can be produced to net or near-net shape; or alternatively they can be produced as billet or ingot material for secondary shaping and processing.

Applications

The combined attributes of metal-matrix composites, together with the costs of fabrication, vary widely with the nature of the material, the processing methods, and the quality of the product. In engineering, the type of composite used and its application vary significantly, as do the attributes that drive the choice of metal-matrix composites in design (Table M.11). For example, high specific modulus, low cost, and high weldability of extruded aluminum oxide particle-reinforced aluminum are the properties desirable for bicycle frames. High wear resistance, low weight, low cost, improved high-temperature properties, and the possibility for incorporation in a larger part of unreinforced aluminum are the considerations for design of diesel engine pistons.

TABLE M.11

Some Composite Components with Proven Potential