Metal Matrix Composites

Metal matrix composites (MMCs) consist of at least two chemically and physically distinct phases, suitably distributed to give properties not obtainable with any one of the phases individually. Generally, there are two phases, say, a fibrous or particulate phase distributed in an appropriate manner in matrix. Examples include continuous alumina fibers in an aluminum matrix composites used in power transmission lines, Nb–Ti filaments in a copper matrix for superconducting magnets, tungsten carbide (WC)/cobalt (Co) particulate composites used as cutting tool and oil drilling inserts.

A wide range of metals and their alloys may be used as matrix materials to make metal matrix composites (MMCs). In this chapter, we review some of the basic concepts and fundamentals of bonding and structure of common metals. Following this, we provide a summary of the characteristics of some of the most common metals that are used as matrix materials in MMCs.

Metal matrix composites can be made by processes involving liquid, solid, or gaseous state. We describe some of the important processing techniques below.

Interface is a very general term used in different fields of science and technology to denote the location where two entities meet. The term in composites refers to a bounding surface between the reinforcement and matrix across which there occurs a discontinuity in a parameter such as chemical composition, elastic modulus, coefficient of thermal expansion (CTE), and thermodynamic properties such as chemical potential. Interface (fiber/matrix or particle/matrix) is very important in all kinds of composites. This is because in most all composites, the interfacial area per unit volume is very large. Also, in most metal matrix composite (MMC) systems, the reinforcement and the matrix will not be in thermodynamic equilibrium, i.e., there will be a present thermodynamic driving force for an interfacial reaction that will reduce the energy of the system. All these items make the interface to have very important influence on the properties of the composite.

In this chapter, we examine a variety of means of calculating elastic and physical constants of metal matrix composites, given the same constants for the individual components and the arrangement of components in the composite, and thermal stresses generated because of mismatch in the coefficient of thermal expansion (CTE) of the components. In fact, most of the material discussed in this chapter is applicable to all kind of composites. Specifically, we provide a micromechanical description of physical properties such as density, thermal expansion coefficients, thermal and electrical conductivity, and various elastic constants. Of particular interest are methods or expressions that predict elastic constants of composites because of the generally high anisotropy found in composites. A description of conventional and microstructure-based finite element techniques to predict the elastic and thermal constants is also provided.

In this chapter we discuss the monotonic strengthening and fracture mechanisms of continuous fiber and discontinuously reinforced (short fiber and particle) metal matrix composites. Cyclic fatigue and creep of MMCs are discussed in Chaps. 8 and 9, respectively.

Fatigue is the phenomenon of mechanical property degradation under cyclic loading. The cyclic loads may be mechanical, thermal, or a combination of the two. Many high-volume applications of composite materials involve cyclic-loading situations, e.g., automobile components and aircraft structures. Below we provide a brief description of the two main approaches that have been used to quantify fatigue behavior of materials. For a more complete description, the reader may consult the texts by Meyers and Chawla (2009) and Suresh (1998).

The creep behavior of MMCs is of great significance, since in many structural and nonstructural applications, these materials will be subjected to constant stress (or strain) for long periods of time, at temperature above half of the homologous temperature (homologous temperature is the temperature of interest divided by melting point, both in K; i.e., T/T_m). Most materials exhibit three distinct stages of creep: (1) primary creep, (2) secondary or steady-state creep, and (3) tertiary creep. In primary creep, the strains are relatively small. In the secondary or steady-state regime, a linear relationship exists between the strain and time (constant strain rate). This is believed to be a result of the combination of hardening and recovery mechanisms during creep. Finally, in the tertiary regime, the material undergoes cavitation and void growth, which is manifested in terms of a very rapid increase in strain with time.

Wear can be defined as the loss of material that occurs when two surfaces rub against each other. Two common forms of wear are:

Metal matrix composites are used in a myriad of applications. The high strength-to-weight ratio, enhanced mechanical and thermal properties over conventional materials, and tailorability of properties make them very attractive in a variety of applications. Increasingly MMCs have been used in several areas including (Evans et al. 2003):