Overview

Metal matrix composites offer a number of advantages compared to their base metals, such as higher specific strengths and moduli, higher elevated temperature resistance, lower coefficients of thermal expansion, and, in some cases, better wear resistance. On the down side, they are more expensive than their base metals and have lower toughness. Metal matrix composites also have some advantages compared to polymer matrix composites, including higher matrix dependent strength and moduli, higher elevated temperature resistance, no moisture absorption, higher electrical and thermal conductivities, and nonflammability. However, metal matrix composites (MMCs) are normally more expensive than even polymer matrix composites, and the fabrication processes are much more limited, especially for complex structural shapes. Due to their high cost, commercial applications for metal matrix composites are sparse. There are some limited uses for discontinuously reinforced MMCs but almost no current applications for continuously reinforced MMCs.

Metal matrix composites can be subdivided according to the type of reinforcement shown in Fig. 9.1. The reinforcement can be particulates (particles which are approximately equiaxed); high strength single crystal whiskers; short fibers which are usually random but can contain some degree of alignment; or long aligned multifilament or monofilament fibers. Particulate reinforced composites (Fig. 9.2), primarily silicon carbide (SiC) or alumina (Al ₂O ₃) ceramic particles in an aluminum matrix, are known as discontinuously reinforced aluminum (DRA). They exhibit high stiffness, low density, high hardness, adequate toughness at volume percentages less than 25%, and relatively low cost. Normal volume percentages are 15 25% with...