Metal Matrix Composites- General Introduction

There are more than 50,000 materials available to engineers for the design and manufacturing of products for various applications. These materials range from ordinary metals(copper, cast iron, brass) that have been available for several hundred years, to the more recently developed, advanced materials e.g., composites, ceramics and other high performance materials. Due to the wide choice of materials, today's engineers are posed with a big challenge for the right selection of a material and the right choice of a manufacturing process for an application. It is difficult to study all of these materials individually; therefore, a broad classification is necessary for simplification and characterization. These materials, depending on their major characteristics (stiffness, strength, density and melting temperature) can be broadly divided into four main categories: (1) metals, (2) plastics, (3) ceramics and (4) composite . Each class contains large number of materials with a range of properties which to some extent results in an overlap of properties with other classes.

A ceramic matrix is usually brittle. Carbon, ceramic, metal and glass fibers are typically used with ceramic matrices in areas where extreme environments (high temperatures, etc.) are anticipated. Glass and glass-ceramic composites usually have elastic modulus much lower than that of the reinforcement. Carbon and metal oxide fibers are the most common reinforcements with glass matrix composites. The best characteristics of glass/ceramic matrix composites are their strength at high service temperatures. The primary applications of glass matrix composites are for heat-resistant parts in engines exhaust systems and electrical components.

Increasing quantities of metal matrix composites (MMCs) are being used to replace conventional materials in many applications, especially in the automobile and recreational industries. The MMCs are aluminum alloys reinforced with ceramic particles and these low-cost composites provide higher strength, stiffness and fatigue resistance with a minimal enhancement in density over the base alloy. The major advantages of Aluminum Matrix Composites (AMCs) include greater strength, improved high temperature properties, controlled thermal expansion coefficient, thermal/heat management, enhanced/tailored electrical performance, improved abrasion/wear resistance,improved damping capabilities, low induced radioactivity under nuclear environments, low stiffness and weight, saving in materials and energy .

The alloy designation for Al is based on four digits corresponding to the principal alloying elements. The most important alloying elements in aluminum alloy systems are copper (2xxx), manganese (3xxx), silicon (4xxx), magnesium (5xxx) and zinc (7xxx). The most commonly used route for fabrication of these composites has been through infiltration of molten metal into preformed and porous ceramic bodies. For example, Al-alloys have been successfully infliltrated with hydraulic or gas pressure into fly-ash,SiC, Al2O3, MgO and AlN performs. It is very interesting to emphasize the influence of volume fraction of reinforcing particle on the mechanical properties of MMCs .