Metal Matrix Composites

Metal Matrix Composites (MMC's) are classified into three broad categories depending on the aspect ratio of the reinforced phase and are not new to the industry. Firstly, unidirectional aligned long fibers are embedded in a matrix. The fiber material could be carbon or a ceramic like silicon carbide, and the matrix can be a metal like titanium/copper/aluminum. Mechanical properties like strength and stiffness are higher in the longitudinal direction than those in the transverse direction of the fibers. Secondly, short fiber reinforced composite is formed, wherein the short fibers are oriented randomly in the matrix material.

Finally, particulate composite in which irregular-shaped particles of second phase are dispersed in a metal matrix. The particle ceramics like silicon carbide, graphite, magnesium oxide, fly ash or aluminum oxide and the matrix could be a metal like aluminum, copper, titanium or magnesium. These composites can be synthesized by vapor phase, liquid phase or solid phase processes.The focus of this work will be on liquid phase processes where the matrix, in the form of a liquid, is mixed with the reinforcements and allowed to solidify to form a composite. Fiber-reinforced composites, where the aspect ratio (ratio of length to diameter) of the second phase lies above 100, are used in applications where higher specific strength and specific modulus are required in a direction parallel to the fiber. In particulate composites, the aspect ratio of the second phase is close to one. This class of materials gives marginal improvement in strength, as compared to the matrix alloy and provides isotropic strength properties and improved wear properties. MMC's have proved to be an important class of materials, with the potential to replace a number of conventional materials being used in automotive, aerospace, defense, and leisure industries, where the demand for lightweight and higher strength components is increasing.