CHAPTER ONE
INTRODUCTION
A composite is considered to be any multiphase material that exhibits a significant proportion of the properties of both constituent phases such that a better combination of properties is realized. This is termed as the ‘principle of combined action’ (2). According to this principle, better combinations are fashioned by the judicious combination of two or more distinct materials. All composites generally have one thing in common: a matrix or binder combined with a reinforcing material, within which is a dispersion of one or more phases of another material.
Metal matrix composites, as we know today have evolved significantly during the past few years. The primary support of the composites has come from the aerospace industry for airframe and spacecraft structures. More recently the automotive, electronics and recreation industries have been working diligently with these composites. The driving force behind the development of most of the existing composites has been their capability to be designed to provide needed types of material behaviour.
The focus of research and development in the metal matrix composites (MMCs) area has recently shifted toward low-cost discontinuously reinforced composites which are targeted for automotive and aerospace applications. The optimum properties of MMCs and the enhanced performance from these materials however depend on the judicious selection of the metallic matrix material, reinforcing phase and the processing technique. The composite fabrication technique is an important consideration. For a given set of constituents, the fundamental link between properties and cost is determined by the fabrication method. Processing in general, is concerned with the introduction of reinforcement into the matrix with a uniform distribution. The major hurdle is the achievement of proper bonding between the matrix and the reinforcement in order to attain good load transfer between phases.
A wide variety of fabrication techniques have been explored for metal matrix composites. These include liquid phase methods, deposition of matrix from a semi solid or vapour phase, and solid state consolidation. Liquid phase processing has attractive economic aspects. Chopped fibres, porous ceramics compacts and particulates have been incorporated into matrix alloys. In some cases, pressure assistance has been used to infiltrate the reinforcement with the molten matrix. These methods result in microstructures dictated by the solidification of the molten metal. The green sand casting technique has been among the simplest and the most economical processes of fabricating the particulate metal matrix composites. However due to poor wetting of the ceramic particles by molten alloy, the introduction and uniform dispersion of the reinforcement into the liquid matrix is difficult.