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This article provides some insight into an area that has here-to-fore been ignored, namely, the matrix microstructure of cast metal matrix composites (MMC) 30 INTRODUCTIONThe great advantage of composites materials is the specificity of their properties, meaning that specific materials can be engineered for specific applications. This advantage, however, has actually retarded composite applications. With such freedom of design, materials engineers have been unable to accumulate a very large data base of mechanical properties. We may never have the production volume or the demand to produce such a data base for many promising systems. In the absence of such a data base, we must have alternative methods of predicting material performance. Thus, the goal of composite materials development should be the complete and fundamental understanding of the relationships between microstructures, properties, and processing parameters. Such an understanding is necessary if these technologies are ever to become an economic reality.It is one of the tenets of materials science that the properties of a material are a direct consequence of the microstructural features of that material. In the case of composite materials, the material microstructure is a function of the fiber microstructure, the fiber volume fraction and distribution, the interface and the metal matrix microstructure. It is becoming increasingly apparent 1 that the final properties of a metal matrix composite are highly dependent on the nature and the properties of the metal matrix and the interface. The metal matrix cannot be viewed merely as a "glue" tying fibers together.At the inception of solidification processing of MMC at MIT, we were surprised to find that so few observations of the structure/properties/processing relationships existed in the open literature. It is possible to design a fiber from the inside out to produce desirable properties. Likewise, it should be possible (through processing control) to design or specify the structure of the matrix and the nature of the interface between the matrix and the fiber. As Chawla so apt'ly observed,l the "matrix should not be regarded as a secondary component. The final in-situ structure of (the) matrix will be a function of variables." Thus, "there is plenty of room for controlling the composite properties." This work demonstrates the range of microstructural control that may be achieved through control of fundamental solidification processing parameters. BACKGROUND

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