Characterization of cast, discontinuous alumina-silicate fiber-reinforced aluminum composites

Metal matrix composites were made by high-pressure infiltration casting (HiPIC) of aluminum alloy A356 into alumino-silicate rigidized fiber preforms. In this process, the molten alloy is infiltrated into the fiber preform under high pressures of the order of 100 MPa. Since microstructure is generally acknowledged to influence the mechanical performance, the composite microstructure was evaluated as a first step in understanding the failure characteristics of this composite system. Optical and scanning electron microscopy were used to examine the composite microstructure for porosity, unfiberized alumina-silicate inclusions (shot), preform compression and fiber clumping, fracture and connectivity of the reinforcement. Shot particles have been imaged, both on and below the surface using scanning acoustic microscopy. Quantitative metallographic techniques, in conjunction with image analysis, were used to characterize the microstructure of the composite in terms of the orientation and size distribution of the fibers and the occurrence of shot. Using ultrasonic C-scans, the composite was nondestructively characterized for processing flaws such as preform crushing, density gradients, uninfiltrated areas and porosity. Micropores on the order of 10-20 μm in diameter were detected with this technique and characterized in more detail by scanning electron microscopy.