Microsegregation during Solidification of Graphitic Fiber-Reinforced Aluminum Alloys under External Heat Sinks

Squeeze casting and melt infiltration were employed in processing continuous graphitic fiber-reinforced aluminum matrix composites. The fiber reinforcements were (1) uncoated carbon fiber (UNC-CF), (2) Ni-coated carbon fiber (NiC-CF), and (3) bare graphite fibers (GRFs), and they were externally cooled to enhance the local solidification of the matrix alloy. The solidified microstructures and their composition profiles were examined using optical microscopy, scanning electron microscopy–energy-dispersive X-ray, and electron probe microanalysis–wavelengh-dispersive X-ray. The resultant microstructures in the UNC-CF and NiC-CF–reinforced composites exhibited significant differences from those found in the GRF-reinforced composite, in terms of solidified morphologies and compositions. It was found that coarse columnar dendrites developed in the fiber-free matrix, fine equiaxed dendrites in the chilled matrix, and columnar-like arms in the fiber-reinforced matrices. In contrast, in bare GRF-reinforced composites, two distinct regions were clearly distinguished: (1) a region consisting of coarse equiaxed dendrites in the fiber-free matrix and (2) a featureless morphology within the fiber reinforcement regions. These distinct microstructures were attributed to preferential heat extraction through the GRFs, which possess a relatively high thermal conductivity. Apparently, heat extraction through the GRFs led to the formation of single α-Al envelopes on the fiber surfaces. In addition, the extent of solute segregation found in the GRF-reinforced alloy composite was relatively small when compared with the CF-reinforced alloy composites.