Generally, fabrication costs of composite materials are high because several processes are involved, such as embedding fibres or particles within the matrix materials followed by various heat treatments. In situ composites have recently attracted attention [1], as their fabrication processes are expected to be effective for reducing the fabrication costs and the desired micro/macro structures of the composite material are obtained during the fabrication process. In the case of Al/Al2O3 composites, a process of direct oxidation of molten Al has