Interface Studies of Aluminum Metal Matrix Composites

In both continuous- and discontinuous-fiber aluminum metal matrix composites (MMC) the interface plays a major role in fracture. The interface chemistry and crystallography of both graphite/aluminum continuous-fiber and silicon carbide/aluminum (SiC/Al) discontinuous-fiber MMCs were observed on ion beam thinned specimens in the transmission electron microscope (TEM). The fracture mode and fracture surface chemistry of SiC/Al MMCs were investigated with the scanning electron microscope (SEM) and Auger electron spectroscopy (AES) combined with inert ion sputtering. The materials investigated were representative of the discontinuous-fiber SiC/Al composite (ARCO, SILAG) and the particulate SiC/Al composites (DWA) at the time of the study.

The TEM results show that an oxide is present at some of the SiC/Al interfaces, often in the form of γ-Al₂O₃ (along with MgAl₂O₄ when the aluminum matrix contains magnesium). The fracture path is not dominated by the interfacial failure but is primarily a matrix failure path. The more ductile the matrix, the less interfacial fracture. The large volume fraction of silicon carbide plays a major role in the fracture behavior by influencing the localized volume of material being deformed. Some of the fracture energy may have been reduced due to prior formation of dislocation networks due to thermal coefficient of expansion mismatch between the silicon carbide and the aluminum matrices. These networks were not apparent in the particulate SiC/Al MMC.

Both fine-grained γ-Al₂O₃ and small amounts of coarse-grained Al₄C₃ were found in the interfaces of the as-received graphite/aluminum composite specimens. During heat treatment, some of the Al₄C₃ phase grows into and along the graphite fiber surface.