Stability and response to rolling of the interfaces in cast Al–SiCp and Al–Mg alloy-SiCp composites

Aluminium matrix composites reinforced with unoxidized or oxidized SiC particles and varying Mg concentrations have been prepared using stir casting technique. The extent of interfacial reactions and composition of the reaction products, formed at the Al–SiC interfaces in as-cast form, and after heat treatments at 893 K (620 °C) or 925 K (652 °C) for 4 or 20 h, have been studied on the surfaces of bulk samples, and those of the SiC particles extracted from the composite by electro-chemical leaching. Alloying of Al matrix with 0.5 or 1 wt.% Mg and its segregation at the interfaces has been found to be effective in restricting the formation of the Al4C3 at the interfaces during casting. Reaction of Mg and Al with SiO2 on the surfaces of oxidized SiC, leads to the formation of MgO and MgAl2O4 crystals, which act as diffusion barrier between Al and SiC at the interfaces. At the same time, it is evident that the depletion of SiO2 film by reaction with Al or Mg leads to the formation of Al4C3 during high temperature exposure. Cold or hot rolling leads to cracking of a large fraction of SiC particles in the as-cast as well as heat-treated composites, while Al–SiC interfaces severely embrittled by formation of Al4C3 also crack, because of inherent weakness to shear.