Abstract
Discontinuously reinforced aluminium (DRA) composites were prepared by a novel ion exchange process in which the reinforcing phase, aluminium oxide, was introduced by exchanging the magnesium ions in magnesium aluminate spinel with aluminium [1]. The exchange reaction occurs by the diffusion of magnesium and aluminium ions across the interface in opposite directions. The process led to the formation of DRA composites with a remarkable resistance to high temperature deformation. We present an ab-initio density functional study of the coherent cube-on-cube interfaces between spinel (MgAl2O4) and Al metal and relevant X-ray diffraction studies of the DRA composite. We find that the presence of surface electric dipoles near the interface, originated by the ion exchange process, leads to a lowering of the Al/MgAl2O4 interface toughness, and an increased residual surface stress, which does not support the hypothesis that the ion exchange process is directly responsible for the increased creep resistance of the DRA composite. We propose instead that deep diffusion of the Al atoms into MgAl2O4, resulting in the formation of γ-Al2O3 nanograins, is the mechanism that impedes dislocation movement and thus increases the interface toughness.