Influence of the content of ceramic phase on the precipitation hardening of Al alloy 7000/AlN nanocomposites

Nanocomposites of 7000 Al alloy reinforced with about 4–30 vol.% ceramic phase (aluminium nitride and oxide particles) were prepared by cryomilling and hot pressing. The effect of the content of a fine dispersion of ceramic phase on the precipitation hardening of the Al-alloy matrix was studied by microhardness measurements and transmission electron microscopy. The results show that for a given ageing treatment, the presence of the ceramic phase leads to an increase of the precipitation kinetics in the matrix. However, the efficiency of the hardening is observed only when the content of ceramic phase is lower than or equal to 10 vol.%, because in nanostructured materials, the hardness is principally due to the large number of grain boundaries.