TEM INVESTIGATIONS OF AL 6061 AND OF COMPOSITES REINFORCED WITH AL2O3PARTICLES

The Al alloy 6061 and metal matrix composites (MMC) of the same alloy as matrix, reinforced with particulate Al2O3 (10 and 20 vol.%), were i nvestigated by transmission electron microscopy (TEM) methods after bo th heat treatments T5 and T6. Al 6061 shows similar mechanical data (y ield strength, tensile strength) after both heat treatments whereas af ter T5 heat treatment the MMC show a lower strength than after T6 heat treatment. A homogeneous distribution of fine precipitates (Mg2Si Gui nier-Preston zones) was found throughout. the specimen after T6 heat t reatment in the alloy Al 6061 and in the matrix of the MMC. Contrary t o this, precipitation-free zones (PFZ) were observed after the T5 heat treatment in both Al 6061 and die MMC. The PFZ occur along grain boun daries (about 200 nm wide) and in addition in the MMC around the ceram ic particles in the areas of high dislocation densities (about 700 nm wide). It is proposed that the PFZ observed after T5 heat treatment ar e caused by the slow air-cooling process after the extrusion and that the PFZ around the ceramic particles are responsible for die poor mech anical data. The slow cooling rate enables the vacancies to diffuse to grain boundaries and dislocations near the ceramic particles where th ey are annihilated. In consequence, the lack of vacancies leads to a r eduction of both diffusion rate and nucleation and therefore to the fo rmation of PFZ in these areas. Spinel crystals were found to cover the Al2O3 Particles in all MMC specimens independent of the two heat trea tments.