5. Consolidation of Al Powder and Colloidal Suspension of Al₂O₃ Nanoparticles After 16 h Ball Milling

In order to have a more homogeneous particle dispersion, a much more severe ball milling process was carried out. A higher ball-to-powder ratio (r = 10:1) was adopted and the attritioning process was performed for 16 h. So far, better dispersion was achieve by using colloidal solution of γ-Al₂O₃ nanoparticles in isopropyl alcohol than by using dry-Al₂O₃. Thus, the composites were prepared again by adding 2 wt% of colloidal alumina to the Al powder. The micrographs of the starting materials correspond to those shown in Figs. 3.​1 and 4.​1. Either ECAP at 400 °C or hot extrusion at 400 °C were performed to consolidate the composite powder after milling. A selection of the extruded billets were then cold rolled down to a cross section of 1 mm² to verify the workability of the material. Pure Al powder was also consolidated and cold worked following the same procedure adopted for the nanocomposites so as to investigate the properties of a reference unreinforced sample.