Nanostructured Al–Al2O3 composite formed in situ during consolidation of ultrafine Al particles by back pressure equal channel angular pressing

Ultrafine pure Al particles were consolidated into fully dense bulk material using back pressure equal channel angular pressing (BP-ECAP). The consolidation was carried out at 400 °C with a back pressure of 200 MPa. A fully dense Al–Al₂O₃ composite consisting of mostly nanocrystalline Al and γ-Al₂O₃, a small fraction of ultrafine Al grains and amorphous alumina was produced after four passes from the freshly formed particles. In contrast, no consolidation was achieved from the aged particles which had been kept for between 18 and 24 months. The formation of the nanostructure was attributed to the interaction between severe shear deformation and in situ oxidation during ECAP. The ultimate strength of the nanostructured material reached ～740 MPa in compression with a plastic strain to fracture of the order of ～1%. It is demonstrated that ultrafine particles can be well consolidated by ECAP when they are sheared to change shape rather than to slide over each other.