Consolidation of mechanically alloyed aluminium matrix composite powders by severe plastic deformation

Materials with refined microstructures continue to attract scientific and technological interest due to the expected advantages in mechanical properties when compared with conventional materials with coarse microstructure. In the recent years many processing alternatives, which result in fine microstructures have been developed and the technique of severe plastic deformation (SPD) is one of them, which can successfully be used to obtain ultra fine-grained metals and composites. Torsion straining under high pressure or severe plastic torsion straining is a SPD method that has proved to be useful in powder consolidation as well as in the formation of nanostructures with high-angle grain boundaries. In the present work, we use high pressure torsion straining to consolidate powders of A1-AA6061 alloy reinforced by 5%AlN particles. The aluminium matrix and reinforcement powders were mixed in a high-energy mill for two different lengths of times: 1.5 and 10 hours. The powders were cold consolidated into disks 0.3-0.5 mm thick and 7 mm wide by high-pressure torsion straining under a stress of 5 to 7.5 GPa. The disks were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). Mechanical properties were evaluated by microhardness measurements. The microstructure and hardness of the consolidated composites are discussed on the basis of the initial condition of the composite powders.