Al-AIN composites consolidated from partially nitride aluminum alloy powders by Equal Channel Angular Pressing (ECAP)

Al-AlN composite powders were fabricated through nitriding aluminium powder mixtures of AA 6061-2%Mg-1%Sn-3%Nylon at 560 °C in nitrogen at atmospheric pressure. Each nitrided particle is enveloped by an in-situ formed AlN shell, which is a nanocomposite of Al-AlN, comprising nanoscale AlN whiskers and an Al matrix. The thickness of the ‘AlN shell’ increases with nitridation time, through which the AlN content can be readily controlled. The partially nitrided powders were consolidated using back pressure-assisted Equal Channel Angular Pressing (ECAP) at 200 °C, with a view to dispersing the nanoscale AlN. It was found that the processing behaviours of the Al-AlN composite powders were significantly affected by the AlN content. The ‘AlN shells’ were difficult to fragment as they deformed readily during ECAP. As a result, agglomeration or segregation of the AlN phase was observed after multi-passes of ECAP, particularly at large volume fractions. The attendant microstructures show shear banding and surface cracking when the AlN content is more than 20 vol.%. However, at a low volume fraction of AlN (8.9%), multi-passes of ECAP resulted in a dense Al-AlN microstructure with a reasonable dispersion of the AlN in the matrix