Highly densified Al2O3 was prepared from a transition (mainly q-phase) Al2O3 powder doped with a-seeds. Two thermal processing steps resulted in the densification and the q- to a-phase transformation were characterized by thermomechanical analysis (TMA), porosimetry, scanning and transmission electron microscopies (SEM and TEM). The evolution of the grain growth and porosity were observed. The results showed that the first-stage densification in association with the q- to a-phase transformation could be controlled to form ultrafine and uniform microstructures, and that would benefit the second-stage densification. The additional a-seeds do not only enhance the transformation rate and give a narrower pore size, but also allow the growth of uniform a-grains to sizes ca. 60 nm. The final densification properties of two seeded samples by thermal treatments were investigated revealing the effects of the transformation in the first stage and the influence of two sintering parameters, e.g. temperature and soaking time on the sintered microstructure.

Keywords: alumina, theta phase, seeding, densification, microstructure