Plastic strain and grain size effect on high-pressure phase transformations in nanostructured TiO2 ceramics

Anatase TiO2 was severely deformed using high-pressure torsion and the effect of plastic strain and grain size on phase transformations was investigated. A high-pressure TiO2-II phase (columbite) with the orthorhombic structure was formed under pressures of 1 and 6 GPa. Fraction of TiO2-II increased with increasing the plastic strain and remained stable at ambient pressure. Microstructural analysis showed that TiO2-II was stabilized in grains with sizes less than similar to 15 nm because of high energy barrier for reverse phase transformation, while larger grains had the anatase structure. Large densities of oxygen vacancies and dislocations were also formed after severe plastic deformation. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.