Retention of nanoscale grain size in bulk sintered materials via a pressure-induced phase transformation

Pressure-induced phase transformations during high pressure (1.5 GPa)/low temperature (similar to 1/3Tm) sintering of the metastable anatase polymorph of nanocrystalline TiO2 is described in this paper. We have found that after sintering the transformed product (rutile) had a smaller grain size than the starting parent phase (anatase) under the same processing conditions (sintering pressure, temperature, and time). We attribute this effect to an increased nucleation rate and lowered diffusion rate brought on by the high sintering pressure. As expected, the grain size of the parent anatase phase increased with sintering temperature; but surprisingly the transformed rutile grain size decreased with temperature. In contrast, previous work with low pressure or pressureless sintering has always shown enhanced grain growth of both the parent and daughter phases with increasing temperature. This anomalous behavior of the transformed rutile grain size is explained in terms of the competition between nucleation and growth processes during the pressure-induced phase transformation. We conclude that a fully dense compact with retained nano-grain size can be obtained by properly controlling the sintering temperature and pressure at which the metastable to stable phase transformation rakes place. (C) 1997 Acta Metallurgica Inc.