First stages of crystallization in a silicate glass: study by high-resolution electron microscopy.

The first stages of crystallization have been investigated for a glass in the system SiO2-Al2O3-Li2O with small amounts of titanium and zirconium as nucleating elements. The studies were performed by electron diffraction and electron microscopy in high-resolution mode. It has been found that the two phases forming the final glass ceramic crystallize successively. The first one gives small crystallites (less than 5 nm) randomly dispersed. The lattice plane images in several orientations are in good agreement with TiZrO4 crystal lattice. At 780 degrees C the average crystal size increases with increasing time of the heat treatments from 2.5 nm at 0.5 h to 4.5 nm at 2 h and then remains nearly constant. The crystallization of the second phase, a silicate solid solution with beta-quartz structure, occurs after complete crystallization of TiZrO4.