Analytical and HREM study of the early stages of SiO2–Al2O3–(Mg, Zn)O glass crystallisation

The transformation from the amorphous into the crystalline structure at temperatures close to temperature of glass transition of 750 °C was investigated in a 47SiO2–22Al2O3–17MgO–2ZnO–12TiO2 glasses with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). The investigations indicated that heat treatment of this glass results in the formation of rod-like crystals of high-temperature quartz structure. At the early stages of crystallisation, the formation of (Mg, Al) titanates was observed. TEM of glasses heat-treated at 725 °C showed nucleation and growth of small dendrite-like precipitates enriched in Ti and depleted in Si. High-resolution transmission electron microscopy (HRTEM) has indicated that the crystallisation is occurring through a local ordering of the atoms in the amorphous glass structure, within areas up to 5 nm. Above 900 °C an enstatite (Mg, Zn)SiO3 phase is formed.