Vitrification and devitrification phenomena in the ternary MgO–Al2O3–B2O3 system

The ternary MgO–Al2O3–B2O3 system was studied with respect to the final product that either amorphous or devitrified glasses in the down middle region of the phase diagram. The developed phases in either the devitrified or the crystallized glasses were investigated. Magnesium borate (Mg2B2O5) and a spinel (MgAl2O4) phases were developed in the devitrified samples during the cooling of the melts, whereas the crystallization of the formed glasses led to the development of magnesium borate (e.g. suanite (Mg2B2O5) and kotoite (Mg3B2O6)) and aluminium borate (Al4B2O9) phases. The microstructures of the studied crystalline glasses were characterized by volume crystallization of either lath- or rounded-like crystals of suanite and Al-borate phases. The grain size of the formed crystals were increased from 10 μm to 500 nm with the domination of suanite or Al-borate phases. The coefficient of thermal expansion (CTE) changed from 69 to 59 × 10−7 °C−1 for glasses and from 81 to 56 × 10−7 °C−1 for the glass-ceramics, in the 20–500 °C range. The data of the glasses were correlated to the local structure changed induced by increasing of Al2O3/MgO and MgO/B2O3 ratios. The results of the glass-ceramics were explained on the basis of the nature and concentration of all phases formed including a residual glassy matrix.