Rapid Crystallization of SiO ₂ ‐Al ₂ O ₃ Glasses

Abstract

The crystallization of Al 2 O 3 ‐rich glasses in the system SiO 2 ‐Al 2 O 3 which were prepared by flame‐spraying and/or splat‐cooling was studied by DTA, electron microscopy, and X‐ray diffraction. Over a wide range of compositions, the crystallization temperature ( T x ) remained near 1000°C, changing smoothly with composition. In all cases crystallization of mullite was detected by X‐ray diffraction. In the low‐Al 2 O 3 region, coarsening of the microstructure during crystallization was observed by electron microscopy. In the high‐Al 2 O 3 region mullite and γ‐Al 2 O 3 cocrystallized; this behavior may be interpreted as evidence of a cooperative process of crystallization at the respective T x 's. The crystallite size of the mullite immediately after rapid crystallization increased continuously with increasing Al 2 O 3 content. In light of the T x data, the adequacy of the evidence for the proposed metastable miscibility gap in the SiO 2 ‐Al 2 O 3 system is questioned.