Crystallization Mechanisms in Glass-Ceramics

Transmission electron microscopy, electron diffraction, and microprobe X-ray analysis were used to study crystallization of glasses in the systems Li₂O-SiO₂, BaO-SiO₂, and Li₂O-A1₂O₃-SO₂. The ternary system, with 4 mol% TiO₂ added to an Li₂O-Al₂O₃-4SiO₂ composition, crystallizes with a simple morphology of equiaxed grains of the β-quartz metastable phase which transforms at higher temperatures to the stable β-spodumene structure. The binary systems exhibit a more complex crystallization morphology dictated by crystal anisotropy, temperature, impurity content, and susceptibility either to intermediate-phase formation (BaO-SiO₂) or to liquid immiscibility (Li₂O-SiO₂). The initial crystal growth units formed in these systems are frequently two-phase branched morphologies many micrometers in diameter. They may be recrystallized to form polycrystalline glass-ceramics with submicrometer grain sizes.