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 Li 2 O‐SiO 2 , BaO‐SiO 2 , and Li 2 O‐A1 2 O 3 ‐SO 2 . The ternary system, with 4 mol% TiO 2 added to an Li 2 O‐Al 2 O 3 ‐4SiO 2 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‐SiO 2 ) or to liquid immiscibility (Li 2 O‐SiO 2 ). 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.