Internal Nucleation of Highly Undercooled Magnesium Metasilicate Melts

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Abstract
Crystallization and vitrification in undercooled, fine magnesium silicate droplets, with compositions ranging from 34.5 ≤ wt% MgO ≤ 39.9, were examined following containerless drop tube processing. From an initial phase assemblage of a mixture of the metasilicate (MgSiO3) polymorphs orthoenstatite and clinoenstatite, three morphological powder types were observed following processing: unmelted shards, glass spheres, and melted/recrystallized spheres. The primary phase in the powders processed at a maximum temperature of ∼1650°C is the high‐temperature metasilicate polymorph protoenstatite, with metastable forsterite (Mg2SiO4) also appearing. The melted/recrystallized spheres have the uniform, submicrometer texture of a glass ceramic, decisively different from the surface crystallization textures normally seen for melts/glasses of these compositions. Transmission electron microscopy results indicate that the glass‐ceramic texture occurs because the process technique allows a liquid‐phase immiscibility to precede crystallization. The phases and textures developed during containerless solidification processing of these metsilicate compositions are analyzed thermodynamically; the minimum amount of undercooling required for amorphous phase separation is evaluated using the metastable extensions of the forsterite + liquid and the silica‐rich, twoliquid miscibility phase boundaries. The application of metastable phase diagram analysis is demonstrated as an effective guide for identifying potential compositions for development of novel glass‐ceramics.

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