INTERNAL NUCLEATION OF HIGHLY UNDERCOOLED MAGNESIUM METASILICATE MELTS

Crystallization and vitrification in undercooled, fine magnesium silicate droplets, with compositions ranging from 34.5 less-than-or-equal-to wt% MgO less-than-or-equal-to 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 approximately 1650-degrees-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 metasilicate 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, two-liquid 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.