Crystallization in a Barium‐Containing Magnesium Aluminosilicate Glass‐Ceramic

Crystallization in a Ba‐containing magnesium aluminosilicate (Ba‐MAS) glass‐ceramic was studied using transmission electron microscopy. Ba‐MAS slabs were hot‐pressed at 1000°C to form dense glassy bodies, which were crystallized by further heating in air or argon between 850° and 1300°C. Heterogeneous nucleation of a metastable highquartz solid solution (μ‐cordierite) occurred at crystallization temperatures below 1100°C, followed by dendritic growth; the interdendrite phase was Ba‐rich silicate glass. The high‐quartz solid solution generally acted as a precursor for the nucleation of the stable β ‐phase, which was the final crystallization product. Surprisingly, slower crystallization kinetics were observed in argon than in air. Further morphological changes at the higher temperatures and longer annealing, times resulted from a Rayleigh type instability, which led to breakup of the interdendrite silicate rods, and simultaneous polygonization of β ‐cordierite.