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-degrees-C to form dense glassy bodies, which were crystallized by further heating in air or argon between 850-degrees and 1300-degrees-C. Heterogeneous nucleation of a metastable high-quartz solid solution (mu-cordierite) occurred at crystallization temperatures below 1100-degrees-C, followed by dendritic growth; the interdendrite phase was a Ba-rich silicate glass. The high-quartz solid solution generally acted as a precursor for the nucleation of the stable beta-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 beta-cordierite.