Crystal structure development during devitrification of quenched mullite

The kinetics, microstructural changes, and crystal structure development for crystallization of amorphous, quenched, mullite composition glass (3Al2O3·2SiO2) were studied between 900 and 1400°C. The phenomena observed were characterized using non-isothermal differential scanning calorimetry (DSC), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), powder X-ray diffraction (XRD, with both a standard laboratory diffractometer, as well as with synchrotron radiation), and Rietveld analysis. Crystallization of amorphous mullite was observed to occur in two steps. The activation energy for crystallization was 892 kJ/mol for the first step and 1333 kJ/mol for the second step. From the amorphous state, the first phase(s) to crystallize were alumina-rich, pseudotetragonal mullite (∼70 mol% Al2O3). These crystals were highly strained and contained numerous nanometer scale inclusions. With increasing temperature, the crystals were observed to incorporate increasing amounts of SiO2, and approach the equilibrium orthorhombic structure. By 1400°C the pseudotetragonal to orthorhombic transition was complete, the strain was eliminated, most of the inclusions had been assimilated, there was ∼67% reduction in grain size, and the crystals had attained the composition of the initial, bulk glass (∼60 mol% Al2O3).