Temporal evolution of crystallization in MgO-Al2O3-SiO2-ZrO2 glass ceramics
From glasses with the composition 51.9 SiO 221.2 MgO21.2 Al 2O 35.7 ZrO 2 (in mol %), high and low-quartz solid solution glass ceramics were formed upon heat treatment at 950 °C for different dwell times. A combination of X-ray diffractometry, transmission electron microscopy, and scanning transmission electron microscopy with annular dark field imaging and energy-dispersive X-ray analysis revealed the temporal evolution of the volume crystallization. It is found that the crystallization of high-quartz solid solution is nucleated either directly at star-shaped ZrO 2 crystallites or within a zone of Zr-depleted glass surrounding the latter and proceeds isotropically into the sample volume, thereby expelling secondary, circular ZrO 2 precipitates. With advancing annealing time, the quartz solid solution is depleted in Mg and Al since indialite (Mg 2Al 4Si 5O 18) and spinel (MgAl 2O 4) are formed as well, thus enabling a change from high-quartz solid solution to the low-quartz modification while sample cooling. Upon analyses of samples heat treated for different soaking times, different states of crystallization were observed. Using electron diffraction and nanoanalytical techniques, a growth model with special emphasis on the early stages of volume crystallization was developed.