Crystallization Mechanism of Amorphous Mullite and the Al2O3-SiO2Phase Diagram

Abstract

ABSTRACTRecent progress in the crystallization of amorphous mullite (3Al2O3•2SiO2) and yttrium aluminate (Y3Al5O12, 5Al2O3•3Y2O3or “YAG”) beads and fibers is reviewed. Studies were made by differential thermal analyses, (DTA, DSC), synchrotron X-ray diffraction and Rietveld profile fitting, HREM, EDS and SEM. Crystallization kinetics, activation energies and microstructures were determined, leading to temperature-time-transformation (T-T-T) curves. The crystallization of mullite from the solid state proceeds by formation of large-grained, pseudo-tetragonal mullite of composition 70 mol% Al2O3, which is highly strained and which contains <10 nm silica-containing inclusions. During further heat treatments, the inclusions react with the first-formed mullite yielding single phase, orthorhombic mullite of 60 mol% Al2O3. Recrystallization also occurs on further heating with a significant reduction of grain size, in a manner similar to recrystallization in brass. These observations (using more modern and sensitive characterization techniques) suggest the presence of a peritectic-like, syntectic or syntectoidal invariant reaction in the Al2O3-SiO2phase diagram. Further work needs to be done to ascertain whether the reaction is an equilibrium or non-equilibrium phenomenon.