Crystallization mechanism of amorphous mullite and the Al₂O₃-SiO₂ phase diagram

Recent progress in the crystallization of amorphous mullite (3Al₂O₃·2SiO₂) and yttrium aluminate (Y₃Al₅O₁₂, 5Al₂O₃·3Y₂O₃ or "YAG") beads an 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-SiO₂ phase diagram. Further work needs to be done to ascertain whether the reaction is an equilibrium or non-equilibrium phenomenon.