Phase separation, crystallization and polyamorphism in the Y₂O₃-Al₂O₃ system

A detailed study of glass formation from aerodynamically levitated liquids in the (Y₂O₃)_x(Al₂O₃)_1-x system for the composition range 0.21≤x≤0.41 was undertaken by using pyrometric, optical imaging and x-ray diffraction methods. Homogeneous and clear single-phase glasses were produced over the composition range 0.27≤x≤0.33. For Y₂O₃-rich compositions (0.33≤x≤0.375), cloudy materials were produced which contain inclusions of crystalline yttrium aluminium garnet (YAG) of diameter up to 40 μm in a glassy matrix. For Y₂O₃-poor compositions around x = 0.24, cloudy materials were also produced, but it was not possible to deduce whether this resulted from (i) sub-micron inclusions of a nano-crystalline or glassy material in a glassy matrix or (ii) a glass formed by spinodal decomposition. For x = 0.21, however, the sample cloudiness results from crystallization into at least two phases comprising yttrium aluminium perovskite and alumina. The associated pyrometric cooling curve shows slow recalescence events with a continuous and slow evolution of excess heat which contrasts with the sharp recalescence events observed for the crystallization of YAG at compositions near x = 0.375. The materials that are the most likely candidates for demonstrating homogeneous nucleation of a second liquid phase occur around x = 0.25, which corresponds to the limit for formation of a continuous random network of corner-shared AlO₄ tetrahedra