Non-equilibrium phase synthesis in Al2O3-Y2O3 by spray pyrolysis of nitrate precursors

The phase evolution in the Al2O3-Y2O3 system has been studied for 4, 10, 15 and 37.5% Y2O3, amorphous powders prepared by spray pyrolysis of nitrate precursor solutions. Two distinct metastable transformation sequences were identified for the amorphous powders upon heat treatment. Crystallisation accompanied by partitioning leads to a mixture of hexagonal YAlO3 and gamma -Al2O3 (spinet structure). Partitionless crystallisation, on the other hand, leads directly to a gamma -Al2O3 solid solution in dilute alloys, and garnet at temperatures as low as 800 degreesC in the stoichiometric composition provided segregation is avoided during decomposition. Further heat treatment of a yttria-supersaturated gamma -Al2O3 leads to the precipitation of the orthorhombic YAlO3 that is stable up to temperatures as high as 1600 degreesC, while hexagonal YAlO3 converts to garnet, Y3Al5O12. A rationalisation of the phase evolution sequence has been attempted on the basis of kinetic considerations, cation coordination and semi-quantitative free energy-composition curves for the various competing phases. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.