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 p recursor solutions. Two distinct metastable transformation sequences were i dentified for the amorphous powders upon heat treatment. Crystallisation ac companied 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 garn et at temperatures as low as 800 degreesC in the stoichiometric composition provided segregation is avoided during decomposition. Further heat treatme nt of a yttria-supersaturated gamma -Al2O3 leads to the precipitation of th e orthorhombic YAlO3 that is stable up to temperatures as high as 1600 degr eesC, 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-comp osition curves for the various competing phases. (C) 2001 Acta Materialia I nc. Published by Elsevier Science Ltd. All rights reserved.