Metastable extension of the fluorite phase field in Y₂O₃-ZrO₂ and its effect on grain growth

Abstract

Pure Y₂O₃ and Y₂O₃-ZrO₂ solid solutions have been prepared by melt atomization and by pyrolysis of nitrate solutions. Extended solubility is readily achieved in both techniques for the entire composition range investigated: melts with 0-30% ZrO₂ and precursors with 0-50% ZrO₂. However, solidification of under cooled droplets yields almost exclusively single phase powders with the structure of cubic yttria (D5₃). In contrast, the pyrolysis route leads to a sequence of metastable microstructures beginning with a nanocrystalline disordered fluorite-based (C1) solid solution. Further heating leads to the evolution of much larger (micron size) flake crystals with a {001} texture, concurrent with partial ordering of the oxygen ions to the sites occupied in the D5₃ structure. The driving force for ordering and the rate of grain growth decrease with increasing ZrO₂ addition. Abrupt heating to high temperatures or electron irradiation can induce ordering without substantial grain growth. There is no significant reduction in porosity during the recrystallization, which with the other observations suggests that grain growth is driven by the free energy available for the ordering transformation from fluorite to the yttria structure. This route offers opportunities for single crystal thin film development at relatively low processing temperatures.