Quantification of High-Temperature Transition Al O and Their Phase Transformations*.

High-temperature treatment of γ-Al O can lead to a series of polymorphic transformations, including the formation of δ-Al O and θ-Al O . Quantification of the microstructure in the range where δ- and θ-Al O are formed represents a formidable challenge, as both phases accommodate a high degree of structural disorder. In this work, we explore the use of an XRD recursive-stacking formalism for the quantification of high-temperature transition aluminas. We formulate the recursive-stacking methodology for modelling of disorder in δ-Al O and twinning in θ-Al O and show that explicitly accounting for the disorder is necessary to reliably model the XRD patterns of high-temperature transition alumina. We also use the recursive stacking approach to study phase transformation during high-temperature (1050 °C) treatment. We show that the two different intergrowth modes of δ-Al O have different transformation characteristics and that a significant portion of δ-Al O is stabilized with θ-Al O even after prolonged high-temperature exposures.