Quantification of High-Temperature Transition Al2 O3 and Their Phase Transformations*.

ABSTRACT

High-temperature treatment of γ-Al2 O3 can lead to a series of polymorphic transformations, including the formation of δ-Al2 O3 and θ-Al2 O3 . Quantification of the microstructure in the range where δ- and θ-Al2 O3 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-Al2 O3 and twinning in θ-Al2 O3 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-Al2 O3 have different transformation characteristics and that a significant portion of δ-Al2 O3 is stabilized with θ-Al2 O3 even after prolonged high-temperature exposures.