Quantification of High Temperature Transition Al2O3 and Their Phase Transformations

High temperature exposure of gamma-Al₂O₃ can lead to a series of polymorphic transformations, including the formation of delta-Al₂O₃ and theta-Al₂O₃. Quantification of the microstructure in the delta/theta-Al₂O₃ formation range represents a formidable challenge as both phases accommodate a high degree of structural disorder. In this work, we explore the use of XRD recursive stacking formalism for quantification of high temperature transition aluminas. We formulate the recursive stacking methodology for modelling of disorder in delta-Al₂O₃ and twinning in theta-Al₂O₃ and show that explicitly accounting for the disorder is necessary to reliably model the XRD patterns of high temperature transition alumina. In the second part, we use the recursive stacking approach to study phase transformation during high temperature (1050 ºC) treatment. We show that the two different intergrowth modes of delta-Al₂O₃ have different transformation characteristics, and that a significant portion of delta-Al₂O₃ is stabilized with theta-Al₂O₃ even after prolonged high-temperature exposures. In discussions, we outline the limitation of the current XRD approach and discuss a possible multimodal XRD and NMR approach which can improve analysis of complex transition aluminas.