Abstract
The microstructural development of oxidizedγ-TiAl is presented with a focus on oxidation inair. The investigations were carried out usingconventional, analytical, and, especially,energy-filtered transmission electron microscopy (EFTEM). Threeimportant points were studied in detail: (1) the“nitrogen effect,” (2) the“surface-finish effect,” and (3) thesubsurface zone. Nitrogen leads to the formation of TiN andTi2AlN at the metal-scale interfaceinterrupting alumina and thereby preventing thedevelopment of a continuous alumina layer. TheAl-depletion layer formed during the oxidation process develops from a single-phaselayer, consisting of a cubic phase, to a two-phaselayer, consisting of the cubic phase andα2-Ti3Al. The cubic phase isnot known in the system Ti-Al-O-N. Oxidation in oxygen depends on the surfacepreparation of the sample with rapid oxidation kineticsfor fine polishing and slow kinetics for a 600-gritSiC-paper finish. The rougher surface finish leads to the development of a recrystallization zonenear the surface and supports the formation of acontinuous alumina layer in the early stages ofoxidation. As for the oxidation in air, the cubic phaseis formed first underneath the oxide scale,followed by α2-Ti3Alformation.
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