Microstructural study of oxidized γ-TiAl

The microstructural development of oxidized gamma - TiAl is presented with a focus on oxidation in air. The investigations were carried out using conventional, analytical, and especially, energy-filtered transmission electron microscopy (EFTEM). Three important points were studied in detail. (1) the "nitrogen effect," (2) the "Surface-finish effect, " and (3) the subsurface zone. Nitrogen leads to the formation of TiN and Ti2AlN at the metal-scale interface interrupting alumina and thereby preventing the development of a continuous alumina layer. The Al-depletion layer formed during the oxidation process develops from a single-phase layer, consisting of a cubic phase, to a two-phase layer, consisting of the cubic phase and alpha 2-Ti3Al The cubic phase is not known in the system Ti-Al-O-N. Oxidation in oxygen depends on the surface preparation of the sample with rapid oxidation kinetics for fine polishing and slow kinetics for a 600-grit SiC-paper finish. The rougher surface finish leads to the development of a recrystallization zone near the surface and supports the formation of a continuous alumina layer in the early stages of oxidation. As for the oxidation in air, the cubic phase is formed first underneath the oxide scale, followed by alpha(2)-Ti3Al formation.