Reaction and diffusion phenomena upon oxidation of a (γ + α2) TiAlNb alloy in air

A two-phase fully lamellar titanium aluminide alloy, Ti46Al8Nb, was exposed to air at temperatures ranging from 700 to 900 degrees C. Isothermal and cyclic oxidation tests were conducted for up to 1000 h. Oxidation kinetics was followed by gravimetric measurements. Post-test examination of specimens comprised light microscopy, scanning electron microscopy/energy dispersive X-ray spectrometry (EDS), transmission electron miscoscopy/EDS, X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, glow discharge spectrometry, nanoindentation and scratch test. Very thin and perfectly adherent scales were composed of nanometric grains of aluminium oxide with a corundum structure on the surface, followed by a compact layer of titanium dioxide with a rutile structure and a fine-grained porous mixed-oxide layer. Chains of voids appeared locally between the compact and the porous part of the scale after extended exposure. The alloy-scale interface was complex, and contained coarse grains of alumina and Nb-rich precipitates embedded in a Ti-rich matrix. Niobium, as the least mobile element, marked the diffusion front. Major transport processes in the near-surface zone were nitrogen and oxygen inward diffusion and aluminium outward diffusion. The scale was not uniform in thickness, which might be related to grain-boundary diffusion, different orientations of the colonies of lamellac and phase composition of the lamellae on the specimen surface.