Formation of Z-Ti50Al30O20 in the sub-oxide zones of γ-TiAl-based alloys during oxidation at 1000°C

The oxidation behaviour of gamma-TiAl and alpha(2)-Ti3Al + gamma-TiAl alloys was studied at 1000 degrees C in pure oxygen up to 250 h. All alloy coupons were polished to a 1 mu m finish prior to oxidation. A series of gamma-Ti48Al52 coupons polished to a 600-grit finish were also prepared. The type of scale which formed during oxidation and the consequent subsurface changes in the alloy were found to depend on the presence and amount of alpha(2) in the starting alloy microstructure, surface finish, and oxidation time. The gamma-alloy with a 600-grit finish formed a protective Al2O3-rich scale during the early stages of oxidation, whereas the same alloy with a 1 mu m finish formed a less protective Al2O3 + TiO2 scale. In the former case, a continuous subsurface layer of Z-Ti50Al30O20 formed from gamma by essentially equal amounts of oxygen enrichment and aluminium depletion; essentially no titanium diffusion was required. With continued oxidation, the continuous Z-layer destabilized at the Z/gamma interface into a mixture of alpha(2)(O) + Z. The presence of alpha(2)(O) + Z in the subsurface zone always corresponded to the growth of a non-protective Al2O3 + TiO2 scale, suggesting that alpha(2)(O) precipitation initiates the breakdown of the initially formed Al2O3-rich scale. The subsurface zone in the gamma alloy with a 1 mu m finish varied, containing either internal Al2O3 and no Z phase or alpha(2)(O) + Z. The alpha(2) + gamma alloys formed a non-protective Al2O3 + TiO2 scale and subsurface zone of a2(0) + Z at all the times studied. The discontinuous distribution of gamma in the two-phase alloys prevented formation of a continuous Z layer. The Z phase was determined to be metastable on the basis of equilibration experiments with Ti-Al-O alloys. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.