High-temperature oxidation behavior of Al2O3/TiAl matrix composite in air

Abstract

The oxidation behavior of Al2O3/TiAl in situ composites fabricated by hot-pressing technology was investigated at 900° in static air. The results indicate that the mass gains of the composites samples decrease gradually with increasing Nb2O5 content and the inert Al2O3 dispersoids effectively increase the oxidation resistance of the composites. The higher the Al2O3 dispersoids content, the more pronounced the effect. The primary oxidation precesses obey approximately the linear laws, and the cyclic oxidation precesses follow the parabolic laws. The oxidized sample containing Ti2AlN and TiAl phases in the scales exhibits excellent oxidation resistance. The oxide scale formed after exposure at 900°C for 120 h is multiple-layered, consisting mainly of an outer TiO2 layer, an intermediate Al2O3 layer, and an inner TiO2+Al2O3 mixed layer. From the outer layer to the inner layer, TiO2+Al2O3 mixed layer presents the transit of Al-rich oxide to Ti-rich oxide mixed layer. Near the substrate, cross-section micrograph shows a relatively loose layer, and micro- and macro-pores remain on this layer, which is a transition layer and transferres from Al2O3+TiO2 scale to substrate. The thickness of oxide layer is about 20 μm. It is also found that continuous protective alumina scales can not be observed on the surface of oxidation scales. Ti ions diffuse outwardly to form the outer TiO2 layer, while oxygen ions transport inwardly to form the inner TiO2+Al2O3 mixed layer. Under long-time intensive oxidation exposure, the internal Al2O3 scale has a good adhesiveness with the outer TiO2 scale. No obvious spallation of the oxide scales occurs. The increased oxidation resistance by the presence of in situ Al2O3 particulates is attributed to the enhanced alumina-forming tendency and thin and dense scale formation. Al2O3 particulates enhance the potential barrier of Ti ions from M/MO interface to O/MO interface, thereby the TiO2 growth rate decreases, which is also beneficial to improve the oxidation resistance. Moreover, the multi-structure of the TiO2+Al2O3 mixed layer decreases the indiffusion of oxygen ions and also avails to improve the high temperature oxidation resistance of the as-sintered composites.