Influence of different annealing processes on the oxidation behaviour of γ-TiAl

Titanium aluminides are an attractive material for aerospace as well as automotive industry particularly due to their low mass density and good mechanical properties in the temperature range between 700°C and 900°C. But above 750°C titanium aluminides show a poor oxidation behaviour caused by fast growing titania. In this study particularly thermally pre-treated γ-TiAl was investigated with regard to the oxidation resistance in comparison to non pre-treated material. The Ti-45Al-8Nb alloy was exposed under various conditions, such as different atmospheres (high vacuum, Ar- and H2-atmosphere), temperature (900°C and 1000°C) and time (1-200h). Under high vacuum conditions (10-6mbar) only phase transformation in the surface region took place. The lamellar structure of the base material changed into single α2-Ti3Al with oxygen enrichment on top. Under Ar-atmosphere (50mbar) mainly nitrides were formed at the surface with an α2-layer and σ-Nb2Al precipitations below. During annealing γ-TiAl in hydrogen under atmospheric pressure (1040mbar) the material oxidized relatively fast. A dense TiO2 layer was formed on top, followed by a mixed oxide scale and a relatively thick region of alumina and σ-Nb2Al precipitations in an α2-Ti3Al matrix. Several samples of these various thermally pre-treated materials were tested by cyclic oxidation at 900°C in air for investigating the oxidation behaviour. The microstructures of the formed oxides of pre-treated and non pre-treated γ-TiAl are comparable. The material pre-treated under high vacuum conditions exhibited the lowest oxidation rate, while the mass gain of the specimens annealed under Ar-atmosphere increased rapidly in the first cycles. All pre-treated specimens exceeded a lifetime of 600 cycles at minimum. The reference material showed spallation already after 520 cycles. The microstructure and phase formation of all annealed and tested samples were investigated by SEM and EDX