Thermodynamic considerations of the beneficial effect of halogens on the oxidation resistance of TiAl-based alloys

The high-temperature oxidation resistance of γ-TiAl alloys can be significantly improved by the addition of small amounts of halogens, e.g. by ion implantation. The mechanism for this so-called halogen effect is based on the exclusive transport and subsequent oxidation of a gaseous aluminium-containing species in the inner region of the initially-formed scale to develop an Al2O3-rich protective layer. The present study verified this mechanism on the basis of thermodynamic calculations in the temperature range 700–1100 °C. The standard Gibbs energies of the most important reactions for the halogens F, Cl, Br, and I were considered and temperature-dependent halogen partial-pressure limits for the occurrence of the beneficial effect were determined. The lower partial-pressure limit was considered to be dictated by the minimum supply rate of aluminium necessary to sustain Al2O3 scale growth, while the upper limit was dictated by the competition between Al2O3 and TiO2 formation. The results from the chlorine calculations were used to estimate the number of chlorine atoms necessary at the alloy surface for the beneficial effect to operate.