Near-threshold fatigue crack growth mechanisms in TiAl alloys

In view of the introduction of TiAl-based alloys into structural components the fatigue crack propagation of these materials must be assessed. However, as for conventional engineering alloys, their fatigue crack propagation resistance results from a complex balance between processes of different nature (metallurgical, chemical, mechanical,...). Besides there are reports in literature about an anomalous temperature dependence of the fatigue crack growth resistance of TiAl alloys. In such cases the higher resistance is observed at elevated temperature above the brittle-to-ductile transition, the lowest just below this transition and the room temperature is intermediate between these two situations. These differences in resistance would be related to different contribution of intrinsic resistance, crack tip shielding by oxide-induced closure and environmental effects. In an attempt to elucidate this point, the present study is tackling the issue of identifying the different micromechanisms involved in the fatigue crack growth process and quantifying their respective influence by conducting a series of tests on a quaternary alloy Ti-48Al-2Cr-2Nb in air at different temperatures (25degreesC, 150degreesC, 500degreesC, 750degreesC and 800degreesC) and under different environmental conditions. The detailed analysis of the results indicates that the nearly temperature-independent fatigue crack growth behavior in ambient air result from a temperature-independent intrinsic resistance in the near-threshold regime associated with a predominant roughness-induced closure mechanism and a nearly constant fatigue crack growth enhancement by water vapor.