Mean stress and environmental effects on near-threshold fatigue crack propagation on a Ti6246 alloy at room temperature and 500 degrees C

The cracking behavior of a Ti6246 alloy under cyclic loading superimposed t o different levels of mean stress is studied, with special attention to the near-threshold fatigue crack propagation regime, and to possible coupled e ffects of corrosion and creep. Tests were conducted at room temperature and 500 degrees C in selected environmental conditions (high vacuum, controlle d atmospheric leak low pressure, controlled partial pressure of water vapor in pure argon) and at different frequencies. The near-threshold crack prop agation at low K-max (i.e. low R ratio) is shown to be highly sensitive to the environment, and a predominant detrimental influence of water vapor is observed, even under very low partial pressure. Conditions for the occurren ce of an abnormal behavior consisting: in the disappearance of the threshol d for sufficiently:high K-max level, are discussed from tests performed at various constant K-max levels in ambient air, high vacuum and humidified ar gon. This effect is observed in air and in vacuum for K-max higher than 52 MPa root m, and is related to an intrinsic creep damage process which appea rs more efficient at room temperature than at 500 degrees C acid more accen tuated in air than in vacuum. The origin of this abnormal near-threshold be havior is discussed in comparison with a similar behavior described in the literature at room temperature on another type of Ti6246 alloy and at 120 d egrees C on an IMIX834, additional experiments conducted at 500 degrees C i n humidified argon have shown a critical K-max level reduced to 22 MPa root m. This behavior is:suspected to be related to a contribution of stress co rrosion cracking induced by water vapor when some conditions favoring a loc alization of the deformation and the attainment of a critical embrittlement are fulfilled.