ENVIRONMENTAL FATIGUE OF AN AL-LI-CU ALLOY .1. INTRINSIC CRACK-PROPAGATION KINETICS IN HYDROGENOUS ENVIRONMENTS

Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates (da/dN) in peak-aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant DELTA-K and K(max) loading. Such rates are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness by aqueous 1 pct NaCl with anodic polarization, pure water vapor, moist air, and NaCl with cathodic polarization. While da/dN depend on DELTA-K4.0 for the inert gases, water vapor and chloride induce multiple power laws and a transition growth rate "plateau." Environmental effects are strongest at low DELTA-K. Crack tip damage is ascribed to hydrogen embrittlement because of accelerated da/dN due to parts-per-million (ppm) levels of H2O without condensation, impeded molecular flow model predictions of the measured water vapor pressure dependence of da/dN as affected by mean crack opening, the lack of an effect of film-forming O2, the likelihood for crack tip hydrogen production in NaCl, and the environmental and DELTA-K-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li2CO3 and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. Small crack effects are not observed for 2090; such cracks do not grow at abnormally high rates in single grains or in NaCl and are not arrested at grain boundaries. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075.