Environment-exposure-dependent fatigue crack growth kinetics for Al–Cu–Mg/Li

Fatigue crack growth rates (da/dN) in under and peak aged Al-Cu-Mg and Al-Cu-Li depend on environmental exposure given by water vapor pressure/loading frequency (PH2O/f). The exposure dependence of da/dN at constant stress intensity range exhibits four regimes, explained based on hydrogen environment embrittlement and three rate-limiting processes that are similar for each alloy and aging condition. Above a threshold environmental exposure (~0.01 Pa s), impeded molecular flow governs increasing da/dN at moderate water vapor exposures to 2 Pa s. At higher exposures, H diffusion limitation and surface reaction saturation reduce the da/dN dependence on PH2O/f, with plateau response above ~500 Pa s. Slip morphology rather than solute or phase reactivity per se controls da/dN for low to moderate exposures, since identical slow growth rates are produced for shearable precipitate or cluster structures that each promote heterogeneous slip-band formation and {1 1 1}-faceted cracking. Alloy design for fatigue crack growth resistance depends on the environmental exposure and strength requirements of the application, as shown by the dramatic difference in da/dN degradation due to peak aging, only for Al-Cu-Mg and only in the low PH2O/f regime.