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

Fatigue crack growth rates (d a/d N) in under and peak aged Al–Cu–Mg and Al–Cu–Li depend on environmental exposure given by water vapor pressure/loading frequency ( P H 2 O / f ) . The exposure dependence of d a/d N 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 d a/d N at moderate water vapor exposures to 2 Pa s. At higher exposures, H diffusion limitation and surface reaction saturation reduce the d a/d N dependence on P H 2 O / f , with plateau response above ∼500 Pa s. Slip morphology rather than solute or phase reactivity per se controls d a/d N 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 d a/d N degradation due to peak aging, only for Al–Cu–Mg and only in the low P H 2 O / f regime.