Effect of artificial aging on the fatigue crack propagation resistance of 2000 series aluminum alloys

Artificial aging performed to impart higher strength generally degrades the fatigue crack propagation (FCP) resistance of naturally aged 2xxx-series aluminum alloys. This behavior is examined for commercial AA2024-T351 and a naturally aged Al–Cu–Mg–Li alloy stressed in high-humidity air. Environmental fatigue crack growth rate decreases with initial-artificial aging, then increases monotonically with increasing aging time. Overaging does not improve cracking resistance. This effect of microstructure is pronounced at low stress intensity factor range and persists for high stress ratio conditions that minimize crack closure. For both alloys, the highest resistance to fatigue crack growth correlates with the presence of artificial aging intensified solute clusters, and absence of distinct precipitates, as evidenced by electron microscopy and small-angle neutron scattering (SANS). Increased crack growth rates correlate with the dissolution of clusters and/or the formation of an increasing amount of S′ precipitates for AA2024 and T1 precipitates for the Li-bearing alloy. The fundamental effects of very fine-scale clusters and precipitates on cyclic-slip mode and environment-sensitive crack tip damage are unresolved.