Coupled influence of microstructure and atmosphere environment on fatigue crack path in new generation Al alloys

The fatigue crack propagation behavior of new generation Al alloys developed for aeronautical applications is studied at moderate ΔK and in the near threshold domain. The crack growth rate and the crack path are shown to depend on alloy composition, aging condition and atmosphere environment, and to be governed by the slip morphology. In absence of environment assistance, a crystallographic (1 1 1) faceted cracking leads to a slow stage I-like propagation in Al–Li–Cu alloys and underaged Al–Cu–Mg alloys with microstructure consisting of shareable precipitates or solute cluster structures that promote heterogeneous slip-band formation, which is in contrast with a ductile transgranular featureless stage II crack path in overaged Al–Cu–Mg. In air at moderate ΔK, an adsorption assisted propagation mechanism is assumed to prevail in both Li and Mg bearing materials, water vapor assistance inducing a transgranular stage II regime associated to homogeneous slip generating a flat-facet and step-like features; in the near threshold domain, the same mechanisms is operating for Al–Cu–Mg alloys while even more accelerated growth rates and lower effective threshold for Al–Cu–Li alloys are attributed to an assistance of hydrogen produce from the dissociation of adsorbed water vapor.