Environmentally Assisted Crack Initiation of High-Strength 7XXX Aluminium Alloys in Humid Air

When aluminium is exposed to humid air, water molecules are reduced at the surface generating atomic hydrogen, which can be absorbed and embrittle the material. High-strength, zinc-containing 7XXX alloys are particularly at risk to suffer from Environmentally Assisted Cracking in water vapour (EAC WV) when mechanically loaded. The main objective of this thesis was to compare the EAC WV performance of alloys representing the conventional and the new generation type within the 7XXX series. The latter are characterized by an elevated zinc content and a higher Zn/Mg ratio to increase mechanical properties and reduce the quench sensitivity. Two materials are studied as representatives for both alloy classes, in particular, the effect of stress level, relative humidity and temperature on their EAC WV performance. The focus of this study was on crack initiation at smooth surfaces as this has been largely neglected in literature so far. Concluding from EAC lifetimes of smooth cylindrical specimens, stressed under uniaxial constant load, the susceptibility to EAC WV failure is significantly higher for the new generation alloy than for the conventional alloy. The analysis of the crack initiation indicates that several sub-processes contribute to the EAC WV mechanism and must be differentiated for the individual alloys.