Tertiary creep and ductile fracture in Al-Zn alloy: a consequence of structure development

The cause of failure in ductile materials has been, for many years, related to the presence of cavities. However, although large cracks or cavities have been associated with final fracture, direct observation of very small cavities throughout the neck development in pure metals or solid solution alloys has never been made. Cavity nucleation requires the existence of large local effective stresses for long enough periods of time. The different stages of necking in an Al-11 wt% Zn alloy were studied since it is known that large effective stresses are produced at sub-boundaries during the secondary stage of creep. No cavities were observed. Instead, the boundaries support large effective stresses which continue to be relaxed by dislocation emission throughout all the neck development. The subgrain size decreases from about 20 μm at the onset of tertiary creep to about 0.6 μm at the tip of the broken specimen at the same time as the stress continuously rises due to the large reduction in section throughout the neck. The presence of coarse slip bands from the beginning of the tertiary stage is related to strain localization as the high effective stresses lead to sub-boundary breakthrough and localized extensive dislocation activity. Fracture of the specimen occurs by shear and microcrack development following the localized softening associated with sub-boundary destruction