The Relationship between Structure and Stress-Corrosion Life in an Al–Zn–Mg Alloy

Abstract

A detailed structural examination has been carried out on extrusions in an Al–Zn–Mg alloy (Zn 4·0, Mg 2·0, Mn 0·33, Cr 0·18, Zr 0·15, Fe 0·26, Si 0·12%). The extrusions had been produced to cover a range of macro structures and so showed varying stress-corrosion lives. The peripheral zones of all the extrusions were recrystallized, and the regions within the cores that appeared to be “fibrous” on a macro scale were composed of a substructure in which sub grains of very similar orientation were situated in layer-like colonies. The particles precipitated along low-angle boundaries during ageing were finer and more numerous than those precipitated on high-angle boundaries, although the associated precipitate-free zones along both types of boundary were of similar width. In addition, some coarse intermetallic particles of a different origin were located preferentially on the high-angle boundaries. Observations of their paths indicated that the stress-corrosion cracks followed high angle boundaries preferentially, and that their progress was impeded when they encountered regions of sub grains. Thus, the stress-corrosion lives of the extrusions were affected by the distribution of high-angle boundaries within them. The reason why the cracks followed high-angle rather than low-angle boundaries must be associated with the nature of the interfaces at the boundaries, and/or with the different distributions of particles along them. The relative roles of susceptibility to mechanical failure and to electrochemical attack are discussed from these aspects.