Interrelation of TEM microstructure, composition, tensile properties, and corrosion resistance of Al-Cu-Mg-Mn alloys

Electron microscopy was used to investigate the effects of alloying element content and quenching rate on the extent of precipitation during quenching and the resulting type of corrosion attack of naturally aged Al-Cu-Mg-Mn, Al-Cu, Al-Cu-Mg and Al-Cu-Mn alloys. Magnesium addition to Al-Cu led to S-phase (Al2CuMg) grain boundary precipitation and development of dislocation loops of the condensed vacancy type. This appears attributable to the greater affinity of magnesium atoms for copper atoms than for vacancies. Manganese, in the presence of copper and magnesium, retarded precipitation, apparently because of its high affinity for vacancies, lowering the number of unassociated vacancies available for long range diffusion. While increasing as-quenched strengths, this association apparently has the effect of developing grain boundary regions with steep electrochemical potential gradients, which appear responsible for susceptibility of the alloy to intergranular corrosion. It is concluded that precipitation phenomena which control the hardening and corrosion behavior of these alloys are related to the relative binding energies between the several solute elements, the affinities of these individual elements for vacancies, and the ratio of the atomic concentrations of the solute elements.