An analytical electron microscopy study of constituent particles in commercial 7075-T6 and 2024-T3 alloys

To better understand the role of constituent particles in pitting corrosion, analytical electron microscopic studies were performed on the constituent particles in commercial 7075-T6 and 2024-T3 alloys. Five phases, namely, Al23CuFe4 and amorphous SiO2 in 7075-T6 and Al2CuMg, Al2Cu, and (Fe,Mn)(x)Si(Al,Cu)(y) in 2024-T3, were identified. The crystal structure and chemistry of the Al23CuFe4, Al2CuMg, and Al2Cu phases in these alloys are in good agreement with the published data. Small deviations from their stoichiometric compositions were observed and are attributed to the influence of alloy composition on the phase chemistry. For the (Fe,Mn)(x)Si(Al,Cu)(y) (approximately, x = 3 and y = 11) phase, a rhombohedral structure, with lattice parameter a = b = c = 1.598 nm and alpha = beta = gamma = 75 deg, was identified and is believed to be a modified form of either Al8Fe2Si or Al10Mn3Si. Information from this study provided technical support for studying the electrochemical interactions between the individual particles (or phases) and the matrix. The corrosion results are reported in a companion article.