Segregation of S precipitates at T/Al interfaces and relevant effects on localized corrosion mechanisms in Al-Cu-Mg alloy

The segregation behavior of S(Al2CuMg) precipitates at T(Al20Cu2Mn3)/Al interfaces during aging process in an Al-Cu-Mg alloy was investigated. It was found that the main alloying elements of Cu and Mg are uniform around T phases after solution treatment. The elemental segregation was not found at T/Al interfaces. During artificial aging, the S precipitates were favorable to nucleate at T/Al interfaces with the specific orientation relationships between T phase and Al matrix. Owing to the S precipitation preferential at T/Al interfaces, Cu/Mg lean zones are formed approaching the segregated S layers, leading to the formation of multishell structure surrounding T phases. The segregated S precipitates at T/Al interfaces were hardly coarsening even undergoing over aging due to the required Cu and Mg element depleted in the surrounding matrix by S precipitation. As a result, the growth of S aggregates is more difficult in comparison with the S phases normally precipitated from the Al matrix. Based on the multishell structure surrounding T phases, the effects of the S and elemental segregation on the localized corrosion behavior around T phases were revealed by STEM characterizations, and the localized corrosion mechanisms were elucidated finally.