Influence of Sc and Zr on Mg precipitation and sensitization resistance in Al-Mg-(Sc-Zr) alloys

Precipitation kinetics of Al 3 Mg 2 in Al-Mg binaries and Al-Mg-Sc-Zr alloys were studied during artificial ageing in the range [170 ∘ C-250 ∘ C], with the aim to determine the influence of Al 3 (Sc, Zr) dispersoids on Mg precipitation and resistance to corrosion. A calibration with model alloys enabled a quantitative determination of the evolution of Mg content in solid solution which was coupled with APT measurements performed on samples aged for a long time. Precipitate morphology and distribution were analyzed via SEM and TEM techniques, identifying two phases, β ’ and β . The β phase is known to form a continuous network of precipitates on grain boundaries, making it sensitive to corrosion. Corrosion tests using nitric acid mass loss on aged samples demonstrated the role of Al 3 (Sc, Zr) dispersoids in enhancing intergranular corrosion resistance. Results showed mass losses 4.5 times higher for the Al-Mg alloy compared to the alloy containing dispersoids. To illustrate the contribution of different nucleation sites and thus their role in corrosion resistance, the competition between homogeneous and heterogeneous nucleation of Al 3 Mg 2 was modeled based on Classical Nucleation and Growth Theories (CNGTs). Overall, this study, shows that Al 3 (Sc, Zr) dispersoids accelerate the precipitation kinetics of Al 3 Mg 2 by a decade by lowering the nucleation energy barrier, thereby improving the sensitization resistance of Al-Mg alloys. • Gorter Nordheim equations are calibrated to determine the evolution of Mg content in solid solution during ageing • Addition of Sc and Zr act as preferential nucleation sites for Mg atoms • Dispersoids increase the sensitization resistance of Al-Mg alloys • Mean-field model helps understanding the competition between each nucleation sites