Study of precipitation in Al-Mg-Si Alloys by atom probe tomography II. Influence of Cu additions

Atom probe tomography (APT) analysis and hardness measurements have been used to characterise the early stages of precipitation in three Al-Mg-Si alloys with different Cu contents (Al-0.51 at.%Mg-0.94 at. %Si, with 0.01 at.%, 0.06 at.%, or 0.34 at.% Cu). A range of single and multi- stage heat treatments were chosen to evaluate the changes in precipitation processes. Three ageing temperatures were investigated, 298 K (natural ageing), 353 K (pre-ageing) and 453 K (automotive paint-bake conditions). The Cu content had significant effects on the microstructural evolution within the alloy. Formation of clusters which can act as precursors of elongated precipitates during paint-baking was found to be enhanced with increasing Cu content. This improved the paint-bake hardening response and mitigated the deleterious effects of natural ageing. Cu was present in all precipitates in the highest Cu-containing alloy. These precipitates were believed to be precursors to the Q' phase. Mechanisms for the effects of Cu on precipitation kinetics are proposed. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.