Investigating the initial cluster formation mechanisms in high-strength 6xxx aluminium alloys by atom probe tomography

Aluminium alloys are seeing wider use in the automotive sector as part of a drive to reduce vehicle weight and improve energy efficiency. In this study two variants of a high strength Al-6xxx alloy were investigated; one with a low Cu content, the other with a high Cu content. The aim was to gain greater understanding of the mechanisms governing precipitation after natural ageing and the effect Cu content has on this precipitation, in relation to improving mechanical properties for automotive crash management systems. The ageing process for the investigated alloy involves two stages of raised temperature ageing; a lower temperature pre-age and a higher temperature final age. Quantification of the smallest solute clusters in Al-6xxx via atom probe tomography (APT) is challenging. As such, novel data analysis techniques were introduced to derive reliable quantitative information from the APT data, with minimal reliance on user selected parameters. In particular the point excess solute, the fraction of solute in excess above the matrix concentration, is introduced and used as a quantitative measure of the degree of solute segregation. APT samples were produced with a range of natural ageing times, the shortest of which was 459 s. No Mg-Mg segregation was found after 30-60 minutes of natural ageing. Si and Cu co-segregation was observed in all samples, even for the shortest natural ageing times, but cannot be confirmed as genuine segregation or an artefact of APT field evaporation process. The amount of solute segregation increased with further natural ageing. No difference in the fraction of solute segregated was found between the low Cu and high Cu alloys. An "immediate" pre-age or a "delayed" pre-age, with a short or long period of natural ageing respectively, was applied to samples of both alloys. Immediate pre-aged samples were found to contain small precipitates, while delayed samples contained solute clusters and a few heterogeneous precipitates nucleated on dislocations. The precipitates found after immediate pre-ageing in the high Cu alloy were smaller and had a higher number density than in the low Cu alloy. These precipitates were also more spherical, while the low Cu alloy contained short rods precipitates. It is surmised from this that Cu is modifying the precipitate-matrix interface, removing the preference for anisotropic precipitates. DSC experiments showed that dissolution of solute clusters occurs in the delayed samples, but not in immediately pre-aged samples. This dissolution occurred in both the high and low Cu alloys. After final ageing elongated rod precipitates formed in all samples. In the low Cu alloy the rods were short and had a high number density after immediate pre-ageing but were larger and had a lower number density after delayed pre-ageing. In the high Cu alloy there was also a higher number density of precipitates in the immediate material. Both high Cu conditions contained a higher number density of precipitates than found in the low Cu alloy. The precipitate number density correlates with the mechanical properties, the high Cu alloy having a higher ultimate tensile strength than the low Cu alloy, as did the immediate samples relative to the delayed. In APT data, rod precipitates were found to have nucleated from small precipitates. These small precipitates are thought to be the precipitates observed after immediate pre-ageing and a remaining population of solute clusters, present after delayed pre-ageing, which did not all dissolve. The number density of larger precipitates corresponded to the number density of small precipitates. The high Cu alloy had a higher number density of small precipitates after final ageing in both immediate and delayed conditions. In the delayed samples it is supposed that more solute clusters do not dissolve during final ageing in the high Cu alloy, due to Cu lowering the solute cluster/precipitate-matrix interfacial energy. A tensile strain was applied to some high Cu alloy samples in between pre-ageing and final ageing. The ultimate tensile strength of the stretched material was increased relative to the un-stretched. The stretching increased the relative Si and Cu content of the solute clusters present after delayed pre-ageing. This did not appear to affect whether these clusters dissolved and the same number density of small precipitates was found after final ageing as without stretching. These small precipitates found after stretching were also found to be acting as nucleation sites for rod precipitates. After stretching low aspect ratio lath precipitates were found, which were not observed without stretching. These are thought to have nucleated on the non-equilibrium dislocations introduced by the cold work. Overall the precipitate number density when fully aged was lower with application of stretch and the increase in strength was thus attributed to change in morphology and type of precipitates formed.