Quantitative studies of clustering and precipitation in Al-Mg-Si(-Cu) alloys

Abstract

In the present work, the clustering and precipitation reactions in age hardenable Al-Mg- Si(-Cu) (6xxx) alloys have been studied by atom probe tomography (APT), transmission electron microscopy (TEM) related methods and density functional theory (DFT) based calculations. In combination, these techniques give chemical and structural information at a sub-nanometre scale, allowing observation and interpretation of all stages of the phase separation process in these alloys.

By APT studies of Al-Mg-Si alloys it was found that all alloys where precipitation is boosted as a result of clustering contain clusters with compositions close to Mg1Si1. This accounts both for clusters produced during pre-aging heat treatment (~ 100°C) and clusters produced in alloys of low solute content during natural aging (NA) at room temperature. If, on the other hand, precipitation is retarded as a result of clustering, the clusters were found to have compositions different from Mg/Si ~ 1, being very Mg-rich or very Si-rich. These clusters form during NA in alloys of high solute content, and they were found to (partly) dissolve upon heat treatment at higher temperatures. This resulted in a strong reduction of the number of nucleation sites – which gave a similar reduction of precipitate number densities. This is commonly referred to as the negative NA effect. Addition of Cu atoms to Al-Mg-Si alloys has been reported to reduce the negative NA effect, and APT studies in the present work reveal that this can be associated with Cu atoms being encapsulated into the NA clusters. This stabilizes them, and prevents them from being dissolved at higher temperatures. A high number of nucleation sites thereby survive, leading to a reduction in the negative NA effect.

Through hardness measurements and TEM studies it was investigated how the various NA effects evolved with time in Al-Mg-Si alloys of different solute content. It was found that the negative NA effect is actually a transient phenomenon, and upon long NA storage times the process is reversed. It was also observed that the same three clustering processes are always taking place during the phase separation – but within different time intervals and with different relative weights. This opens for new evaluations of the earliest stages of phase separation in age hardenable alloys.

A number of metastable precipitate phases in Al-Mg-Si-Cu alloys were also investigated by APT and TEM: It was found that the semi-coherent Q’ phase contains 50% less Cu than the incoherent equilibrium Q to which it is isostructural. Further DFT investigations of the C phase, which coexists with Q’ in the alloy microstructure, revealed that reductions in the Cu content is necessary in order for these phases to grow coherently along Al type directions. This implies that the coherency criterion dictates not only the structure of metastable phases – but also their composition.