  Room temperature solute clustering in aluminium alloys, or natural ageing,
despite its industrial relevance, is still subject to debate, mostly due to its
experimentally challenging nature. To better understand the complex
multi-constituents' interactions at play, we have studied ternary and
quaternary subsystems based on the Al-Cu alloys, namely Al-Cu-Mg, Al-Cu-Li and
Al-Cu-Li-Mg. We used a recently introduced correlative technique using
small-angle neutrons and X-ray scattering (SANS and SAXS) to extract the
chemically resolved kinetics of room temperature clustering in these alloys,
which we completed with differential scanning calorimetry (DSC) and
micro-hardness measurements. The comparison of the clustering behaviours of
each subsystem allowed us to highlight the paramount role of Mg as a trigger
for diffusion and clustering. Indeed, while a strong natural ageing was
observed in the Al-Cu-Mg alloy, virtually none was shown for Al-Cu-Li. A very
slight addition of Mg (0.4%) to this system, however, drastically changed the
situation to a rapid formation of essentially Cu-rich hardening clusters, Mg
only joining them later in the reaction. This diffusion enabling effect of Mg
is discussed in terms of diffusion mechanism and complex interactions with the
quenched-in vacancies.
