  The influence of hydrogen on the structural evolutions of an Al-Mg-Si alloy
during natural and artificial ageing was investigated experimentally. The aim
of the study was a better understanding of interactions between hydrogen and
crystalline defects and especially vacancies. Experimental data demonstrate
that during natural ageing in hydrogen environment, the hardening response is
delayed. This is attributed to a slower recovery of excess vacancies linked to
a lower mobility. To confirm and quantify the influence of hydrogen on the
vacancy migration energy, artificial ageing was carried out in conditions where
the vacancy concentration is constant. Hence, long-time annealing treatments
were carried out to investigate the influence of hydrogen on the coarsening of
rod-shaped precipitates. Using transmission electron microscopy and atom probe
tomography, it was demonstrated that the precipitate volume fraction and
composition are unchanged under H2 atmosphere but the coarsening kinetic is
significantly reduced. This leads to a delayed softening, in good agreement
with theoretical estimates. Thus, even a low concentration of hydrogen in solid
solution significantly affects the mobility of alloying elements in the
aluminium matrix. This is the result of hydrogen-vacancy interactions that lead
to an increase of the vacancy migration energy. Based on classical coarsening
theories, it was possible to demonstrate that this increase is of about 5% for
a concentration of hydrogen close to the vacancy concentration.
