  The consequences of hydrogen on the formation and growth of GP zones during
natural ageing in an Al-5Cu alloy is investigated experimentally down to the
atomic scale and numerically using ab initio calculations. As observed through
scanning/transmission electron microscopy, the hardening kinetic is delayed due
to a slower growth of GP zones during natural ageing when hydrogen is
incorporated. According to ab initio calculations, the delayed hardening
results from hydrogen trapped in vacancies that reduce significantly the
diffusion coefficient of copper and the self diffusion of aluminium. This
reduction of the diffusion coefficient is either due to hydrogen being in the
path of the atom that exchange with the vacancy (and increasing the energy
barrier) or due to hydrogen being dissociated from the vacancy (and leading to
a less stable state).
