Hydrogenated vacancies lock dislocations in aluminium

Due to its high diffusivity, hydrogen is often considered a weak inhibitor or even a promoter
of dislocation movements in metals and alloys. By quantitative mechanical tests in an
environmental transmission electron microscope, here we demonstrate that after exposing
aluminium to hydrogen, mobile dislocations can lose mobility, with activating stress more
than doubled. On degassing, the locked dislocations can be reactivated under cyclic loading
to move in a stick-slip manner. However, relocking the dislocations thereafter requires a
surprisingly long waiting time of ~10³s, much longer than that expected from hydrogen
interstitial diffusion. Both the observed slow relocking and strong locking strength can be
attributed to superabundant hydrogenated vacancies, verified by our atomistic calculations.
Vacancies therefore could be a key plastic flow localization agent as well as damage agent in
hydrogen environment.