Hydrogen at symmetric tilt grain boundaries in aluminum: segregation energies and structural features.

Aluminum is envisioned to be an important material in future hydrogen-based energy systems. Here we report an ab initio investigation on the interactions between H-atoms and common grain boundaries (GBs) of fcc Al: Σ9, Σ5, Σ11 and Σ3. We found that upon segregation to the GBs, single H-atoms can cause displacement of Al-atoms. Increasing their concentration revealed large cooperative effects between H-atoms that favor the segregation when other H-atoms are bound at neighboring sites. This makes these GBs able to accommodate high concentrations of H-atoms with considerable segregation energies per atom. Structural analyses derived from Laguerre-Voronoi tessellations show that these GBs have many interstitial sites with higher symmetry than the bulk tetrahedral interstitial site. Many of those sites have also large volumes and higher coordination numbers than the bulk sites. These factors are the increased driving force for H-atom segregation at the studied GBs in Al when compared to other metals. These GBs can accommodate a higher concentration of H-atoms which indicates a likely uniform distribution of H-atoms at GBs in the real material. This suggests that attempting to mitigate hydrogen uptake solely by controlling the occurrence of certain GBs may not be the most efficient strategy for Al.