  Ever more stringent regulations on greenhouse gas emissions from
transportation motivate efforts to revisit materials used for vehicles.
High-strength Al-alloys often used in aircrafts could help reduce the weight of
automobiles, but are susceptible to environmental degradation. Hydrogen (H)
"embrittlement" is often pointed as the main culprit, however, the mechanisms
underpinning failure are elusive: atomic-scale analysis of H inside an alloy
remains a challenge, and this prevents deploying alloy design strategies to
enhance the materials' durability. Here we successfully performed near-atomic
scale analysis of H trapped in second-phase particles and at grain boundaries
in a high-strength 7xxx Al-alloy. We used these observations to guide atomistic
ab-initio calculations which show that the co-segregation of alloying elements
and H favours grain boundary decohesion, while the strong partitioning of H
into the second-phases removes solute H from the matrix, hence preventing
H-embrittlement. Our insights further advance the mechanistic understanding of
H-assisted embrittlement in Al-alloys, emphasizing the role of H-traps in
retarding cracking and guiding new alloy design.
