Quantitative insights into hydrogen embrittlement in two-step aged 7xxx aluminum alloys

Nanoscopic η-MgZn₂ precipitates are crucial for strengthening aluminum alloys but has recently been reported to be a primary source of hydrogen embrittlement. Two-step over-aging has been used to mitigate this issue, traditionally attributed to η precipitate evolution. However, recent studies highlight the hydrogen-trapping capability of T-phase precipitates, suggesting their role in practical alloys has been historically overlooked. The present study confirms and quantifies T-phase precipitates in both one-step and two-step over-aged high-Zn 7xxx Al alloys, aiming to isolate their effect on hydrogen embrittlement from industrial aging treatments. Results show that at the same strength level, two-step aged alloys exhibit higher hydrogen resistance than their one-step aged counterparts, which cannot be explained by η precipitate coarsening alone. Instead, this improvement is attributed to an increased fraction of hydrogen-trapping T-phase precipitates, as supported by high-resolution transmission electron microscopy. This finding creates opportunities for optimizing industrial aging techniques to develop next-generation hydrogen-resistant aluminum alloys.