Abstract
Ever more stringent regulations on greenhouse gas emissions from transportation motivate efforts to revisit materials used for vehicles1. High-strength aluminium alloys often used in aircrafts could help reduce the weight of automobiles, but are susceptible to environmental degradation2,3. Hydrogen ‘embrittlement’ is often indicated as the main culprit4; however, the exact mechanisms underpinning failure are not precisely known: atomic-scale analysis of H inside an alloy remains a challenge, and this prevents deploying alloy design strategies to enhance the durability of the materials. Here we 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, and the strong partitioning of H into the second-phase particles 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 minimizing cracking and guiding new alloy design.
Highlights
The enduring question remains of where the H is located in the microstructure and how such traps facilitate catastrophic failure
Peak-aged specimens were electrochemically charged with D for subsequent atom probe tomography (APT) analysis after validating that H and D show a similar embrittling effect on mechanical properties (Extended Data Fig. 2)
D-charged specimens were prepared by plasma focused-ion beam (PFIB) at cryogenic temperatures to limit the introduction of H29, and immediately analysed by APT using voltage pulsing to minimize residual H from APT28,29
Summary
〈110〉Al phases (the S phase[47] and Al3Zr dispersoid). The colour schemes reflect the microstructures where specific APT analyses were performed. The alloy was exposed to the same H-charging and tensile test conditions, but no sign of H embrittlement was found, neither in the tensile test results nor in the metallographic fractography (Extended Data Fig. 10) These findings support the result that the co-segregation of Mg and H to free surfaces provides the driving force for the embrittlement of GBs. Generally, avoiding the ingress of H in the first place is extremely unlikely to work, and the best approach to mitigate H embrittlement is to control its trapping to maximize the in-service lifetime of the components. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/
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