Influence of hydrogen on strain localization and fracture behavior in Al-Zn-Mg-Cu aluminum alloys

Hydrogen-induced dislocation motion is characterized in terms of the microscopic strain distribution in Al-Zn-Mg-Cu aluminum alloys. Hydrogen-induced strain localization was visualized in 3D using X-ray tomography and related microstructural tracking techniques. The strain localization was observed as a form of obliquely aligned shear bands. The strain localization becomes more intense with an increase in holding time at each loading step, indicating that more internal hydrogen is partitioned to the strain localization regions with holding time. In addition, the concentration of hydrostatic strain is observed in the strain localization region. Numerous nano voids were generated after deformation and were determined from the precise interpretation of the measured hydrostatic tension. Direct observation of the nano voids was then successfully performed by employing high-angle annular dark-field (i.e., HAADF) scanning transmission electron microscopy imaging and imaging-type computed tomography (CT) techniques. It is assumed that nano voids can serve a dual role as a fracture origin site and a hydrogen trap site. However, no evidence for hydrogen embrittlement originating from nano voids was observed. Instead, it can be assumed that the most hydrogen was partitioned to nano voids in strain localization regions during deformation due to its high density. A hydrogen embrittlement model was proposed based on these findings, where in-situ hydrogen repartitioning, which is necessary for hydrogen embrittlement to occur, is considered. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.