Revisiting stress-corrosion cracking and hydrogen embrittlement in 7xxx-Al alloys at the near-atomic-scale

TL;DR

This study investigates hydrogen embrittlement and stress-corrosion cracking in 7xxx series aluminum alloys at the near-atomic scale, revealing microstructural mechanisms and solute transport processes affecting crack propagation.

Contribution

It provides new insights into hydrogen segregation, oxide formation, and microstructural effects on SCC in industrial aluminum alloys, moving beyond model systems.

Findings

Hydrogen segregates to dislocations and grain boundaries.

Mg-rich amorphous oxide forms on crack surfaces.

Oxygen ingress exceeds solubility limits near the oxide-metal interface.

Abstract

Hydrogen embrittlement (HE) affects all major high-strength structural materials and as such is a major impediment to lightweighting e.g. vehicles and help reduce carbon-emissions and reach net-zero. The high-strength 7xxx series aluminium alloys can fulfil the need for light, high strength materials, and are already extensively used in aerospace for weight reduction purposes. However, depending on the thermomechanical and loading state, these alloys can be sensitive to stress-corrosion cracking (SCC) through anodic dissolution and hydrogen embrittlement. Here, we study at the near-atomic-scale the intra- and inter-granular microstructure ahead and in the wake of a propagating SCC crack. Moving away from model cases not strictly relevant to application, we performed an industry-standard test on an engineering Al-7XXX alloy. H is found segregated to planar arrays of dislocations and to grain boundaries that we can associate to the combined effects of hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanisms. We report on a Mg-rich H-rich amorphous oxide on the corroded crack surface and evidence of Mg-related diffusional processes leading to dissolution of the strengthening eta-phase precipitates ahead of the crack. We show ingress of up to 1 at% O, i.e. well above the solubility limit of O in Al, near the oxide-metal interface, while no increased level of H is found in the matrix. We provide an array of discussion points relative to the interplay of structural defects, transport of solutes, thereby changing the resistance against crack propagation, which have been overlooked across the SCC literature and prevent accurate service life predictions.