Influence of dislocations on hydrogen diffusion and trapping in an Al-Zn-Mg aluminium alloy

Abstract

Cu-lean Al-Zn-Mg alloys are known to be susceptible to hydrogen embrittlement (HE), which currently limits their use in automotive industry. Several works suggested that the resulting loss of mechanical properties was related to hydrogen trapping in different metallurgical sites. The present work attempts to provide a better understanding of the hydrogen-dislocations interactions to evaluate their influence on the loss of mechanical properties of hydrogen-embrittled Al-Zn-Mg alloys. Pre-strained samples of 7046 aluminium alloy (AA7046) were therefore prepared in order to increase the density of motionless dislocations. Tensile samples, pre-strained or not, were then corroded in 0.6 M NaCl and mechanically tested to evaluate their HE susceptibility and the role of dislocations on hydrogen diffusion. Results highlighted a significant improvement of the HE resistance of the alloy with the increase in the density of motionless dislocations induced by the pre-strain step. This was attributed to preferential hydrogen trapping on motionless dislocations leading to a decrease in the hydrogen amount in the grain boundaries. The measurements of hydrogen penetration depth by Scanning Kelvin Probe Force Microscopy (SKPFM) for cathodically charged samples provided further evidence to support these assumptions.