Hydrogen induced amorphisation around nanocracks in aluminium

Abstract

The environment generally increases the rate of fatigue crack growth in aluminium alloys by an order of magnitude due to environmental moisture that dissociates on the fracture surface to release atomic hydrogen causing some form of hydrogen assisted cracking or embrittlement. However, to date there is still uncertainty in the mechanism by which hydrogen increases the rate of crack growth. To further investigate this, classical molecular dynamic simulations were performed on a nanoscale crack in a block of pure aluminium with two hydrogen pre-charged configurations, namely, in-plane and adsorbed atomic hydrogen subjected to either cyclic unidirectional and hydrostatic strains. It was found that in-plane hydrogen combined with high stress produced a hydrogen filled amorphous zone at the crack tip resulting in quasi-cleavage like separation. Our results reveal that the production of the amorphous zone under cyclic loading significantly increased the rate of crack growth, indicative of hydrogen embrittlement, while adsorbed hydrogen had little effect on the extension of these nanocracks.