Simulating intergranular hydrogen enhanced decohesion in aluminium using density functional theory

Materials modelling at the atomistic scale provides a useful way of investigating the widely debated fundamental mechanisms of hydrogen embrittlement in materials like aluminium alloys. Density functional theory based tensile tests of grain boundaries (GBs) can be used to understand the hydrogen enhanced decohesion mechanism (HEDE). The cohesive zone model was employed to understand intergranular fracture from energies obtained in electronic structure calculations at small separation increments during ab initio tensile tests of an aluminium Σ11 GB supercell with variable coverages of H. The standard rigid grain shift test and a quasistatic sequential test, which aims to be faster and more realistic than the rigid grain shift method, were implemented. Both methods demonstrated the effects of H on the cohesive strength of the interface. The sequential method showed discrete structural changes during decohesion, along with significant deformation in general compared to the standard rigid approach. H was found to considerably weaken the GB, where increasing H content led to enhanced embrittlement such that, for the highest coverages of H, GB strength was reduced to approximately 20% of the strength of a pure Al GB - it is proposed that these results simulate HEDE. The possibility of finding H coverages required to induce this effect in real alloy systems is discussed in context by using calculations of the heat of segregation of H.