Simulating intergranular hydrogen enhanced decohesion in aluminium using density functional theory

Abstract

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.

Mechanisms of Environmentally Induced Crack Initiation in Humid Air for New-Generation Al-Zn-Mg-Cu Alloys

“…It should be noted that recent DFT simulations have demonstrated that HEDE can probably occur more readily by fracture of the interface between η phase precipitates with the Al matrix than by the grain boundaries themselves [55], but the GBs also rapidly lose their cohesive strength with increasing local hydrogen concentrations. There also appears to be no limit to the concentration of hydrogen that can segregate to a GB, or the η interface, which with sufficient hydrogen present can drastically reduce the cohesive strength [25]. It is therefore noteworthy that the area fraction of GB η precipitates in these alloys (in the T7651 temper) is extremely high and has been measured to be ~ 18% and 52% for AA7085 and AA7449 respectively.…”

“…size, shape and aspect ratio) as well as second phase H trap sites also play a role [23,24]. The composition, structure, and the nature of interface with the matrix of the GBPs can further affect their susceptibility to interfacial segregation of H and their decohesion behaviour, with recent atomistic modelling suggesting that H can reduce the η GBP-matrix interface cohesive strength more dramatically than that of the Al grain boundaries themselves, with and without GB segregation present [25,26].…”

Adsorption structure and properties of Ni/Fe electrodeposition interface: a DFT study

“…These techniques provide avenues for studying surface composition, structure, adsorption sites, and surface energy [23], but they cannot be studied at the atomic level. In addition, during the experimental process, surface structures are often prone to damage, introducing uncertainties and obstacles to surface structure analysis [24,25]. Therefore, theoretical calculations have become an irreplaceable research method in surface science.…”

Evaluating the Effect of Hydrogen on the Tensile Properties of Cold-Finished Mild Steel

“…Reduced cohesion between metal atoms, particularly at critical stress concentrators such as inclusions, voids, grain boundaries, and residual stresses from processing, promotes crack initiation and propagation at lower stress levels than in non-embrittled steel, which can lead to both intergranular and transgranular [19] fractures, which are more brittle in nature and contribute to a lower macroscopic and microscopic tensile strength [10]. Figure 1 shows various hydrogen traps which capture and localize hydrogen atoms, intensifying their negative effects on material properties.…”