Feilong Yang

Stability and clusterization of hydrogen-vacancy complexes in B2-FeAl: insight from hydrogen embrittlement

Little is known about hydrogen-vacancy interactions and their contributions to hydrogen embrittlement (HE) in iron aluminides. H-induced vacancy formation, stability and clusterization of hydrogen-vacancy complexes (VFeHn) in B2-FeAl were studied via density functional theory (DFT) and thermodynamic formalism. The presence of an interstitial H atom in FeAl forms superabundant Fe-vacancies. The H atoms are more likely to be trapped around the Fe-vacancies than diffuse from one octahedral interstitial site to another. One Fe-vacancy can trap at most six H atoms to form VFeHn complexes with H atoms occupying the six first-nearest-neighbor (1NN) Oct2Fe–4Al sites of VFe one by one; the H–H distances are 1.920–2.785 A. The VFeH6 complex is the major complex under ambient conditions and prefers to grow larger by clusterization of V2FeH12 units along 〈100〉 and {100} with internal H2 molecules closely associated with the crack along the {100} planes. Thus we propose a mechanism of isotropic hydrogenated vacancy-cluster induced HE: hydrogen addition-induced isotropic V2FeH12〈100〉 clusters of line and planar shapes are embryos for the formation of cracks and H2 bubbles. This grows ever bigger as a function of H concentration and eventually leads to the macroscopic failure observed experimentally.

Nucleation and growth of H blisters in stacking fault on B2–FeAl {100} planes

In the present work, the H accumulating behaviors at the stacking fault (SF) on {100} planes in B2–FeAl are studied by first-principles calculations. It is concluded that the SF on B2–FeAl {100} planes can trap H atoms, which serve as nucleation sites for H bubbles. When the areal density (the number of H atoms per the cross-sectional area of the SF) of the trapped H is as high as 5.9 × 1015 atoms per cm2, hydrogen recombines into molecules. With further increasing trapped H atoms, H bubbles grow gradually, yielding a hydrogen pressure of 3.4 GPa and strikingly elongating Al–Al bonds near the SF by 70% which implies the initiation of a crack, and eventually leads to a macroscopic fracture and crack of {100} type observed experimentally with the build-up of high pressures of hydrogen gases. This provides theoretical evidence for a HE mechanism of hydrogen blisters in iron aluminides.

H/He interaction with vacancy-type defects in α-Al2O3 single crystals studied by positron annihilation

To probe the interaction of H and He, produced by tritium decay, with vacancy-type defects of α-Al2O3 as a tritium permeation barrier (TPB) in fusion reactors, α-Al2O3 single crystals were treated in pure Ar gas, D2 gas and T2 gas with subsequent tritium aging, respectively, and then their positron annihilation lifetimes and the type of defects that may contribute to the observed positron lifetime components were studied, in combination with DFT results. More monovacancies and vacancy clusters were formed in the thermally hydrogenated samples when compared to the fresh and Ar-annealed samples, indicating the stabilizing effect of hydrogen; this was consistent with the Fermi level position of α-Al2O3 moving towards the conduction band minimum (CBM) in the presence of hydrogen impurities, resulting in VAl3− and [VAl3−–H+]2− becoming more stable, as observed by DFT calculations. The monovacancies were slightly eliminated when the samples were thermally annealed and then aged in T2 gas at room temperature, indicating that He filled the vacancies. This was consistent with it being favourable for He atoms to occupy Al vacancies, with HeAl3− forming most readily, whilst more vacancy clusters were continuously induced, suggesting that Al–O bonds weakened and thus nano-hardness decreased with an external load. This study provides the first evidence that Al vacancies can be stabilized by H and filled with He, which will provide further novel TPB design opportunities.