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

Abstract

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 (V_FeH_n) 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 V_FeH_n complexes with H atoms occupying the six first-nearest-neighbor (1NN) Oct_2Fe–4Al sites of V_Fe one by one; the H–H distances are 1.920–2.785 Å. The V_FeH₆ complex is the major complex under ambient conditions and prefers to grow larger by clusterization of V_2FeH₁₂ 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 V_2FeH₁₂〈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.