Synergetic effect of H and He impurities in Ti3AlC2: first principles calculations

First principles calculations have been performed to investigate the synergetic effect of H and He impurities with vacancies in Ti3AlC2. The configurations and energetics of H-n-H-e-V-Al complexes (n <= 4) and He-He/He-H/H-H interactions have been studied. It is found that the impurity H atom prefers to occupy the tetrahedral interstitial site (I-tetr-3), but the He atom prefers to occupy the octahedral interstitial site (I-oct-4) in perfect Ti3AlC2. Within a pre-existing Al vacancy, the most favorable site for a He atom is close to tetr-site, meanwhile the H atom preferentially deviates from the vacancy center with the separation 1.3 angstrom along the < 001 > direction. He-H and He-He show a weakly attractive interaction, but weak repulsion occurs in the H-H interaction, which is different from the case of Ti3SiC2. The He-V-Al complex plays an important role in the trapping of H atoms. The He-V-Al cluster can trap up to three H atoms in the absence of H-2 molecules, which leads to the formation of a H-He hybridized bubble. Thus, the He atom can subsequently suppress further aggregation of H atoms and block hydrogen embrittlement and volume swelling.