Monovacancy-hydrogen interaction in pure aluminum: Experimental and ab-initio theoretical positron annihilation study

We report here on hydrogen-vacancy interactions in high purity aluminum by employing positron annihilation spectroscopy (PAS) analysis of hydrogen-loaded samples, aiming to study the mobility of vacancies. The samples were heat treated at 893 K in an atmosphere consisting of a mixture of H2 and Ar gas and, thus, loaded with hydrogen. The samples were then quenched to ice water and subsequently measured in-situ at different tem-peratures. In parallel we performed ab-initio density functional theory (DFT) calculations of lifetimes of positrons trapped in vacancies associated with 1-8 H atoms. Our experimental results suggest in comparison with the ab-initio calculations that complexes of vacancies with one hydrogen atom (V-H pairs) were formed in Al samples annealed in a mixture of H2 and Ar gas. Furthermore, hydrogen absorbed in aluminum immobilizes vacancies, i. e. the recovery of vacancies is delayed from 220 K up to around 280 K. At that temperature, V-H complexes start to dissociate, and hydrogen atoms previously bound to vacancies are released. In contrast, for Al samples not loaded with hydrogen isolated monovacancies become mobile around 220 K. In both cases mobile vacancies start to form vacancy clusters. From our experimental data we determined that the formation energy of mono-vacancies in Al is 0.62 +/- 0.01 eV. This value is in very good agreement with 0.63 eV obtained by our ab-initio DFT calculations.