Computer simulations on the microscopic mechanisms for the nucleation of hydrogen bubbles in aluminum

Abstract

Trapping of multiple hydrogen (H) atoms inside vacancies in aluminum (Al) is related to the formation of H bubbles, which ultimately results in embrittlement of the material. However, the formation mechanisms of H bubbles are still not well known. In this work, we perform systematical ab initio simulations to study the trapping of H atoms inside vacancies, and its influence on the growth of vacancies. It is found that monovacancy can trap as many as 8 H atoms, forming Vac1H8. H atoms prefer the off-center positions close to tetrahedral sites in vacancy. The shortest H–H bond length in vacancy is 2.10 Å among H atoms, which suggests that it is impossible to form H2 molecule in monovacancy. The accumulation of H atoms reduces the vacancy formation energy closest to Vac1Hm, promoting the formation of new vacancy. The newly produced vacancy can bind tightly with Vac1Hm and form Vac2Hm. With the similar way, the continuous formation of new vacancies and aggregation of H atoms may ultimately result in growth of VacnHm and formation of superabundant vacancies and H bubbles in Al.