First-principle study of initial irradiation damage in aluminum

Aluminum and its alloy play an important role in nuclear industry, where irradiation damage continually occurs and significantly affects the structures and physical properties of materials: especially long-term irradiation can lead to the formation of helium bubbles and holes in the substrate. During the initial irradiation damage, point defects are the major defects.Studying the point defects is of great significance for understanding the irradiation damages and the mechanism of defect development. In this paper, three possible intrinsic point defects (Al vacancies, Al tetrahedral interstitials and Al octahedral interstitials) and three possible helium defects (substituted He, He tetrahedral interstitials and He octahedral interstitials) produced by initial irradiation damage in aluminum are studied by the first-principle plane wave pseudo-potential method within the framework of density functional theory. The formation of the defects and their effects on the stability of the system are compared through crystal structure, formation energy and binding energy. Besides, the electronic mechanism is analyzed from the point of view of density of states (DOS), partial density of states (PDOS), electron density difference and charge populations. It is shown that for the same type of defects, the greater the lattice distortions, the lower the stability of system is and the more difficult the formation of defects. For the formation of the same type of defects, the extent of difficulty in forming defects is in the following order: vacancies (substituted atoms), octahedral interstitials, and tetrahedral interstitials. However, for the same sites, although the intrinsic defects cause greater lattice distortions than the helium defects, they are in fact relatively easier to form, which indicates that the difference between the bonding performances of Al and He plays a leading role in determining the interaction between defects and the aluminum substrate. Besides, the results of binding energy and optimization show that interstitials readily combine with vacancies, and Al has stronger combining ability than He. On the whole, interstitials mainly exist in octahedral interstices, and both octahedral Al and He can cause some electrons to transfer to higher energy levels, lead to some weakening of the covalent interaction between atoms nearest to the interstitials, and eventually reduce the stability of the system. And further study shows that the bond between interstitial Al and its nearest atom features a strongly covalent state, while the interaction between He and its nearest atom is dominated by van der Walls force with weak ionic bond, which accounts for the lower stability of system doped with helium defects.