Al辐照损伤初期的第一性原理研究

First-principle study of initial irradiation damage in aluminum

采用密度泛函理论框架下的第一性原理平面波赝势方法,对Al辐照损伤初期产生的本征点缺陷和He缺陷进行了研究.通过晶体结构、缺陷形成能和结合能,分析比较了缺陷形成的难易程度及对晶体稳定性的影响,并从态密度、差分电荷密度和电荷布居的角度,分析了其电子机理.结果表明:对于同类型的缺陷,其造成的晶格畸变越大,体系稳定性越低,缺陷形成的难度越大.同类型缺陷形成的难易程度由易到难依次为空位(置换位原子)、八面体间隙原子和四面体间隙原子,但相同位置的本征缺陷的形成难度小于He缺陷.间隙原子容易与空位结合,且Al原子与空位结合的能力强于He原子.间隙Al原子和He原子主要存在于八面体,且缺陷原子引起部分电子向更高能级转移,并导致与其最邻近的Al原子之间的共价作用减弱,从而降低了体系稳定性.间隙Al原子与最邻近的Al原子之间产生了强烈的共价作用,而He原子和最邻近Al原子之间主要为范德瓦耳斯力和较弱的离子键,这是含He缺陷的体系稳定性更低的重要原因.

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 a 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.