氦团簇及氦间隙原子在钨中的稳定性研究

首先采用分子动力学方法研究了在钨中预存氦-空位团簇（He_nV₂₂）后氦原子结合能与氦-空位比的关系。研究发现:当氦-空位比小于4.5时，氦原子结合能随氦-空位比呈线性减小趋势；当氦-空位比大于4.5时，氦原子的结合能随氦-空位比出现剧烈振荡的现象，这种现象是由于钨中预存氦-空位团簇随机挤出位错环使体系能量骤降所导致的。与此同时，氦-空位团簇周围出现了一些处于亚稳态的fcc结构和hcp结构的钨。为了研究氦团簇周围压强对钨基体相变的影响，本文利用第一性原理对钨的三种结构进行了高压相变计算，发现静水压力不能使钨的三种结构互相转变。另外，通过对bcc钨和fcc钨中四面体间隙氦原子和八面体间隙氦原子电荷密度差的计算，发现bcc钨中四面体间隙氦原子的稳定性高于八面体间隙氦原子的稳定性，而在fcc钨中四面体间隙氦原子的稳定性弱于八面体间隙氦原子的稳定性。

Study on the Stability of Helium Clusters and Interstitials of Helium in Tungsten

In this paper, the molecular dynamics method was used to study the binding energy of a helium atom with the helium-vacancy cluster (He_nV₂₂) in tungsten. As a result, when the heliumvacancy ratio was less than 4.5 the binding energy decreased with the helium-vacancy ratio linearly. When the heliumvacancy ratio was larger than 4.5 the binding energy appeared vibrating extremely. After analyzing kinetic processes, it was found that the phenomenon was due to the helium-vacancy cluster extruding dislocation loop form the tungsten body randomly which leading to the energy of the system droping sharply. At the same time, the helium-vacancy cluster was surrounded by some metastable fcc-tungsten and hcptungsten. Trying to explain the phenomenon, we employed the firstprinciples calculation to study the phase transformation of tungsten under high-pressure. We found that the phase transformation can not occur under hydrostatic pressure. Moreover after analyzing the charge density difference, we found that the stability of the tetrahedral interstitial helium atom was higher than that of the octahedral interstitial helium atom in bcc-tungsten, whereas the stability of the tetrahedral interstitial helium atom was weaker than the stability of the octahedral interstitial helium atom in the fcc-tungsten