Yang Zhuocen, Hammond Karl D
Nuclear Engineering Program, University of Missouri, Columbia, MO 65211, United States of America.
J Phys Condens Matter. 2018 Aug 15;30(32):325002. doi: 10.1088/1361-648X/aad0bc. Epub 2018 Jul 3.
We present the results of an atomistic modeling study of small helium cluster migration in the plane of symmetric tilt grain boundaries. The relevant migration pathways and energies were determined by way of temperature accelerated dynamics and the nudged elastic band method. We find that small helium clusters show much higher migration energies when bound to the grain boundary than in the bulk for all types of grain boundaries, indicating strongly-impeded helium transport behavior. Larger helium clusters (up to three helium atoms) tend to have higher migration energies compared with smaller clusters. Longer-distance migrations also tend to have higher migration energies, but helium cluster migration is highly affected by the structure of the grain boundary. The binding energy of the grain boundaries studied is high enough that helium clusters would be unlikely to leave the grain boundary plane. However, vacancy migration energies are relatively low compared to the bulk, and are also much lower than helium cluster migration energies on the grain boundary plane. This suggests that helium cluster migration on the grain boundary is actually governed by the rate of vacancy migration: in the bulk, helium clusters are mobile, but they become bound to and immobilized by grain boundaries, forming bubbles. Bubbles, however, are likely more mobile on the grain boundary than they are in the bulk due to the increased rate of vacancy migration on the grain boundary. We expect similar migration behavior for other types of grain boundaries because of the increased excess volume found near all grain boundaries.
我们展示了关于小氦团簇在对称倾斜晶界平面内迁移的原子尺度建模研究结果。通过温度加速动力学和推挤弹性带方法确定了相关的迁移路径和能量。我们发现，对于所有类型的晶界，小氦团簇在与晶界结合时的迁移能量比在体相中高得多，这表明氦的传输行为受到强烈阻碍。与较小的团簇相比，较大的氦团簇（多达三个氦原子）往往具有更高的迁移能量。长距离迁移也往往具有更高的迁移能量，但氦团簇的迁移受到晶界结构的高度影响。所研究晶界的结合能足够高，以至于氦团簇不太可能离开晶界平面。然而，与体相相比，空位迁移能量相对较低，并且也远低于晶界平面上氦团簇的迁移能量。这表明晶界上氦团簇的迁移实际上受空位迁移速率的控制：在体相中，氦团簇是可移动的，但它们会与晶界结合并被固定，形成气泡。然而，由于晶界上空位迁移速率增加，气泡在晶界上可能比在体相中更易移动。由于在所有晶界附近都发现过量体积增加，我们预计其他类型的晶界也会有类似的迁移行为。
