分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-08-23
摘要： We present systematic first-principles studies of occupancy, diffusion, and segregation of He atoms, as well as underlying evolution mechanism of He clusters in Ta, Re and Os precipitates to help to understand the effect of alloying solutes on He retention and bubble formation. It is found that single He atom prefers tetrahedral (T) site in Ta and basal octahedral (BO) site in Re and Os. Single He atom prefers the diffusion path from one T site to the closest T site in Ta, and the diffusion path from one BO site to the next BO site passing an octahedral site along $\langle$0001$\rangle$ direction in Re and Os. The diffusion of single He atom in Ta, Re and Os is relatively difficult than that in W. He atoms are energetically favorable to segregate at interstitial sites forming He$_{n}$ clusters in Ta, Re and Os, and form close-packed He monolayer structure between (110) planes in Ta. The binding strength of He$_{n}$ cluster in Ta, Re and Os is relatively small than that in W (W>Os>Re>Ta). Compared with interstitial sites, He atoms are more energetically favorable to aggregate in vacancy cluster forming VacmHen. The binding strength order of VacmHen cluster in the systems obeys W>Os>Re>Ta. We thus expect that the presence of Ta and Re may contribute to reduce He retention and bubble formation in W.
分类： 物理学 >> 核物理学 分类： 核科学技术 >> 核聚变工程技术 提交时间： 2024-02-27
摘要： The helium bubbles induced by 14 MeV neutron irradiation can cause intergranular fractures in reduced activation ferritic martensitic (RAFM) steel, which is a candidate structural material for fusion reactors. In order to elucidate the susceptibility of different grain boundaries (GBs) to helium-induced embrittlement, the tensile fracture processes of 10 types of GBs with and without helium bubbles in body-centered cubic (bcc) iron at the relevant service temperature of 600 K were investigated via molecular dynamics methods. The results indicate that in the absence of helium bubbles, the GBs studied here can be classified into two distinct categories: brittle GBs and ductile GBs. The atomic scale analysis shows that the plastic deformation of ductile GB at high temperatures originates from complex plastic deformation mechanisms, including the Bain/Burgers path phase transition and deformation twinning, in which the Bain path phase transition is the most dominant plastic deformation mechanism. However, the presence of helium bubbles severely inhibits the plastic deformation channels of the GBs, resulting in a significant decrease in elongation at fractures. For bubble-decorated GBs, the ultimate tensile strength increases with the increase of the misorientation angle. Interestingly, the coherent twin boundary Ʃ3{112} was found to maintain relatively high fracture strength and maximum failure strain under the influence of helium bubbles.
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