分类： 材料科学 >> 材料科学（综合） 提交时间： 2022-06-20
摘要：
The behavior of nano-voids composed of vacancies (Vs) at grain boundaries (GBs) is fundamental to the design of the radiation tolerance of poly-crystalline metals (PCs) via GB engineering. In this study, based on differential evolution, a framework for determining the stable structure of GB nano-voids is developed. Combining the framework with multiscale simulations, we elucidate the vacancy-accumulation and GB void
formation mechanism under irradiation. A GB-structure dependent picture is revealed. At special coincidence-site-lattice (CSL) GBs of Ʃ5(310) and Ʃ5(210) with a medium V-GB binding energy, the V could be reemitted from the GB and also has driving force to be clustered at the GB, developing particularly stable V-clusters from a linear configuration to a platelet and finally to three-dimensional void that has large strain fields in iron with small bulk modulus and a bulk-void alike structure in the GB with large bulk modulus. A group of vacancies reconstruct their positions during the growth. The ripening is also mediated by the mobility of small V-clusters in addition to free Vs. General high-angle and low-angle GBs trap Vs efficiently, where V-clusters only align one-dimensionally or hardly nucleate. Based on the bonding among the vacancies and their neighboring atoms of a nano-void, we propose a high-accuracy predictive linear energetic model applied to the nano-void both at the iron/molybdenum/tungsten GBs and in the grain interior. The model captures the anisotropic feature of a nano-void and reproduces the oscillated vacancy energy level near a nano-void, showing distinct advantages over conventional continuum model and Wulff construction based energy model. Finally, the collective behavior of multiple GBs plays a role in the GB void formation. The present work offers fundamental mechanistic insights to GB nano-void formation and growth and sets a key step towards GB-void prevention in PCs by reducing the fraction of special CSL-GBs.
分类： 材料科学 >> 材料科学（综合） 提交时间： 2017-08-23
摘要： Nanoporous metals have been shown to exhibit radiation-tolerance due to the trapping of the defects by the surface. However, the behavior of vacancy clusters near the surface is not clear which involves the competition between the self-trapping and segregation of small vacancy clusters (Vn) nearby the surface. In this study, we investigated the energetic and kinetic properties of small vacancy clusters near tungsten (0 0 1) surface by combining molecular statics (MS) calculations and object Kinetic Monte Carlo (OKMC) simulations. Results show that vacancies could be clustered with the reduced formation energy and migration energy of the single vacancy around a cluster as the respective energetic and kinetic driving forces. The small cluster has a migration energy barrier comparable to that for the single vacancy; the migration energy barriers for V1–5 and V7 are 1.80, 1.94, 2.17, 2.78, 3.12 and 3.11 eV, respectively. Clusters and become unstable near surface (0 0 1) and tend to dissociate into the surface. At the operation temperature of 1000 K, the single vacancy, V2, V3 and V4 were observed to segregate to the surface within a time of one hour. Meanwhile, larger clusters survived near the surface, which could serve as nucleating center for voids near the surface. Our results suggest that under a low radiation dose, surface (0 0 1) could act as a sink for small vacancy clusters, alleviating defect accumulation in the material under a low radiation dose. We also obtained several empirical expressions for the vacancy cluster formation energy, binding energy, and influence radius as a function of the number of vacancies in the cluster.
分类： 材料科学 >> 金属与冶金 提交时间： 2017-08-23
摘要： Under nuclear fusion environments, displacement damage in tungsten (W) is usually caused by neutrons irradiation through producing large quantities of vacancies (Vs) and interstitials (SIAs). These defects not only affect the mechanical properties of W, but also introduce trap sites for implanted hydrogen isotopes and helium. Nano-structured W with high fraction of free surfaces has been developed to mitigate the radiation damage. However, the mechanism of the surface reducing defects accumulation is not well understood. Using multiscale simulation methods, we investigated the interaction of the SIA and V with different surfaces at across length and time scales. We found that, at a typical operation temperature of 1000K, surface (110) preferentially heals radiation damage of W compared with surface (100) and boundary (310). On surface (110), the diffusion barrier for the SIA is only 0.68eV. The annihilation of the SIA-V happens via the coupled motion of the V segregation towards the surface from the bulk and the two dimensional diffusion of the SIA on the surface. Such mechanism makes the surface (110) owe better healing capability. On surface (100), the diffusion energy barrier for the SIA is 2.48eV, higher than the diffusion energy barrier of the V in bulk. The annihilation of the SIA-V occurs via the V segregation and recombination. The SIA was found to migrate one dimensionally along a boundary (310) with a barrier of 0.21eV, leading to a lower healing efficiency in the boundary. This study suggested that the on-surface process plays an important role in healing radiation damage of NP W in addition to surface-enhanced diffusion and annihilation near the surface. A certain surface structure renders nano-structured W more radiation-tolerant.
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