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 [ Time:2014/3/6 ]
Researchers at the institute of Solid State Physics make new progress in simulating radiaiton resistance of nano-crystals tungsten and iron
Author :LI Xiangyan

 Recently, researchers at the Institute of Solid State Physics, collaborating with those at the Institute of Plasma physics and the Institute of Modern Physics, make new progress in simulating radiation resistance of nano-crystals tungsten and iron. Relevant results are published in authorized journals in fusion fields-Nuclear Fusion and Journal of Nuclear Materials.
Radiation damage in tungsten and its alloys is a big issue in development of fusion energy. Radiation-induced changes in microstructures not only degrade materials properties, but also change retention behaviors of H/He in matrix materials. Recent studies show that nano-crystals and nano-oxide dispersion strengthened materials generally reduce accumulation of radiation-created defects in materials, making materials show highly radiation-tolerance. However, the mechanisms at atomic level remain unknown. Researchers at the Institute of Solid State Physics employ molecular dynamics and molecular statics, explore the energetic and kinetic role of grain boundaries in healing radiation damage in tungsten. Results show that, during irradiation, grain boundaries preferentially absorb interstitials over vacancies, forming a special defect structure with vacancies rich near grain boundaries and interstitials highly localized at the grain boundary after the cascade cools down. Grain boundaries (GBs) serve as sinks for radiation-produced defects by reducing vacancy/interstitial formation energy and diffusion barriers near the GB; particularly, interstitials near the GB can barrier-freely migrate into the GB. The interstitials that preferentially diffuse into the GB can annihilate with vacancies near the GB via a low barrier process (Figure 1). Researchers also find that the range within which vacancy diffusion and recombination are enhanced is rather limited to 1-1.5 nm, leading to a small GB volume fraction (only several percents). This indicates that by introducing a high density of dispersoids may be more effective in improving radiation performance of tungsten than by using fine grains of tungsten. Relevant results are published in Nuclear Fusion (Nuclear Fusion, 2013, 53, 123014)
 For the issue of radiation damage in ferritic-martensitic steels, researchers use combined techniques of molecular dynamics, molecular statics and temperature accelerated dynamics, investigating energetic and kinetic behaviors of small vacancy clusters near iron GBs. Calculations show that, GBs also serve as sinks for small vacancy clusters with the vacancy number 1-9, besides for point defects (vacancies and interstitials). Vacancy clusters near the GB have high mobility, undergoing three stages: the location of the cluster in the bulk, partially at the GB and completely at the GB (Figure 2). Relevant results are published in Journal of Nuclear Materials (Journal of Nuclear Materials, 2013, 440, 250-256).
To further explore the influence of GBs on defects production and evolution, researchers using molecular-statics, investigate principal physical parameters characterizing the binding of vacancies and interstitials with grain boundaries (GBs), and their annihilation near GBs in iron, molybdenum and tungsten. Interaction parameters are obtained, including defects formation energy, segregation energy and diffusion-annihilation barriers, and corresponding range. These parameters serve as important reference for analyzing radiation performance and designing new radiation-resistant materials. Relevant results are published in Journal of Nuclear Materials (Journal of Nuclear Materials, 2014, 444, 229-236). The publications receive great attention of fellow researchers (see Article Usage Dashboard provided by Elsevier Science & Technology Journals, Figure 3).
Above work is financially supported by the National Magnetic Confinement Fusion Program, the National Natural Science Foundation of China, and the Strategic Priority Research Program of the Chinese Academy of Sciences. The Center for Computation Science, Hefei Institutes of Physical Sciences, is thanked for computational support.
Articles links: 
1. Nuclear Fusion (Nuclear Fusion, 2013, 53, 123014)
2. Journal of Nuclear Materials (Journal of Nuclear Materials, 2013, 440, 250-256)
3. Journal of Nuclear Materials (Journal of Nuclear Materials, 2014, 444, 229-236)

Figure 1. Annihilation of Frenkel pairs near the GB.
(a) Change in the system energy along the annihilation path.
(b) Normalized displacements of the involved atoms in the annihilation. d is the atomic displacements during the annihilation divided by the maximum displacements of the corresponding atom.
(c) Illustration of the annihilation process. V represents the vacancy. A, B, C and D are the atoms involved in the annihilation which displace more than 0.6 Å. C is the interstitial trapped within the GB. The axes X and Y are along [3 1 0] and [ 3 0], respectively.

Figure 2. The formation energy for a subset of the structural evolution near the GB (a) and the defect configuration near the GB (b) and at the GB (c). The distance between A and the GB is about 7.5 Å. E is located at the GB.

Figure 3. Usage reports of two articles published in Journal of Nuclear Materials.

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