Molecular Dynamics Study for the Stress Effect on the Radiation-Induced Defect of Refractory Materials : Molecular Dynamics Study for the Stress Effect on the Radiation-Induced Defect of Refractory Materials
W, Mo, V materials and radiation damage quantification under stress condition

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Oda, Takuji\
공과대학 에너지시스템공학부
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서울대학교 대학원
학위논문 (박사)-- 서울대학교 대학원 : 공과대학 에너지시스템공학부, 2019. 2. Oda, Takuji\.
Tungsten (W), molybdenum (Mo), and vanadium (V) as candidate materials for plasma devices and fusion reactors are subjected to several sources of strain during operating conditions and these strain levels will affect the defects generation when the materials under collision cascades. To reveal the strain effects on radiation defects formation and evolution behaviors, we have evaluated the energy of defects formation, the number of defects generation, cascade pattern, and subcascade propagation under the effect of strain. At first, we evaluated the threshold displacement energy (TDE) as it is an important quantity used theoretically to determine the number of defects formed by irradiation of high-energy particles and it is well known to be used for the NRT model of damage quantification. However, TDE evaluated for the typical material of our study, i.e. W, have been reported with different values and then used in previous studies, which has caused inconsistencies in calculated damage amounts. At first, for our thesis objective, we evaluated accurately the free strained TDE using molecular dynamics (MD) method, where the TDE is defined as the average value of the minimum displacement energies over several sets of recoil directions.

To determine the TDE accurately, the effects of calculation settings, such as the simulation cell size, the number of sampled recoil directions, the incremental step of the recoil energy in searching the threshold energy, and the thermal vibration of atoms, were analyzed. A TDE of 85 eV was obtained for tungsten with a correlation factor of 4.5%. This TDE value is close to the one recommended by the American Society for Testing and Materials (ASTM), 90 eV. Consequently, we conclude that 90 eV is a reasonable choice for the TDE of tungsten. In the same manner, the Mo and V were evaluated following the recommended setting as 78 eV for Mo and 57 for V respectively. After we got the right free strained values and after we checked our MD results compared with the reported and experimental result we applied the strain condition and evaluated the strained TDE.

We tested the influence of hydrostatic and uniaxial strains on Frenkel pair formation energy (FPE) and TDE for W, Mo, and V. Under applied strain the self-interstitial atom formation energy decreases significantly, while the vacancy formation energy slightly increases, which causes concurrent decreases in FPE and TDE. The opposite responses are observed under compression strain. This result indicates that the radiation defect formation is enhanced by a tensile strain, while suppressed by a compressive strain. The strain effects on TDE and FPE are determined mainly by the volume change in the deformed crystal regardless of the strain mode. Both TDE and FPE under strain conditions are described by linear functions of the volume change. After successfully evaluated deformed TDE we can estimate roughly the amount of defects generated of the specific materials by applying the NRT model. However theoretical results expected to be different than the real MD results of cascade collision, especially at larger PKA energies and under larger strain levels.

Simulation of different PKA energies between 1, 6 and 10 keV, under applied strain, was conducted at a fixed temperature. Simulation results showed that as the atomic displacement cascade proceeds under strains, the peak and surviving number of Frenkel pair defects increases with increasing tension
however, these increments were more prominent under larger volume changing due to dislocation segment formation at around 1.6 % of hydrostatic strain and under 10 keV of PKA energy. The rate of increase/decrease in the number of Frenkel pairs, their clustering, and their cluster sizes under expansion/compression strain conditions were higher for higher PKA energy. Clusters formed of vacancies and interstitials were both larger under tensile strain conditions.

We added further analysis in the appendix. A, for clusters evolution, as under strain condition the clusters from (1~3) SIA show stability under larger strain values. For the mobility, the diffusivity of the SIA clusters has a gradual transition from three dimensional (3D) to one dimensional (1D) path at the saturated strain. The 1D transition was observed for large clusters and large strain while the 3D transition was for small clusters and lower strains. Under cascade collision, defects generation and their mobility, growth rate, and distribution is the cornerstone for understanding the structural evolution of a material used under irradiation conditions. Overall, the present results suggest that strain effects should be considered carefully in radiation damage environments, specifically for conditions of low temperature and high radiation energy. Compressive strain conditions could be beneficial for materials used in nuclear reactor power systems.
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Energy Systems Engineering (에너지시스템공학부)Theses (Ph.D. / Sc.D._에너지시스템공학부)
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