| Energy shortage is one of the most important factors that restrict the development of human society.Searching for new alternative energy resources is the main task at present.As one of the most ideal clean and safe energies,nuclear fusion will be bound to become the first choice for the most stable source of energy.However,there are still many problems in realization of nuclear fusion energy.In particular,the choice of the structural materials,which are always exposed to extreme radiation conditions in fusion reactors,is concerned with the safety and the durability of fusion reactors.The structural materials are irradiated by different particles,especially neutrons with energy as high as 14.1 MeV.These particles induce large amounts of impurities(such as H and He)in the structural materials through transmutation reactions,which can significantly change physical and mechanical properties of the structural materials.Due to the low activation property and excellent performance at high temperature,vanadium(V)and V-based alloys are regarded as promising candidates of the structural materials in fusion reactors.Experimental results demonstrate that the impurities such as H,He,C,N and O have a great impact on the performance of V.In order to improve the performance of V under high temperature and high radiation significantly,the alloying elements titanium(Ti)and chromium(Cr)are particularly added.Therefore,the study of the effects of transmutation products,impurities and alloying elements Ti and Cr on the structure and properties of V is of great significance for the selection of structural materials.In this thesis,by employing the first-principles method based on density functional theory,we studied the effects of impurities(H,He,C,N and O)and alloy elements Ti and Cr on the structure and mechanical properties of V systematically.Moreover,we also studied the effects of impurity C on the behavior of N/He/O in V.Firstly,we investigated the effect of the alloying element Ti on the thermodynamic stability and electronic structure of H/He in perfect V.The results show that:(i)a single H atom prefers to reside in a tetrahedral interstitial site(T-site)in dilute V-Ti binary alloy systems;(ii)H atoms tend to cluster in the vacancy;a mono-vacancy is shown to be capable of trapping three H atoms;(iii)the presence of Ti can increase the trapping energy of H in perfect V and reduce the H trapping capability of the vacancy defects.That indicates that doping with Ti to form dilute V-Ti binary alloys can restrain the dissolution of H the solution for H,and thus suppress the retention of H;(iv)the presence of Ti increases the formation energies of He clusters and He-vac cluster in perfect V,suggesting that Ti can restrain the dissolution of He and the aggregation of He in the vacancy;(v)He and Hen-1-vacm clusters are attractive in the vicinity of Ti,for a given m value,the interaction between the interstitial He atoms and small clusters with the number of He atoms increases;for a given n value,the interaction between interstitial He atoms and small clusters becomes stronger with the increasing number of vacancies,which indicates that the stress from multiple He atoms gathered near Hen-vacm clusters.Secondly,we explored the effect of the presence of impurity C on the behavior of N/He/O in bulk V.Impurities C and N tend to occupy the O-site and the formation energy of N is lower,comparing with that of C.For the C-C,C-N and N-N pairs,the two adjacent planes of the O-site are occupied and paired along the<111>direction.When there are vacancy defects in the supercell,the impurity C and N atoms do not tend to occupy the vacancy center,but occupy the O-site near the vacancy.It means that the impurity atoms are easy to form vanadium carbide or vanadium nitride with the V atom of the substrate.When interstitial C,He impurities exist synchronously in bulk V,the solution energy has a minimum value,3.05 eV while C-He distance is 2.39 A.When C-He the distance less than 2.39 A,there is repulsion between C and He,which makes the He atom tend to stay away from the C atom.When there is mono-vacancy in bulk V,the C-vac cluster is prone to trap He atoms and form C-vac-Hen cluster.With the number of He atoms increasing,the adjacent V atom will be crowed out to release the local stress of C-vac-Hen cluster,then a Frankel defect pair will be formed.The new formed C-vac-Hen large clusters could capture more He atoms.The mono-vacancy can accommodate up to two O atoms.Furthermore,the most stable configuration of the O atoms is arranged along the<100>direction.The trapping energy of vac+C clusters on O atom is lower than that of mono-vacancy on O.Finally,we studied the effect of alloying element Cr on the stability of H/He in binary V-Cr alloy.We found that the alloying element Cr can increase the solution energy of H and He in V.Mono-vacancy can trap as many as four H atoms in V-Cr alloy while the mono-vacancy can capture up to six H atoms in the intrinsic V.In the V-Cr alloy,the He atoms tend to occupy the O-site near Cr and the He-vac cluster can trap as many as six H atoms.In this thesis,our results revealed the effects of alloying elements Ti and Cr on the behavior of impurities H,He,C,N and O in V.These results shall provide useful references for designing V as a candidate structural material in the future fusion reactors. |
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