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  • 1.
    Li, Ruihuan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Dalian University of Technology, China.
    First-principles study of the multiple He trapping in defects in vanadium and SiC2015Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In fusion environment, large amount of helium (He) atoms are produced by transmutation along with structural damage in the structural materials, causing materials swelling and degrading of physical properties. In this thesis, using first-principles method, I examined the microscopic mechanism of He trapping in vacancies and voids in structural materials (vanadium solid and 6H–SiC composites). In vanadium, a single He atom located in the tetrahedral interstitial site (TIS) turned out to be more stable than that in the octahedral interstitial site (OIS). Helium atoms were placed one by one into the vacancy defects (monovacancy and void) from the remote TISs, and we calculated the trapping energies as a function of the number of He atoms inside the vacancy defects. We found that, the monovacancy and void (about 0.6 mn in diameter) can host up 18 and 66 He atoms, respectively, in vanadium solid. The induced internal pressure by He bubbles in monovacancy and small void increased up to 7.5 GPa and 19.3 GPa, respectively. In vacancy defect, the He–He equilibrium distances decreased with the amount of He atoms incorporated in monovacancy and small void, and the host lattice expanded dramatically. The atomic structures of selected He clusters trapped in vacancies were compared with the gas-phase clusters. In complex 6H–SiC, there are ten kinds of interstitial sites for a single He atom. According to the calculated formation energy, the most stable site is the. R site. [1], where R site alternates with hexagonal interstitial sites. We explored the interactions between an interstital He atom and HenVam (Va stands for vacancy) clusters (n, m = 1 – 4). We found that the binding energy between He and the HenVam clusters increases with the number of vacancies (e.g., the binding energy is 1.3 eV for He2Va3, and 1.7 eV for He2Va4, respectively). The small void (about 0.55 nm in diameter) in 6H–SiC can accommodate up to 14 He atoms and the corresponding internal pressure is estimated to be 2.5 GPa. The maximum density of He atoms in a small He bubble is about 50 atoms/nm3, which is of the same magnitude as the experimental value 10 atoms/nm 3. Compared to vanadium, a small nanosized void in the 6H–SiC host lattice has a weak tendency for trapping He. When trapped seventy He atoms in small void in vanadium, the nearest vanadium bond expands 22–28 %, and the volume of the void expands by 80%. At the same time, with fourteen atoms encapsulated in a small void in 6H–SiC, the local Si–C bonds explans 1–5%, and the volume of the small void expands about 7%. We suggest that the differences in the cohesive energies in these two systems are responsible for the different He trapping behavior.

  • 2.
    Li, Ruihuan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Dalian University of Technology, China.
    Li, W.
    Zhang, C.
    Zhang, P.
    Fan, H.
    Liu, D.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Zhao, J.
    He-vacancy interaction and multiple He trapping in small void of silicon carbide2015Ingår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 457, s. 36-41Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In fusion environment, large amounts of helium (He) atoms are produced by transmutation along with structural damage in the structural materials, causing material swelling and degrading of physical properties. To understand the microscopic mechanism of He trapping in vacancies and voids, we explored He-vacancy interactions in HenVam (Va for vacancy) clusters (n, m = 1-4) and multiple He trapping in a 7-atom void of silicon carbide (SiC) by first-principles calculations. The binding energy between He and the HenVam clusters increases with the number of vacancies, while the vacancy binding energy gradually increases with the number of He atoms. Furthermore, a small cavity of about 0.55 nm in diameter can accommodate up to 14 He atoms energetically and the corresponding internal pressure is estimated to be 2.5 GPa. The tendency of He trapping in small voids provides an explanation for the experimentally observed He bubble formation at vacancy defects in SiC materials.

  • 3.
    Li, Ruihuan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Dalian University of Technology, China.
    Lu, Song
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. University of Turku, Finland.
    Kim, Dongyoo
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Schönecker, Stephan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Zhao, Jijun
    Kwon, Se Kyun
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Uppsala University, Sweden.
    Stacking fault energy of face-centered cubic metals: thermodynamic and ab initio approaches2016Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 28, nr 39, artikel-id 395001Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The formation energy of the interface between face-centered cubic (fcc) and hexagonal close packed (hcp) structures is a key parameter in determining the stacking fault energy (SFE) of fcc metals and alloys using thermodynamic calculations. It is often assumed that the contribution of the planar fault energy to the SFE has the same order of magnitude as the bulk part, and thus the lack of precise information about it can become the limiting factor in thermodynamic predictions. Here, we differentiate between the interfacial energy for the coherent fcc(1 1 1)/hcp(0 0 0 1) interface and the 'pseudo-interfacial energy' that enters the thermodynamic expression for the SFE. Using first-principles calculations, we determine the coherent and pseudo-interfacial energies for six elemental metals (A1, Ni, Cu, Ag, Pt, and Au) and three paramagnetic Fe-Cr-Ni alloys. Our results show that the two interfacial energies significantly differ from each other. We observe a strong chemistry dependence for both interfacial energies. The calculated pseudo-interfacial energies for the Fe-Cr-Ni steels agree well with the available literature data. We discuss the effects of strain on the description of planar faults via thermodynamic and ab initio approaches.

  • 4.
    Li, Ruihuan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Dalian University of Technology, China.
    Lu, Song
    Kim, Dongyoo
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Schönecker, Stephan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Zhao, Jijun
    Kwon, Se Kyun
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Stacking fault energy of face-centered cubic metals: thermodynamic andab initio approaches2015Manuskript (preprint) (Övrigt vetenskapligt)
  • 5.
    Li, Ruihuan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Dalian University of Technology, China.
    Zhang, P.
    Li, Xiaojie
    Ding, Jianhua
    Wang, Yuanyuan
    Zhao, Jijun
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Effects of alloying elements (Cr, W) on the He behavior in bcc Fe: a first-principles study2015Manuskript (preprint) (Övrigt vetenskapligt)
  • 6.
    Li, Ruihuan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Dalian University of Technology, China.
    Zhang, Pengbo
    Li, Xiaojie
    Ding, Jianhua
    Wang, Yuanyuan
    Zhao, Jijun
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Uppsala University, Sweden.
    Effects of Cr and W additions on the stability and migration of He in bcc Fe: A first-principles study2016Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 123, s. 85-92Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Density functional theory calculations have been performed to study the effects of alloying Cr and W on the stability and diffusivity of interstitial He impurity in body-centered cubic (bcc) Fe host lattice. The interaction between two close Cr/W atoms is repulsive. The relative stable position for an interstitial He remains the tetrahedral interstitial site in the presence Cr. Energetically, He prefers to locate far away from W inside Fe host lattice due to the strong repulsive interaction between He and W. On the other hand, the He migration barrier becomes lower in the presence of Cr (0.026 eV) and W(0.049 eV), as compared to 0.059 eV for pure Fe. Addition of Cr is benefit for He self-trapping, while W is against. The effective diffusivity of He decreases with increasing Cr and W concentrations. Moreover, the additions of Cr and W slightly hinder He being trapped by monovacancy.

  • 7.
    Li, Ruihuan
    et al.
    Dalian University of Technology, China.
    Zhang, Pengbo
    Zhang, Chong
    Huang, Xiaoming
    Zhao, Jijun
    Vacancy trapping mechanism for multiple helium in monovacancy and small void of vanadium solid2013Ingår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 440, nr 1-3, s. 557-561Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using first-principles methods, we have investigated the microscopic mechanism for He trapping in two kinds of vacancy defects (monovacancy and 9-atom void) inside vanadium host lattice. In the monovacancy, single He prefers to occupy the octahedral site near vacancy rather than vacancy center. Inside vacancy defects, the He-He equilibrium distances range in 1.6-2.2 angstrom. After more He atoms are incorporated, the magnitude of trapping energy decreases and the host lattice expand dramatically. A monovacancy and 9-atom void can host up to 18 and 66 He atoms, respectively, with internal pressure up to 7.5 and 19.3 GPa. The atomic structures of selected He clusters trapped in vacancies are compared with the gas-phase clusters. The strong tendency of He trapping at vacancies and 9-atom voids provides an explanation for experimentally observed He bubble formation at vacancy defects in metals.

  • 8.
    Li, Xiaojie
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Dalian University of Technology, China.
    Schonecker, Stephan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Li, Ruihuan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Li, Xiaoqing
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Wang, Yuanyuan
    Zhao, Jijun
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Department of Physics and Astronomy, Division of Materials Theory, Sweden.
    Vitos, Levente
    Department of Physics and Astronomy, Division of Materials Theory, Sweden; Wigner Research Center for Physics, Hungary.
    Ab initio calculations of mechanical properties of bcc W-Re-Os random alloys: effects of transmutation of W2016Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 28, nr 29, artikel-id 295501Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    To examine the effect of neutron transmutation on tungsten as the first wall material of fusion reactors, the elastic properties of W1-x-yRexOsy (0 <= x, y <= 6%) random alloys in body centered cubic (bcc) structure are investigated systematically using the all-electron exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA). The calculated lattice constant and elastic properties of pure W are consistent with available experiments. Both Os and Re additions reduce the lattice constant and increase the bulk modulus of W, with Os having the stronger effect. The polycrystalline shear modulus, Young's modulus and the Debye temperature increase (decrease) with the addition of Re (Os). Except for C-11, the other elastic parameters including C-12, C-44, Cauchy pressure, Poisson ratio, B/G, increase as a function of Re and Os concentration. The variations of the latter three parameters and the trend in the ratio of cleavage energy to shear modulus for the most dominant slip system indicate that the ductility of the alloy enhances with increasing Re and Os content. The calculated elastic anisotropy of bcc W slightly increases with the concentration of both alloying elements. The estimated melting temperatures of the W-Re-Os alloy suggest that Re or Os addition will reduce the melting temperature of pure W solid. The classical Labusch-Nabarro model for solid-solution hardening predicts larger strengthening effects in W1-yOsy than in W1-xRex. A strong correlation between C' and the fcc-bcc structural energy difference for W1-x-yRexOsy is revealed demonstrating that canonical band structure dictates the alloying effect on C'. The structural energy difference is exploited to estimate the alloying effect on the ideal tensile strength in the [0 0 1] direction.

  • 9.
    Lu, Song
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Li, Ruihuan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Kádas, K.
    Zhang, H.
    Tian, Y.
    Kwon, S. K.
    Kokko, K.
    Hu, Q. -M
    Hertzman, S.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Stacking fault energy of C-alloyed steels: The effect of magnetism2017Ingår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 122, s. 72-81Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    First-principles calculations have been performed to study the effect of C on the stacking fault energy (SFE) of paramagnetic γ-Fe and Fe[sbnd]Cr[sbnd]Ni austenitic steel. In these systems, the local magnetic structure is very sensitive to the volume in both fcc and hcp structures, which emphasizes the importance of the magnetovolume coupling effect on the SFE. The presence of C atom suppresses the local magnetic moments of Fe atoms in the first coordination shell of C. Compared to the hypothetical nonmagnetic case, paramagnetism significantly reduces the effect of C on the SFE. In the scenario of C being depleted from the stacking fault structure or twin boundaries, e.g., due to elevated temperature, where the chemical effect of C is dissipated, we calculate the C-induced volume expansion effect on the SFE. The volume induced change in the SFE corresponds to more than ∼ 50% of the total C effect on the SFE obtained assuming uniform C distribution. © 2016 Acta Materialia Inc.

  • 10.
    Ruihuan, Li
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    First-principles study of defects instructural materials2016Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In this thesis, first we focus on the Helium (He) and He bubbles behavior in three kinds of the most promising candidate structural materials for future fusion reactor. These materials are vanadium, silicon carbide (SiC) composites, and reduced activation ferritic-martensitic (RAFM) steels. Second we investigate the intrinsic stacking fault of face-centered cubic (fcc) metals and alloys, with special emphasis on the interfacial energy between fcc and hexagonal close packed (hcp) phases. The present research has been carried out using modern ab initio quantum mechanical tools based on Density Functional Theory.

    The microscopic mechanism of He trapping in vacancies and voids in structural materials has been examined using first-principles calculations based on pseudopotential method as implemented in the Vienna ab initio Simulation Package (VASP). For body-centered cubic (bcc) vanadium (paper I), the trapping energies for multiple He atoms in monovacancy and 9-atom small void (about 0.6 nm in diameter) have been investigated. It is found that monovacancy and 9-atom void capture at least 18 and 66 He atoms, respectively. The corresponding internal pressure caused by He cluster is as large as 7.5 and 19.3 GPa. The He-He distance constrained in small void is shorter than in gas-phase Hen clusters. This finding is consistent with the results obtained for the radial distribution function. For hexagonal 6H–SiC (paper II), the interactions between a He (in one vacancy, Va) and HenVam clusters (n, m = 1 – 4) have been investigated. For a specified vacancy number (i.e. m fixed) in HenVam, the bind energy decreases with increasing He atoms, meaning that it becomes increasingly difficult for trapping more He atoms due to the He-He repulsion. This phenomenon is further confirmed by the attractive interaction between a vacancy and HenVam that expands the void space to release He-He repulsive interaction. However, bulk 6H–SiC has a weak capacity to capture He atoms (14 He atoms) due to its brittle property. The estimated internal pressure (2.5 GPa) has the same order of magnitude as the experimental value (0.8 GPa). For ferromagnetic bcc iron (Fe) (paper III), we concentrate on the effect of chromium (Cr) and tungsten (W) alloying elements on the He stable interstitial position, migration energy and trapping energy. The formation energies of He in tetrahedral interstitial site (T-site) and octahedral interstitial site (O-site) with different number of Cr and W atoms have been studied. The He formation energy trends with increasing Cr and W content are non-linear, respectively. It is found that the antiferromagnetic Cr-Cr coupling in bcc Fe transforms to ferromagnetic coupling, and the repulsion between He and W is larger than in pure W host lattice. The He migration energy and the number of He atoms trapped by monovacancy become lower compared to pure Fe due to the additional Cr and W. It is found that Cr and W lead to higher trapping energies for multiple He and slightly hamper He trapping in vacancy compared to pure bcc Fe.

    In the second part of the thesis (paper IV) the stacking fault energy (SFE) and interfacial energy of six fcc metals and Fe-Cr-Ni alloys have been studied. SFE γ plays an important role in determining the plastic deformation mechanism of fcc metals and thus is a fundamental parameter describing and understanding the mechanical properties of high-technology alloys. Small SFE favors twinning, and high SFE favors dislocation slip. The formation energy of the interface between fcc(111)/hcp(0001) is a key parameter in determining the SFE when using standard thermodynamic approaches. In this thesis, two other models that are commonly used in the ab initio calculation of the SFE are considered. One is based on the supercell technique with one intrinsic stacking fault pure unit cell, and the other on the axial interaction model. Due to the different conditions for hcp structures in entering the thermodynamic model and the above ab initio models, we differentiate between the actual interfacial energy σ for the coherent fcc(111)/hcp(0001) interface and the "pseudo-interfacial energy (σ∗)", the latter appearing in the thermodynamic expression for the SFE. Using the first-principles exact muffin-tin orbitals method (EMTO) in combination with the coherent potential approximation (CPA), we investigated the coherent and pesudo-interfacial energy for six fcc metal (Al, Ni, Cu, Ag, Pt, and Au) and three Fe-Cr-Ni alloys. It is found the two interfacial energies remarkable differ from each other. Our results form the first systematic first-principles data for the interfacial energies of monoatomic fcc metals and austenitic stainless steels and are expected to be used in future thermodynamic predictions.

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