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First-principles study of defects instructural materials
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.ORCID iD: 0000-0001-5676-418X
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , x, 66 p.
National Category
Other Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-182124ISBN: 978-91-7595-856-9 (print)OAI: oai:DiVA.org:kth-182124DiVA: diva2:903547
Public defence
2016-03-10, Sal B2, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20160218

Available from: 2016-02-18 Created: 2016-02-16 Last updated: 2016-03-09Bibliographically approved
List of papers
1. Vacancy trapping mechanism for multiple helium in monovacancy and small void of vanadium solid
Open this publication in new window or tab >>Vacancy trapping mechanism for multiple helium in monovacancy and small void of vanadium solid
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2013 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 440, no 1-3, 557-561 p.Article in journal (Refereed) Published
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.

Keyword
Equilibrium distances, First principles method, Gas-phase clusters, Microscopic mechanisms, Octahedral sites, Trapping energy, Vacancy Defects, Vacancy trapping
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-159219 (URN)10.1016/j.jnucmat.2013.03.068 (DOI)000323396600070 ()2-s2.0-84885295185 (Scopus ID)
Conference
NuMat Conference, OCT 22-25, 2012, Osaka, Japan
Note

QC 20150126

Available from: 2015-01-26 Created: 2015-01-26 Last updated: 2017-12-05Bibliographically approved
2. He-vacancy interaction and multiple He trapping in small void of silicon carbide
Open this publication in new window or tab >>He-vacancy interaction and multiple He trapping in small void of silicon carbide
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2015 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 457, 36-41 p.Article in journal (Refereed) Published
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.

Keyword
Atoms, Binding energy, Calculations, Silicon, Silicon carbide, Swelling, Vacancies, Crystal atomic structure, First-principles calculation, Large amounts, Microscopic mechanisms, SiC materials, Silicon carbides (SiC), Small cavities, Structural damages, Vacancy Defects
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-159217 (URN)10.1016/j.jnucmat.2014.10.062 (DOI)000349169100005 ()2-s2.0-84910628889 (Scopus ID)
Note

QC 20150126

Available from: 2015-01-26 Created: 2015-01-26 Last updated: 2017-12-05Bibliographically approved
3. Effects of alloying elements (Cr, W) on the He behavior in bcc Fe: a first-principles study
Open this publication in new window or tab >>Effects of alloying elements (Cr, W) on the He behavior in bcc Fe: a first-principles study
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2015 (English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-182197 (URN)
Note

QS 2016

Available from: 2016-02-17 Created: 2016-02-17 Last updated: 2016-02-18Bibliographically approved
4. Stacking fault energy of face-centered cubic metals: thermodynamic andab initio approaches
Open this publication in new window or tab >>Stacking fault energy of face-centered cubic metals: thermodynamic andab initio approaches
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2015 (English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-182198 (URN)
Note

QS 2016

Available from: 2016-02-17 Created: 2016-02-17 Last updated: 2016-02-18Bibliographically approved

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