| 1. |
- Toijer, Elin, et al.
(författare)
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Ab initio modelling of intergranular fracture of nickel containing phosphorus : Interfacial excess properties
- 2021
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Ingår i: Nuclear Materials and Energy. - : Elsevier BV. - 2352-1791. ; 28
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Tidskriftsartikel (refereegranskat)abstract
- In the present work, the impact of phosphorus impurities on the grain boundary strength of nickel has been investigated by means of density functional theory (DFT) modelling. Owing to different outcomes and trends previously reported in the literature, it is unclear whether P is strengthening or weakening the Ni grain boundary. To address this issue, we utilize three different DFT based methods: the excess-energy approach, rigid grain separation, and Rice-Wang's thermodynamic approach. The results show that the commonly used rigid model predicts P to have an increasing effect on the peak stress of Ni of up to 14%, as opposed to a reduction, which is indicated by the excess-energy approach. Employment of the Rice-Wang approach, on the other hand, displays a slight reduction in work of separation. The results show that the discrepancies between previous works can be attributed not so much to the physics of the system, but to the applied model, the partition scheme and the interpretation of the outcomes. This underlines the importance of a proper description of the fracture process, and shows that common simplifications can have a decisive impact on the observed trends.
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| 2. |
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| 3. |
- Olsson, Pär A T, 1981-, et al.
(författare)
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All-atomic and coarse-grained molecular dynamics investigation of deformation in semi-crystalline lamellar polyethylene
- 2018
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Ingår i: Polymer. - : Elsevier Ltd. - 0032-3861 .- 1873-2291. ; 153, s. 305-316
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Tidskriftsartikel (refereegranskat)abstract
- In the present work we have performed classical molecular dynamics modelling to investigate the effects of different types of force-fields on the stress-strain and yielding behaviours in semi-crystalline lamellar stacked linear polyethylene. To this end, specifically the all-atomic optimized potential for liquid simulations (OPLS-AA) and the coarse-grained united-atom (UA) force-fields are used to simulate the yielding and tensile behaviour for the lamellar separation mode. Despite that the considered samples and their topologies are identical for both approaches, the results show that they predict widely different stress-strain and yielding behaviours. For all UA simulations we obtain oscillating stress-strain curves accompanied by repetitive chain transport to the amorphous region, along with substantial chain slip and crystal reorientation. For the OPLS-AA modelling primarily cavitation formation is observed, with small amounts of chain slip to reorient the crystal such that the chains align in the tensile direction. This force-field dependence is rooted in the lack of explicit H-H and C-H repulsion in the UA approach, which gives rise to underestimated ideal critical resolved shear stress. The computed critical resolved shear stress for the OPLS-AA approach is in good agreement with density functional theory calculations and the yielding mechanisms resemble those of the lamellar separation mode. The disparate energy and shear stress barriers for chain slip of the different models can be interpreted as differently predicted intrinsic activation rates for the mechanism, which ultimately are responsible for the observed diverse responses of the two modelling approaches. © 2018 Elsevier Ltd
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| 4. |
- Frostenson, Carl Mikael, 1992, et al.
(författare)
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Hard and soft materials: Putting consistent van der Waals density functionals to work
- 2022
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Ingår i: Electronic Structure. - : IOP Publishing. - 2516-1075. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- We present the idea and illustrate potential benefits of having a tool chain of closely related regular, unscreened and screened hybrid exchange-correlation (XC) functionals, all within the consistent formulation of the van derWaals density functional (vdW-DF) method (Hyldgaard et al (2020 J. Phys.: Condens. Matter 32 393001)). Use of this chain of nonempirical XC functionals allows us to map when the inclusion of truly nonlocal exchange and of truly nonlocal correlation is important. Here we begin the mapping by addressing hard and soft material challenges: Magnetic elements, perovskites, and biomolecular problems.We also predict the structure and polarization for a ferroelectric polymer. To facilitate this work and future broader explorations, we present a stress formulation for spin vdW-DF and illustrate the use of a simple stability-modeling scheme. The modeling supplements density functional theory (DFT) (with a specific XC functional) by asserting whether the finding of a soft mode (an imaginary-frequency vibrational mode, ubiquitous in perovskites and soft matter) implies an actual DFT-based prediction of a low-temperature transformation.
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| 5. |
- Gajdek, Dorotea, et al.
(författare)
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Structural Changes in Monolayer Cobalt Oxides under Ambient Pressure CO and O 2 Studied by In Situ Grazing-Incidence X-ray Absorption Fine Structure Spectroscopy
- 2022
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:7, s. 3411-3418
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Tidskriftsartikel (refereegranskat)abstract
- We have used grazing incidence X-ray absorption fine structure spectroscopy at the cobalt K-edge to characterize monolayer CoO films on Pt(111) under ambient pressure exposure to CO and O2, with the aim of identifying the Co phases present and their transformations under oxidizing and reducing conditions. X-ray absorption near edge structure (XANES) spectra show clear changes in the chemical state of Co, with the 2+ state predominant under CO exposure and the 3+ state predominant under O2-rich conditions. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that the CoO bilayer characterized in ultrahigh vacuum is not formed under the conditions used in this study. Instead, the spectra acquired at low temperatures suggest formation of cobalt hydroxide and oxyhydroxide. At higher temperatures, the spectra indicate dewetting of the film and suggest formation of bulklike Co3O4 under oxidizing conditions. The experiments demonstrate the power of hard X-ray spectroscopy to probe the structures of well-defined oxide monolayers on metal single crystals under realistic catalytic conditions.
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| 6. |
- Hiremath, Praveenkumar, et al.
(författare)
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Effects of interatomic potential on fracture behaviour in single- and bicrystalline tungsten
- 2022
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 207, s. 1-18
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Tidskriftsartikel (refereegranskat)abstract
- In the present work, we have evaluated the performance of different embedded atom method (EAM) andsecond-nearest neighbour modified embedded atom method (2NN-MEAM) potentials based on their predictive capabilities for modelling fracture in single- and bicrystalline tungsten. As part of the study, a new 2NN-MEAM was fitted with emphasis on reproducing surface, unstable stacking fault and twinning energies as derived fromdensity functional theory (DFT) modelling. The investigation showed a systematic underestimation of surfaceenergies by most EAM potentials, and a significant variation in unstable stacking and twinning fault energies.Moreover, the EAM potentials in general lack the ability to reproduce the DFT traction–separation (TS) curves. The shorter interaction length and higher peak stress of the EAM TS curves compared to the 2NN-MEAM and DFT TS curves result in one order of magnitude higher lattice trapping than for cracks studied with 2NN-MEAM. These differences in lattice trapping can lead to significant qualitative differences in the fracture behaviour. Overall, the new 2NN-MEAM potential best reproduced fracture-relevant material properties and its results were consistent with fracture experiments. Finally, the results of fracture simulations were compared with analytical predictions based on Griffith and Rice theories, for which emerging discrepancies were discussed.
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| 7. |
- Hiremath, Praveenkumar, et al.
(författare)
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Phosphorus driven embrittlement and atomistic crack behavior in tungsten grain boundaries
- 2024
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 244, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the role of phosphorus (P) impurities on the fracture toughness and underlying failure mechanisms by means of classical atomistic modeling for a set of ⟨110⟩ symmetric tilt tungsten grain boundaries (GBs). This entailed the utilization of a quasi-static mode I displacement-controlled setup with cohesive zone volume elements (CZVEs) to study failure mechanisms and evaluate the fracture toughness of the GB cracks. The fracture toughness was estimated using three approaches: computing (i) the individual and (ii) the average energy release rate of CZVEs along the fractured surfaces and using them as inputs for the Griffith model, and(iii) relating the fracture toughness to crack propagation initiation. The cracks in all the pristine GBs evolved in a brittle fashion, occasionally forming facetted cleavage planes. Upon introduction of impurities, other mechanisms such as void formation and crack-tip transformation were also observed. Depending on the GBproximity of the occupied segregation sites, local strengthening was seen occasionally for individual CZVEs and at the crack-tip, which was triggered by local impurity-induced crack deflection onto planes with higher cohesion. But when the fracture toughness from the averaged energy release rate was considered, an overall reduction with increasing impurity segregation was found, although to a varying degree for different GBs. This indicates an overall increased degree of embrittlement with increasing P-segregation at the GBs, which concurs with most experimental results reported in the literature.
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| 8. |
- Krause, Andreas M., et al.
(författare)
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Interstitial diffusion of hydrogen in M7C3 (M=Cr,Mn,Fe)
- 2023
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 218
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Tidskriftsartikel (refereegranskat)abstract
- To increase the understanding of the role of carbide precipitates on the hydrogen embrittlement of martensitic steels, we have performed a density functional theory study on the solution energies and energy barriers for hydrogen diffusion in orthorhombic M7C3 (M = Cr, Mn, Fe). Hydrogen can easily diffuse into the lattice and cause internal stresses or bond weakening, which may promote reduced ductility. Solution energies of hydrogen at different lattice positions have systematically been explored, and the lowest values are -0.28, 0.00, and 0.03 eV/H-atom for Cr7C3, Mn7C3, and Fe7C3, respectively. Energy barriers for the diffusion of hydrogen atoms have been probed with the nudged elastic band method, which shows comparably low barriers for transport via interstitial octahedral sites for all three systems. Analysis of the atomic volume reveals a correlation between low solution energies and energy barriers and atoms with large atomic volumes. Furthermore, it shows that the presence of carbon tends to increase the energy barrier. Our results can explain previous experimental findings of hydrogen located in the bulk of CrC precipitates and provide a solid basis for future design efforts of steels with high strength and commensurable ductility.
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| 9. |
- Kroon, Martin, et al.
(författare)
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Anisotropic Elastic-Viscoplastic Properties at Finite Strains of Injection-Moulded Low-Density Polyethylene
- 2018
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Ingår i: Experimental mechanics. - : Springer New York LLC. - 0014-4851 .- 1741-2765. ; 58:1, s. 75-86
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Tidskriftsartikel (refereegranskat)abstract
- Injection-moulding is one of the most common manufacturing processes used for polymers. In many applications, the mechanical properties of the product is of great importance. Injection-moulding of thin-walled polymer products tends to leave the polymer structure in a state where the mechanical properties are anisotropic, due to alignment of polymer chains along the melt flow direction. The anisotropic elastic-viscoplastic properties of low-density polyethylene, that has undergone an injection-moulding process, are therefore examined in the present work. Test specimens were punched out from injection-moulded plates and tested in uniaxial tension. Three in-plane material directions were investigated. Because of the small thickness of the plates, only the in-plane properties could be determined. Tensile tests with both monotonic and cyclic loading were performed, and the local strains on the surface of the test specimens were measured using image analysis. True stress vs. true strain diagrams were constructed, and the material response was evaluated using an elastic-viscoplasticity law. The components of the anisotropic compliance matrix were determined together with the direction-specific plastic hardening parameters. © 2017 The Author(s)
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| 10. |
- Mirzai, Amin, et al.
(författare)
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First-principle investigation of doping effects on mechanical and thermodynamic properties of Y2SiO5
- 2021
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Ingår i: Mechanics of materials. - : Elsevier. - 0167-6636 .- 1872-7743. ; 154
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the variation of elastic stiffness moduli and the thermodynamic properties of yttrium orthosilicate (Y2SiO5, YSO) under various doping concentrations of Eu3+ ions. The model is based on a low temperature approximation (T « θD), and the plane-wave density functional theory (DFT) is used to carry out the calculations. The results show that the ions primarily occupy the Y1 site of the basic molecule for all applied concentrations. The overall shear, bulk, and Young’s moduli exhibit a decreasing trend with increasing concentration. The overall anisotropy shows a very small increase with increasing concentration. The Debye temperature as well as the Grünesien parameter for each concentration are predicted. Lastly, the predicted heat capacity at constant volume is calculated and compared to experimental values. Our study reveals that there is almost linear relationship between concentration and mechanical properties of YSO. The decrease of the Grünesien parameter with concentration increase might decrease the anharmonic effects in YSO, although this effect is small. In addition, the change in heat capacity with concentration rise is negligible.
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| 11. |
- Nigro, Claudio F., Dr.
(författare)
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Phase-field modeling of stress-induced precipitation and kinetics in engineering metals
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The formation of brittle compounds in metals operating in corrosive environments can be a tremendous source of embrittlement for industrial structures and such phenomenon is commonly enhanced in presence of stresses. To study this type of microstructural change modeling is preferred to experiment to reduce costs and prevent undesirable environmental impacts. This thesis aims at developing an engineering approach to model stress-induced precipitation, especially near stress concentrators, e.g. crack tips, for multi-phase and polycrystalline metals, with numerical efficiency.In this thesis, four phase-field models are developed and applied on stress-induced hydride precipitation in zirconium and titanium alloys. The energy of the system is minimized through the time-dependent Ginzburg-Landau equation, which provides insights to the kinetics of the phenomenon. In these models, the driving force for precipitation is the coupling between the applied stress and the phase transformation-induced dilatation of the system. Models 1-3 implicitly incorporate near crack-tip stress fields by using linear elastic fracture mechanics so that only the phase-field equation is solved numerically with the finite volume method, reducing the computational costs. Phase transformation is investigated for intragranular, intergranular and interphase cracks in single- and two-phase materials by considering isotropy and some degrees of anisotropy, grain/phase boundary energy, different transition orders and solid solubility limit. Model 4 allows representing anisotropy connected to lattice mismatch and the orientation of the precipitates influenced by the applied stress. The model is employed through the finite element program Abaqus, where the fully coupled thermo-mechanical solving method is applied to the coupled mechanical/phase-field problem. Hydride growth is observed to follow the near-crack tip hydrostatic stress contours and can reach a steady state for specific conditions. The relation between hydride formation kinetics and material properties, and stress relaxation are well-reflected in the results.With the presented approaches, precipitation kinetics including different kinds of defects, multi-phase microstructures, phase/grain boundaries, order transitions and loading modes can be successfully captured with low computational costs. They could therefore contribute to the numerical efficiency of multi-scale environment-assisted embrittlement prediction schemes within commercial software serving engineering projects.
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| 12. |
- Olsson, Pär A T, 1981-, et al.
(författare)
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Ab initio investigation of monoclinic phase stability and martensitic transformation in crystalline polyethylene
- 2018
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Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 2:7, s. 7-13
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Tidskriftsartikel (refereegranskat)abstract
- We study the phase stability and martensitic transformation of orthorhombic and monoclimic polyethylene by means of density functional theory using the nonempirical consistent-exchange vdW-DF-cx functional [Phys. Rev. B 89, 035412 (2014)]. The results show that the orthorhombic phase is the most stable of the two. Owing to the occurrence of soft librational phonon modes, the monoclimic phase is predicted not to be stable at zero pressure and temperature, but becomes stable when subjected to compressive transverse deformations that pin the chains and prevent them from wiggling freely. This theoretical characterization, or prediction, is consistent with the fact that the monoclimic phase is only observed experimentally when the material is subjected to mechanical loading. Also, the estimated threshold energy for the combination of lattice deformation associated with the T1 and T2 transformation paths (between the orthorhombic and monoclimic phases) and chain shuffling is found to be sufficiently low for thermally activated back transformations to occur. Thus, our prediction is that the crystalline part can transform back from the monoclimc to the orthorhombic phase upon unloading and/or annealing, which is consistent with experimental observations. Finally, we observe how a combination of such phase transformations can lead to a fold-plane reorientation from {110} to {100} type in a single orthorhombic crystal.
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| 13. |
- Olsson, Pär A T, 1981-
(författare)
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Ab initio thermo-elasticity of δ-MHx (M=Zr, Ti)
- 2023
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 218
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Tidskriftsartikel (refereegranskat)abstract
- In the present work, we report the results of a systematic ab initio study of the thermo-elastic properties of -MH1.5 (M=Zr, Ti). This investigation serves three purposes: (i) Elucidate the fully anisotropic temperature dependent elastic constants of hydrides, (ii) address discrepancies in thermal expansion data reported in the literature and (iii) provide input data for thermodynamic-based phase-transformation modelling. Due to a reduced contribution from the vibrational free energy to the strain energy, in agreement with experimental observations we find that the temperature dependent stiffness of hydrides vary to a much lesser degree than the matrix. For -ZrH1.5, we further find that Zener’s anisotropy ratio varies with temperature. Regarding the linear thermal expansion, our results indicate that it is highly temperature dependent. With the exception of a few outliers, our DFT data concurs well with experimental data, if the temperature range over which it was measured is taken into account.
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| 14. |
- Olsson, Pär A T, 1981-, et al.
(författare)
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Atomistic investigation of functionalized polyethylene-alumina interfacial strength and tensile behaviour
- 2021
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 187
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Tidskriftsartikel (refereegranskat)abstract
- We study the adhesion and tensile behaviour of bi-layer interfaces comprising polyethylene, doped with carbonyl and hydroxyl functional groups emanating from ozone treatment, and α" role="presentation">-Al2O3" role="presentation"> by means of density functional theory and classical atomistic modelling. The results show that the deformations are localized within the polymer and comprise chain slip, disentanglement and detachment from the substrate, where only the latter is notably affected by the doping. The binding energies and excess forces associated with the detachment of functional groups from the alumina substrate are of the order of 1.7 eV and 1 nN, respectively, for both types. Although such forces do not affect the maximum peak stress notably, they give rise to spikes in the traction-separation curves following the fibril formation and promote increased total work of fracture.
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| 15. |
- Olsson, Pär A T, 1981-, et al.
(författare)
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Grain Size-Dependent Thermal Expansion of Nanocrystalline Metals
- 2023
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Ingår i: Materials. - : MDPI. - 1996-1944. ; 16:14
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Tidskriftsartikel (refereegranskat)abstract
- In the present work, we have used classical molecular dynamics and quantum mechanical density functional theory modeling to investigate the grain size-dependent thermal expansion coefficient (CTE) of nanocrystalline Cu. We find that the CTE increases by up to 20% with a gradually decreasing grain size. This behavior emerges as a result of the increased population of occupied anti-bonding states and bond order variation in the grain boundary regions, which contribute to thereduced resistance against thermally-induced bond stretching and dictate the thermal expansion behavior in the small grain size limit. As a part of the present work, we have established a procedure to produce ab initio thermal expansion maps that can be used for the prediction of the grain size dependent CTE. This can serve as a modeling tool, e.g., to explore the impact of grain boundary impurity segregation on the CTE.
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| 16. |
- Starikov, Sergei, et al.
(författare)
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Angular-dependent interatomic potential for large-scale atomistic simulation of W-Mo-Nb ternary alloys
- 2024
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 233
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Tidskriftsartikel (refereegranskat)abstract
- We present a new classical interatomic potential designed for simulation of the W-Mo-Nb system. The angular-dependent format of the potential allows for reproduction of many important properties of pure metals and complex concentrated alloys with good accuracy. Special attention during the development and validation of the potential was paid to the description of vacancies, screw dislocations and planar defects, as well as thermo-mechanical properties. Here, the applicability of the developed model is demonstrated by studying the temperature dependence of the elastic moduli and average atomic displacement in pure metals and concentrated alloys up to the melting point.
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| 17. |
- Vesti, Anders, et al.
(författare)
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First-principles study on thermal expansion of W-Re sigma and chi phases
- 2024
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Ingår i: Nuclear Materials and Energy. - : Elsevier BV. - 2352-1791. ; 39
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Tidskriftsartikel (refereegranskat)abstract
- We investigate how the Re content affects the coefficient of thermal expansion (CTE) of the non-stoichiometric W-based and phases, forming upon neutron irradiation of W, to explore and quantify its mismatch between precipitates (W-Re) and matrix (W). To this end, we have conducted first-principles calculations using two approaches: the Debye-Grüneisen (DG) model and the quasi-harmonic approximation (QHA). The two approaches yield different results: the QHA, which is deemed to be the most accurate of the two, predicts substantial changes with Re content, while the acoustic-modes based DG model does not. The CTE of the σ and χ at stable Re contents is compared to experimental values for bcc-W and bcc-W-Re containing 25 at.% Re. Taking bcc-W as a reference, we find a significant mismatch in CTE of up to 37% and 62% for σ and χ, respectively, which may contribute to thermal stress buildup in the material at elevated temperatures. The mismatch is shown to increase with the temperature and Re content for both phases. The produced data are used to fit a temperature and Re concentration-dependent analytical function of the CTE for both phases, which can be employed as input for continuum mechanical modeling.Previous article in issue
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