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Now showing items 81 - 90 of 90

  • Simulation of threshold displacement energies in FeCr

    Juslin, N.   Nordlund, K.   Wallenius, J.   Malerba, L.  

    We have studied the role of chromium on threshold displacement energies in FeCr for the fusion reactor steel relevant concentration 10% Cr. We have used molecular dynamics simulations in order to determine whether the observed Cr-content dependence of macroscopic properties can be due to the defect production. We compare FeCr-alloys with pure iron and chromium, employing two different potential sets for the Fe-Cr system. We find that there are no significant differences between pure iron and FeCr with 10% Cr for the 100, 110 and 111 directions and the average threshold energy. (c) 2006 Elsevier B.V. All rights reserved.
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  • Improved ERO modelling of beryllium erosion at ITER upper first wall panel using JET-ILW and PISCES-B experience

    Borodin, D.   Romazanov, J.   Pitts, R.A.   Lisgo, S.W.   Brezinsek, S.   Borodkina, I.   Eksaeva, A.   Safi, E.   Nordlund, K.   Kirschner, A.   Linsmeier, Ch.  

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  • Adaptive molecular decomposition: Large-scale quantum chemistry for liquids

    Jarvi, T.T.   Mayrhofer, L.   Polvi, J.   Nordlund, K.   Pastewka, L.   Moseler, M.  

    We present a linear-scaling method based on self-consistent charge non-orthogonal tight-binding. Linear scaling is achieved using a many-body expansion, which is adjusted dynamically to the instantaneous molecular configuration of a liquid. The method is capable of simulating liquids over large length and time scales, and also handles reactions correctly. Benchmarking on typical carbonate electrolytes used in Li-ion batteries displays excellent agreement with results from full tight-binding calculations. The decomposition slightly breaks the Hellmann-Feynman theorem, which is demonstrated by application to water. However, an additional correction also enables dynamical simulation in this case.
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  • Bond order potential for gold

    Backman, M.   Juslin, N.   Nordlund, K.  

    We develop an analytic bond order potential for modelling of gold. The bond order formalism includes bond angularity and offers an alternative approach to the embedded atom type potentials frequently used to describe metallic bonding. The advantage of the developed potential is that it can be extended to describe interactions with covalent materials. Experimental and ab initio data of gold properties is used to fit the potential and a good description of bulk and defect properties is achieved. We use the potential to simulate melting of nanoclusters and find that the experimentally observed size dependent melting behaviour is reproduced qualitatively.
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  • Cluster ion-solid interactions from meV to MeV energies RID B-8547-2008

    Nordlund, K.   Jaervi, T. T.   Meinander, K.   Samela, J.  

    The nature of cluster ion-surface interactions changes dramatically with the kinetic energy and mass of the incoming cluster species. In this article we review some recent work on the nature of cluster-surface interactions spanning an energy range from a few tens of meV/atom to several MeV/cluster and cluster sizes in the range of 1-300000 atoms/cluster. We describe five possible distinct outcomes of a single cluster impact event: (i) deposition into a non-epitaxial configuration, (ii) deposition into an epitaxial configuration, (iii) crater formation by liquid flow, (iv) crater formation by hydrostatic pressure, (v) implantation.
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  • Atomistic simulations of Be irradiation on W: mixed layer formation and erosion

    Lasa, A.   Heinola, K.   Nordlund, K.  

    In the recent ITER-Like Wall experiment at JET, tungsten (W) and beryllium (Be) are used as the first wall plasma-facing materials. Due to the plasma-wall interactions, these materials will erode, be transported, re-deposit and mix. We present the first computational, atomistic, systematic study on the W-Be material mixing under fusion-relevant conditions. To this end, W surfaces were irradiated by Be, varying the impacting energy and angle, followed by annealing the mixed W-Be layers. At low energies, a Be layer is deposited on W, suppressing the W erosion. The materials mix as the W atoms migrate towards the Be layer due to the heat of mixing. Be-2 and BeW molecules eroded, both physically (dimer sputtering) and chemically (sputter etching). All the mixed layers show an underlying hcp-like Be structure and the Be : W ratios are close to those in the intermetallic phases (Be2W-Be12W). However, no crystalline alloy structure formed, even after annealing. Further, we present a geometrical model for the angular dependence of the Be reflection, which strongly affects the W sputtering.
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  • Interaction of dislocations with carbides in BCC Fe studied by molecular dynamics

    Granberg, F.   Terentyev, D.   Nordlund, K.  

    In this study, the atomic processes involving the interaction of an edge dislocation with carbide precipitates in an iron matrix are investigated by molecular dynamics, utilizing two interatomic potentials (Hepburn, 2008 and Henriksson, 2013). The carbides investigated were.Fe3C and M23C6, where M was either Fe or Cr. The results from spherical precipitates were compared with rod shaped obstacles, to investigate the effect of climb in the unpinning process and the stress related to this process. The rod simulations showed a higher unpinning stress for all investigated sizes and temperatures, which indicates that climb will play a role in the unpinning phenomenon. The results showed, as previous studies, a decrease of the unpinning stress with increasing temperature and that a larger obstacle yields a higher unpinning stress. The Orowan process of dislocation unpinning was observed with both potentials as an increase in the needed unpinning stress in consecutive interactions with the same obstacle. The results showed that the structure of the obstacle does not affect the unpinning stress, as much as temperature, for obstacles >=3D 2 nm. Comparison of obstacles with the same structure but with different composition, Fe23C6 and Cr23C6, showed that the small shearable chromium carbides were stronger than the corresponding iron carbides, pointing to the importance of the chemical composition of the carbide. (C) 2015 Elsevier B.V. All rights reserved.
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  • The nature of high-energy radiation damage in iron

    Zarkadoula, E.   Daraszewicz, S. L.   Duffy, D. M.   Seaton, M. A.   Todorov, I. T.   Nordlund, K.   Dove, M. T.   Trachenko, K.  

    Understanding and predicting a material's performance in response to high-energy radiation damage, as well as designing future materials to be used in intense radiation environments, requires knowledge of the structure, morphology and amount of radiation-induced structural changes. We report the results of molecular dynamics simulations of high-energy radiation damage in iron in the range 0.2-0.5 MeV. We analyze and quantify the nature of collision cascades both at the global and the local scale. We observe three distinct types of damage production and relaxation, including reversible deformation around the cascade due to elastic expansion, irreversible structural damage due to ballistic displacements and smaller reversible deformation due to the shock wave. We find that the structure of high-energy collision cascades becomes increasingly continuous as opposed to showing sub-cascade branching as reported previously. At the local length scale, we find large defect clusters and novel small vacancy and interstitial clusters. These features form the basis for physical models aimed at understanding the effects of high-energy radiation damage in structural materials.
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  • Improved ERO modelling for spectroscopy of physically and chemically assisted eroded beryllium from the JET-ILW

    Borodin, D.   Brezinsek, S.   Borodkina, I.   Romazanov, J.   Matveev, D.   Kirschner, A.   Lasa, A.   Nordlund, K.   Bjorkas, C.   Airila, M.   Miettunen, J.   Groth, M.   Firdaouss, M.  

    Physical and chemical assisted physical sputtering were characterised by the Be I and Be II line and BeD band emission in the observation chord measuring the sightline integrated emission in front of the inner beryllium limiter at the torus midplane. The 3D local transport and plasma-surface interaction Monte-Carlo modelling (ERO code [18]) is a key for the interpretation of the observations in the vicinity of the shaped solid Be limiter. The plasma parameter variation (density scan) in limiter regime has provided a useful material for the simulation benchmark. The improved background plasma parameters input, the new analytical expression for particle tracking in the sheath region and implementation of the BeD release into ERO has helped to clarify some deviations between modelling and experiments encountered in the previous studies [4,5]. Reproducing the observations provides additional confidence in our 'ERO-min' fit for the physical sputtering yields for the plasma-wetted areas based on simulated data. (C) 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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  • Mechanism of vacancy formation induced by hydrogen in tungsten

    Liu, Yi-Nan   Ahlgren, T.   Bukonte, L.   Nordlund, K.   Shu, Xiaolin   Yu, Yi   Li, Xiao-Chun   Lu, Guang-Hong  

    We report a hydrogen induced vacancy formation mechanism in tungsten based on classical molecular dynamics simulations. We demonstrate the vacancy formation in tungsten due to the presence of hydrogen associated directly with a stable hexagonal self-interstitial cluster as well as a linear crowdion. The stability of different self-interstitial structures has been further studied and it is particularly shown that hydrogen plays a crucial role in determining the configuration of SIAs, in which the hexagonal cluster structure is preferred. Energetic analysis has been carried out to prove that the formation of SIA clusters facilitates the formation of vacancies. Such a mechanism contributes to the understanding of the early stage of the hydrogen blistering in tungsten under a fusion reactor environment. (C) 2013 Author( s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
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