High energy collision cascades in tungsten: dislocation loops structure and clustering scaling laws

Recent experiments on in-situ high-energy self-ion irradiation of tungsten (W) show the occurrence of unusual cascade damage effects resulting from single ion impacts, shedding light on the nature of radiation damage expected in the tungsten components of a fusion reactor. In this paper, we investigate the dynamics of defect production in 150 keV collision cascades in W at atomic resolution, using molecular dynamics simulations and comparing predictions with experimental observations. We show that cascades in W exhibit no subcascade break-up even at high energies, producing a massive, unbroken molten area, which facilitates the formation of large defect clusters. Simulations show evidence of the formation of both 1/2 and interstitial-type dislocation loops, as well as the occurrence of cascade collapse resulting in vacancy-type dislocation loops, in excellent agreement with experimental observations. The fractal nature of the cascades gives rise to a scale-less power law type size distribution of defect clusters.Comment: 6 pages, 3 figure

Cooperative mixing induced surface roughening in bilayer metals: a possible novel surface damage mechanism

Molecular dynamics simulations have been used to study a collective atomic transport phenomenon by repeated Ar$^+$ irradiations in the Ti/Pt interfacial system. The ion-induced injection of surface atoms to the bulk, the ejection of bulk atoms to the top layers together with surface erosion is strongly enhanced by interfacial mixing. This process leads to a dense interfacial material, and broadening of the interface region. The process scales with the relative difference of the atomic masses. We find that surface roughening and interfacial mixing is strongly coupled via an enhanced counterflow material transport normal to the surface which might be a novel surface damage mechanism. This cooperative phenomenon is active when the bilayer system is subjected to a high dose ion irradiation (multiple ion irradiations) and leads to surface cavity growth.Comment: 6 pages, 6 figures. accepted in Nucl. Instrum. Meth.

Historical review of computer simulation of radiation effects in materials

In this Article, I review the development of computer simulation techniques for studying radiation effects in materials from 1946 until 2018. These developments were often closely intertwined with associated experimental developments, which are also briefly discussed in conjunction with the simulations. The focus is on methods that either deal directly with the primary radiation damage generation event, or with such defects or phase changes that typically occur due to radiation. The methods discussed at some length are, in order of historical appearance: Reaction rate theory or rate equations (RE), Monte Carlo neutronics calculations (MCN), Metropolis Monte Carlo (MMC), Molecular Dynamics (MD), Binary Collision Approximation (BCA), Kinetic Monte Carlo (KMC), Discrete Dislocation Dynamics (DDD), Time-Dependent Density Functional Theory (TDDFT), and Finite Element Modelling (FEM). For each method, I present the origins of the methods, some key developments after this, as well as give some opinions on possible future development paths. (C) 2019 The Author. Published by Elsevier B.V.Peer reviewe

Does the thermal spike affect low-energy ion-induced interfacial mixing?

Molecular dynamics simulations have been used to obtain the three-dimensional distribution of interfacial mixing and cascade defects in Ti/Pt multilayer system due to single 1 keV $Ar^+$ impacts at grazing angle of incidence. The Ti/Pt system was chosen because of its relatively high heat of mixing in the binary alloy and therefore a suitable candidate for testing the effect of heat of mixing on ion-beam mixing. However, the calculated mixing profile is not sensitive to the heat of mixing. Therefore the thermal spike model of mixing is not fully supported under these irradiation conditions. Instead we found that the majority of mixing occurs after the thermal spike during the relaxation process. These conclusions are supported by liquid, vacancy as well as adatom analysis. The interfacial mixing is in various aspects anomalous in this system: the time evolution of mixing is leading to a phase delay for Ti mixing, and Pt exhibits an unexpected double peaked mixing evolution. The reasons to these effects are discussed.Comment: 7 pages, 12 figures, Nucl. Instr. Meth. B211, 524. (2003

Channeling maps for Si ions in Si : Assessing the binary collision approximation

Simulations based on the binary collision approximation (BCA) are in principle less accurate than molecular dynamics (MD) simulations. In this work, we present a comprehensive comparison between BCA and MD for Si ions impinging on a (001)-Si surface by comparing "channeling maps", i.e., projected ranges of the ions as a function of incidence direction in a representative part of the angular space. We find quantitative differences to develop as the energy decreases below similar to 100 eV, but find qualitative agreement down to similar to 10 eV. Moreover, the quality of the BCA channeling maps depends on the implementation of the BCA, which is explained in terms of double-hits and missed collisions.Peer reviewe

Sputtering yields exceeding 1000 by 80keV Xe irradiation of Au nanorods

Using experiments and computer simulations, we find that 80 keV Xe ion irradiation of Au nanorods can produce sputtering yields exceeding 1000, which to our knowledge are the highest yields reported for sputtering by single ions in the nuclear collision regime. This value is enhanced by more than an order of magnitude compared to the same irradiation of flat Au surfaces. Using MD simulations, we show that the very high yield can be understood as a combination of enhanced yields due to low incoming angles at the sides of the nanowire, as well as the high surface-to-volume ratio causing enhanced explosive sputtering from heat spikes. We also find, both in experiments and simulations, that channeling has a strong effect on the sputtering yield: if the incoming beam happens to be aligned with a crystal axis of the nanorod, the yield can decrease to about 100

Molecular dynamics simulations of cascades in strained carbide inclusions embedded in alpha-iron

The effect of strain on the amount of point defects created in Fe and Cr carbide inclusions embedded in ferrite has been investigated. The spherical carbide inclusions consisted of either Fe3C or Cr23C6. Recoil energies from 100 eV to 3 keV and strains from -0.15 (compressive) to 0.01 (tensile) were used. The overall tendency is that the number of point defects - such as antisites, vacancy and interstitials - inside the carbide is lowered when the strain grows more negative (compressive). Outside the carbides, the number of defects is markedly higher for strongly compressive strains than for e.g. zero strain, especially at high energies. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.Peer reviewe