A.E. Sand

Direct observation of size scaling and elastic interaction between nano-scale defects in collision cascades

Using in-situ transmission electron microscopy, we have directly observed nano-scale defects formed in ultra-high purity tungsten by low-dose high energy self-ion irradiation at 30K. At cryogenic temperature lattice defects have reduced mobility, so these microscope observations offer a window on the initial, primary damage caused by individual collision cascade events. Electron microscope images provide direct evidence for a power-law size distribution of nano-scale defects formed in high-energy cascades, with an upper size limit independent of the incident ion energy, as predicted by Sand et al. [Eur. Phys. Lett., 103:46003, (2013)]. Furthermore, the analysis of pair distribution functions of defects observed in the micrographs shows significant intra-cascade spatial correlations consistent with strong elastic interaction between the defects.

Multiscale modelling of plasma-wall interactions in fusion reactor conditions

The interaction of fusion reactor plasma with the material of the first wall involves a complex multitude of interlinked physical and chemical effects. Hence, modern theoretical treatment of it relies to a large extent on multiscale modelling, i.e. using different kinds of simulation approaches suitable for different length and time scales in connection with each other. In this review article, we overview briefly the physics and chemistry of plasma–wall interactions in tokamak-like fusion reactors, and present some of the most commonly used material simulation approaches relevant for the topic. We also give summaries of recent multiscale modelling studies of the effects of fusion plasma on the modification of the materials of the first wall, especially on swift chemical sputtering, mixed material formation and hydrogen isotope retention in tungsten.

Surface effects and statistical laws of defects in primary radiation damage: Tungsten vs. iron

We have investigated the effect of surfaces on the statistics of primary radiation damage, comparing defect production in the bcc metals iron (Fe) and tungsten (W). Through molecular dynamics simulations of collision cascades we show that vacancy as well as interstitial cluster sizes follow scaling laws in both bulk and thin foils in these materials. The slope of the vacancy cluster size distribution in Fe is clearly affected by the surface in thin foil irradiation, while in W mainly the overall frequency is affected. Furthermore, the slopes of the power law distributions in bulk Fe are markedly different from those in W. The distinct behaviour of the statistical distributions uncovers different defect production mechanisms effective in the two materials, and provides insight into the underlying reasons for the differing behaviour observed in TEM experiments of low-dose ion irradiation in these metals.

Improving atomic displacement and replacement calculations with physically realistic damage models

Atomic collision processes are fundamental to numerous advanced materials technologies such as electron microscopy, semiconductor processing and nuclear power generation. Extensive experimental and computer simulation studies over the past several decades provide the physical basis for understanding the atomic-scale processes occurring during primary displacement events. The current international standard for quantifying this energetic particle damage, the Norgett−Robinson−Torrens displacements per atom (NRT-dpa) model, has nowadays several well-known limitations. In particular, the number of radiation defects produced in energetic cascades in metals is only ~1/3 the NRT-dpa prediction, while the number of atoms involved in atomic mixing is about a factor of 30 larger than the dpa value. Here we propose two new complementary displacement production estimators (athermal recombination corrected dpa, arc-dpa) and atomic mixing (replacements per atom, rpa) functions that extend the NRT-dpa by providing more physically realistic descriptions of primary defect creation in materials and may become additional standard measures for radiation damage quantification.The Norgett−Robinson−Torrens displacements per atom model is the benchmark to assess radiation damage in metals but has well-known limitations. Here, the authors use molecular dynamics to introduce material-specific modifications to describe radiation damage more realistically.

Cascade fragmentation: deviation from power law in primary radiation damage

ABSTRACT The sizes of defect clusters, produced in materials by energetic ion or neutron impacts, are critically important input for models describing microstructural evolution of irradiated materials. We propose a model for the distribution of sizes of vacancy and self-interstitial defect clusters formed by high-energy impacts in tungsten, and provide new data from in situ ion irradiation experiments to validate the model. The model predicts the statistics of sub-cascade splitting and the resulting distribution of primary defects extending over the entire range of cluster sizes, and is able to provide initial conditions for quantitative multi-scale simulations of microstructural evolution. GRAPHICAL ABSTRACT IMPACT STATEMENT We present a model, parameterized for tungsten, for the distribution of defect sizes in primary radiation damage, as an essential step in multi-scale modelling of microstructural evolution in irradiated materials.

Subcascade formation and defect cluster size scaling in high-energy collision events in metals

Please note that terms and conditions apply. You may also be interested in: High-energy collision cascades in tungsten: Dislocation loops structure and clustering scaling laws A. E. Sand, S. L. Dudarev and K. Nordlund Cascade morphology transition in bcc metals Wahyu Setyawan, Aaron P Selby, Niklas Juslin et al. Simulation of defects in fusion plasma first wall materials Troev T, Nankov N and Yoshiie T Primary damage in tungsten using the binary collision approximation, molecular dynamic simulations and the density functional theory A De Backer, A Sand, C J Ortiz et al. Electronic excitations in atomistic models of radiation damage C P Race, D R Mason, M W Finnis et al. Electronic effects in high-energy radiation damage in tungsten E Zarkadoula, D M Duffy, K Nordlund et al. Variability in atomic collision cascade distributions

Cascade debris overlap mechanism of 〈100〉 dislocation loop formation in Fe and FeCr

Two types of dislocation loops are observed in irradiated α-Fe, the 1/2〈111〉 loop and the 〈100〉 loop. Atomistic simulations consistently predict that only the energetically more favourable 1/2〈111〉 loops are formed directly in cascades, leaving the formation mechanism of 〈100〉 loops an unsolved question. We show how 〈100〉 loops can be formed when cascades overlap with random pre-existing primary radiation damage in Fe and FeCr. This indicates that there are no specific constraints involved in the formation of 〈100〉 loops, and can explain their common occurrence.

Experimental observation of the number of visible defects produced in individual primary damage cascades in irradiated tungsten

We present a new analysis of nanoscale lattice defects observed after low-dose in situ self-ion irradiation of tungsten foils at cryogenic temperature. For decades, defect counts and size-frequency histograms have been the standard form of presenting a quantitative analysis of the nanoscale “black-dot” damage typical of such irradiations. Here we demonstrate a new statistical technique for generating a probability distribution for the number of defects produced in a single cascade. We show that while an average of fewer than one defect is observed per incident ion, the number of cascades with two or more visible defects produced is significant. Introduction. – In situ irradiation of TEMtransparent foils offers great control over damage production and thermal history, and becomes a very valuable tool for nuclear materials research when we can extract data comparable to our simulations of physical radiation damage processes. For decades, quantitative analyses of irradiation damage in the form of nanoscale defects as been presented as defect counts and size-frequency histograms [1–3], but the position correlation of the defects has been under-utilised, due to the time-consuming nature of identifying a sufficient number of spots by eye to make such an analysis practical. The importance of this data, present in micrographs but unexploited, has recently become clearer. Primary damage cascades are inherently random processes, and their subsequent microstructural evolution may well be dominated by rare events [4–7]: the evolution of point defects and small clusters will be strongly skewed if a large dislocation loop is also generated during the heat spike phase [8], and if multiple large loops are in close proximity then their elastic interaction can lead to self-trapping [9]. Recently we have developed techniques [10, 11] for automating the analysis of black-dot damage, which produces a reproducible list of spot positions and sizes in a few minutes. We have been able to verify that primary damage cascade events in ultra-high purity tungsten foil produce a power-law size-frequency distribution of defects [10], and have accounted for observed deviations from power-law behaviour due to subcascade branching [12]. We have also been able to show, from analysis of the pairwise radial distribution function, that the characteristic size of individual cascades is of order one nanometer [11]. In this letter we perform a new type of analysis, to find the number of visible defects produced in a single cascade. In situ TEM experiments. – Experimental data for the count of visible defects per cascade is generated from in situ self-ion irradiations of high purity tungsten foils, performed at cryogenic temperature at the IVEMTandem Facility at Argonne National Laboratory. It is known that the collapse of displacement cascades in selfion irradiated tungsten produces large, nanometre-scaled ( and therefore TEM visible ) dislocation loops [9, 10]. We perform experiments at cryogenic temperature (30K), where the mobility of irradiation-induced defects is reduced, though we acknowledge that Brownian motion of defects due to quantum fluctuations of atomic positions associated with their zero point motion will still be present [13] and so loop loss to the surface can still occur. 3mm discs were cut from ultra-high purity tungsten