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

Recoil energy dependence of primary radiation damage in tungsten from cascade overlap with voids

Models of radiation damage accumulation often assume a constant rate of additional primary damage formation during prolonged irradiation. However, molecular dynamics simulations have shown that the presence of pre-existing radiation-induced defects modifies the numbers of additional defects formed from individual cascades. In this work, we study the formation of defects in tungsten for a range of pri-mary recoil energies, for cascades that fully overlap with pre-existing voids of different sizes. We extend a recent model describing defect production in the presence of pre-existing damage to also account for the recoil energy dependence, and parametrize the extension based on our simulation data. We also an-alyze the morphology of the primary damage from cascades overlapping with voids, and show that the in-cascade formation of ( 1 0 0 ) dislocation loops in such events is more dependent on the size of the pre-existing void, than on the energy of the primary recoil. (c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )Peer reviewe

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

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Heavy ion ranges from first-principles electron dynamics

The effects of incident energetic particles, and the modification of materials under irradiation, are governed by the mechanisms of energy losses of ions in matter. The complex processes affecting projectiles spanning many orders of magnitude in energy depend on both ion and electron interactions. Developing multi-scale modeling methods that correctly capture the relevant processes is crucial for predicting radiation effects in diverse conditions. In this work, we obtain channeling ion ranges for tungsten, a prototypical heavy ion, by explicitly simulating ion trajectories with a method that takes into account both the nuclear and the electronic stopping power. The electronic stopping power of self-ion irradiated tungsten is obtained from first-principles time-dependent density functional theory (TDDFT). Although the TDDFT calculations predict a lower stopping power than SRIM by a factor of three, our result shows very good agreement in a direct comparison with ion range experiments. These results demonstrate the validity of the TDDFT method for determining electronic energy losses of heavy projectiles, and in turn its viability for the study of radiation damage.Peer reviewe

CASCADE DEBRIS OVERLAP MECHANISM OF <100> DISLOCATION LOOP FORMATION IN Fe AND FeCr

Two types of dislocation loops are observed in irradiated alpha-Fe, the 1/2 loop and the loop. Atomistic simulations consistently predict that only the energetically more favourable 1/2 loops are formed directly in cascades, leaving the formation mechanism of loops an unsolved question. We show how 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 loops, and can explain their common occurrence. Copyright (C) EPLA, 2017Peer reviewe

Effects of cascade-induced dislocation structures on the long-term microstructural evolution in tungsten

In recent years, a number of systematic investigations of high-energy collision cascades in tungsten employing advanced defect analysis tools have shown that interstitial clusters can form complex non-planar dislocation structures. These structures are sessile in nature and may potentially have a strong impact on the long-term evolution of the radiation microstructure. To clarify this aspect, we selected several representative primary damage states of cascades debris and performed annealing simulations using molecular dynamics (MD). We found that immobile complexes of non-planar dislocation structures (CDS) evolve into glissile and planar shaped 1/2 loops with an activation energy of similar to 1.5 eV. The CDS objects were implemented in an object kinetic Monte Carlo (OKMC) model accounting for the event of transformation into 1-D migrating loops, following the MD data. OKMC was then used to investigate the impact of the transformation event (and the associated activation energy) on the evolution of the microstructure.Peer reviewe