Surface damage in TEM thick α-Fe samples by implantation with 150 keV Fe ions

We have performed molecular dynamics simulations of implantation of 150 keV Fe ions in pure bcc Fe. The thickness of the simulation box is of the same order of those used in in situ TEM analysis of irradiated materials. We assess the effect of the implantation angle and the presence of front and back surfaces. The number and type of defects, ion range, cluster distribution and primary damage morphology are studied. Results indicate that, for the very thin samples used in in situ TEM irradiation experiments the presence of surfaces affect dramatically the damage produced. At this particular energy, the ion has sufficient energy to damage both the top and the back surfaces and still leave the sample through the bottom. This provides new insights on the study of radiation damage using TEM in situ.This work was supported by the European Fusion Development Agreement (EFDA), the VII EC framework through the GETMAT and MATISSE projects, and the Generalitat Valenciana PROMETEO2012/011

Molecular dynamics simulations of irradiation of α-Fe thin films with energetic Fe ions under channeling conditions

Using molecular dynamics simulations with recent interatomic potentials developed for Fe, we have studied the defects in thin films of pure bcc Fe induced by the displacement cascade produced by Fe atoms of 50, 100, and 150 keV impinging under a channeling incident angle of 6° to a [001] direction. The thin films have a thickness between 40 and 100 nm, to reproduce the thickness of the samples used in transmission electron microscope in-situ measurements during irradiation. In the simulations we focus mostly on the effect of channeling and free surfaces on damage production. The results are compared to bulk cascades. The comparison shows that the primary damage in thin films of pure Fe is quite different from that originated in the volume of the material. The presence of near surfaces can lead to a variety of events that do not occur in bulk collisional cascades, such as the production of craters and the glide of self-interstitial defects to the surface. Additionally, in the range of energies and the incident angle used, channeling is a predominant effect that significantly reduces damage compared to bulk cascades.This work was supported by the FPVII projects FEMaS, GETMAT and PERFECT and by the MAT-IREMEV program of EFDA. We acknowledge the support of the European Commission, the European Atomic Energy Community (Euratom), the European Fusion Development Agreement (EFDA) and the Forschungszentrum Jülich GmbH, jointly funding the Project HPC for Fusion (HPC-FF), Contract number FU07-CT-2007-00055

Insights from atomistic models on loop nucleation and growth in α-Fe thin films under Fe+ 100 keV irradiation

The question of how loops nucleate and grow in α-Fe under irradiation is addressed using object kinetic Monte Carlo with parameters from molecular dynamics and density functional theory calculations. Two models are considered for the formation of loops, both based on recent atomistic simulations. In one model loops are formed by the interaction between ½ loops. In a second model small interstitial clusters, nucleated in the collision cascade, can grow as or ½ loops. Comparing results from the calculations to experimental measurements of loop densities, ratios and sizes produced by Fe+ 100 keV irradiation of UHP Fe thin films at room temperature, the validity of the models is assessed. For these experimental conditions, the reaction model does not seem to be very efficient in the production of loops due to the fast recombination of ½ loops to surfaces. Therefore, in our thin film simulations (at very low carbon concentrations) most loops are a result of the nucleation model. In bulk simulations this effect could change since the probability of interactions between ½ loops would increase. Moreover, simulations show that total visible cluster concentration depends strongly on sample thickness and carbon content, while crystal orientation does not seem to have a significant role. Finally, the ratio of to ½ visible clusters changes with increased carbon concentration.This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The research leading to these results is partly funded by the European Atomic Energy Communitys (Euratom) Seventh Framework Programme FP7/2007e2013 under grant agreement No. 604862 (MatISSE project) and in the framework of the EERA (European Energy Research Alliance) Joint Programme on Nuclear Materials

Surface-induced vacancy loops and damage dispersion in irradiated Fe thin films

Transmission electron microscopy (TEM) in situ ion implantation is a convenient way to study radiation damage, but it is biased by the proximity of the free surfaces of the electron transparent thin sample. In this work this bias was investigated by performing irradiation of Fe in thin foil and bulk form with ions of energies between 50 keV and 100 keV using molecular dynamics simulations. The damage resulting from the subsequent displacement cascades differs significantly between the two sample geometries. The most remarkable difference is in the resulting 〈1 0 0〉 vacancy loops. Both their size and frequency are much greater in thin films, with loops reaching 4 nm in size. This is due to an imbalance between the number of vacancies and self-interstitials produced, since the faster self-interstitials can escape to the surfaces and remain there as ad-atoms. In addition, the self-interstitial clusters are smaller for thin foils and there is a larger dispersion of the induced damage in terms of defect number, defect clustering and defect morphology. The study discusses the impact of these results on the study of radiation effects during in situ experiments.MJA thanks the UA for support through an institutional fellowship. The research leading to these results is partly funded by the European Atomic Energy Community’s (Euratom) Seventh Framework Programme FP7/2007–2013 under Grant agreement No. 604862 (MatISSE project) and in the framework of the EERA (European Energy Research Alliance) Joint Programme on Nuclear Materials and the Generalitat Valenciana PROMETEO2012/011. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 under Grant agreement No. 633053