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Neutrons and high energy ions create displacement cascades in materials, which have been simulated using Molecular Dynamics, for many decades now. The breakthrough of this work is to explore in a large statistics of more than 7000 cascades the relation between early cascade morphology and the final primary damage using a multivariate multiple regression analysis. For two energies in Fe and W, the total number of defects, the number of SIA and vacancy clusters and their full size distributions have been characterized using a multivariate multiple linear regression analysis based on 7 descriptors of the primary damage and 3 morphology descriptors. We find that the combination of the volume and the sphericity is significant. This analysis highlights several cascade properties, among them, that the large and spherical cascades create less defects and in particular, less mono defects than small and fragmented ones. 13 interatomic potentials differing either by their equilibrium part or the way they were hardened have been included in this study and the multivariate analysis shows that the choice of potential has a limited influence on the total number of defects but a large one on the number of mono vacancies. On average, soft potentials create cascades of larger volume, smaller sphericity and producing more defects than hard potentials. Finally, the formation of vacancy clusters is different in Fe than in W. In Fe, the fraction of vacancies in clusters is larger than that of SIAs and larger vacancy clusters are created than SIA clusters. In W, it is the opposite. The reasons are the differences of stopping power and threshold displacement energies, which result in different spatial distributions of open volumes that form during the expansion stage of the cascade., QC 20211215

Radiation-induced segregation (RIS) of solutes in materials exposed to irradiation is a well-known problem. It affects the lifetime of nuclear reactor core components by favoring radiation-induced degradation phenomena such as hardening and embrittlement. In this work, RIS tendencies in face centered cubic (fcc) Ni-X (X = Cr, Fe, Ti, Mn, Si, P) dilute binary alloys are examined. The goal is to investigate the driving forces and kinetic mechanisms behind the experimentally observed segregation. By means of ab initio calculations, point-defect stabilities and interactions with solutes are determined, together with migration energies and attempt frequencies. Transport and diffusion coefficients are then calculated in a mean-field framework, to get a full picture of solute-defect kinetic coupling in the alloys. Results show that all solutes considered, with the exception of Cr, prefer vacancy-mediated over interstitial-mediated diffusion during both thermal and radiation-induced migration. Cr, on the other hand, preferentially migrates in a mixed-dumbbell configuration. P and Si are here shown to be enriched, and Fe and Mn to be depleted at sinks during irradiation of the material. Ti and Cr, on the other hand, display a crossover between enrichment at lower temperatures, and depletion in the higher temperature range. Results in this work are compared with previous studies in body centered cubic (bcc) Fe, and discussed in the context of RIS in austenitic alloys., QC 20210215

The energetics, defect volume and magnetic properties of single SIAs and small SIA clusters up to size 6 have been calculated by DFT for different configurations like the parallel 〈110〉 dumbbell, the non parallel 〈110〉 dumbbell and the C15 structure. The most stable configurations of each type have been further analyzed to determine the influence on their stability of various solute atoms (Ti, V, Cr, Mn, Co, Ni, Cu, Mo, W, Pd, Al, Si, P), relevant for steels used under irradiation. The results show that the presence of solute atoms does not change the relative stability order among SIA clusters. The small SIA clusters investigated can bind to both undersized and oversized solutes. Several descriptors have been considered to derive interesting trends from results. It appears that the local atomic volume available for the solute is the main physical quantity governing the binding energy evolution, whatever the solute type (undersized or oversized) and the cluster configuration (size and type)., Export Date: 13 February 2018; Article; CODEN: JNUMA; Correspondence Address: Becquart, C.S.; Univ.Lille, CNRS, INRA, ENSCL, UMR 8207, UMET, Unité Matériaux et TransformationsFrance; email: charlotte.becquart@univ-lille1.fr; Funding details: 2016153636, PRACE, Partnership for Advanced Computing in Europe AISBL. QC 20180327

This work investigates the nanostructural evolution under ion irradiation in both a binary Fe-C alloy and an Fe-9%Cr-C alloy at 300°C, using an object kinetic Monte Carlo model originally developed to simulate neutron irradiation in Fe-(Cr)-C alloys. We studied the effect of specificities of ion irradiation that are expected to have an impact on the evolution of radiation-induced defect clusters as compared to neutron irradiation. The most relevant impact is due to the difference in dose-rate between neutron and ion irradiation. Moreover, the effect of two different ion irradiation regimes (continuous versus pulsed) on the nanostructure evolution material was also investigated: the presence of Cr was seen to lead to a different response over accumulated dose, the reason having been identified in the defect cluster recombination mechanism.

The damage produced by primary knock-on atoms (PKA) in W has been investigated from the threshold displacement energy (TDE) where it produces one self interstitial atom-vacancy pair to larger energies, up to 100 keV, where a large molten volume is formed. The TDE has been determined in different crystal directions using the Born-Oppenheimer density functional molecular dynamics (DFT-MD). A significant difference has been observed without and with the semi-core electrons. Classical MD has been used with two different empirical potentials characterized as 'soft' and 'hard' to obtain statistics on TDEs. Cascades of larger energy have been calculated, with these potentials, using a model that accounts for electronic losses (Sand et al 2013 Europhys. Lett. 103 46003). Two other sets of cascades have been produced using the binary collision approximation (BCA): a Monte Carlo BCA using SDTrimSP (Eckstein et al 2011 SDTrimSP: Version 5.00. Report IPP 12/8) (similar to SRIM www.srim.org) and MARLOWE (RSICC Home Page. (https://rsicc.ornl.gov/codes/psr/psr1/psr-137.html) (accessed May, 2014)). The comparison of these sets of cascades gave a recombination distance equal to 12 angstrom which is significantly larger from the one we reported in Hou et al (2010 J. Nucl. Mater. 403 89) because, here, we used bulk cascades rather than surface cascades which produce more defects (Stoller 2002 J. Nucl. Mater. 307 935, Nordlund et al 1999 Nature 398 49). Investigations on the defect clustering aspect showed that the difference between BCA and MD cascades is considerably reduced after the annealing of the cascade debris at 473 K using our Object Kinetic Monte Carlo model, LAKIMOCA., QC 20161017

The threshold displacement energy in iron is determined using ab initio molecular dynamics. This is the most fundamental input parameter for radiation damage assessments. The predictions agree well with the available experiments and provide a significantly lower average value for iron than the standard one. This result impacts radiation damage assessments in iron alloys and steels and especially so for dose estimations and conditions close to the threshold. The importance of using an appropriate description of the core and valence electrons is highlighted. Energy loss simulations provide important fitting parameters for improved interatomic potentials. IMPACT STATEMENT Ground-breaking ab initio calculations of the threshold displacement energies in iron show significant differences in the angular anisotropy and predicted average value with respect to previous literature., QC 20161110

The first step towards the understanding and the modelling of the Fe-Cr alloy kinetic properties consists in estimating the migration energies related to the processes that drive the microstructure evolution. The vacancy's migration barrier is expected to depend on the vacancy-migrating atom pair atomic environment as pointed out by Nguyen-Manh et al. or Bonny et al. In this paper, we address the issue of the dependence on the vacancy's local atomic environment of both the vacancy migration energy and the configurational energy change Delta E that occurs when the vacancy jumps towards one of its nearest neighbour sites. A DFT approach is used to determine the ground state energy associated to a given configuration of the system. The results are interpreted in the light of the chromium-chromium and chromium-vacancy binding energies as well as the substitutional chromium atoms magnetic properties., QC 20140822

The influence of the local environment on vacancy and self-interstitial formation energies has been investigated in a face-centered-cubic (fcc) Fe-10Ni-20Cr model alloy by analyzing an extensive set of first-principle calculations based on density functional theory. Chemical disorder has been considered by designing special quasirandom structures and four different collinear magnetic structures have been investigated in order to determine a relevant reference state to perform point defect calculations at 0 K. Two different convergence methods have also been used to characterize the importance of the method on the results. Although our fcc Fe-10Ni-20Cr would be better represented in terms of applications by the paramagnetic state, we found that the antiferromagnetic single-layer magnetic structure was the most stable at 0 K and we chose it as a reference state to determine the point defect properties. Point defects have been introduced in this reference state, i.e., vacancies and Fe-Fe, Fe-Ni, Fe-Cr, Cr-Cr, Ni-Ni, and Ni-Cr dumbbell interstitials oriented either parallel or perpendicular to the single layer antiferromagnetic planes. Each point defect studied was introduced at different lattice sites to consider a sufficient variety of local environments and analyze its influence on the formation energy values. We have estimated the point defect formation energies with linear regressions using variables which describe the local environment surrounding the point defects. The number and the position of Ni and Cr first nearest neighbors to the point defects were found to drive the evolution of the formation energies. In particular, Ni is found to decrease and Cr to increase the vacancy formation energy of the model alloy, while the opposite trends are found for the dumbbell interstitials. This study suggested that, to a first approximation, the first nearest atoms to point defects can provide reliable estimates of point defect formation energies., QC 20140317

The long-term objective of the European Fusion Development Agreement (EFDA) fusion materials programme is to develop structural and armor materials in combination with the necessary production and fabrication technologies for reactor concepts beyond the International Thermonuclear Experimental Reactor. The programmatic roadmap is structured into four engineering research lines which comprise fabrication process development, structural material development, armor material optimization, and irradiation performance testing, which are complemented by a fundamental research programme on "Materials Science and Modeling." This paper presents the current research status of the EFDA experimental and testing investigations, and gives a detailed overview of the latest results on materials research, fabrication, joining, high heat flux testing, plasticity studies, modeling, and validation experiments., QC 20131212

The current magnetic confinement nuclear fusion power reactor concepts going beyond ITER are based on assumptions about the availability of materials with extreme mechanical, heat, and neutron load capacity. In Europe, the development of such structural and armour materials together with the necessary production, machining, and fabrication technologies is pursued within the EFDA long-term fusion materials programme. This paper reviews the progress of work within the programme in the area of tungsten and tungsten alloys. Results, conclusions, and future projections are summarized for each of the programme's main subtopics, which are: (1) fabrication, (2) structural W materials, (3) W armour materials, and (4) materials science and modelling. It gives a detailed overview of the latest results on materials research, fabrication processes, joining options, high heat flux testing, plasticity studies, modelling, and validation experiments., QC 20130205

All the recent DEMO design studies for helium cooled divertors utilize tungsten materials and alloys, mainly due to their high temperature strength, good thermal conductivity, low erosion, and comparably low activation under neutron irradiation. The long-term objective of the EFDA fusion materials programme is to develop structural as well as armor materials in combination with the necessary production and fabrication technologies for future divertor concepts. The programmatic roadmap is structured into four engineering research lines which comprise fabrication process development, structural material development, armor material optimization, and irradiation performance testing, which are complemented by a fundamental research programme on "Materials Science and Modeling". This paper presents the current research status of the EFDA experimental and testing investigations, and gives a detailed overview of the latest results on fabrication, joining, high heat flux testing, plasticity, modeling, and validation experiments., QC 20120109

The objective of the FP6 Perfect Project was to develop a first example of integrated multiscale computational models, capable of describing the effects of irradiation in nuclear reactor components, namely vessel and internals. The use of ab initio techniques to study, in the most reliable way currently possible, atomic-level interactions between species and defects, and the transfer of this knowledge to interatomic potentials, of use for large scale dynamic simulations, lie at the core of this effort. The target materials of the Project were bainitic steels (vessel) and austenitic steels (internals), i.e. iron alloys. In this article, the advances made within the Project in the understanding of defect properties in Fe alloys, by means of ab initio calculations, and in the development of interatomic potentials for Fe and Fe alloys are overviewed, thereby providing a reference basis for further progress in the field. Emphasis is put in showing how the produced data have enhanced our level of understanding of microstructural processes occurring under irradiation in model alloys and steels used in existing nuclear power plants., QC 20120124

The performance of four recent semi-empirical interatomic potentials for iron, developed or used within the FP6 Perfect Project, is evaluated by comparing them between themselves and with available experimental or, more often, density functional theory data. The quantities chosen for the comparison are of specific interest for radiation damage studies, i.e. they concern mainly properties of point-defects and their clusters, as well as dislocations. For completeness, an earlier, widely used (also within the Project) iron potential is included in the comparison exercise as well. This exercise allows conclusions to be drawn about the reliability of the available potentials, while providing a snapshot of the state-of-the-art concerning fundamental properties of iron, thereby being also useful as a kind of handbook and as a framework for the validation of future semi-empirical interatomic potentials for iron. It is found that Mendelev-type potentials are currently the best choice in order to "extend density functional theory" to larger scales and this justifies their widespread use, also for the development of iron alloy potentials. However, a fully reliable description of self-interstitial atom clusters and dislocations with interatomic potentials remains largely an elusive objective, that calls for further effort within the concerned scientific community., QC 20120124

The evolution of the microstructure of dilute Fe alloys under irradiation has been modelled using a multiscale approach based on ab initio and atomistic kinetic Monte Carlo simulations In these simulations both self interstitials and vacancies isolated or in clusters are considered Isochronal annealing after electron irradiation experiments have been simulated in pure Fe Fe-Cu and Fe-Mn dilute alloys focusing on recovery stages I and II The parameters regarding the self interstitial - solute atom interactions are based on ab initio predictions and some of these interactions have been slightly adjusted without modifying the interaction character on isochronal annealing experimental data The different recovery peaks are globally well reproduced These simulations allow interpreting the different recovery peaks as well as the effect of varying solute concentration For some peaks these simulations have allowed to revisit and re-interpret the experimental data In Fe-Cu the trapping of self interstitials by Cu atoms allows experimental results to be reproduced although no mixed dumbbells are formed contrary to the former inter pretations Whereas in Fe-Mn the favorable formation of mixed dumbbell plays an important role in the Mn effect., QC 20120217

The primary state of damage obtained in molecular dynamics (MD) simulations of displacement cascades in alpha-Fe, particularly the fraction of point-defects in clusters, depends on the interatomic potential used to describe the atomic interactions. The differences may influence the microstructural evolution predicted in damage accumulation models which use results from MD cascade simulations as input. In this work, a number of displacement cascades of energy ranging from 5 to 40 keV have been simulated using the same procedure with four different interatomic potentials for alpha-Fe, each of them providing, among other things, varying descriptions of self-interstitial atoms (SIA) in this metal. The behaviour of the cascades at their different phases and the final surviving defect population have been studied and compared applying the same cascade analysis criteria for all potentials. The outcome is discussed trying to identify the characteristics of the potential that have the largest influence on the predicted primary state of damage., QC 20100525 QC 20111004.