Molecular dynamics simulations of irradiation of alpha-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 degrees 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. (C) 2014 Elsevier B. V. All rights reserved.

Analysis of nano-sized irradiation-induced defects in Fe-base materials by means of small angle neutron scattering and molecular dynamics simulations

Gang Yu

Thermonuclear fusion of light atoms is considered since decades as an unlimited, safe and reliable source of energy that could eventually replace classical sources based on fossile fuel or nuclear fuel. Fusion reactor technology and materials studies are important parts of the fusion energy development program. For the time being, the most promising materials for structural applications in the future fusion power reactors are the Reduced Activation Ferritic/Martensitic (RAFM) steels for which the greatest technology maturity has been achieved, i.e., qualified fabrication routes, welding technology and a general industrial experience are almost available. The most important issues concerning the future use of RAFM steels in fusion power reactors are derived from their irradiation by 14 MeV neutrons that are the product, together with 3.5 MeV helium ions, of the envisaged fusion reactions between deuterium and tritium nuclei. Indeed, exposure of metallic materials to intense fluxes of 14 MeV neutrons will result in the formation of severe displacement damage (about 20-30 dpa per year) and high amounts of helium, which are at the origin of significant changes in the physical and mechanical of materials, such as hardening and embrittlement effects, for instance. This PhD Thesis work was aimed at investigating how far the Small Angle Neutron Scattering (SANS) technique could be used for detecting and characterizing nano-sized irradiation-induced defects in RAFM steels. Indeed, the resolution limit of Transmission Electron Microscopy (TEM) is about 1 nm in weak beam TEM imaging, and it is usually thought that a large number of irradiation-induced effects have a size below 1 nm in RAFM steels and that these very small defects actually contribute to the irradiation-induced hardening and embrittlement of RAFM steels occurring at irradiation temperatures below about 400°C. The aim of this work was achieved by combing SANS experiments on unirradiated and irradiated specimens of RAFM steels with Molecular Dynamics (MD) simulations of main expected nano-sized defects in irradiated pure Fe and Fe-He alloys, as model materials for RAFM steels, and simulations of their corresponding TEM images and SANS signals. In particular, the SANS signal of various types of defects was simulated for the first time. The methodology used in this work was the following: SANS experiments were performed by applying a strong saturating magnetic field to unirradiated and irradiated specimens of three types of RAFM steels, namely the European EUROFER 97, the Japanese F82H and the Swiss OPTIMAX A steels. The available irradiated specimens included specimens which had been irradiated with 590 MeV protons in the Proton IRradiation EXperiment (PIREX) facility at the Paul Scherrer Institute (PSI) at temperatures in the range of 50-350°C to doses in the range of 0.3-2.0 dpa. SANS spectra as well as values of the so-called A ratio, which represents the ratio of the total scattered intensity to the nuclear scattered intensity, were obtained for the various irradiation doses and temperatures investigated. MD simulations of atomic displacement cascades in pure Fe and in Fe-He alloys were performed using Embedded Atom Method (EAM) many-body interatomic potentials. The main nano-sized defects that should be produced in RAFM steels under irradiation were created by means of MD in pure Fe. These included dislocation loops of various types, voids, helium bubbles with various He concentration and Cr precipitates. TEM images of cascade damage and all the defects created by MD were simulated in the dark field/weak beam imaging modes by using the Electron Microscopy Software (EMS) developed by P.A. Stadelmann (EPFL) and analyzed in terms of variations of contrast intensities versus depth inside the specimen. The SANS signal provided by cascade damage and all the defects created by MD was simulated by using a slightly modified version of EMS, accounting for neutrons instead of electrons. The SANS technique has been proven in this work to be a very powerful tool for detecting nano-sized irradiation-induced defects and a tool well complementary to TEM for characterizing such very small irradiation-induced defects. Indeed, TEM appears most adapted to investigate structural defects, such as dislocation loops and helium bubbles with high helium concentration, which yield significant lattice deformation of the surrounding matrix, while SANS is most adapted to investigate phase defects, such as voids, helium bubbles with low helium concentration and Cr precipitates. By combining the results of SANS experiments with those of MD simulations, TEM image simulations and SANS signal simulations, the nano-sized irradiation-induced defects were tentatively identified as small helium bubbles. While the radiation hardening measured for RAFM steels cannot be explained by accounting only for the defects observed in TEM, it could be successfully modeled by accounting also for a reasonable number density of the nano-sized defects evidenced using the SANS technique.

EPFL2008

Multiscale Modelling of Irradiation Induced Effects on the Plasticity of Fe and Fe-Cr

Seyed Masood Hafez Haghighat

Degradation of mechanical properties due to nanometric irradiation induced defects is one of the challenging issues in designing ferritic materials for future nuclear fusion reactors. Various types of defects, namely dislocation loops, voids, He bubbles and Cr precipitates may be produced in ferritic materials due to high doses of irradiation with the 14 MeV neutrons stemming from the deuterium-tritium fusion reaction. Multiscale modelling methods, namely molecular dynamics (MD) and dislocation dynamics (DD) methods, are used here to study the impact of radiation-induced defects on the mechanical properties of model ferritic material. MD simulation is used to study the state of a nanometric helium bubble in α-Fe as a function of temperature, 10 to 700 K, and He content, 1 to 5 He atoms per vacancy. It appears that up to moderate temperatures the Fe lattice can confine He to solid state, in good agreement with known solid-liquid transition diagram of pure He. However, while for a given temperature and He density range where an fcc structure is expected, He in the bubble forms an amorphous phase. He bubble forms a polyhedron whose morphology depends on either the surface energy or elastic-plastic properties of Fe at either low or high pressure, respectively. At high He contents the bubble surface breaks down at the mechanical stability limit of the Fe crystal, leading to a pressure decrease in the bubble. The basic mechanisms of the interaction in α-Fe between a moving dislocation and a nanometric defect, as a function of temperature, interatomic potentials, interaction geometry and size are investigated using MD simulation. The stress-strain responses are obtained under imposed strain rate and using different interatomic potentials for Fe-Fe and Fe-He interactions. It appears that voids and He bubbles are strong obstacles to dislocation, which induce hardening and loss of ductility. A nanometric void is a stronger obstacle than a He bubble at low He contents, whereas at high He contents, the He bubble becomes a stronger obstacle. It also appears that different potentials give different strengths and rates of decrease of obstacle strength with increasing temperature. Temperature eases the dislocation release, due to the increased mobility of the screw segments appearing on the dislocation line upon bowing from the void or He bubble. Concerning the obstacle size at low content He bubble, where it is penetrable defect, size increase from 1 to 5 nm make them harder in agreement with the elasticity of continuum. At high He contents a size dependent loop punching is observed, which at larger bubble sizes leads to a multistep dislocation-defect interaction. Atomistic simulations reveal that the He bubble induces an inhomogeneous stress field in its surroundings, which strongly influences the dislocation passage depending on the geometry of the interaction. Fe-Cr alloys are also studied using MD simulations as model alloys for the ferritic base steels. Studying the flow stress of a moving edge dislocation in Fe(Cr) shows that the flow stress is not sensitive to temperature and strain rate, while it increases with Cr concentration. The flow stress of a screw dislocation is temperature dependant and its value is much higher than that of edge dislocation. Interaction in a pure Fe matrix of an edge dislocation with a nanometric Cr precipitate having various Cr content inside (Cr/Fe ratio) indicates that with the Cr/Fe ratio the obstacle strength increases, with a strong sensitivity to the short-range order. Temperature induces a monotonous decrease of the strength for all studied Cr/Fe ratios. DD calculation coupled to finite element method (FEM) is used to simulate the interaction of an edge dislocation with a void. DD calculations present a good match to the MD simulation results for the impact of image forces on the dislocation due to the free internal surface of the void. Using DD simulation, hardening due to the presence of nanometric defects, as immobile dislocation segments and spherical defect, are considered. The results of MD simulations for the strength of different defects are introduced in DD simulation. It appears that the presence of both immobile dislocation segments and nanometric defects may lead to the hardening of α-Fe. However, the defect density has more significant importance on the DD simulation, obtained here, than the strength of a single defect. TEM in-situ straining is used to validate simulations, in particular on the dislocation interaction mechanisms. Experiments were performed in ultra high purity Fe specimens. It appears that the microstructure of the material consists mainly in elongated screw dislocations. The in-situ straining tests led to the observation of the formation of screw dislocation dipoles in consistency with the MD simulation results. The interaction between a dislocation and an obstacle, as nanometric defect or immobile dislocations, leading to their shear or to Orowan loop formation were successfully observed. This work shows that using of different simulation methods can assist in the understanding of nano- and meso-scale phenomena occurring due to the presence of irradiation induced defects upon deformation of α-Fe. The information obtained from various microstructure mechanisms at lower scale simulations can be passed to higher scale simulation methods in order to realize the simulated phenomena. Experimental observations may validate what is observed in simulations provided the simulation scale is reachable in experimental observations.

EPFL2010

Molecular Dynamics Simulations of Helium Atoms Clustering in bcc Iron

Ning Gao

Helium atoms are known to have a significant impact on materials used in fission and fusion reactors. In particular, the presence of helium atoms can change the mechanical properties and degrade the lifetime of reactors. In order to develop the helium-resistance materials, the underlying interactions between helium atoms and matrix atoms have to be understood. In the past, atomistic simulations have contributed a lot to the basic energy states of helium atoms in different defects of materials. Especially, insights into the formation energies of helium atoms at simple defects have been calculated by ab initio and molecular dynamics. The substitional helium has lower formation energy than the tetrahedral and octahedral interstitial helium atom. Additionally, the binding energies of helium with small HenVm cluster have also been found to be all positive. However, all these simulations have been performed without showing the kinetic formation process of such helium defects. The interactions of defect with larger number of helium atoms are also not explored in detail. In the first part of this thesis, a new relaxation-fitting method has been developed for cross potential of Fe-He system based on more energy data of helium-defect clusters from ab initio calculations and by including the migration energy of single free helium atom in the fitting process. Besides, the recently fitted magnetic potentials of Fe have been used to describe the Fe-Fe interactions for such cross potential. The new fitted Fe-He potentials have predicted well not only the properties of simple helium defect, but also the properties of helium clusters and diffusion process of helium atoms. Based on the new Fe-He potentials, the interactions between helium atoms and Fe with the effect of single vacancy have been simulated utilizing the classical molecular dynamics (MD) method in the single bcc Fe lattice at room temperatures by helium atoms up to 18. The binding energies of helium atom with HenV cluster are computed. By increasing number of helium atoms, the binding energy decreases first, then slowly increases up to n = 4. It is then almost constant between n = 5 to 15 but increases strongly at n = 16 to form a peak. This last increase is related to the athermal self-interstitial atom (SIA) emission in the form of dumbbell which relaxes the restrained system to He16V2 cluster. The second binding energy peak occurs at n = 18 by the formation of the second SIA to form He18V3 cluster. Close inspection of the local configuration shows the formed SIA combines with the helium-defect cluster. Calculation of the pressure of the helium-defect cluster shows local peak normal stress and shear stress values up to 9 GPa and 4 GPa, respectively. The local configurations of helium-defect cluster suggest that with increasing helium content, some symmetrical structures can be formed. By increasing the number of helium atoms inserted in single bcc Fe lattice, the clustering process of the formed SIAs with presence of helium cluster has been simulated with MD methods at room temperatures with the new fitted Fe-He potential. The first SIA is formed after 6 He atoms being inserted without vacancy. The simulations indicate that the SIA has mainly two configurations as either a dumbbell or a crowdion, which can be exchanged in the interface between helium bubble and matrix atoms. The SIA cluster can change direction and absorb more SIAs to grow. Small dislocation loops have been observed to form with Burgers vector b = 1/2 a0 on a {111} habit plane. The kinetic process of formation and growth of dislocation loop explored by MD simulations includes: (1) the formation of a self-interstitial atom by the high pressure of the helium cluster/bubble; (2) the diffusion of these self-interstitial atoms on the interface between helium bubble and matrix; (3) the clustering of the self-interstitial atoms in the form of a dislocation loop and its diffusion away from bubble when the number of SIAs in loop reaches a critical number. Moreover, the effect of grain boundary (GB) on the formation of helium clusters has been explored with MD simulations in symmetrical GBs and normal nano-grain boundary samples. Helium atoms can be trapped by GBs at free spaces. The local atomic excess free volume (LAEFV) is found to the one of the main parameters not only related with the energy state of GB but also with the nucleation and growth of He atoms in the GB.

EPFL2011