Dynamics Simulations Of Displacement Cascades Research Articles

Molecular Dynamics Simulations of Displacement Cascades in BCC-Fe: Effects of Dislocation, Dislocation Loop and Grain Boundary

The interactions between displacement cascades and three types of structures, dislocations, dislocation loops and grain boundaries, in BCC-Fe are investigated through molecular dynamics simulations. Wigner–Seitz analysis is used to calculate the number of point defects induced in order to illustrate the effects of three special structures on the displacement cascade. The displacement cascades in systems interacting with all three types of structure tend to generate more total defects compared to bulk Fe. The surviving number of point defects in the grain boundary case is the largest of the three types of structures. The changes in the atomic structures of dislocations, dislocation loops and grain boundaries after displacement cascades are analyzed to understand how irradiation damage affects them. These results could reveal irradiation damage at the microscale. Varied defect production numbers and efficiencies are investigated, which could be used as the input parameters for higher scale simulation.

Kinetics of radiation defect formation in iron under high displacement rate irradiation

In this study, the kinetics of vacancy and interstitial formation has been simulated in iron at a temperature of 600 K under irradiation at high radiation defect production rate 6.5·10-3,3.25·10-3,1.63·10-3dpa/s and displacement energies of 10 keV, 15 keV, 20 keV. The reaction-diffusion equations and molecular dynamics simulation of displacement cascades have been combined to calculate the kinetics of radiation defect formation. We have introduced the criterion to estimate homogeneity of distribution of radiation defects in the system. The kinetics of radiation defect formation has been calculated up to doses of ~5·10−5 dpa, whereas stationary values of their concentration are achieved at ~10−5 dpa. It has been shown, that high defect production rate of irradiation causes inhomogeneous distribution of the radiation defects with a finite life-time. Also, we consider the effect of high defect production rate and displacement energy on kinetics of radiation defect formation and the self-diffusion mechanism in iron at the considered temperature.

Atomistic simulations for the effects of stacking fault energy on defect formations by displacement cascades in FCC metals under Poisson’s deformation

We performed molecular dynamics simulations of displacement cascades in FCC metals under Poisson’s deformation using interatomic potentials differing in stacking fault energy (SFE), in order to investigate the effect of tensile strain on the SFE dependence of defect formation processes. There was no clear SFE dependence of the number of residual defects and the size distribution of defect clusters under both no strain and the applied strain, while the strain enhanced the defect formation to a certain extent. We also observed that the strain affected the formations of self-interstitial atom (SIA) clusters depending on their size and the Burgers vector. These results were consistent with the analysis based on the defect formation energies. Meanwhile, the number of SIA perfect loops was higher at lower SFE under both no strain and the applied strain, leading to an increase in the ratio of glissile SIA clusters with a decrease in SFE. Further, the absolute number of SIA perfect loops was increased by the applied strain, while the SFE dependence of the number of SIA perfect loops was not affected. These findings were associated with the difference in formation energy between an SIA perfect loop and an SIA Frank loop. The insights extracted from this study significantly contribute to the modeling of microstructural evolution in nuclear materials under irradiation, especially for low SFE metals such as austenitic stainless steels.

Role of pre-existing point defects on primary damage production and amorphization in silicon carbide (β-SiC)

Abstract Molecular dynamics simulations of displacement cascades were conducted to study the effect of point defects on the primary damage production in β-SiC. Although all types of point defects and Frenkel pairs were considered, Si interstitials and Si Frenkel pairs were unstable and hence excluded from the cascade studies. Si (C) vacancies had the maximum influence, enhancing C (Si) antisites and suppressing C interstitial production, when compared to the sample without any defects. The intracascade recombination mechanisms, in the presence of pre-existing defects, is explored by examining the evolution of point defects during the cascade. To ascertain the role of the unstable Si defects on amorphization, simulations involving explicit displacements of Si atoms were conducted. The dose to amorphization with only Si displacements was much lower than what was observed with only C displacements. The release of elastic energy accumulated due to Si defects, is found to be the amorphizing mechanism.

Extreme ion irradiation of oxide nanoceramics: Influence of the irradiation spectrum

Oxide nanoceramics combine the enhanced radiation tolerance of nanocrystalline materials with the chemical inertness of oxides, and are promising materials for highly corrosive and intense radiation environments. In this work, nanocrystalline Al2O3 thin films are irradiated at 600 °C with either 12 MeV Au5++18 MeV W8+ or 4 MeV Ni2+ ions. The radiation damage exposure exceeds 450 displacements per atom. A comprehensive analysis of the irradiated samples is accomplished by X-Ray Diffractometry (XRD), Transmission Electron Microscopy (TEM) and Scanning-TEM (STEM). Results are compared in an effort to establish correlations between the irradiation spectrum and the response of this class of materials to radiation environments. The results show that grain growth is the main microstructural change induced by ion irradiation in the material, regardless of the ion utilized in this work. The phase evolution may be depth-dependent, and depends strongly on the ion utilized and on the irradiation spectrum. 12 MeV Au5++18 MeV W8+ irradiations favor the formation of γ-Al2O3 and α-Al2O3, while 4 MeV Ni2+ irradiations yield mainly δ-Al2O3, accompanied by small α-Al2O3 centers. Molecular dynamics simulations of displacement cascades are used to support discussions on the mass effect brought about by the different ions.

Establishing the isotropy of displacement cascades in UO 2 through molecular dynamics simulation

Simulation of displacement cascades is a valuable approach in furthering our understanding of how the physical properties of nuclear fuel evolve. Molecular dynamics simulations of displacement cascades in uranium dioxide have been performed at three different primary knock-on atom energies. Various properties of the cascade (such as the spatial extent and total number of defects) are monitored as the cascade progresses. Both the statistical variation of these properties and the dependence on the crystallographic direction of the primary knock-on atom are investigated in order to determine the isotropy of these events.

Identification and characterization of defects produced in irradiated fused silica through molecular dynamics

We present molecular dynamics simulations of displacement cascades due to energetic recoils in amorphous silica, a candidate material for fusion applications. We have performed a statistical study of the different kinds of defects produced as a function of primary knock-on atom (PKA) energy. The range of energies studied is from 0.4 to 3.5 keV. We measure how the concentration of different kinds of defects vary with recoil energy and we catalogue these defect according to their potential energy, morphology and coordination. Our calculations show mainly four types of defects, Si 3, Si 5, O 1 and O 3 where the numbers denote their coordination. The production of these defects increases with PKA energy except for the case of Si 5. A faster increase in the production rate with energy is observed for O 1 and O 3 types of defects with respect to Si 3. Results are correlated to known experimentally observed defects.

Simulations of cascades in pure and alumina doped magnesia

Atomic scale molecular dynamics simulations of displacement cascades are carried out in an initially defect-free MgO lattice and in a lattice where 0.2% of the Mg2+ ions have been replaced by Al3+ ions, charge compensated by magnesium vacancies. Results show that the properties of the cascade in the perfect lattice exhibit considerable anisotropy with respect to the direction of the incident primary knock-on atom (PKA). We find that a different anisotropy of equivalent magnitude occurs as a consequence of the (random) distribution of defects in the vicinity of the cascade. This suggests that the details of damage production during a cascade event can depend sensitively on the distribution of dopants and impurity atoms around the PKA.

Molecular dynamics simulation of primary irradiation defect formation in Fe–10%Cr alloy

Molecular dynamics simulations of displacement cascades in Fe and Fe–10%Cr have been performed for primary knock-on energies from 1 to 20 keV using two different Finnis–Sinclair style interatomic potentials. The different potentials were fit to describe the extremes of positive (attractive) and negative (repulsive) binding between substitutional Cr atoms and Fe self-interstitial atoms. As expected, the effect of Cr on the collisional stage of cascade evolution and on the number of point defects and point defect clusters produced is quite minimal. However, the quantity of mixed Fe–Cr dumbbells produced is sensitive to the choice of potential.

Overview of the U.S. Fusion Materials Sciences Program

Highlights of recent U.S. fusion materials research activities are summarized, including multiscale materials modeling and experimental results. Recent first principles atomistic calculations on vanadium and iron-helium have found that previous interatomic potentials incorrectly predict several important point defect properties. Molecular dynamics simulations of displacement cascades are now approaching energies equivalent to 14 MeV fusion neutrons. Considerable effort is being devoted to understanding the fundamental mechanisms of low temperature radiation hardening and embrittlement. Work is also in progress to determine the allowable temperature and dose operating regimes for candidate reduced activation structural materials (including transmutant helium effects). New compositions of reduced activation steels and vanadium alloys with potential for significantly improved properties are being investigated. Due to recent improvements in SiC/SiC ceramic composites, engineering-relevant mechanical property tests are being introduced to replace historical qualitative screening tests. Materials research in support of the ITER burning plasma physics machine is briefly described.

Molecular dynamics simulations of phase formation and stability in the Al(Ni) system under irradiation

Molecular dynamics simulations of displacement cascades in aluminium, aluminium–nickel solid solutions and inhomogeneous samples containing amorphous or crystalline Al3Ni precipitates are performed. It is shown that no segregation occurs inside the displacement cascade and that the nickel atoms are in an interstitial position after the thermal spike. The number of defects created by the cascade increases with the nickel initial composition until this concentration reaches 20 at% Ni, at which point a complete amorphization is observed. Furthermore, it is observed that precipitates are dissolved or remain stable under cascade conditions depending on their size.

In-cascade interstitial cluster formation in concentrated ferritic alloys with strong solute–interstitial interaction: a molecular dynamics study

Molecular dynamics simulations of displacement cascades up to 40keV have been performed using an EAM potential that mimics Fe–Cr alloys, by favouring interstitial configurations containing solute atoms. Both pure α-Fe and Fe–10at.%Cr alloys were studied. The interstitial population at the end of the cascade was analysed from the standpoint of clustered fraction and chemical nature of the interstitial atoms. For pure α-Fe the results obtained are comparable with previous work. Two are the main effects of the presence of a high concentration of solute atoms strongly interacting with interstitials. First, a large number of solute atoms in interstitial position, mostly isolated, are found, in a proportion far above the average solute concentration in the alloy. Secondly, interstitial clusters containing a fraction of solute interstitial atoms always larger than the average solute concentration in the alloy are produced and this solute aggregation inside clusters may be a thermal spike effect. However, the total number of interstitial atoms produced in the cascade and even the in-cascade interstitial clustered fraction do not appear to be significantly influenced by the presence of solute atoms in high concentration, possibly because the masses of Fe and Cr are very similar.

Molecular dynamics simulation of displacement cascades in Fe–Cr alloys

An embedded atom method (EAM) empirical potential recently fitted and validated for Fe–Cr systems is used to simulate displacement cascades up to 15 keV in Fe and Fe–10%Cr. The evolution of these cascades up to thermalisation of the primary damage state is followed and quantitatively analysed. Particular attention is devoted to assessing the effect of Cr atoms on the defect distribution versus pure Fe. Using the Wigner–Seitz cell criterion to identify point defects, first results show that the main effect of the presence of Cr in the system is the preferential formation of mixed Fe–Cr dumbbells and mixed interstitial clusters, with expected lower mobility than in pure Fe.

Kinetic Monte Carlo simulations of void lattice formation during irradiation

Over the last decade, molecular dynamics simulations of displacement cascades have revealed that glissile clusters of self-interstitial crowdions are formed directly in cascades and that they migrate one-dimensionally along close-packed directions with extremely low activation energies. Occasionally, under various conditions, a crowdion cluster can change its Burgers vector and glide along a different close-packed direction. The recently developed production bias model (PBM) of microstructure evolution under irradiation has been structured specifically to take into account the unique properties of the vacancy and interstitial clusters produced in the cascades. Atomic-scale kinetic Monte Carlo (KMC) simulations have played a useful role in understanding the defect reaction kinetics of one-dimensionally migrating crowdion clusters as a function of the frequency of direction changes. This has made it possible to incorporate the migration properties of crowdion clusters and changes in reaction kinetics into the PBM. In the present paper we utilize similar KMC simulations to investigate the significant role that crowdion clusters can play in the formation and stability of void lattices. The creation of stable void lattices, starting from a random distribution of voids, is simulated by a KMC model in which vacancies migrate three-dimensionally and self-interstitial atom (SIA) clusters migrate one-dimensionally, interrupted by directional changes. The necessity of both one-dimensional migration and Burgers vectors changes of SIA clusters for the production of stable void lattices is demonstrated, and the effects of the frequency of Burgers vector changes are described.

The role of Cu in displacement cascades examined by molecular dynamics

Molecular dynamics simulations of displacement cascades in pure Fe, Fe–0.2 at.% Cu and Fe–2 at.% Cu have been done with different primary knocked-on atom (PKA) energies. The typical defects (vacancies and interstitials) appear during the cascade process. Most of the defects recombine in the first few picoseconds. The presence of Cu atoms does not seem to influence the primary damage in the time span covered by MD. Neither Cu precipitates, nor dilute atmospheres have been observed to form in the course of the MD simulations. However, a tendency to form mixed objects (vacancies or interstitials gathered with Cu atoms) is noticed in the Fe–2 at.% Cu. Our feeling is that the numerous vacancies left at the end of the cascade recombination phase will interact with the Cu atoms and will act as attractive centers. The role of the interstitials is less clear for the moment.

Molecular Dynamics simulations of displacement cascades: role of the interatomic potentials and of the potential hardening

ABSTRACTThe role of the interatomic potentials on the primary damage has been investigated by Molecular Dynamics (MD) simulations of displacement cascades with three different interatomic potentials dedicated to α-Fe. The primary damage, caused by the neutron interaction with the matter, has been found to be potential sensitive. We have investigated the equilibrium parts of the potential as well as the “short distance interactions” which appear to have a strong influence on the cascade morphology and defects distribution at the end of the cascade. The static properties as well as dynamical (thermal) characteristics of the potentials have been considered; the kinetic and potential energy transfers during the collisions have also been studied.

Molecular dynamics simulation of displacement cascades in matrix of french light water reactor waste containment glass

A simplified nuclear glass containing the major elements of the French light water reactor containment glass was prepared from a molecular dynamics standpoint using Born-Mayer-Hugins potentials. Accelerating an atom in the simulation cell with an energy below 1.5 keV was the first step in addressing the problem of ballistic damage induced along the path of a recoil nucleus in the glass. Among the observations made for these simulations, it was noted that displaced alkali metal atoms were found around the periphery of the damage zone, that collective displacements occurred in the glass network and were in fact more energy-effective than individual displacements. Finally, a phenomenon of general reduction of network connectivity in the damage zones was revealed.

Computer simulations study of iron–copper alloy

The vessel steels of pressurised water reactors are embrittled by neutron irradiation. The changes of mechanical properties are commonly supposed to result from the formation of point defects, dislocation loops, voids and Cu-rich precipitates. The composition of such precipitates, specially the existence of vacancies, is not accessible through experiments. It is suggested that two mechanisms promote the formation of Cu-rich features in pressure vessel steels during neutron irradiation. When the Cu content is lower than 0.1%, solute-rich atmospheres can be formed in displacement cascades. For higher Cu contents, in addition to the latter phenomenon, a mechanism of accelerated precipitation can also induce the formation of Cu-rich clusters or precipitates. In order to understand all the mechanisms, Molecular Dynamics and Monte Carlo simulations have been carried out in pure Fe, and Fe–Cu alloys. Inter-atomic potentials of the Embedded Atom Method type, found in literature for pure Fe and pure Cu were used. We built a cross potential for the Fe–Cu interactions. It was fitted to thermodynamics and crystallographic properties of Fe–Cu alloys. The structure and properties of Cu–Vacancy(ies) clusters and Cu precipitates in bcc Fe have been studied by Monte Carlo simulations. Molecular Dynamics simulations of displacement cascades with different Primary Knocked-on Atom energies (1–20 keV) were carried out. Clusters containing Fe, Cu and vacancies were observed, located at the centre of the cascade. From our results, it appears that displacement cascades influence Cu clustering.

Non-steady-state conditions and incascade clustering in radiation damage modeling

The results of molecular dynamics simulations of displacement cascades indicate that a large fraction of the point defects that survive after intracascade annealing may be found in clusters, rather than as individual point defects. In addition, the mobility of these small point defect clusters may be relatively high. The impact of these clusters is discussed in the framework of a radiation damage model for austenitic stainless steel using the chemical reaction rate theory. Although this theory has been broadly applied in models simulating such radiation-induced phenomena as void swelling, irradiation creep, and embrittlement, such models have generally not fully accounted for incascade clustering. Many of these models have also focused on the assumed steady state behavior and tended to neglect the explicit dose and temperature dependence of the sink structure. A previously-developed model has been modified to permit a more extensive investigation of the time (dose) dependence of the point defect and extended defect concentrations and of the impact of incascade clustering. A preliminary evaluation of the so-called production bias was also carried out with this model. The results indicate that defect behavior is complex and that the limiting behavior predicted by simple analytical solutions is frequently not achieved. The comparison of the production bias and the conventional dislocation/interstitial bias found that two were comparable if examined separately at 500°C, but that the production bias effect was minimized in the presence of a reasonable dislocation/interstitial bias.

Molecular dynamics simulation of displacement cascades in Cu and Ni: Thermal spike behavior

Molecular dynamics simulations of energetic displacement cascades in Cu and Ni were performed with primary-knock-on-atom (PKA) energies up to 5 keV. The interatomic forces were represented by the Gibson II (Cu) and the Johnson-Erginsoy (Ni) potentials. Our results indicate that the primary state of damage produced by displacement cascades is controlled basically by two phenomena: replacement collision sequences during the ballistic phase, and melting and resolidification during the thermal spike. The thermal-spike phase is of longer duration and has a more marked effect in Cu than in Ni. Results for atomic mixing, defect production, and defect clustering are presented and compared with experiment. Simulations of “heat spikes” in these metals suggest a model for “cascade collapse” based on the regrowth kinetics of the molten cascade core.