E Alonso

Comparative study of radiation damage accumulation in Cu and Fe

Bcc and fcc metals exhibit significant differences in behavior when exposed to neutron or heavy ion irradiation. Transmission electron microscopy (TEM) observations reveal that damage in the form of stacking fault tetrahedra (SFT) is visible in copper irradiated to very low doses, but that no damage is visible in iron irradiated to the same total dose. In order to understand and quantify this difference in behavior, we have simulated damage production and accumulation in fcc Cu and bcc Fe. We use 20 keV primary knock-on atoms (PKAs) at a homologous temperature of 0.25 of the melting point. The primary damage state was calculated using molecular dynamics (MD) with empirical, embedded-atom interatomic potentials. Damage accumulation was modeled using a kinetic Monte Carlo (kMC) algorithm to follow the evolution of all defects produced in the cascades. The diffusivities and binding energies of defects are input data for this simulation and were either extracted from experiments, the literature, or calculated using MD. MD simulations reveal that vacancy clusters are produced within the cascade core in the case of copper. In iron, most of the vacancies do not cluster during cooling of the cascade core and are available for diffusion. In addition, self-interstitial atom (SIA) clusters are produced in copper cascades but those observed in iron are smaller in number and size. The combined MD/kMC simulations reveal that the visible cluster densities obtained as a function of dose are at least one order of magnitude lower in Fe than in Cu. We compare the results with experimental measurements of cluster density and find excellent agreement between the simulations and experiments when small interstitial clusters are considered to be mobile as suggested by recent MD simulations.

Highly optimized empirical potential model of silicon

We fit an empirical potential for silicon using the modified embedded atom (MEAM) functional form, which contains a nonlinear function of a sum of pairwise and three-body terms. The three-body term is similar to the Stillinger-Weber form. We parametrized our model using five cubic splines, each with 10 fitting parameters, and fitted the parameters to a large database using the force-matching method. Our model provides a reasonable description of energetics for all atomic coordinations, Z, from the dimer (Z = 1) to fcc and hcp (Z = 12). It accurately reproduces phonons and elastic constants, as well as point defect energetics. It also provides a good description of reconstruction energetics for both the 30° and 90° partial dislocations. Unlike previous models, our model accurately predicts formation energies and geometries of interstitial complexes - small clusters, interstitial-chain and planar {311} defects.

Simulation of damage production and accumulation in vanadium

Energetic atoms which have been knocked-off their lattice sites by neutron or ion irradiation leave a trail of vacancies and interstitials in their wake. Most of these defects recombine with their opposites within their own collision cascade. Some fraction, however, escape to become freely migrating defects (FMD) in the bulk of the material. The interaction of FMD with the microstructure has long been linked to changes in the macroscopic properties of materials under irradiation. We calculate the fraction of FMD in pure vanadium for a wide range of temperatures and primary knock-on atom (PKA) energies. The collision cascade database is obtained from molecular dynamics (MD) simulations with an embedded atom method (EAM) potential. The actual FMD calculation is carried out by a kinetic Monte Carlo (kMC) code with a set of parameters extracted either from the experimental literature or from MD simulations. Annealing each individual cascade at different temperatures allows the mobile species to escape and account for FMD. We also analyze damage accumulation in a specimen irradiated at low dose rate in the presence of impurities. At the temperature studied, beginning of stage V, we observe that only vacancies are free to move whereas most interstitials are stopped by impurities. We also analyze the role of impurities on damage accumulation and on the concentration of mobile defects.

Comparative study of damage accumulation in iron under magnetic and inertial fusion conditions

Abstract We present results of kinetic Monte Carlo (KMC) simulations of damage accumulation in Fe under conditions relevant to inertial (IFE) and magnetic fusion energy (MFE), with input obtained from molecular dynamics (MD). MD simulations provide information on the primary state of damage and were carried out for cascades with primary knock-on atom (PKA) energies ranging from 100 eV to 50 keV. These were used as input for a KMC simulation in which pulsed IFE irradiation and continuous MFE irradiation were simulated and compared. The MD collision cascades were introduced into the KMC simulation reproducing a recoil spectrum of 14 MeV neutrons. For pulsed irradiation, we discuss the manner in which damage accumulates depending on temperature and pulse rate. At low temperature, we show that there is no significant difference between pulsed and continuous irradiation when the integrated dose rate is the same.

Heavy ion irradiation and annealing of lead: atomistic simulations and experimental validation

Abstract We simulated the evolution of the defect microstructure resulting from irradiation of lead with 180 keV Er+ at liquid Nitrogen temperature followed by an isochronal anneal up to 350 K. The results were validated by comparison with experiments. The simulation consists of a coupled molecular dynamics (MD) and kinetic Monte Carlo (KMC) calculation that follows the production and migration of defects during irradiation and subsequent isochronal annealing. Defect diffusivities and cluster energetics were calculated by MD simulations with and embedded atom-like potential for Pb, or obtained from experiments whenever available. The primary stage of the damage produced by the energetic recoils was also calculated using MD. These calculations show the formation of dense interstitial and vacancy clusters after the cooling of the cascade. The ions are implanted at a temperature of 94 K and the damage is annealed by increasing the temperature on a stepwise fashion by 50 K every 5 min. The time–temperature evolution of the density of point defects and defect clusters is calculated in this simulation. The results are compared with experimental observations for the same irradiation and annealing conditions. The experiments consist of a set of transmission electron microscope (TEM) micrographs taken at different times during the anneal. The micrographs show the presence of loops after irradiation at 94 K, the increase in loop density with temperature, and the disappearance of all the loops at temperatures of ∼340 K. These results are in good agreement with the simulations, which help us understand the underlying processes occurring during irradiation and annealing.

Simulation of damage evolution and accumulation in vanadium

Energetic atoms which have been knocked off their lattice sites by neutron or ion irradiation leave a trail of vacancies and interstitials in their wake. Most of these defects recombine with their opposites within their own collision cascade. Some fraction, however, escape to become freely migrating defects (FMD) in the bulk of the material. The interaction of FMD with the microstructure has long been linked to changes in the macroscopic properties of materials under irradiation. We calculate the fraction of FMD in pure vanadium for a wide range of temperatures and primary knock-on atom (PKA) energies. The collision cascade database is obtained from molecular dynamics (MD) simulations with an embedded atom method (EAM) potential. The actual FMD calculation is carried out by a kinetic Monte Carlo (kMC) code with a set of parameters extracted either from the experimental literature or from MD simulations. We take two different approaches to the problem and compare them. The first consists of an idealized simulation for single cascades. Annealing each cascade at different temperatures allows the mobile species to escape and account for FMD. The second analyzes bulk diffusion and damage.