Showing papers by "David Bacon published in 1998"

The effects of electron-phonon coupling on defect production by displacement cascades in -iron

Abstract: Molecular dynamics (MD) simulations have been used to investigate the effects of electron-phonon coupling on defect production and clustering produced in the primary cascade state of neutron radiation damage in -Fe. A simplified physical model of electron-phonon coupling has been incorporated with the hybrid continuum/MD code developed by Gao et al to allow heat transfer from phonons to electrons, and applied to study defect generation as a function of the strength of electron-phonon coupling for cascade energy of 2, 5 and 10 keV. The number of point defects produced in the primary damage state increases with increasing strength of electron-phonon coupling. Most of the additional defects form in the cascade core, but the fraction of interstitials in clusters remains almost constant. These effects arise from reduced mobility of atoms in the shorter thermal spike. The quenched-in clustering of vacancies plays a role in the increase of the vacancy clustering fraction with increasing strength of electron-phonon coupling. This mechanism is predicted to inhibit the formation of dislocation vacancy loops and the strong coupling, therefore, reduces the probability of loop formation.

Kinetic Monte Carlo Annealing Simulation of Damage Produced by Cascades in Alpha-Iron

Abstract: The mobile defects created in displacement cascades can either interact within the cascade region or undergo long-range diffusion in the crystal. The kinetic Monte Carlo code ALSOME has been used in the present work to carry out annealing simulations of electron irradiation and single cascades with energy in the range of 2 to 40 keV in α-Fe as a function of temperature. Isochronal annealing of electron irradiation shows a temperature-dependence of the recovery stages that is reasonably close to experiment, but Stage I is controlled by the rotation energy of the dumbbell to the crowdion. The annealing of single cascades has demonstrated that nearly 60% of SIAs formed in the primary state of cascade damage escape from the cascade at temperatures above stage I. Most of the escaping SlAs are in clusters, for only 10% of them are mono-interstitials. Although the number of escaping defects increases with increasing cascade energy, the relative fraction is almost constant for the recoil energies considered. The results are compared with those for copper obtained using the same code [1].

Glissile and Sessile Vacancy and Self-Interstitial Clusters in BCC and FCC Metals

Abstract: The production bias model of radiation damage has focused attention on the production of glissile clusters (GCs) in the displacement cascades. The formation of glissile SIA clusters in molecular dynamics simulations of cascades has been reported frequently whereas information about sessile clusters (SCs) is rather sparse. Comprehensive models of microstructure evolution in metals under irradiation conditions should include all possible defects (and clusters) and interactions between them. Detailed information of this nature can be obtained by means of atomic-scale computer simulation. In the present paper we review some recent results of atomistic simulation of sessile and glissile defect clusters in metals, dealing with their mechanisms of formation and their properties. In particular, we discuss the formation of SCs and GCs in the cascade region and also their creation in bulk metal as a result of cluster growth.

Mobility of Self-Interstitials in FCC and BCC Metals

Abstract: Diffusion of self-interstitial atoms (SIAs) has been studied in bcc-Fe and fcc-Cu using molecular dynamics and interatomic potentials of different types. The Fe potentials describe SIA configurations of different stability. The temperature dependence of the SIA diffusion mechanisms is qualitatively similar for both potentials. At high temperature the diffusion is three-dimensional via the dumbbell mechanism. The contribution of one-dimensional mechanism via the crowdion increases when temperature decreases. At low temperature ( dumbbell. The migration mechanism is mainly a three-dimensional random walk via this dumbbell with small contributions from the crowdion at high temperature and a two-dimensional caging mechanism at low temperature.

Point-defect properties and threshold displacement energies in Cu3Au

Abstract: Molecular dynamics computer simulations have been employed with a Finnis-Sinclair form of interatomic potential to calculate the properties of point defects in Cu3Au and to establish the threshold displacement energy, E d, for both Cu and Au primary knock-on atoms in this alloy. The most stable interstitial is the Cu-Cu ⟨100⟩ dumbbell centred on a Cu site in the {100} plane which contains only Cu atoms, and all configurations of a Au interstitial are unstable with respect to the formation of a Cu antisite defect plus a Cu-Cu ⟨100⟩ interstitial. The results are compared with those of Spaczer, Caro and Victoria, who used embedded-atom potentials and whose data for antisite defects are different from those found here. E d for Au recoils has a lower value on average than that for Cu recoils, in contrast with the threshold energy of recoils of the oversized species Al in Ni3Al. For Cu recoils, however, the E d values are very similar to those found for Ni recoils in Ni3Al, particularly for orientations in whic...

On The Mechanism of Interstitial Cluster Migration in α-Fe

Abstract: Recent molecular dynamics (MD) computer simulations have shown that clusters consisting of up to a few tens of self-interstitial atoms (SIAs) are highly mobile along closed-packed crystallographic directions in pure copper and iron. This effect has important consequences for microstructure evolution in irradiated metals and so it is desirable to investigate the mechanisms of the cluster motion. In the present paper the results of MD modelling of the thermally-activated motion of clusters of 3, 9 and 17 SIAs in α-Fe in the temperature range from 90 to 1400 K are analyzed. The extensive MD data has enabled the migration of clusters, as well as that of individual SIAs in the clusters, to be treated with high statistical accuracy. The correlation between the motion of the centre of gravity of a cluster and the jumps of individual SIAs in the cluster is revealed. It is found that the SIAs in a cluster jump almost independently and their jump frequency depends on the number of SIAs in the cluster. This leads to a simple relationship between the jump frequency of a cluster and the number of SIAs in it. The cluster jump frequency exhibits a deviation from the Arrhenius relationship. The reason for this is discussed.

Monte Carlo Investigation of Cascade Damage Effects in Metals Under Low Temperature Irradiation

Abstract: The Monte Carlo (MC) method is used to study cascade damage effects in damage accumulation in pure metals at temperatures below stage III, when vacancies and their clusters are immobile. The irradiation is modelled by sequential introduction of collections of defects representing the primary damage state of cascades placed randomly in the simulation volume. The cascades generated in molecular dynamics simulations for recoil energies from 2 to 20keV are used. Concentrations of point defects as well as defect cluster densities are monitored as a function of dose up to 0.02dpa. The results are compared with those obtained in the mean-field approximation. Factors responsible for the difference in damage accumulation under homogeneous and cascade irradiation conditions are revealed. The effects of temperature and recoil energy are studied. Problems in MC modelling of one-dimensional diffusion of interstitial clusters connected with the limited volume of the simulation box are revealed.

MD Investigation of Thermal Spike Effects on Defect Production and Disordering by Displacement Cascades in Ni3Al

Abstract: Molecular dynamics (MD) simulations are used to obtain detailed information on defect production and disordering produced in the primary cascade state of radiation damage in Ni3AI as a function of irradiation temperature, Tirr Although the number of Frenkel pairs decreases with increasing Tirr, the size of interstitial clusters and the interstitial clustering fraction increase with Tirr. The number of antisites defects per cascade increases by about 20% as Tir increases from 100 to 600K, and then by about 90% from 600 to 900K due to the increase in the intensity and lifetime of the thermal spike. The average size of disordered zones at different Tir, is compared with experimental transmission electron microscopy data and the results are in reasonable agreement. The long-range order parameter in the cascade region is found to be consistent with values obtained experimentally on Ni3A1 irradiated by ions to a similar dose. The chemical short-range order parameter is about 0.6 over the whole range of Tirr, considered, suggesting that the cascade core retains some short-range order at high temperature due to local reordering.

Atomistic Simulation of Mobile Defect Clusters in Metals

Abstract: The structure, stability and thermally-activated motion of interstitial and vacancy clusters in Fe and Cu have been studied using atomic scale computer simulation. All studied interstitial clusters and perfect interstitial loops (PILs) in Fe are mobile whereas their mobility in Cu can be suppressed at large sizes (bigger than 49-61 self-interstitials depending on the temperature) due to dissociation. A comparative study of relaxed configurations has shown that the structure of small perfect dislocation loops of vacancy and self-interstitial nature is very similar. Molecular dynamics simulation has demonstrated that small perfect vacancy loops (PVLs) in Fe consisting of more than 37 vacancies are stable over a wide temperature range and produce atomic displacements by a thermally-activated movement in the direction of the Burgers vector. The mechanism is qualitatively similar to that of SIA clusters studied earlier. Motion of vacancy loops in Cu does not occur because they transform into sessile configurations similar to stacking fault tetrahedra. These results point to the possibly important contribution of vacancy loop mobility to the difference in radiation damage between bcc and fcc metals, and between fcc metals with different stacking fault energy.

Computer Simulation of Defect Production and Behaviour in Displacement Cascades in Metals

Abstract: Recent research using molecular dynamics to simulate radiation damage due to displacement cascades in metals is reviewed. It includes results dealing with the effect on defect formation of primary knock-on atom energy and irradiation temperature. Clear dependencies and trends have emerged in these areas. In terms of the development of models to describe the evolution of radiation damage microstructure, the important parameters are not only the total number of Frenkel defects but also the distribution of their population in clusters and the form and mobility of these clusters. Results on these aspects are reviewed and it is shown that computer simulation is providing detailed information that paves the way for successful development of models of the evolution of damage beyond the stage of the cascade process.