Tomas Diaz de la Rubia, Brian Wirth, and Eduardo Alonso,
Lawrence Livermore National Laboratory

Research Objectives

We are studying the interaction of defects produced during irradiation or deformation of a metal with the microstructure of that particular material, such as dislocations and grain boundaries. In particular we are studying the interaction of dislocations with interstitial loops and stacking fault tetrahedra, and the production of displacement cascades close to dislocations and grain boundaries. The data obtained from these simulations will be used as input to diffusion models and dislocation dynamics models.

Computational Approach

	Damage produced by a 20 keV recoil atom in copper. Red spheres represent the location of interstitial atoms; green spheres represent vacancy sites. The damage produced in this face-centred cubic material due to irradiation results in large clusters of interstitials and vacancies. The production of a large interstitial cluster that migrates along a <110> direction is observed in this simulation.

Mostly we employ molecular dynamics simulations using empirical interatomic potentials. Parinello-Rahman boundary conditions are used to apply stress to study dislocation motion. The link cell method is used to efficiently calculate the neighbors of the atoms in the computational box. The partition into link cells is also used to distribute the atoms across the nodes in the parallel machine. Defect diffusion is studied using kinetic Monte Carlo models. The reaction rates for defect interaction and defect dissolution are input for this method, as well as the defect distribution, obtained from molecular dynamics simulations.

Accomplishments

We have simulated the formation of stacking fault tetrahedra (SFT) in copper both from the collapse of a vacancy plane and directly during irradiation. The presence of SFT after irradiation of copper has been observed experimentally for many years; however, molecular dynamics simulations have failed until now to reproduce this observation. We were able to simulate the production of SFT during irradiation of copper by using high energy irradiation and long relaxation times.

We have also simulated the damage produced by self-irradiation of copper, for energies between 200 eV and 20 keV. Several cascades were obtained for each energy (maximum 12 and minimum 5). The defect production as a function of energy was compared to that obtained from the Kinchin-Pease model. The production rate is comparable to the Kinchin-Pease only for low energies, reaching a constant value of 0.2 × Kinchin-Pease at energies ~5 keV. The database extracted from these simulations was used to study defect accumulation and diffusion using kinetic Monte Carlo. One of the cascades was followed for 100 picoseconds. An interstitial cluster containing 38 defects was produced in this cascade. This interstitial is highly mobile, and its migration is followed in this simulation. The next step in our simulations is to study the interaction of these types of clusters with dislocations.

Significance

Publications

M.-J. Caturla, N. Soneda, E. Alonso, B. D. Wirth, T. Diaz de la Rubia, and J. M. Perlado, "Comparative study of radiation damage accumulation in Cu and Fe," J. Nucl. Mater. (submitted).

E. Alonso, M.-J. Caturla, T. Diaz de la Rubia, and J. M. Perlado, "Simulation of damage production and accumulation in vanadium," J. Nucl. Mater. (submitted).

N. Soneda and T. Diaz de la Rubia, "Defect production, annealing kinetics and damage evolution in alpha-Fe: An atomic-scale computer simulation," Philosophical Magazine A 78, 995 (1998).