Defect production, annealing kinetics and damage evolution in α-Fe: An atomic-scale computer simulation

Abstract

Radiation-induced microstructural and compositional changes in solids are governed by the interaction between the fraction of defects that escape their nascent cascade and the material. We use a combination of molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations to calculate the damage production efficiency and the fraction of freely migrating defects in α-Fe at 600 K. MD simulations provide information on the nature of the primary damage state as a function of recoil energy, and on the kinetics and energetics of point defects and small defect clusters. The KMC simulations use as input the MD results and provide a description of defect diffusion and interaction over long time and length scales. For the MD simulations, we employ the analytical embedded-atom potential developed by Johnson and Oh for α-Fe, including a modification of the short-range repulsive interaction. We use MD to calculate the diffusivities of point defects and small defect clusters and the binding energy of small vacancy and interstitial clusters. We show that, at temperatures below about 600 K, small interstitial clusters form prismatic dislocation loops which migrate in one dimension with a very low activation energy E _a ≈ 0.1eV. We also present results of MD simulations of displacement cascades at energies up to 20keV. The results show that, for recoil energies above 5keV, interstitials are produced in the form of small prismatic loops with a high probability, but vacancies are not. The MD results are then combined with a KMC simulation of defect interaction and diffusion, which includes the one-dimensional glide of small interstitial loops. The results provide a clear picture of the damage annealing process and show that for 20keV cascades the escape probability for both vacancies and interstitials is about 65%. This results in a freely migrating defect production efficiency of 20% of the total defect production predicted by the modified Kinchin-Pease model (the displacements per atom standard). The capability of the hybrid MD-KMC method for carrying out long length and time scale simulations of damage evolution in irradiated materials is emphasized.