Defect production, annealing kinetics and damage evolution in alpha-Fe: anatomic-scale computer simulation

Radiation-induced microstructural and compositional changes in solids are g overned by the interaction between the fraction of defects that escape thei r nascent cascade and the material. We use a combination of molecular dynam ics (MD) and kinetic Monte Carlo (KMC) simulations to calculate the damage production efficiency and the fraction of freely migrating defects in alpha -Fe at 600 K. MD simulations provide information on the nature of the prima ry damage state as a function of recoil energy, and on the kinetics and ene rgetics 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 em ploy the analytical embedded-atom potential developed by Johnson and Oh for alpha-Fe, including a modification of the short-range repulsive interactio n. We use MD to calculate the diffusivities of point defects and small defe ct clusters and the binding energy of small vacancy and interstitial cluste rs. We show that, at temperatures below about 600 K, small interstitial clu sters form prismatic dislocation loops which migrate in one dimension with a very low activation energy E-a approximate to 0.1 eV. We also present res ults of MD simulations of displacement cascades at energies up to 20 keV. T he results show that, for recoil energies above 5 keV, interstitials are pr oduced in the form of small prismatic loops with a high probability, but va cancies 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 dam age annealing process and show that for 20 keV cascades the escape probabil ity for both vacancies and interstitials is about 65%. This results in a fr eely migrating defect production efficiency of 20% of the total defect prod uction predicted by the modified Kinchin-Pease model (the displacements per atom standard). The capability of the hybrid MD-KMC method for carrying ou t long length and time scale simulations of damage evolution in irradiated materials is emphasized.