Rigid-lattice Monte Carlo study of nucleation kinetics in dilute bcc Fe-Cu alloys using statistical sampling techniques

The chemical composition and nucleation kinetics of Cu precipitates in thermally aged bcc Fe-Cu alloys is investigated with statistical sampling techniques combined with Monte Carlo simulation. The analysis of nucleation dynamics is performed by a rigid-lattice Monte Carlo algorithm based on the vacancy-exchange mechanism. At elevated aging temperatures from 450 to 650 °C, the critical cluster size and the corresponding nucleation barrier are evaluated using an Umbrella Sampling technique. The nucleation rate of Cu precipitates is calculated on basis of nucleation trajectories harvested by Forward Flux Sampling (FFS). The fastest nucleation in dilute bcc Fe-Cu alloys is predicted to occur at 550–600 °C, which is in good agreement with experimental findings. The critical Cu clusters at 450–650 °C are found to contain 10–40 atoms with an increasing nucleation energy barrier from 10 to 23 kBT. For small clusters with less than 30 atoms, a significant shape anisotropy is observed during nucleation. Special emphasis is placed on the Cu concentration profiles of critical nuclei, where we observe that the first formed Cu clusters contain a substantial amount of iron at lower annealing temperatures. However, the Cu content of the clusters increases rapidly to almost unity during aging. Growth of the clusters follows mostly after the nucleus has substantially enriched in Cu content.