Combined atomic-scale modelling and experimental studies of nucleation in the solid state

The process of solid-state nucleation in highly supersaturated solid solutions has been investigated on the atomic scale by a combination of three-dimensional atom probe analysis and atomistic modelling using dynamical Ising models. In binary Cu-Co alloys, a simple atom-exchange model with a single thermodynamic parameter derived from phase-diagram data was able to reproduce the atomic-scale microstructures observed in the atom probe, and also match the measured peak precipitate density. Modelling solute effects in complex copper-bearing steels required a more sophisticated model based on a vacancy-hopping mechanism and a larger number of thermodynamic and kinetic parameters derived from independent experimental data and theoretical calculations. The model gave an excellent match to the experimentally observed microstructures, and it reproduced features such as the clustering of Ni and Mn before the precipitation of Cu. The model also allowed time-dependent behaviour to be investigated, and it showed that solute clustering of Ni and Mn occurs during the cooling of the alloy. These clusters then act as heterogeneous nucleation sites for the formation of copper precipitates. Understanding such complex solute interaction effects through combined experiment and modelling is an essential step to controlling nucleation and hence the fine-scale microstructures in advanced engineering alloys.