Atomic-scale characterisation of precipitation in copper-cobalt alloys

Abstract Cu-1.0at%Co, Cu-2.0at%Co and Cu-2.6at%Co alloys, aged at 723 K for various times up to 120 h, have been examined using FIM-AP and PoSAP. The alloys were chosen such that the free-energy barrier to decomposition was small (less than ≈ 10 kT ). Three-dimensional atomic reconstructions of the alloys are presented, and the resulting fine-scale (sub-nanometre) microstructures are analysed in terms of: degree of phase separation; cluster sizes and densities; matrix and cluster compositions; and cluster morphology. The experimental results from the PoSAP are compared with three-dimensional Monte Carlo simulations, the sub-regular solution model and the classical nucleation model. There is a good qualitative match between the Monte Carlo simulations and the experimental data: the simulations exhibit nucleation and growth, coarsening and Ostwald ripening behaviour. In addition, a quantitative comparison between experiment and simulation has been undertaken and there is good agreement for quantities such as cluster densities and matrix compositions, as a function of ageing time.