Pre-precipitate clustering in aged aluminium alloys

Although a generalised thermodynamic description of precipitate nucleation in alloys has been available for many years, the actual atomic processes that are involved have remained largely unknown. This situation is now changing, however, through the combined use of high resolution transmission electron microscopy and atom probe field ion microscopy. Details are given of the application of these techniques to the study of nucleation processes in alloys based on the Al-Cu-Mg and Al-Mg-Si systems that have been quenched and aged at elevated temperatures, and in which clustering of different atom species has been detected. Special attention is paid to the Al-Cu-Mg-Ag system in which an increased understanding has been obtained of the role of silver in stimulating nucleation of the Ω phase.