Applications of atom-probe tomography to the characterisation of solute behaviours

In many materials, mechanistic understanding of material microstructure/property relationships requires knowledge of alloy structures at the atomic scale. This remains one of the main challenges of materials science. Historically, because of insufficient spatial resolution of available microstructural techniques, theories relating the role of alloying elements to materials properties were inferred from phenomenological studies. More recently, with the advent of techniques such as atom-probe tomography the spatial resolution limits have been dramatically improved. For instance, since the speculation by Cottrell and Bilby in 1949, it was generally believed that solute segregation at dislocations leads to strain hardening, but the direct proof came only recently when Blavette et al. (Nature, 1999) directly imaged a solute atmosphere at an edge dislocation, using atom-probe tomography. The recent progress in atom-probe tomography (both in dataset size and materials that can be analysed) enables atomic-scale studies of the structures of alloys and, more specifically, the analysis of solute behaviour which is a crucial issue in all areas of materials science. Clustering, ordering, site occupancy and solute/defect interactions are topics that are relevant to all classes of materials and directly affect the mechanical, electrical, magnetic, transport, etc., properties of materials. The real space information from atom-probe tomography provides a direct comparison with atomic simulations. Furthermore experimental data from the early stages of phase transformation can now be used to help validate the energetics used in atomistic modelling. This review will highlight: (1) the current limits of spatial and chemical resolution of atom-probe tomography and the resulting limits of data interpretation, (2) the data analysis tools that have been developed so far and the possible future paths for development and (3) examples where atom-probe tomography has provided mechanistic insight on the behaviour of solutes, in particular clustering, ordering and interactions with defects, and their effect on material properties.