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A possible description of the ideal microscope would be an instrument which was able to reconstruct, with atomic resolution and in 3 dimensions, both the position and the chemical identity of atoms in a material. The 3-dimensional atom probe (3DAP) is the technique which comes closest to this goal.The position-sensitive atom probe (PoSAP) was the first 3DAP. In the PoSAP, the high magnification of the field-ion microscope is combined with the time-of-flight mass spectroscopy of the atom probe, and position-sensitive detection based on a wedge-and-strip anode, Fig.1. This combination allows the chemical identity and the original surface position to be determined for single atoms removed from a field-ion specimen by pulsed field evaporation. Continued field evaporation and analysis builds up a 3D image of the distribution of all the atomic species originally present in the material, Fig. 2.

3-dimensional atom probe (3DAP) analysis enables 3D reconstruction of the chemical variations inside materials with near atomic resolution. This technique has enabled important insights into the phase chemistry and microstructure of a range of complex engineering materials. Although individual planes can be resolved when analysing in a direction close to a low-index pole, the spatial resolution of this technique has not been sufficient for reconstruction of all the original lattice positions. The spatial resolution is thought to be fundamentally limited by small lateral displacements of the atom on the surface of the specimen during the field evaporation process due primarily to interactions with the neighbouring surface atoms. It has therefore been considered that it will be impossible to correctly reconstruct the crystal lattice from 3DAP data. Recently, we have found that a 3DAP reconstruction of a region near a grain boundary in pure tungsten (an element renown for its good imaging and evaporation behaviour) revealed several different sets of crystallographic planes visible at angles up to 45° from the direction of analysis.

Steels represent by far the most widely used class of engineering alloys, and comprise almost 95% of total world metal production. Yet despite their familiarity and ubiquitous distribution, many fundamental questions remain unanswered regarding the nature of the solid state chemical reactions occurring during the heat treatment of steels, the mechanisms by which alloy additions affect the kinetics of these processes, and the role of trace impurities on properties.The difficulties involved in studying such processes experimentally arise from two main sources. Firstly, the effects involved are often on an extremely fine scale. Secondly, one of the key elements involved is carbon, which is extremely difficult to study by conventional microanalytical techniques.Remarkable progress has been achieved in these areas through the application of the atom probe method. Much of this work has been reviewed in a recent monograph by Miller et al.