Although reliable position and composition data are obtained with the Tomographic Atom Probe, the procedure of position calculation by charge centroiding fails when the detector receives two or more ions with close spaced positions and the same mass-to-charge ratio. As the charge clouds of the ions overlap, they form a unique charge pattern on the multianode detector. Only one atom is represented and its position is biased. In order to estimate real positions, we have developed a correction method. The spatial distribution of charges inside a cloud issued from one impact is modelled by a Gaussian law. The particular properties of the Gaussian enable the calculation of exact positions of the two impacts of the overlapped charge patterns and charges of corresponding clouds. The calculation may be generalized for more than two overlapped clouds. The method was tested on a plane-by-plane analysis of a fully ordered Cu3Au alloy performed on a (100) pole.

The tomographic atom probe (TAP) is the first 3D atom probe based around the use of a parallel position encoding system. This contribution will be focused on the implementation and the study of the TAP position sensitive detector. In the first part, it will be shown why this detector necessarily has to be sensitive to simultaneous events and the detector principle will be described. Then performance and limitations of the TAP will be discussed and related to the detector performance itself. Critical points such as spatial resolution, mass resolution or detection efficiency will be quantified.

A set of low alloy model reactor pressure vessel steels, with systematic variations in their Mn, Ni, and Si contents, were neutron-irradiated to high fluence (1.4 × 1020 n/cm2) in the Advanced Test Reactor at Idaho at 290°C and a flux of 3.6 × 1012 n/cm2s. The alloys were analysed using atom probe tomography and solute clusters were observed in each alloy, including in one alloy that contained low nominal levels of Mn (0.04 at. %) and Si (0.06 at. %). Changes in the mechanical properties of the alloys were correlated with cluster volume fractions. Whilst the effect of nominal composition was observed to influence cluster composition, cluster nucleation site was not observed to affect composition. Several grain boundaries were also analysed and the segregation behaviour of certain elements is discussed.

Data collected with 3D atom probes have to be carefully analysed in order to give reliable composition data precisely positioned in the probed volume. Indeed, the large analysed surfaces of 3D atom probes require the development of reconstruction methods taking into account the tip geometry. When the analysis does not take place in the close vicinity of the tip axis, the analysis direction is no longer perpendicular to the evaporated surface. The influence of this effect on the local magnification and atom positioning must be taken into account. The proposed procedure will be validated by studying the effects of calculations on a long-range-ordered phase.

The tomographic atom prob (TAP) is the most advanced of the three‐dimemsional atom probes currently in use. Atom probes are unique in that both the lateral and the depth resolution are high. The genera; development of atom probes and the principle of operation ot the TAP, in which the sample is field evaporated atomic loayer by atomic layer, are outlined. The analysis of a nickel base superalloy is given as an example of an application in metallurgy.

The tomographic atom probe is a new instrument which enables a small volume of a metallic material to be reconstructed in 3D on a near-atomic scale. The basic principles on which the tomographic atom probe relies are briefly described. The performance of this new generation of apparatus is illustrated on the ground of some specific experiments. The intrinsic resolution of the spatial detector that was designed and developed is estimated. Several 3D atomic reconstructions of materials are provided. Images related to the investigation of precipitation processes in two-phase nickel-base superalloys, grain-boundary segregation effects as well as G-phase formation related to the spinodal decomposition of the ferrite in duplex stainless steels are given as illustrations. The quantitativity of composition measurements and the mass resolution of the instrument are discussed.

Duplex stainless steels are subject to embrittlement after long term aging at temperatures below 500°C. This embrittlement is usually attributed to the α-α' phase separation occuring in the ferrite, though the role of G-phase particles, when present, may not be negligible. In order to try to restore their mechanical properties to that of the as-quenched conditions, specimens were reannealed above the Fe-Cr miscibility gap. The mechanical effect of various reannealing treatments is reported, and correlated with transmission electron microscopy and atom probe field ion microscopy microstructural investigations.