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Atom probe tomography (APT) has shown to be a quantitative instrument for the measurement of phase composition in solids including metals, oxides, and semiconductors. It has become now a key approach in Nanosciences. The first French prototype (the tomographic atom probe-TAP) was designed and set up in the lab in the early nineties and subsequently marketed by CAMECA [1]. More recently a laser-assisted TAP has been developed so that non conducting materials (oxides, semiconductors) can be analysed [2]. APT is the only analytical microscope able to map out the 3D distribution of chemical species at the atomic scale. The volume analysed is close to 50x50x100 nm3. The composition in a small selected volume (1 nm3) within this volume can be measured. The spatial resolution of the instrument is 0.1 nm in depth and a fraction of a nm at the specimen surface. APT is therefore a suitable technique to investigate number of key issues in material science including the nucleation of precipitates and the segregation of impurities to crystal defects in metals, or the distribution of dopants in semiconductors. Potential of APT will be shown through selected illustrations dealing with structural materials (steels, nickel base superalloys, Al base alloys) and functional materials (spin valves, magnetic semiconductors).

Focus will be made on nucleation. The composition and structure of nuclei during the early stages of solid state phase separation in binary systems is a challenging problem both from a theoretical and an experimental point of view and is of utmost importance for applications (e.g. metallurgy). In systems where the equilibrium phase has a different structure as the parent solid solution, the kinetics pathway often proceed through the nucleation of coherent isostructural nuclei (the well known GP zones in Al base alloys). APT investigations showed that these transient nuclei are sometimes observed as diffuse zones with a solute concentration smaller than the equilibrium phase (e.g. SiB, CuCr, FeCu, FeCr below the spinodal line). In this presentation, APT experiments will be discussed and confronted to theoretical predictions (non-classical nucleation theories).

[1] D. Blavette, A. Bostel, J.M. Sarrau, B. Deconihout and A. Menand, 1993, Nature 363, 432 [2] B. Gault, F. Vurpillot, A. Vella, M. Gilbert, A. Menand, D. Blavette, B. Deconihout, Design of a femto-second laser assisted Tomographic Atom Probe , Rev. Sci. Instr. 77, 043705 (2006)

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