Atom Probe Tomography.

  1. 3D reconstruction of a small volume analyzed by atom probe tomography in a nuclear vessel steel (Novovoronesch power plant – Russia). On this image, only alloying elements are displayed to exhibit solute clusters and a segregation along a dislocation. These features result from the severe irradiation conditions (neutron irradiation during 20 years at a temperature of 290°C), and they significantly affect the mechanical properties of the material.

  2. 3D reconstruction of a small volume analyzed by atom probe tomography in a FeCu0.2% alloy aged during 90min at 500°C. Fe atoms are displayed in black and Cu atoms in red. Four nanoscaled Cu precipitates that have nucleated during the aging treatment are clearly exhibited. Vertical atomic planes are (110) planes from the bcc ferrite phase.

  3. Nano-clusters in an AlMgZn alloy:

    η’ precipitate in a 7108.5 alloy (Al-Zn-Mg system). Such precipitate, which exhibit a disk morphology, grow along {111} planes of the Al-matrix. They are responsible, together with η-MgZn2 precipitates, for the peak hardness of the 7108.5 alloy.

    Characterisation of the composition and volume fraction of η’ and η precipitates in an Al-Zn-Mg alloy by the combination of Atom Probe, Small-Angle X-ray Scattering and Transmission Electron Microscopy, M. Nicolas, W. Lefebvre, B. Doisneau-Cottignies, A. Deschamps, Acta materialia, 2005, vol. 53, 2881-2892.

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  4. Structural defects imaged by field ion microscopy:

    Metal crystals are formed by the regular and symmetrical lattice, arrangement of individual atoms. As observed in the field ion microscope, a perfect crystal shows an image, such as the one presented, where individual atoms are resolved. The high symmetry of the pole figure (different concentric rings) reflects the lattice symmetries. Nevertheless, real crystals are not perfect (the nature of the defects being the basis of the mechanical properties of real materials). Several types of defects can be directly observed at the atomic scale using the field ion microscope.

    • Linear defects (1D), such as dislocation lines, locally modify the symmetry of the poles around the core of the dislocation.
    • Planar defects (2D), such as grain boundaries, modify the general symmetry of the micrograph. A rupture line, separating two individual grains with own symmetry, marks the location of the grain boundary.
    • Volume defects (3D), such as precipitates, usually give rise to changes in the local contrast, more cohesive phases appearing in clear contrast.

    ©F. Danoix– CNRS UMR 6634
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  5. Atomic scale characterization of phase separation in the Fe-Cr system:

    The binary Fe-Cr system is the basis of all stainless steels. In order to understand the properties of stainless steels, it is essential to understand the phase transitions occurring in the binary Fe-Cr system, among which the mechanism known as '475°C embrittlement'. Field ion microscopy and atom probe microanalysis, because of their ability to identify the phases formed during ageing, give access to the microstructure of these materials, down to the atomic scale. They give the possibility to investigate the time evolution of the microstructure, and to clearly identify the two possible modes of phase separation, i.e. nucleation and growth on one side, and spinodal decomposition on the other. In the first case, small nanometre scale Cr rich precipitates form and develop. In the second, a double interconnected network of Cr rich and Fe rich regions develops, spatially and in term of concentration variation amplitude. These two modes are the two possible paths to reach equilibrium, constituted of large Cr rich and Fe rich domains.

    ©F. Danoix & S. Novy – CNRS UMR 6634
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