Atom Probe Field Ion Microscopy and High Resolution Electron Microscopy: two complementary methods for atomic scale characterisation

Summary form only given. Atom probe tomography is now a well recognized technique for both imaging and analyzing metallic materials at the subnanometer scale. The limitations of this techniques are mostly related to the smallness of the investigated volume, and to the limited microstructural information it can access. APT investigations are limited to volumes of about 100times100times100 nm3, which may not be fully representative of the materials microstructure. Therefore APT investigations are mostly combined with electron microscopy studies. On the other hand, APT offers a true subnanometer resolution. As compared to TEM related techniques, the major advantage of APT lies in its depth resolution, which overcomes the problem of depth sampling. Another major advantage of APT is that it also offers composition analysis capabilities with the same resolution, which can usually not be challenged by electron microscopy related techniques. On the basis of some selected metallurgical problems, the complementarity of both techniques will be highlighted, both in terms of structure and composition information: First, the nanoscale shell structure of Al3(ZrSc) precipitates is investigated. The combination of APFIM and HREM clearly proves the existence of a Zr rich shell surrounding a Zr depleted core, explaining the growth and coarsening behavior of these precipitates during annealing at 475degC. Second, a partially nanocrystallized amorphous Al92Sm8 (at.%) alloy obtained by a rapid solidification process will be investigated. A unique characterization of the nanostructure combined with the atomic species distribution of the nanocomposite materials will be presented. The absence of excessive solute concentration at the interface nanocrystallites/amorphous matrix and the particle size (about 50-60 nm), higher than the one usually observed in similar alloys, can explain the excellent observed ductility of this nanocrystallized alloy. At last, the first experimental evidence of GP zones in a very dilute Fe-Nb-C-N model high-strength low alloy (HSLA) will be given. The precipitation sequence, from a random solid solution under the simple action of temperature, will be shown. In particular, we will focus on the evidence of monolayered NbN precipitates. A tentative structure will be proposed, based on the information derived from APT and HREM