Application of state-of-the-art characterization techniques will be a central part of our research. We will particularly apply Atom Probe Tomography (APT) as a powerful nano-analytical tool for characterizing complex materials. The principle of APT is based on the controlled field evaporation and ionization of surface atoms from a fine needle-shaped emitter (radius of curvature typically below 100 nm). The field evaporated ions are accelerated towards a position-sensitive detector that records their time-of-flight and impact positions. A three-dimensional elemental map is reconstructed from the raw data, using a back projection algorithm. The great advantage of APT is its high spatial resolution (near-atomic) and high chemical sensitivity (ppm level), making it an ideal tool for studying local phase transformations and nano-scale chemical fluctuations in complex engineering alloys.
Although APT even yields structural information for specific materials such as pure metals, dilute solid solutions, and highly ordered inter metallic phases, structural information cannot alway be accessed with this technique. To acquire both chemical and structural information, other methods need to be applied in conjunction with APT. Experimental techniques, which are particularly powerful in this respect, are transmission (TEM) and scanning electron microscopy (SEM), electron back scatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). In combination with focused-ion-beam (FIB) milling it is nowadays possible to select and jointly analyze specific regions of interest of the sample (such as grain and phase boundaries) with these techniques.
(D. Tytko, P. Choi, J. Klower, A. Kostka, G. Inden, D, Raabe. Acta Materialia 60 (2012) 1731)
A technique, which has been proven be particularly powerful, is correlative APT-TEM microscopy as shown above. The example shows the distribution of boron atoms (visualized as red dots in the left bottom figure) within a nickel-based superalloy. Using correlative characterization, it could be shown that boron atoms are segregated at grain boundaries and thus have a strong influence on the microstructure and mechanical properties of this alloy.