Transmission electron microscopy (TEM) images are 2-dimensional projections of analyzed 3-dimensional objects. In many cases determination of microstructural parameters using stereological rules may lead to inaccurate conclusions. Electron tomography enables 3-dimensional visualization of a material microstructure by using sets of projections obtained at different tilt angles of the specimen (tilt series). This method provides a possibility to avoid uncertainties in determination of size, shape and 3-dimensional distribution.

In order to utilize electron tomography, applied imaging technique should generate contrast that has a fully monotonic relationship with thickness. For crystalline specimens, bright-field and dark-field imaging are not suitable for tomography due to no monotonic contrast generated by diffraction effects or Fresnel fringes. An alternative technique is Z-contrast imaging, obtained by High Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), which shows little or no diffraction effects. Its intensity is approximately proportional to Z² and thickness of a specimen. Those images may be used for reconstruction of the 3-dimensional space.

Main goal of the performed investigation was identification and visualization of nanoparticles in two structural materials: oxide dispersion strengthened (ODS) alloy, INCOLOY MA956 and nickel based superalloy, IN718.

For electron tomography investigation of oxide nanoparticles (dispersoids) in INCOLOY MA956 extraction double replicas were used. STEM-HAADF tilt series were applied for investigation of shape and chemical composition of dispersoids. The results showed that MA956 alloy contains several Y-Al oxide phases and the shape of nanoparticles is related to their crystal structure and chemical composition.

IN718 superalloy is strengthened by coherent nanoparticles of g^’ and g^’’ intermetallic phases. To investigate 3-dimensional distribution of g^’ and g^’’ nanoparticles, energy filtered transmission electron microscopy (EFTEM) was used to collect series of images at different tilt angles. Due to a similarity of chemical composition of  g^’ and g^’’ phases, elemental maps of Cr, which is dissolved in g matrix, were recorded. Analysis of the particles shape showed two classes of precipitates present in IN718 superalloy: spherical–like and asymmetric shaped, elongated through the XY plane.

Acknowledgements: The financial support from the European Union under the Framework 6 program, contract for Integrated Infrastructure Initiative. Reference 026019 ESTEEM

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