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Application of electron tomography to analysis of nanoparticles in structural alloys

Grzegorz Cempura 1Adam Kruk 1Beata Dubiel 1Juan C. Hernandez 2Aleksandra Czyrska-Filemonowicz 1

1. AGH University of Science and Technology (AGH), al. Mickiewicza 30, Kraków 30-059, Poland
2. University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom

Abstract

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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Related papers

Presentation: Oral at E-MRS Fall Meeting 2009, Symposium H, by Grzegorz Cempura
See On-line Journal of E-MRS Fall Meeting 2009

Submitted: 2009-05-25 14:05
Revised:   2009-06-07 00:48