Characterization of grain structure in nanocrystalline gadolinium by high-resolution transmission electron microscopy
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Xiaoyan Song College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, People’s Republic of China
Andrey Chuvilin and Ute Kaiser Electron Microscopy Group of Materials Science, University of Ulm, D-89081 Ulm, Germany
Markus Rettenmayr Institute of Materials Science and Technology, Friedrich Schiller University, D-07747 Jena, Germany (Received 15 April 2008; accepted 8 August 2008)
A method is presented for recognition of nanograins in high-resolution transmission electron microscope (HRTEM) images of nanocrystalline materials. We suggest a numerical procedure, which is similar to the experimental dynamic hollow cone darkfield method in transmission electron microscopy and the annular dark-field method in scanning transmission electron microscopy. The numerical routine is based on moving a small mask along a circular path in the Fourier spectrum of a HRTEM image and performing at each angular step an inverse Fourier transform. The procedure extracts the amplitude from the Fourier reconstructions and generates a sum picture that is a real space map of the local amplitude. From this map, it is possible to determine both the size and shape of the nanograins that satisfy the selected Bragg conditions. The possibilities of the method are demonstrated by determining the grain size distribution in gadolinium with ultrafine nanocrystalline grains generated by spark plasma sintering. I. INTRODUCTION
Nanocrystalline structures exhibit properties that are different from conventional coarse-grained structures. The unique properties result from both the reduced grain size and the large fraction of grain boundaries. Grain size and the grain size distribution (GSD) are important parameters for characterizing the structure of the nanoscale materials. To evaluate the detailed relation between properties and structure of nanocrystalline materials, it is necessary to determine the GSD. Nanocrystalline bulk metals prepared by spark plasma sintering of nanoscaled powders of rare earth metals1,2 show a significant change in mechanical and physical properties, as compared with the polycrystalline rare earth metals. However, the GSD features in the nanocrystalline metal bulks have rarely been characterized in literature,3–5 which should be necessary to correlate the microstructure characteristics with the properties of the nanomaterials. In general, there are several techniques in transmission electron microscopy (TEM) and x-ray diffraction (XRD) to determine the GSD in nanocrystalline materials, each a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0071
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J. Mater. Res., Vol. 24, No. 2, Feb 2009
with its own advantages and drawbacks. Estimation of the GSD from an XRD-pattern is sensitive to the applied analysis method.6 Assumptions have to be made about the shape of the grains and the specific shape of the function of the GSD.7 Therefore an approximated GSD needs to be compared with TEM results. Without knowledge about the shape of the di
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