Characterization of grain structure in nanocrystalline gadolinium by high-resolution transmission electron microscopy Martin Seyring a) Institute of Materials Science and Technology, Friedrich Schiller University, D-07747 Jena, Germany 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 dark- field 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 para- meters for characterizing the structure of the nanoscale materials. To evaluate the detailed relation between prop- erties and structure of nanocrystalline materials, it is nec- essary to determine the GSD. Nanocrystalline bulk metals prepared by spark plasma sintering of nanoscaled powders of rare earth metals 1,2 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 nec- essary 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 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 func- tion of the GSD. 7 Therefore an approximated GSD needs to be compared with TEM results. Without knowledge about the shape of the distribution function, it is not possi- ble to measure the GSD in nanocrystalline Gd via XRD. TEM offers high-resolution TEM (HRTEM) 8 and also several dark-field techniques to measure the nanograin size directly, such as centered dark field (CDF), 6 annular dark field (ADF), or high angle annular dark field (HAADF) in scanning transmission microscopy (STEM). 9 HRTEM was chosen to study the nanocrystal- line grain structure and measure the GSD. HRTEM images show directly the translational symme- try of the crystal lattice in a material. The nanograins are identified and discriminated by the emergence of lattice fringes 10 [Fig. 1(a)]. The appearance of too many features in the image (e.g., generation of moire ´ pattern by overlap of grains 11 ) make it difficult to determine the contours of the nanograins from a simple evaluation of the image. Hy ¨tch and Gandais developed a technique to extract nanograins by Fourier filtering of HRTEM images. 10 The method decomposes the HRTEM lattice image in a) Address all correspondence to this author. e-mail: martin.seyring@uni-jena.de DOI: 10.1557/JMR.2009.0071 J. Mater. Res., Vol. 24, No. 2, Feb 2009 © 2009 Materials Research Society 342