Transmission electron microscopy crystal structure study of the Cr-rich phase in a laser-clad Ni alloy
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I.
INTRODUCTION
THE crystal structure determination for a new phase is usually approached in two steps. First, literature is reviewed to see if any crystal structure data with a similar chemical composition have been established. If such crystal structure data are available, the identification will proceed by comparing the experimental results such as X-ray powder patterns or electron diffraction patterns with the referenced data. If an acceptable agreement is achieved, the job is done. In case there is no referenced crystal structure data or the matching between the experimental result and the referenced crystal structure data is unsatisfactory, a complete structure determination procedure should be employed. This determination would involve the following: (1) point group, (2) unit cell type and dimension, (3) space group, and (4) coordinates of the atoms in the unit cell. Whether the whole process can be completed depends on the quality and quantity of the material available and how complicated the structure is. X-ray, electron, and neutron diffraction are standard techniques. In laser-processed materials, the microstructure is usually very fine due to high cooling rates. The grain size usually varies from nanometers to microns. Electron diffraction is preferable to X-ray diffraction because of the ability to focus down to a very small size (typically 5 to 20 nm). The present article presents the results of crystal structure determination of the Cr-rich phase in a Ni-alloy clad. A detailed description of the cladding process was given in another article, m The microstructure and crystal structure of the clad were examined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Nine different morphologies were differentiated from each other by their geometrical appearance in the micrographs and energy-dispersive Y. LIU, Visiting Research Associate for the Center for Laser Aided Materials Processing, and J. MAZUMDER, Professor of Mechanical and Materials Engineering and Director of the Center for Laser Aided Materials Processing, are with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. K. SHIBATA, Senior Research Scientist, is with Nissan Motor Co. Ltd., Nissan Research Center, Yokosuka 237, Japan. Manuscript submitted June 7, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
X-ray (EDX) compositional analysis. Seven phases in the clad and interface, including three new ones, were identified with convergent beam electron diffraction (CBED) and selected area diffraction (SAD) techniques. The Cr-rich phase is one of the three new phases in the Ni alloy clad. The point group and space group identified by CBED, unit cell dimensions deduced by SAD, and the atomic coordinates in the unit cell determined by high-resolution electron microscopy (HREM) images will be presented. II.
EXPERIMENTAL
An AVCO HPL 10 kW continuous wave CO2 laser with an oscillating mirror system was used as the energy source. An oscillating frequen
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