HRTEM study of Co7W6 and its typical defect structure

The tetrahedrally close-packed structure of Co7W6 (mu-phase) was studied by HRTEM. A massive presence of random sub-unit cell twins was detected. Thes e planar defects were characterized in detail and the twinning probability at potential mirror planes was established. High-precision image simulation of the bulk structure and defect region was performed, using a simulated e volution optimization strategy to refine the imaging conditions. Ab initio calculations were carried out in order to determine the energy involved in twinning, as well as the distortion induced in the structure. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.