Stacking Faults Created by Mechanical Milling in Nanostructured WC-Co Composite Powder
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Stacking Faults Created by Mechanical Milling in Nanostructured WC-Co Composite Powder Yang Zhimin, Mao Changhui, Du Jun General Research Institute for Non-ferrous Metals, Beijing 100088, P. R. China Michel Daniel, Champion Yannick, Hagège Serge and Hÿtch Martin Centre d’Etudes de Chimie Métallurgique / CNRS, 94407 Vitry-sur-Seine, France ABSTRACT Nanostructured WC-Co powders obtained by mechanical milling were investigated by combination of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques. HRTEM image analysis shows that in the as-milled nanostructured powder, many WC grains contain stacking faults lying on the plane{10.0}. Analysis of phase images showed that these defects were nearly periodically ordered along the [10.0] direction. Based on these observations, a structural model is proposed for the WC grains with ordered stacking faults, which is in fact equivalent to a superstructure of WC with space group Amm2. When this model is introduced together with the normal WC structure (space group P 6 m2) into the Rietveld refinement, a much better agreement between the calculated and experimental XRD profiles is obtained. This study allowed obtaining the lattice parameters, grain size, microstrain and other structural information on the as-milled powders. Keyword: structural properties, mechanical milling, WC-Co 1 INTRODUCTION Nanocrystalline materials exhibit a crystallite size in the range of a few nanometers (typically 5-20 nm). It was reported that the mechanical properties of nanocrystalline WC- 10%Co cemented carbides are much better than conventional materials [1]. A lot of processes have been used for synthesizing nanocrystalline materials. Among them, high-energy mechanical milling was widely adopted because of its low cost, efficiency and simplicity of processing. In previous studies [2], nanocrystalline WC-10%Co powder mixtures were prepared by high-energy ball milling and studied by XRD and HRTEM. The lattice parameters, size and strain were analyzed by line profile analysis using the Halder-Wagner plot [3]. Recently, another study by Ungar et al [4] allowed determining the particle size, the size distribution and dislocation density in a nanocrystalline tungsten carbide using a modified Williamson-Hall and Warren-Averbach method. In both previously mentioned studies, peaks were fitted separately and the information on the relative intensities of each peak was not considered. In this work, Rietveld (whole-profile) analysis was tried for the refinement of the experimental XRD patterns [4]. It rapidly appears that the XRD whole profiles could not be satisfactorily fitted using only the modeled whole pattern of WC including line broadening. In fact, intensity residues, either by excess or defect, were systematically found close to the Bragg positions. These discrepancies between calculated and observed intensities indicate that the WC structure introduced in the refinement can not represent correctly the crystal structure of the material and that significant str
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