Dislocation Configurations in Nanocrystalline FeMo Sintered Components
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ONE of the challenges in metal powder sintering technology is how to transfer the peculiar properties of nanocrystalline materials to compacted components. This has become of special interest in recent years, in which extensive research has been devoted to the production of nanocrystalline, heavily deformed metal powders and their compaction by advanced sintering techniques.[1–4] The best processing conditions—highest (i.e. theoretical) density in the sintered component, while preserving as much as possible the fine grain size and high lattice defect density of the starting nanopowder—can hardly be achieved by conventional sintering.[2,5] Within the large family of field-assisted sintering techniques (FAST) or electric current activated sintering (ECAS), spark plasma sintering (SPS) has been studied in some detail.[6] Despite the good results reported in the literature,[7,8] sintering time is still on the order of minutes and the process is controlled in temperature, with an electronic feedback, thus allowing for a certain degree of thermal equilibrium to be reached. Despite undisputed improvements with respect to traditional techniques, the relatively high sintering temperatures required to reach high densities tend to anneal the microstructure, thus losing at least part of the properties of the starting powders.[1,2] Although suitable additions may be used to stabilize the nanostructure and to reduce the damaging effect of high temperature/long process PAOLO SCARDI, Professor, and MIRCO D’INCAU and MATTEO LEONI, Doctors, are with the Department of Materials Engineering and Industrial Technologies, University of Trento, 38100 Trento, Italy. Contact e-mail: [email protected] ALESSANDRO FAIS, Doctor, is with the Department of Materials Science and Chemical Engineering, Turin Polytechnic, 10129 Torino, Italy. This article is based on a presentation given in the symposium ‘‘Neutron and X-Ray Studies of Advanced Materials,’’ which occurred February 15–19, 2009, during the TMS Annual Meeting in San Francisco, CA, under the auspices of TMS, TMS Structural Materials Division, TMS/ASM Mechanical Behavior of Materials Committee, TMS: Advanced Characterization, Testing, and Simulation Committee, and TMS: Titanium Committee. Article published online September 15, 2009 1196—VOLUME 41A, MAY 2010
time,[9] substantial improvements should probably involve a different approach. Key factors such as rapidity and effectiveness of the sintering process seem to be peculiar to another family of FASTs, known as single-pulse techniques, or electrodischarge sintering (EDS). Electrodischarge sintering requires a single, short pulse of electromagnetic energy to sinter the powder compacts while applying mechanical pressure.[10] A recently developed EDS technique, named capacitor discharge sintering (CDS), is based on a single pulse of low voltage, high current electromagnetic energy, delivered in a short time (tens of milliseconds) to powders in a conducting mold previously loaded with mechanical pressure. Capacitor discharge sinterin
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