Comparative microstructural characterization of a friction-stir-welded aluminum alloy using TEM and SEM-based techniques
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I. INTRODUCTION
UNTIL recently, the transmission electron microscope was the first (and only) choice of instrument for characterizing deformation structures.[1,2,3] For many years, on account of its limited spatial resolution, the scanning electron microscope could hardly compete with the transmission electron microscope when detailed images of dislocation boundary structures were required. Recent advances in instrumental development, such as the appearance of field-emission gun (FEG) scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) techniques have resulted in a much more widespread use of SEM for microstructural characterization of deformed materials, nearly replacing traditional transmission electron microscopy (TEM) in many research laboratories. The advantages of SEM-based techniques over TEM are obvious both in terms of specimen preparation and image collection, but the SEM-based diffraction techniques still suffer from a lower angular resolution. Because very low misorientations cannot be distinguished from the orientation noise associated with the inaccuracy of orientation indexing,[4,5,6] a certain population of dislocation boundaries (usually those 1.5 to 2 deg) are ignored in EBSD analysis, which inevitably results in overestimated cell-size data and a wrong assessment of the proportions of boundaries within a given misorientation-angle range. Data postprocessing by use of a modified Kuwahara filtering procedure has been applied to improve the angular resolution of the EBSD technique. It has been reported that such a filtering procedure can improve the angular resolution to 0.5 deg.[7] It should be noted that most investigations of the effect of orientation filtering have been carried out either O.V. MISHIN, Research Engineer, and L. ÖSTENSSON, Engineer, are with Sapa Technology, SE-61281 Finspång, Sweden. A. GODFREY, Professor, is with the Department of Materials Science and Engineering, Tsinghua University, 100084 Beijing, P.R. China. Contact e-mail: [email protected] Manuscript submitted May 5, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
on artificially constructed maps of idealized microstructures, or based on a necessarily qualitative visual assessment of cells in EBSD maps obtained from real microstructures. There is an obvious lack of experimental data where one region is analyzed using a variety of different techniques. In this work, we present images collected from one region using four different TEM- and SEM-based techniques. The latter includes SEM images obtained using conventional orientation contrast mechanisms, gallium-enhanced microscopy (GEM) developed for Al alloys,[8,9,10] and EBSD maps, both in the form available from commercial software and postprocessed using a Kuwahara filter. A sample processed using friction-stir welding (FSW) was used in the present study because this industrial process generates structures with a large variety of misorientations.[11,12] Interestingly, quite different conclusions on the type and origin of the microstructur
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