Several homogenisation treatments were applied to direct chill (DC) cast ingots of aluminium alloy 6063, in order to analyse the resulting microstructures developed from these diverse conditions and their effects on the hot ductility of this alloy. Imaging was performed using scanning electron microscopy (SEM) and a focused ion beam (FIB) instrument. These techniques identified variations in distribution and morphology of second phase particles (AlFeSi and Mg 2 Si). FIB results for the various AlFeSi particles correctly identify their shapes in three dimensions (3D). The particles were identified by energy dispersive spectroscopy (EDS) in the SEM, and by X-ray diffraction (XRD) for bulk samples. Hot tensile testing (HTT) was conducted between 470 and 600uC to asses the hot ductility for each condition. The inferior ductility of as cast samples was due to the poor bond strength of the b AlFeSi phase at the grain boundaries. Homogenised samples, which contain a AlFeSi, exhibited improved ductility. Samples that were water quenched following homogenisation were absent of Mg 2 Si precipitates, when these elements remained in solid solution. These exhibited the highest ductility.
For years, there has been a strong desire to acquire reliable strain measurements from a material using electron backscatter diffraction (EBSD). The quality of the EBSD pattern (EBSP) has been identified as one possible indicator to provide this information. Local dislocations deteriorate the uniformity of the diffracted signal, and thus the EBSP contrast reflects the perfection of the crystal lattice. The pattern quality (PQ) at each point can be represented in a grayscale or color map; an example is presented in Figure 1, which shows recrystallized grains in a deformed matrix.In general, pattern quality (also referred to as image quality [IQ] or band contrast [BC]) is a measure of the intensity of the Kikuchi bands, or correspondingly, the heights of the detected peaks in the Hough transform. It is dependent upon many factors other than strain: beam conditions, geometrical set-up, material phases, video processing of the EBSP (including acquisition time), and parameters used for the Hough transform. All of these variables preclude the possibility of using PQ as an absolute measurement for different scans; however, in a given map, they will remain unchanged. Therefore, pattern quality can serve adequately as a qualitative descriptor of the relative strain distribution in a microstructure. This paper demonstrates that this concept must be used with caution, as the PQ also depends upon orientation, which is not constant within a mapped area (except for a highly textured material). PQ is a function of which Kikuchi bands appear in the pattern (different bands diffract with varying intensities) as well as their position (length fraction of the bands and their location relative to the intensity distribution of the EBSP signal). While this orientation dependence is known, it has not been widely reported, nor has any quantitative evaluation been found in the literature.A (100) silicon single crystal was rotated about its normal while recording EBSPs at regular intervals. Figure 2 presents a plot of PQ as a function of rotational angle. As shown, minimum and maximum values exist at 0 and 45 degrees respectively, and reoccur every 90 degrees thereafter. The results display a periodicity (frequency) consistent with the fourfold crystallographic symmetry.Additional experiments varied the number of Hough peaks used for indexing, to measure this parameter's influence. These results are shown in Figure 2. Since the Kikuchi bands are detected in order of decreasing intensity, when utilizing a greater number of Hough peaks (8 vs. 5), the average intensity, and hence PQ, necessarily decreases; however, the deviation (amplitude) also decreases. Figure 3 displays EBSPs selected from the extrema and inflection points. They display various Kikuchi bands, which rotate about the normal in accordance with the experiment. Due to symmetry, note the mirror images of B and D; the same bands are present in similar vertical positions resulting in comparable PQ values. In conclusion, this work proves that it is important to consider the ori...
Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.
Electron backscatter diffraction (EBSD) was employed to analyze the microstructure and texture of four different Alloy 82H gas tungsten arc welds that previously underwent static and dynamic modulus testing. The Young's moduli were shown to differ among the various welds, but also within each weld, dependent upon the direction measured. These differences were attributed to anisotropy of the crystallographic textures, which were described using inverse pole figures with respect to each of the weld's three orthogonal axes. The Young's modulus demonstrated a strong correlation with the texture, consistent with single crystal experiments. Sample directions containing a large population of {100} orientations had the lowest Young's modulus, while those with {111} grains possessed the highest. Microstructures with {110} textures were closer to the average modulus value of 207 GPa (30.0 Msi). X-ray diffraction texture measurements on four samples were used to verify the EBSD results.
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