Focused ion beam milling: A practical method for preparing cast Al-Si alloy samples for transmission electron microscopy
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S. SHANKAR, Research Scientist, Advanced Casting Research Center, Y.W. RIDDLE, Research Scientist, Advanced Casting Research Center, and M.M. MAKHLOUF, Associate Professor of Mechanical Engineering, and Director, Advanced Casting Research Center, are with the Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609. Contact e-mail: [email protected] Manuscript submitted July 3, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Schematic illustration of the FIB sample preparation technique. A desired microstructure on the initial electropolished surface of the sample is located and masked by a tungsten strip. The rest of the initial sample bulk around this desired location is milled out by a Ga ion beam resulting in an electron-transparent sample cross section.
Fig. 2—Microstructure of the Al-7 pct Si sample showing the location (white rectangular box) where the sample for TEM work was produced by FIB milling. The white rectangular box is illustrated in the adjoining schematic.
TEM sample preparation commences. This saves considerable time compared to the “hit-or-miss” approach of conventional TEM sample preparation techniques. Figures 2 through 5 illustrate the high quality of cast AlSi alloy samples produced via the FIB milling technique. Figures 2 and 5 are SEM images of Al-7 wt pct Si and Al7 wt pct Si-0.022 wt pct Sr, showing the locations in the microstructure where samples for TEM analysis were prepared by FIB milling. The TEM samples were prepared such that both an aluminum dendrite and a eutectic region were visible in the same sample. Figures 3(a) and (b) are lowmagnification bright-field TEM and high-magnification secondary electron SEM images of the Al-7 wt pct Si sample, respectively. Figure 3 compares the TEM and SEM images of the desired location shown in Figure 2. The incident electron beam is not aligned to a specific zone axis in the sample. Boundaries not visible in the SEM are apparent in the TEM. Figure 4 shows higher magnification TEM images at various silicon and aluminum boundaries. Figure 4 also shows [100]Al convergent beam electron diffraction patterns (CBED) from the two primary aluminum dendrites surrounding the eutectic phase as well as the Al in the eutectic region. This demonstrates FIB as a viable method for creating TEM samples from precisely selected locations. Furthermore, the quality of these samples is sufficient to allow conventional TEM analysis. Figure 6 shows high-magnification secondary electron SEM images of the FIB milled desired locations in the cast
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Fig. 5—Microstructure of the Al-7 pct Si-0.02 pct Sr sample showing the location (white rectangular box) where the sample for TEM work was produced by FIB milling. The schematics illustrate the microstructure in the white box.
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Fig. 3—Comparison of TEM and SEM micrographs of Al-7 pct Si at the desired sample location: (a) low-magnification TEM image of the Al-7 pct Si sample and (b) high-magnification SEM image of the Al-7 pct Si sample.
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