1086 Dai Tang et al.
Figure 6. Measured effect of beam current on achievable differ- ence in brightness of backscattered electron (BSE) image between steel matrix and CaO–Al2O3–MgO inclusion (for maximum contrast setting); ASPEX at 10 kV.
on the accelerating voltage through its effect on the size of the interaction volume. The simulated line scans in Figure 5 show the broader transition area between the steel matrix brightness and inclusion brightness at the higher accelerating voltage—as expected from the larger interaction volume. Spatial resolution also depends on how narrowly the electron beam is focused. Beam diameter is affected by changing the “spot size” (which also directly affects beamcurrent and hence signal intensity, as discussed above). The spatial resolution was manually measured (via line scans through the centers of inclusions; see Fig. 7a) as the distance over which the brightness changed from 25 to 75% of the difference between an inclusion and the steel matrix. A disadvantage of performing resolution measurements on actual inclusions is the unknown shape and size of the inclusions below the polished surface, which would affect the resolution measurement somewhat (as illustrated by the simulations shown in Fig. 5). However, the advantage is that steel samples containing oxide inclusions are readily avail- able, and such measurements are directly applicable to the practical purpose of inclusion detection and analysis. The measured spatial resolution for different beam
currents is given in Figures 7b and 7c, for the two instru- ments at 10 and 20 kV. In general, the field-emission instrument has a better resolution than the tungsten fila- ment instrument as expected. For both instruments, spatial resolution is worse at the higher accelerating voltage (20 kV) (reflecting the larger interaction volume). At 20 kV, spatial resolution is nearly independent of the beam current. At 10kV, spatial resolution is better at smaller beam currents.
BSE Images: False Positives and Negatives in Inclusion Detection False positives represent erroneous detection of noninclu- sion features (Type I errors); false negatives occur when target inclusions are not detected (Type II errors). False
Figure 7. Effect of accelerating voltage and beam current (chan- ged by adjusting spot size) on spatial resolution for backscattered electron (BSE) imaging of CaO–Al2O3–MgO inclusions, at 10 and 20 kV. a: Schematic of the procedure to measure spatial resolution; (b) measured spatial resolution (ASPEX); (c) measured spatial resolution (XL30).
tial resolution) would influence the detection of inclusions through causing false positives or false negatives. This effect
positives would increase analysis time—since incorrectly detected “features” would be analyzed by EDX; false nega- tives would cause the true inclusion concentration to be under-analyzed. The quality of the BSE image (noise, contrast, and spa-
was simulated (using MATLAB), based on BSE images of five inclusions (with apparent sizes 2–4 μm2) exposed on the surface of steel (image size 512 ×512 pixels, pixel size 0.3 ×0.3 μm; see Fig. 8). For different dwell times, noise was added based on measurements for the ASPEX instrument (Fig. 3a). “Features” were detected when pixels were darker than the threshold (filtering out features of two pixels or smaller, as is typically done during actual analyses). The results show that dwell times smaller than 4 µs (Fig. 9a; threshold of 160) caused many false positives. Increasing the threshold level (as would be necessary to detect brighter inclusions such as sulfides) also increased false positives (Fig. 9b). Practical measurements demonstrated the effect of
false positives and false negatives: analyses were repeated on the same area of a single steel sample for different thresholds (Table 2), with everything else being equal (ASPEX instrument, 20kV accelerating voltage and spot size 40%; contrast difference between Fe and Al increased to 130 to allow more thresholds to be tested). Table 2 lists the number
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