Automated Inclusion Microanalysis in Steel by SEM 1087


Figure 8. Simulated backscattered electron image of five inclusions embedded in steel, for dwell time of 1 μs per pixel (left) and 32 μs per pixel (right); image size: 512×512 pixels, each pixel corresponds to 0.3×0.3 μm. The area of inclu- sions is from 2 μm2 to 4 μm2. The simulation was based on the ASPEX instrument, with a beam current 0.44nA (spot size 40%), accelerating voltage 10 kV, and the brightness difference between Fe and inclusion 100 (on the dimensionless 0–255 brightness scale).


where %M is the mass percentage of element M found by EDX analysis. Table 2 shows that fewer oxide inclusions were detected


if the threshold was low (false negatives happened). With higher thresholds far more features were detected (false positives). The choice of accelerating voltage also affects the inci-


Figure 9. Number of “false positive” features detected in simulated backscattered electron (BSE) images for (a) different pixel dwell times (fixed threshold of 160); (b) different thresholds (fixed dwell time of 8μs/pixel); features of 2 pixels or smaller filtered out; noise at different pixel dwell times was based on the ASPEX instrument: 0.44nA beam current (spot size 40%), 10 kV accelerating voltage, andBSE brightness setas190 forFeand 90 forinclusions(on the dimensionless 0–255 brightness scale).


of CaO–Al2O3–MgO (oxide) inclusions that were identified by EDX analysis among the potential inclusions (“detected features”) identified in the BSE images. Oxide inclusions were identified by applying a compositional filter after analyzing the detected features by EDX; the criterion was that %Fe<90%, %Mn<10%, and (%Ca+%Mg+%Al)>5%,


dence of false negatives: Figure 10 compares the measured (apparent) size distribution of inclusions (strictly speaking, the distribution of apparent diameters of inclusions on the polished sections), for analyses performed at 10 and 20 kV, all other instrument settings being equivalent. Evidently, the agreement is good for larger inclusions, but for apparent diameters of 1 µm or less, substantially fewer inclusions are detected at 20 kV (note that no user-defined morphology rule is applied in this case; the smallest features that could be detected are 0.2-µm wide, one pixel). The difference is not caused by differences in spatial resolution: for the conditions used here (40% spot size at 10 kV), the spatial resolution is approximately the same at 10 and 20kV (see Figs. 2 and 7b). Instead, the likely cause is the dependence of contrast on inclusion size and shape for analyses performed at 20 kV: as Figure 5 illustrates, at 20kV shallower (or smaller) inclusions are “washed out,” decreasing the contrast between inclusion and steel.


Table 2. Number of CaO–Al2O3–MgO Inclusions Detected, and Ratio of Detected CaO–Al2O3–MgO Inclusions To Total Detected Features for Automated Inclusion Microanalysis Repeated for Different Thresholds.


Threshold Total number of detected features Number of CaO–Al2O3–MgO inclusions detected Ratio of detected CaO–Al2O3–MgO inclusions to total detected features 45


214 169 0.79


65


529 421 0.80


90


739 577 0.78


120


1,048 694 0.66


CaO–Al2O3–MgO inclusions had an average brightness of 35 (Fe brightness 170, Al brightness 40). Instrument: ASPEX; accelerating voltage 20 kV; spot size 40%; total analyzed area 8.4mm2.


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