1088 Dai Tang et al.


Figure 10. Comparison of number of inclusions found with ana- lyses performed at 10 and 20 kV, illustrating that some inclusions (1 µm and smaller) are not detected at 20kV (ASPEX instrument).


Inclusion EDX: Signal Intensity at 10kV


For bulk materials, the intensity (or count rate) of char- acteristic X-rays generated in the specimen increases with the increase of beam current and of overvoltage (the ratio of incident beam energy to the excitation energy) (Goldstein et al., 2003). Both effects suggest that the X-ray count rate would inevitably be lower at 10kV than at 20 kV. However, this is not what is found for micro-inclusions, as illustrated by Figure 11. For lighter elements (such as Mg and Al), the count rate was similar at 10 and 20kV (with the same spot size and live time). This effect persisted for different spot sizes (beam currents): Figure 12 shows that for bulk oxides (CaO and Al2O3), the count rates were higher at 20kV than at 10kV for both Ca and Al Kα peaks (the figure also shows the expected linear increase in X-ray count rates with the increase in beam current). When analyzing an inclusion in steel the Ca Kα count rate was also higher at 20kV than at 10kV (and the same beam current); however, for Al Kα, the count rate at 20kV was lower than 10 kV. This effect is attributed to the difference between the


interaction volume and inclusion size. When the inclusion depth is much larger than the interaction volume, the effect of accelerating voltage would be similar to that for bulk specimens: Al and Ca characteristic X-rays are generated and propagated within the inclusion itself so the surrounding steel matrix would not have an effect. Smaller inclusions, whose depth is comparable to or smaller than that of the interaction volume, would not fully occupy the interaction volume; the interaction volume would extend into the sur- rounding steel matrix. The generated X-rays would inevi- tably be partly absorbed by the surrounding steel matrix. Increasing the accelerating voltage from 10 to 20kV leads to a threefold increase in the depth of the interaction volume, and hence incident electrons would be more likely to gen- erate X-rays from the steel rather than micron-sized inclu- sions. X-ray absorption by the surrounding matrix would also be more severe at the higher voltage. These effects were confirmed by PENEPMA simula-


tions, which were used to calculate X-ray yield (specifically, the ratio of X-ray yield at 20kV to that at 10 kV) for inclusions with different shapes and depths (see results in Fig. 13a). The inclusion shapes were the same as those considered in the simulations of Figure 5: spherical caps,


Figure 11. Comparison of total counts (after 30 s live time) for different characteristic X-rays, for 10 and 20kV electron beams (spot size 5, Schottky field-emission XL30 instrument) incident on a calcium aluminate inclusion (with CaS shell) in steel. The Al and Mg Kα counts are similar for both cases, despite the smaller beam current at 10 kV.


Figure 12. Measured effect of accelerating voltage and beam current on X-ray count rates for bulk oxides and an CaO–Al2O3– MgO inclusion (1.5 µm apparent diameter) in steel. Al Kα count rates: (a) bulk Al2O3;(b) inclusion. Ca Kα count rates: (c) bulk CaO; (d) inclusion. Each point is labeled with the corresponding spot size.


Figure 13. Simulated (a) and measured (b) characteristic X-ray yield ratio (count rate at 20kV divided by that at 10 kV) for Al Kα and Ca Kα from inclusions with different depths, in a steel matrix. Simulations were conducted by PENEPMA. Horizontal broken lines on the right give the ratios for bulk CaO and bulk Al2O3.


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