Automated Inclusion Microanalysis in Steel by SEM 1085
Table 1. Measured Stage Current and Beam Current, and Relative Backscattered Electron (BSE) Detector Gain for XL30 (Spot Size 5) and ASPEX (Spot Size 40%) Instruments.
Accelerating Voltage 10kV
Philips XL30 Stage current (nA) Beam current (nA)
ASPEX Explorer Stage current (nA) Beam current (nA)
Relative BSE detector gain (G)
1.26 2.4
0.23 0.44 1.0
20kV
1.83 2.94
1.17 1.75 2.0
(GIb)0.5 against the reciprocal of the square root of pixel dwell time; Figures 3c and 3d showthat the relationships for 10 and 20kV do coincide for each microscope—thedifferencein BSE image noise-to-signal ratios is indeed adequately explained by differences in beam current and detector efficiency. For improved BSE image spatial resolution (discussed
below) and smaller systematic errors in inclusion composition analysis, the lower accelerating voltage (10 kV) is preferred. The larger noise-to-signal ratio associated with the lower accelerating voltage could be remedied by using a longer pixel dwell time, or by using a larger BSE detector, or a BSE detector placed closer to the sample surface. For theASPEXinstrument (at spot size 40%), lowering the accelerating voltage from 20 kV to 10 kV would require an ~8 times longer pixel dwell time to achieve the same level of BSE image noise. The corre- sponding factor is ~4 for XL30, due to a smaller effect of vol- tage on beam current for that instrument.
BSE Images: Contrast
Detection of inclusions and distinguishing phases in complex (multiphase) inclusions also requires adequate BSE image contrast. BSE image contrastmainly arises fromdifferences in mass-averaged atomic number; see Figure 4 (the values in Fig. 4 were obtained by simulation because not all of these compounds are readily available in pure form). Figure 4 illus- trates that, for bulk phases, there would be substantial BSE image contrast between oxides, sulfides (such asCaS andMnS) and titanium nitrides. The accelerating voltage does not affect therelativeBSE imagebrightness of bulk phases. However, contrast between inclusions and the steel matrix
is affected not only by the difference in mass-averaged atomic number, but also by inclusion size and shape—because, for micron-sized inclusions, the interaction volume may extend beyond the edges of the inclusion into the surrounding steel. In such a case, some electrons would be backscattered from the steel even when the beam is incident on the inclusion— increasing image brightness, and reducing contrast. Such an effect is more severe at higher accelerating voltages, as illustrated by Figure 5: shallower (spherical-cap) inclusions would appear much brighter than deeper inclusions at an accelerating voltage of 20kV; in comparison, the effect of inclusion morphology on
Figure 4. Calculated backscattered electron (BSE) image bright- ness of different bulk phases, normalized such that Fe has a brightness of 170 and Al a brightness of 40 (on the dimensionless 0–255 brightness scale). Calculated from Monte Carlo simula- tions, performed with PENEPMA (Salvat et al., 2007), for 10kV and 20 kV. Originally published by AIST in the AISTech 2015 Proceedings (Tang & Pistorius, 2015).
Figure 5. Calculated backscattered electron (BSE) image line scans (simulated with PENEPMA) across spherical Ca4Al4MgO11–CaS inclusions with 1µm apparent diameter, for three different morpho- logies, and two accelerating voltages. The three morphologies are sun- ken spheres (center of sphere below the polishing plane by the dis- tance r/2; r is the inclusion radius), hemispheres (sphere centered on the polishing plane), and sphericalcaps(centerofinclusion abovethe polishing plane by the distance r/2). The horizontal broken line gives the BSE brightness of bulk oxide. Originally published by AIST in the AISTech 2015 Proceedings (Tang & Pistorius, 2015).
differences in brightness is smaller at 10kV. (In Fig. 5, it is predicted that the image would be brighter at the steel edges adjacent to the inclusion; the reason is that some BSE generated within the steel matrix escape through the neighboring less- dense inclusion, so avoiding absorption within the steel.) The achievable contrast between inclusions and sur-
rounding steel matrix also depends on the beamcurrent and BSE detector gain (and hence on accelerating voltage and spot size). Figure 6 shows the strong effect of beam current on achievable contrast (ASPEX instrument; 10 kV). The achievable contrast is approximately proportional to the beam current.
BSE Images: Spatial Resolution
The spatial resolution in BSE images limits the smallest feature that can be detected; the spatial resolution depends
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