XEOL and EPMA–CL for On-Line and On-Site Analysis of Inclusions in Steel 1147


Figure 4. a: Cathodoluminescence (CL) image of the model sample and (b) photograph of the model sample before bombard- ment with electrons using the portable CL spectrometer.


observed. It was proposed from the result of XEOL analysis on the same model sample that the particles producing green and blue luminescence were MgAl2O4 spinel and Al2O3, respectively. The number of particles producing green luminescence was larger than that of particles producing blue luminescence for the same reason as we discussed in the XEOL analysis. The illuminated area in Figure 4a corresponded to the particles observed in Figure 4b, although all the particles did not produce luminescent colors. This is because the beam current of the portable CL spectrometer, which is dozens of nanoamperes, was not sufficient for particles with smaller size to produce visible luminescent colors. These particles could be detected by increasing the beam current for the same reason as we dis- cussed in the XEOL analysis. We could obtain a larger beam current by optimizing the parameters of the portable CL spectrometer such as the heating temperature of the LiTaO3 crystal, its cooling rate, the distance between the crystal and the model sample, and the pressure inside the sample cham- ber. The number of particles that produced luminescent colors was, however, larger than that in the XEOL image. This is consistent with studies that show that a laboratory-scale CL spectrometer can produce as much luminescent intensity as a synchrotron X-ray beam (Dalby et al., 2010; Lobacheva et al., 2012). This result indicates that the portable CL spectrometer is more suitable for on-site analysis of nonmetallic inclusions in steel than an XEOL analyzer. We subsequently carried out elemental analysis of the


particles that produced luminescent colors using the portable EPMA. Figures 5a, 5b show EDX spectra obtained by bombardment with an electron beam in the vicinity of the particles producing blue (particle 1 in Fig. 4a) and green


Figure 5. Energy-dispersive X-ray spectra obtained by bombard- ment with electron beam in the vicinity of the particles producing (a) blue (particle 1 in Fig. 4a) and (b) green (particle 2 in Fig. 4a) lumi- nescence using the portable electron probe microanalyzer (EPMA).


(particle 2 in Fig. 4a), respectively. Cu L, Si K, and Cu K lines were detected in both spectra. The Si K line was attributed to the insulating materials coated on the Pt0.8Ir0.2 wire and the needle holder (Imashuku & Wagatsuma, 2017). The char- acteristic X-rays of copper (Cu L and Cu K) originated from copper in the model sample. In addition to the Cu L, Si K, and Cu K lines, only the Al K line was detected by electron beam bombardment near the particle producing blue lumi- nescence, while both Mg K and Al K lines were detected by electron beam bombardment near the particles producing green luminescence. These results support our observation in the XEOL analysis that the particles producing green and blue luminescence were MgAl2O4 spinel and Al2O3, respec- tively. The intensity of the Al K line in the Al2O3 particle producing blue luminescence (Fig. 5a) was ~10 times higher than that in the MgAl2O4 particle (Fig. 5b). This might be because the Al2O3 particle was larger than the MgAl2O4 particle or the electron beam was irradiated off-center of the MgAl2O4 particle.Nevertheless, these results suggest that the portable EPMA can distinguish MgAl2O4 spinel and Al2O3 particles with sizes of more than 20 μm. Portability is the key advantage of the portable EPMA–


CL analyzer over the conventional analysis such as EPMA and spark-induced atomic emission spectrometry, although the quantitative accuracy of the portable EPMA–CL analyzer is less than that of the conventional analysis. Thus, the portable EPMA–CL analyzer has potential in on-site analysis for screening nonmetallic inclusions in defective steel products before the conventional analysis. In addition, the portable EPMA–CL analyzer does not require knowledge of


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