MS-4-O-3057 Imaging of Li and He distribution inside steel using low-loss EELS

Low-loss electron energy loss spectroscopy (EELS) was used to detect and study the spatial distribution of He and Li in boron-alloyed steel after neutron irradiation. Both products of the ¹⁰B(n, α)⁷Li nuclear reaction have a low solubility in the steel Fr/Cr matrix. As a consequence, they form nano-sized precipitates or bubbles, which influence the steel’s mechanical properties. Their detailed characterization is an important step towards understanding microstructural changes in steels caused by both products of the transmutation reaction.

Previous TEM studies of He detection and distribution revealed that the He 1s-2p line from the He filled bubble is clearly detectable on the plasmon matrix signal [1], whereas the detection of metallic Li precipitates in metals was not described in the literature. We present the new method for the direct detection of Li and He in the Fe/Cr matrix based on the analysis of EELS spectra in the low-loss range. Calculation of the Fe/Cr plasmon structure using the Gaussian function or, in some complex cases, linear extrapolation and separation of the Li plasmon (9eV) and He 1s-2p line (22.4eV) allow for the generation of elemental maps showing the spatial distribution of Li and He (Figs. 2-3).

Fig. 1a shows a plasmon spectrum of the Fe/Cr matrix. Fig. 1b presents the spectrum of a bubble containing both Li and He lines. The intense He line at 21.7eV and the same line after background subtraction (insert) are visible in Fig. 1b. Left from the He line, an intense Li plasmon peak at 12eV (9.5eV - 10.5eV after subtraction of the Fe plasmon) can be observed clearly.

In Fig. 2 a Li drop of 22 nm in size is located in a 50 nm large He-filled bubble. The Li fills approximately ¼ of the bubble volume. This is clearly visible in (b) the He map and (c) the Li map. The map obtained by the 2.5eV window positioned at 7.5 eV shows the increased plasmon intensity on the drop’s surface (part d), whereas the map obtained at 11eV (part e) reveals nearly the same spatial distribution as the entire Li plasmon shown in part (c).

Fig. 3 presents an area with numerous bubbles that appear dark in the HAADF image. Several bubbles do not contain He, because they are located on the specimen surface – for example, the large bubble at the bottom right of the scanned area. The observation of numerous bubbles with Li drops or Li/He-filled cavities shows that the width of the Li plasmon peak is variable.

The spatial distribution of Li inside the Fe matrix on the nano-scale level was detected and investigated by means of EELS plasmon spectra. Some bubbles are half filled with Li and half with He.

[1] S. Fréchard, et.al J. Nucl. Mater. 393 (2009) 102

[2] M. Klimenkov, et.al Micron 46 (2013) 51–56