3D Elemental Mapping of Non‐metallic Inclusions in Japanese Sword with FIB ‐ SEM / EDS System

Abstract

Japanese swords are made of raw steel produced by smelting iron sand. The raw steel made by the Tatara method contains less phosphorus and sulphur, and higher concentrations of non‐metallic inclusions than modern steel. By analyzing non‐metallic inclusions the source of the iron sand and the heat process used during processing Japanese swords have been investigated [1,2]. The purpose of this study is to reveal the 3D distribution of non‐metallic inclusions to understand the solidification process. The 3D distribution was observed with a Focused Ion Beam ‐ Scanning Electron Microscope (FIB‐SEM). The sample was repeatedly sliced with an FIB‐SEM to expose a new surface which was then automatically analyzed using EDS. The 3D distribution was reconstructed from the acquired EDS data to analyze solidification processes of the non‐metallic inclusions in the edge. There are structures in which aluminum and calcium‐rich areas are wrapped in silicon‐rich areas [3]. In this study, structures of non‐metallic inclusions in back side of a Japanese sword were analyzed. The sample was a Japanese sword with the signature of Bizen Osafune Katsumitsu (property of M. Kitada). It was made in Japan in the 16th century. The 3D distribution of non‐metallic inclusions was determined using a JIB‐4610F (FIB‐SEM, JEOL Ltd.) and an EDS (by Thermo Fisher Scientific). The SEM conditions were as follows; accelerating voltage: 10 kV, probe current: 10 nA, whereas the FIB processing conditions; accelerating voltage: 30 kV, probe current: 10 nA, and ion dose: 100 nC/μm 2 . Cross‐sectional macrograph of the sword after etching is shown in Figure 1 a). The secondary electron image (SEI) of the analysis area indicated by the red square in Figure 1 a) is shown in Figure 1 b). Figure 1 c) is an example of backscattered electron image (BEI) of the non‐metallic inclusion (red circle of Figure 1 b)) analyzed with the FIB‐SEM / EDS. An image reconstructed from BEIs is shown in Figure 2 a). 3D reconstructed images of the non‐metallic inclusions and interspace in the non‐metallic inclusions extracted by volume rendering are shown in Figures 2 b) and c), respectively. The size of the analyzed volume was about X: 15μm, Y: 44 μm, and Z: 15 μm. The slice pitch was 100 nm. Each analysis took 20.5 minutes. BEI and EDS elemental maps of oxygen, aluminum, silicon, calcium, titanium, and iron obtained from the 90th slice are shown in Figure 3 a). A superimposed 3D elemental map of oxygen (green) and iron (black) is shown in Figure 3 b). Therefore, grains in the non‐metallic inclusion are iron oxide. Figure 3 c) is a superimposed 3D elemental map of aluminum (green), silicon (yellow), calcium (cyan) and titanium (magenta). The elemental distribution in the non‐metallic inclusions was clearly observed three dimensionally as shown in Fig.3 c). Elements excepting Fe and O described above in the non‐metallic inclusion are unevenly distributed as shown in Fig. 3 c). That is, many oxides containing Al, Si, Ca and Ti are distributed around iron oxide grains. And interspace exists between the oxides. The back side of a Japanese sword is comprised of core steel. Unlike the blade known as edge steel, core steel is not heavily forged. Therefore, the non‐metallic inclusions in the back side of the Japanese sword keeps the constitution of raw materials in some degree. And the shape of the non‐metallic inclusion also keeps and remains the same as the original material. Interspaced distances between inclusions also remain the same. As to the distribution of the elements, many iron oxides of several micrometers were encapsulated by the non‐metallic inclusions comprised of Al, Si, Ca and Ti oxides. The titanium distribution in the non‐metallic inclusions suggests that the source of the iron sand contained an iron titanate; for example, ilumenite (FeTiO3).