Nondestructive analysis of internal crystallographic structures of Japanese swords using neutron imaging

Crystallographic information and the internal structure of Japanese swords are crucial for understanding their metallurgical characteristics and exploring their making process. By accumulating such information from various swords made in different regions, eras and by different swordsmiths, we believe it will greatly aid in the comprehensive understanding of Japanese swords. We have been investigating Japanese swords using nondestructive analysis methods of neutron Bragg-edge transmission (BET) imaging and neutron tomography. BET provides insights into the crystalline structure of the steel, while the neutron tomography offers cross-sectional views of the sword. In this study, we examined three swords: Kashu Kiyomitsu, Nankaitaro Tomotaka, and Hosokawa Masanori, crafted between the Edo era (ca. 1603–1850) and the Meiji era (1868–1912).

Kashu Kiyomitsu, from the early Edo era, exhibited a complex internal structure, with crystallite size of 0.5 µm or less around the cutting tip. Furthermore, other regions with crystallite sizes of 1 µm or larger were observed, extending from the middle area of the blade to the vicinity of the tang. The cross-sectional features of Kashu Kiyomitsu suggest that the sword has a complex structure probably combining low-carbon steel Kawagane and Shingane with a dedicated high-carbon steel for the cutting edge, Hagane. The martensite phase, indicating the quenched area, extended evenly over approximately 4 mm. In contrast, both Nankaitaro Tomotaka, from the late Edo era, and Hosokawa Masanori, from the Meiji era, exhibited relatively uniform crystallographic structures throughout the entire blade. In Nankaitaro Tomotaka, the martensite phase extended over a width of 4–6 mm from the cutting edge, confirming the arch-like boundary between martensite and ferrite/pearlite within the blade. In Hosokawa Masanori, the martensite phase extended over approximately 8–10 mm from the cutting edge, with a wavy pattern along the longitudinal direction. These observations suggest that Nankaitaro Tomotaka and Hosokawa Masanori were made using simpler sword-making techniques compared to Kashu Kiyomitsu.