Effect of Inhomogeneous Microstructure on Strength and Fracture Resistance in Sharp Edge of Japanese Swords

Abstract

Microtensile and microfracture tests were performed on the sharp-edge regions of Japanese swords fabricated in the Muromachi and Showa periods, which are called old sword (OS) and modern sword (MS), respectively, to correlate the mechanical properties with the inhomogeneous microstructures. The hardness of the sharp-edge regions was characterised by the distribution of fine pearlite mixed in martensite microstructures. The OS containing a large fraction of fine pearlite exhibited a low hardness compared to the MS. Microtensile tests using sharp-edge specimens revealed a positive correlation between their tensile strength and strain-to-failure, as opposed to the common tendency in conventional carbon steels made by modern iron-making technology. The fracture surfaces of the sharp-edge specimens were composed of intergranular and dimple fracture features. The tensile strength and dimple fracture area fraction were higher in the OS than in the MS. These findings suggest that the fine pearlite microstructure contributes to increased strength in the sharp-edge region through inhibiting the linkage of intergranular cracking owing to local plastic deformation. Microfracture tests using the sharp-edge specimens revealed that the intrinsic fracture resistance of both OS and MS was determined by the intergranular fracture, whereas the fine pearlite microstructure increased the resistance to crack propagation. The micromechanical testing study indicates that in the sharp-edge regions, their strength and fracture toughness are simultaneously enhanced by the presence of fine pearlite although depending on its distribution.