Showing papers by "Kaneaki Tsuzaki published in 2021"

Microstructure Refinement by Low-Temperature Ausforming in an Fe-Based Metastable High-Entropy Alloy

Abstract: The effects of ausforming in an Fe30Mn10Cr10Co high-entropy alloy on the microstructure, hardness, and plastic anisotropy were investigated. The alloy showed a dual-phase microstructure consisting of face-centered cubic (FCC) austenite and hexagonal close-packed (HCP) martensite in the as-solution-treated condition, and the finish temperature for the reverse transformation was below 200 °C. Therefore, low-temperature ausforming at 200 °C was achieved, which resulted in microstructure refinement and significantly increased the hardness. Furthermore, plasticity anisotropy, a common problem in HCP structures, was suppressed by the ausforming treatment. This, in turn, reduced the scatter of the hardness.

Roles of Hydrogen and Plastic Strain Distribution on Delayed Crack Growth in Single-crystalline Fe–Si alloy

Abstract: Effects of hydrogen on macroscopic and microscopic features of crack growth in thin specimens were investigated using a thin sheet of single-crystal Fe-3wt%Si alloy. Center-cracked specimens were tested under a sustained load in a hydrogen environment, and under continuous stretching in an air environment. The fracture features were compared to elucidate the role of hydrogen in hydrogen-induced delayed crack growth. In both air and hydrogen environments, the crack growth mode of the thin specimens was the same as that of the thick specimens, despite the significantly reduced thickness. Surprisingly, the crack grew discontinuously, and left striations on the fracture surface, in which shorter striation spacing was observed in hydrogen. In addition, there was a similarity in the deformation microstructures beneath the fracture surface, that is, both microstructures were composed of three distinct layers characterized by different plastic strain gradients and dislocation densities. In the hydrogen environment, the hydrogen-enhanced localized plasticity (HELP) mechanism is believed to be relevant to the crack growth process. Also, HELP was supposed to cause different characteristics (the magnitude of plastic strain, the plastic strain gradient, and dislocation structure) of the three layers in the hydrogen compared to those in the air. Reverse plastic deformation occurred in the regions behind the crack front during crack growth, which is speculated to contribute not only to enlarge the crack tip opening angle (CTOA) but also to blunt the crack tip.

Potential Effects of Short-Range Order on Hydrogen Embrittlement of Stable Austenitic Steels—A Review

Abstract: Here, we present a review of the hydrogen embrittlement behavior of face-centered cubic (FCC) alloys with short-range order (SRO) of solute atoms. In this paper, three types of FCC alloys are introduced: Fe–Mn–C austenitic steels, high-nitrogen steels, and CoCrFeMnNi high-entropy alloys. The Fe–Mn–C austenitic steels show dynamic strain aging associated with Mn–C SRO, which causes deformation localization and acceleration of premature fracture even without hydrogen effects. The disadvantageous effect of dynamic strain aging on ductility, which is associated with the deformation localization, amplify plasticity-assisted hydrogen embrittlement. Cr–N and Co–Cr–Ni SRO effects in high-nitrogen austenitic steels and high-entropy alloys enhance the dislocation planarity, which causes stress concentration in the grain interior and near the grain boundaries. The stress concentration coupled with hydrogen effects causes quasi-cleavage and intergranular fractures.

Microscopic examination of striation spacing during ductile crack growth in Fe-3wt%Si single-crystalline thin plates in air and hydrogen

Abstract: Microscopic features of crack growth in thin plates of single-crystalline Fe-3wt%Si alloy in both air and hydrogen environments were investigated by electron channeling contrast imaging (ECCI), electron backscattering diffraction (EBSD), as well as scanning electron microscopy fractography. The goal was to elucidate the discontinuous crack growth as well as the constant unit distance of crack extension (striation spacing). Center-cracked specimens were tested under a sustained load in a hydrogen environment, while they were under continuous stretching in the air environment. The following results were obtained. (1) Striation is formed by extensive slips emitted from the crack tip, mainly contributed from specific ( 1 1 ‾ 2 )[ 1 ‾ 11 ] and ( 1 ‾ 12 )[ 1 ‾ 1 1 ‾ ] slip systems. The discontinuous crack growth is mainly caused by interaction of the crack and ( 1 1 ‾ 2 )[ 1 ‾ 11 ] and ( 1 ‾ 12 )[ 1 ‾ 1 1 ‾ ] slip bands/cell walls formed ahead of the crack tip. These slip bands/cells show that the spacing between slip bands/cells is constant and independent of the crack length. Hence, the striation spacing is the same as that of the slip bands/cells ahead of the crack tip. (2) Hydrogen may affect the slip behavior by reducing the spacing between slip bands/cells ahead of the crack tip compared to that in the air environment.