京都大学 · 材料科学
Kazuho Okada教授の研究室では、低炭素マルテンサイト鋼における水素破壊のメカニズムを、微細組織と結晶学的要因の観点から解明しています。特に、炭素セグレゲーションによる前処理状態の界面強化が水素脆化に与える影響や、水素存在下での微小な破壊メカニズム（例：準クリープ破壊、クリープ亀裂の始発機構）を、電子顕微鏡、X線回折、EBSDなどの先端分析技術を用いて詳細に解析しています。研究の柱は、高強度鋼の信頼性向上に資する微視的破壊挙動の解明です。
Figures are computed from collected data and may differ slightly.
This study challenged to improve the resistance against hydrogen embrittlement by increasing the concentration of carbon segregated at prior austenite grain boundary (PAGB), XPAGB, in low-carbon martensitic steels. The specimens with/without carbon segregation treatment (Non-seg and Seg specimens, respectively) had almost the same microstructure, other than higher XPAGB in the Seg specimen. While the uncharged Non-seg and Seg specimens exhibited similar mechanical properties, the maximum stress
In this study, the deformation microstructure of hydrogen-charged ferritic-pearlitic 2Mn-0.1C steel was characterized using SEM-BSE, SEM-EBSD, TEM, and neutron diffraction. The microscopic mechanism of hydrogen-related quasi-cleavage fracture along the {011} planes was also discussed. It was found that hydrogen increased the relative velocity of screw dislocations to edge dislocations, leading to a tangled dislocation morphology, even at the initial stage of deformation (e = 3%). In addition, th
The present paper investigated characteristics of fatigue fracture behavior, particularly initiation stage of fatigue fracture (Stage I), in an as-quenched martensitic steel from microstructural and crystallographic points of view. The detailed crystallographic orientation analysis using EBSD revealed that block boundaries in lath martensite structure were the most preferential initiation sites for fatigue cracks. We found that incompatibility of plastic strains between adjacent blocks was the o
The present paper investigated the characteristics of serrated markings on the hydrogen-related quasi-cleavage fracture in an as quenched low-carbon martensitic steel. The serrated markings corresponded to the ridges between {011} micro-facets. The ridges were formed along lath/block/packet boundaries or nearly parallel to 〈211〉 or 〈011〉 directions within the lath. In addition, the angle between {011} micro-facet and tensile axis was the largest of the six crystallographically equivalent {011} p
The present study aimed at strengthening prior austenite grain boundary (PAGB) cohesive energy using carbon segregation and investigated the effect of carbon segregation at PAGB on the microscopic crack propagation behavior of hydrogen-related intergranular fractures in high-strength martensitic steels. At the low hydrogen content (below 0.2 wt. ppm), the fracture initiation toughness ( J IC ) and tearing modulus ( T R ), corresponding to crack growth resistance, were significantly improved by c
A typical hydrogen-related transgranular fracture, namely quasi-cleavage fracture, is usually accompanied by serrated markings on the resultant fracture surfaces in steels with body-centered cubic phases. The present paper investigated the microscopic three-dimensional morphology and crystallographic feature of serrated markings in a 2Mn-0.1C steel mainly composed of ferrite microstructure. The serrated markings corresponded to the corners of the step-like morphologies which consisted of microsc
To achieve widespread application of high-strength steels, such as martensitic steels, understanding the origin of the trade-off relationship between tensile strength and hydrogen embrittlement resistance is crucial. This study aimed to clarify the origin of the trade-off from a perspective of sensitivity to the hydrogen-induced intergranular fracture, which mainly occurs along prior austenite grain boundaries (PAGBs) among several types of high-angle boundaries in martensitic structures. Applyi
Achieving superior fracture resistance under cyclic loading-specifically, a high-fatigue limit-is crucial for ensuring structural safety and supporting a sustainable society. This study demonstrates a breakthrough in overcoming the conventional fatigue limit ceiling in high-strength as-quenched martensitic steel by enhancing resistance to crack initiation. In the as-heat-treated state, high-angle boundaries with large elastic misfits and plastic incompatibility served as precursory sites for int
A typical hydrogen-related transgranular fracture, namely quasi-cleavage fracture, is usually accompanied by serrated markings on the resultant fracture surfaces in steels with body-centered cubic phases. The present paper investigated the microscopic three-dimensional morphology and crystallographic feature of serrated markings in a 2Mn-0.1C steel mainly composed of ferrite microstructure. The serrated markings corresponded to the corners of the step-like morphologies which consisted of microsc
The fatigue limit of steel generally increases with tensile strength; however, when tensile strength exceeds ~1.4 GPa, further increases in tensile strength fail to elevate the fatigue limit or may even decrease it (fatigue limit ceiling). This ceiling poses a challenge for the widespread application of ultrahigh-strength steels. In this study, we successfully achieved a significant improvement in the fatigue limit through pre-fatigue deformation, thereby breaking through the fatigue limit ceili
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