||Osamu TAKAKUWA, Yuhei OGAWA, Ryunosuke MIYATA, Antagonistic fatigue crack acceleration/deceleration phenomena in Ni-based superalloy 718 under hydrogen-supply, Scientific Reports, 13, 6804, 2023.06, Mechanical properties of structural alloys, including Ni-based superalloy 718 (Alloy718), are degraded when hydrogen (H) is supplied: hydrogen embrittlement (HE). The presence of H notably deteriorates fatigue crack growth (FCG) property, which renders the growth rate much higher and shortens the lifetime of the components operating in the hydrogenating environment. Hence, the mechanisms behind such acceleration phenomenon in FCG should be understood comprehensively toward developing promising alloys resistant to hydrogen occlusion. In particular, Alloy718 has a meager resistance to HE, even regularly displaying superior mechanical and physical performances. Notwithstanding, the present study unveiled that the FCG acceleration by dissolved H in Alloy718 can be negligible. An abnormal deceleration of FCG can instead be pronounced by optimizing the metallurgical state, a hopeful prospect in Ni-based alloys applied to the hydrogenating environment..
||Yuhei OGAWA, Osamu TAKAKUWA, Kaneaki TSUZAKI, Solid-solution hardening by hydrogen in Fe-Cr-Ni-based austenitic steel: temperature and strain effects, Materials Science and Engineering A, 879, 145281, 2023.07.
||Yuhei OGAWA, Kohei Noguchi, Osamu TAKAKUWA, Criteria for hydrogen-assisted crack initiation in Ni-based superalloy 718, Acta Materialia, 229, 2022.05, The tensile mechanical properties of a Ni-based superalloy 718 uniformly precharged with ≈ 90 mass ppm hydrogen were investigated under a wide range of temperatures to shed light on the long-standing uncertainties surrounding the H-related embrittlement mechanisms of the material. The detrimental effect of H on ductility was found to be substantial in the near-ambient to high-temperature range, up to 300 °C, stemming from H-assisted microcrack initiations along annealing twin boundaries (ATBs) and crystallographic slip planes (SPs). Dynamic H-dislocation interaction, which has been thought to be a prerequisite for the onset of embrittlement, was found to be unimportant, as demonstrated by employing supplemental tests that incorporated prestraining and intermediate temperature changes. By combining the insights gained from the successfully designed test program, a new model for the nucleation process of H-induced fracturing was established..
||Yuhei Ogawa, Osamu Takakuwa, Saburo Okazaki, Yusuke Funakoshi, Saburo Matsuoka, Hisao Matsunaga, Hydrogen-assisted fatigue crack-propagation in a Ni-based superalloy 718, revealed via crack-path crystallography and deformation microstructures, Corrosion Science, 10.1016/j.corsci.2020.108814, 174, 2020.09, Fatigue crack-growth (FCG) of Ni-based superalloy 718 was investigated under gaseous hydrogen environment (external hydrogen) and uniformly pre-charged state (internal hydrogen). Under external hydrogen, intergranular fracture predominated, whereas dislocation slip-band or twin boundary fracture were prevalent under internal hydrogen. This failure mode divergence encompassed unique characteristics of macroscale FCG response, leading to both cycle- and time-dependent cracking. The intergranular cracking was ascribed to short-circuit diffusion of hydrogen along grain boundaries. Meanwhile, the material's inherently inhomogeneous deformation mode exerts harmfulness when hydrogen was uniformly distributed inside the specimen, causing slip-bands or twin boundaries to become the weakest links for fracture..
||Osamu Takakuwa, Yuhei Ogawa, Saburo Okazaki, Masami Nakamura, Hisao Matsunaga, A mechanism behind hydrogen-assisted fatigue crack growth in ferrite-pearlite steel focusing on its behavior in gaseous environment at elevated temperature, Corrosion Science, 10.1016/j.corsci.2020.108558, 168, 2020.05, Hydrogen-assisted fatigue crack growth in gaseous environment was comparatively examined at room temperature (RT) and 423 K, based on analysis of the deformation structure evolution around crack-wakes using scanning electron microscopy techniques. In hydrogen-gas at RT, the propagating crack displayed weakly-evolved dislocation arrangement, accompanied by a significant acceleration of fatigue crack growth. However, in hydrogen-gas at 423 K, the crack-wake plasticity was well-evolved and analogous to that observed in an inert environment. This apparent recovery of deformation micro structure coincided with suppressed crack growth acceleration, the rationale for which can be interpreted by the trapping/de-trapping equilibrium between hydrogen and dislocations..
||Y. Ogawa, O. Takakuwa, Saburo Okazaki, Koichi Okita, Yusuke Funakoshi, H. Matsunaga, Saburo Matsuoka, Pronounced transition of crack initiation and propagation modes in the hydrogen-related failure of a Ni-based superalloy 718 under internal and external hydrogen conditions, Corrosion Science, 10.1016/j.corsci.2019.108186, 2019.01, The role of hydrogen in tensile ductility loss and on the fracture behaviours of Ni-based superalloy 718 was investigated via tensile tests under hydrogen-charged conditions (internal hydrogen) or in gaseous hydrogen environments (external hydrogen), in combination with post-mortem analyses of fractured samples using electron microscopy techniques. Whereas intergranular fracture was responsible for material degradation under external hydrogen, the failure modes under internal hydrogen conditions were primarily dominated by cracking along slip planes or twin boundaries. The mechanisms of crack initiation and propagation are extensively discussed in terms of hydrogen distribution, intrinsic deformation character of the material and hydrogen-modified dislocation behavior..
||Osamu Takakuwa, Yuhei Ogawa, Junichiro Yamabe, Hisao Matsunaga, Hydrogen-induced ductility loss of precipitation-strengthened Fe-Ni-Cr-based superalloy, Materials Science and Engineering A, 10.1016/j.msea.2018.10.040, 739, 335-342, 2019.01, A brittle-like faceted morphology of a precipitation-strengthened Fe-Ni-Cr-based superalloy after charging via exposure to high-pressure hydrogen gas (100 MPa) at elevated temperature (543 K) was interpreted based on multiple electron microscopy observations: scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) imaging. The observation results revealed that the brittle-like facets were derived from intergranular cracking accompanied by hydrogen-assisted microvoid nucleation at the grain boundaries (GBs). Deformation twinning also played a crucial role in triggering the final grain boundary separation due to local stress concentration at its intersection with the GBs after severe strain hardening; such a process has not yet been considered to explain the hydrogen-induced ductility loss of this type of alloy..
||Saburo Matsuoka, Osamu Takakuwa, Saburo Okazaki, Michio Yoshikawa, Junichiro Yamabe, Hisao Matsunaga, Peculiar temperature dependence of hydrogen-enhanced fatigue crack growth of low-carbon steel in gaseous hydrogen, Scripta Materialia, 10.1016/j.scriptamat.2018.05.035, 154, 101-105, 2018.09, The peculiar temperature dependence of hydrogen-enhanced fatigue crack growth (HEFCG) of low-carbon steel in hydrogen gas was successfully interpreted in terms of ‘trap-site occupancy’ of hydrogen. HEFCG decreased with increasing temperature in hydrogen gas at 0.7 MPa and 298 to 423 K due to lower occupancy of trap sites at higher temperatures. In hydrogen gas at 90 MPa, HEFCG was insensitive to the temperature because most of the trap sites were occupied by hydrogen, regardless of the temperature. Trap sites with a binding energy of 47 kJ/mol, corresponding approximately to the dislocation core, dominated the temperature dependence of HEFCG..
||Osamu Takakuwa, Junichiro Yamabe, Hisao Matsunaga, Yoshiyuki Furuya, Saburo Matsuoka, Comprehensive Understanding of Ductility Loss Mechanisms in Various Steels with External and Internal Hydrogen, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-017-4323-3, 48, 11, 5717-5732, 2017.11, Hydrogen-induced ductility loss and related fracture morphologies are comprehensively discussed in consideration of the hydrogen distribution in a specimen with external and internal hydrogen by using 300-series austenitic stainless steels (Types 304, 316, 316L), high-strength austenitic stainless steels (HP160, XM-19), precipitation-hardened iron-based super alloy (A286), low-alloy Cr-Mo steel (JIS-SCM435), and low-carbon steel (JIS-SM490B). External hydrogen is realized by a non-charged specimen tested in high-pressure gaseous hydrogen, and internal hydrogen is realized by a hydrogen-charged specimen tested in air or inert gas. Fracture morphologies obtained by slow-strain-rate tensile tests (SSRT) of the materials with external or internal hydrogen could be comprehensively categorized into five types: hydrogen-induced successive crack growth, ordinary void formation, small-sized void formation related to the void sheet, large-sized void formation, and facet formation. The mechanisms of hydrogen embrittlement are broadly classified into hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). In the HEDE model, hydrogen weakens interatomic bonds, whereas in the HELP model, hydrogen enhances localized slip deformations. Although various fracture morphologies are produced by external or internal hydrogen, these morphologies can be explained by the HELP model rather than by the HEDE model..
||Osamu Takakuwa, Takuya Fujisawa, Hitoshi Soyama, Experimental verification of the hydrogen concentration around a crack tip using spot X-ray diffraction, International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2016.10.083, 41, 48, 23188-23195, 2016.12, We employed X-ray diffraction using collimated X-rays to quantitatively evaluate the local hydrogen concentration behavior in metals. Hydrogen concentrating around a crack tip significantly accelerates crack propagation, i.e., hydrogen embrittlement. In order to clarify the mechanism leading to this, the local hydrogen concentration behavior, i.e., at a crack tip, was evaluated by numerical analysis and experimental measurements. Although thermal desorption analysis can be used to evaluate the total hydrogen content in metals, it cannot be applied to local areas. Microprint methods, which use chemical reactions between hydrogen and coated elements cannot quantitatively evaluate the hydrogen content. The present study takes account of hydrogen-induced strain, and X-ray diffraction in a confined area was employed to detect variations in lattice spacing before and after hydrogen charging. Using X-ray diffraction applied to a small area, we demonstrate that the hydrogen concentrates in the vicinity of the crack, i.e., at the elastic–plastic boundary..