| 1. |
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.. |
| 2. |
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.. |
| 3. |
Kevinsanny, Saburo Okazaki, Osamu Takakuwa, Koichi Okita, Yusuke Funakoshi, Junichiro Yamabe, Saburo Matsuoka, Hisao Matsunaga, Effect of defects on the fatigue limit of Ni-based superalloy with different grain sizes, Fatigue & Fracture of Engineering Materials & Structures, 10.1111/ffe.12989, 42, 5, 1203-1213, 2019.05. |
| 4. |
Osamu Takakuwa, Yuhei Ogawa, Junichiro Yamabe, Hisao Matsunaga, Hydrogen-induced ductility loss of precipitation-strengthened Fe-Ni-Cr-based superalloy, Materials Science & Engineering A, 739, 335-342, 2019.01. |
| 5. |
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.. |
| 6. |
Yuhei Ogawa, Domas Birenis, Hisao Matsunaga, Osamu Takakuwa, Annett Thøgersen, Øystein Prytz, Junichiro Yamabe, The role of intergranular fracture on hydrogen-assisted fatigue crack propagation in pure iron at a low stress intensity range, Materials Science & Engineering A, 733, 316-328, 2018.08. |
| 7. |
Domas Birenis, Yuhei Ogawa, Hisao Matsunaga, Osamu Takakuwa, Annett Thøgersen, Øystein Prytz, Junichiro Yamabe, Interpretation of hydrogen-assisted fatigue crack propagation in BCC iron based on dislocation structure evolution around the crack wake, Acta Materialia, 156, 245-253, 2018.07. |
| 8. |
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, 154, 101-105, 2018.05. |
| 9. |
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, 48, 11, 5717-5732, 2017.11. |
| 10. |
Yuhei Ogawa, Domas Birenis, Annett Thøgersen, Øystein Prytz, Hisao Matsunaga, Osamu Takakuwa, Junichiro Yamabe, Multi-scale observation of hydrogen-induced, localized plastic deformation in fatigue-crack propagation in a pure iron, Scripta Materialia, 140, 13-17, 2017.07. |
| 11. |
Hisao Matsunaga, Osamu Takakuwa, Junichiro Yamabe, Saburo Matsuoka, Hydrogen-enhanced fatigue crack growth in steels and its frequency dependence, Philosophical Transactions of the Royal Society A, 10.1098/rsta.2016.0412, 375, 2098, 1-14, 2017.06. |
| 12. |
Junichiro Yamabe, Osamu Takakuwa, Hisao Matsunaga, Hisatake Itoga, Saburo Matsuoka, Hydrogen diffusivity and tensile-ductility loss of solution-treated austenitic stainless steels with external and internal hydrogen, International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2017.04.055, 42, 18, 13289-13299, 2017.05. |
| 13. |
Osamu Takakuwa, Fumio Takeo, Mitsuru Sato, Hitoshi Soyama, Using cavitation peening to enhance the fatigue strength of duralumin plate containing a hole with rounded edges, Surface & Coatings Technology, 10.1016/j.surfcoat.2016.08.087, 307, 200-205, 2016.12. |
| 14. |
Osamu Takakuwa, Takuya Fujisawa, Hitoshi Soyama, Experimental verification of the hydrogen concentration behavior around a crack tip using a spot X-ray diffraction, International Journal of Hydrogen Energy, 41, 23188-23195, 2016.11. |
| 15. |
Osamu Takakuwa, Masaaki Nakai, Kengo Narita, Mitsuo Niinomi, Kazuhiro Hasegawa, Hitoshi Soyama, Enhancing the durability of spinal implant fixture applications made of Ti-6Al-4V ELI by means of cavitation peening, International Journal of Fatigue, 10.1016/j.ijfatigue.2016.07.021, 92, 360-367, 2016.11. |
