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Journal Articles

The Two-step nucleation of G-phase in ferrite

Matsukawa, Yoshitaka*; Takeuchi, Tomoaki; Kakubo, Yuta*; Suzudo, Tomoaki; Watanabe, Hideo*; Abe, Hiroaki*; Toyama, Takeshi*; Nagai, Yasuyoshi*

Acta Materialia, 116, p.104 - 113, 2016/09

https://doi.org/10.1016/j.actamat.2016.06.013
 Times Cited Count：73 Percentile：95.76(Materials Science, Multidisciplinary)

Atom probe tomography (APT) and TEM were combined for identifying the stage at which solute clusters transform into compounds crystallographically distinct from the matrix, in the precipitation of the G-phase (Ni$$_{16}$$Si$$_{7}$$Mn$$_{6}$$) from ferrite solid solution subjected to isothermal annealing at 673 K. Based on a systematic analysis of solute clusters as a function of annealing time, the nucleation of the G-phase was found to occur via a two-step process. Moreover, the structural change was found to occur via another two-step process. There was a time lag between the end of cluster growth to become a critical size and the start of the structural change. During the incubation period solute enrichment occurred inside the clusters without further size growth, indicating that the nucleation of the G-phase occurs at the critical size with a critical composition. Judging from the results of APT, TEM and the simulation of electron diffraction patterns, the critical composition was estimated to be Ni$$_{16}$$Si$$_{3.5}$$(Fe,Cr)$$_{3.5}$$Mn$$_{6}$$.

Journal Articles

Effects of thermal aging on microstructure and hardness of stainless steel weld-overlay claddings of nuclear reactor pressure vessels

Takeuchi, Tomoaki; Kakubo, Yuta*; Matsukawa, Yoshitaka*; Nozawa, Yasuko*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Nishiyama, Yutaka; Katsuyama, Jinya; Yamaguchi, Yoshihito; Onizawa, Kunio; et al.

Journal of Nuclear Materials, 452(1-3), p.235 - 240, 2014/09

https://doi.org/10.1016/j.jnucmat.2014.04.003
 Times Cited Count：40 Percentile：95.16(Materials Science, Multidisciplinary)

Microstructures and hardness of stainless steel weld overlay cladding of reactor pressure vessels subjected to the thermal aging at 400 $$^{circ}$$C for 100-10,000 h were investigated using atom probe tomography and nanoindentation technique. The Cr concentration fluctuation in the $$delta$$-ferrite phase caused by spinodal decomposition rapidly progressed by the 100 h aging while NiSiMn clusters increased in number density at 2,000 h and coarsened at 10,000 h. The hardness of the $$delta$$-ferrite phase also rapidly increased at the short aging time. The Cr concentration fluctuation and the hardness were in good correlation with the degree of the Cr concentration fluctuation rather than the formation of the NiSiMn clusters. These results strongly suggested that the dominant factor of the hardening of the $$delta$$-ferrite phase by the thermal aging was Cr spinodal decomposition.

Journal Articles

Effects of neutron irradiation on microstructures and hardness of stainless steel weld-overlay cladding of nuclear reactor pressure vessels

Takeuchi, Tomoaki; Kakubo, Yuta*; Matsukawa, Yoshitaka*; Nozawa, Yasuko*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Nishiyama, Yutaka; Katsuyama, Jinya; Yamaguchi, Yoshihito; Onizawa, Kunio

Journal of Nuclear Materials, 449(1-3), p.273 - 276, 2014/06

https://doi.org/10.1016/j.jnucmat.2014.01.004
 Times Cited Count：19 Percentile：81.57(Materials Science, Multidisciplinary)

Microstructural changes and hardness of stainless steel weld overlay cladding of reactor pressure vessels subjected to the neutron irradiation with a dose of 7.2 $$times$$ 10$$^{19}$$n cm$$^{-2}$$ (E $$>$$ 1 MeV) and a flux of 1.1 $$times$$ 10$$^{13}$$n cm$$^{-2}$$ s$$^{-1}$$ at 290$$^{circ}$$C were investigated by atom probe tomography and with nanoindentation technique. In order to isolate the effect of the irradiation, we compared the results of the measurements of the irradiated sample with that of the aged one at 300$$^{circ}$$C for the time equivalent to the irradiation. The Cr concentration fluctuation was enhanced in the $$delta$$-ferrite phase of the irradiated sample. In addition, the enhancement of the concentration fluctuation of Si, which was not observed in the aged sample, was observed. The hardening at the $$delta$$-ferrite phase was occurred by both the irradiation and the aging. However, the former was more than that expected from the Cr concentration fluctuation, which suggested that the Si concentration fluctuation and irradiation-induced defects were possible origins of the additional hardening.

Journal Articles

Effect of neutron irradiation on the microstructure of the stainless steel electroslag weld overlay cladding of nuclear reactor pressure vessels

Takeuchi, Tomoaki; Kakubo, Yuta*; Matsukawa, Yoshitaka*; Nozawa, Yasuko*; Nagai, Yasuyoshi*; Nishiyama, Yutaka; Katsuyama, Jinya; Onizawa, Kunio; Suzuki, Masahide

Journal of Nuclear Materials, 443(1-3), p.266 - 273, 2013/11

https://doi.org/10.1016/j.jnucmat.2013.07.035
 Times Cited Count：16 Percentile：76.68(Materials Science, Multidisciplinary)

Investigation on irradiation effects of weld-overlay claddings is necessary for safety assessment of reactor pressure vessels. We investigated microstructural changes in the cladding, which was composed of about 90% austenite and 10% $$delta$$-ferrite phases, subjected to the neutron irradiation to 7.2$$times$$10$$^{19}$$ n/cm$$^{2}$$ at 290$$^{circ}$$C, by 3D atom probe tomography technique. In the ferrite phase, the amplitude of the Cr and Si concentration fluctuation was increased by the irradiation and Ni and Mn concentration fluctuations were newly occurred. In the austenite phase, $$gamma$$'(Ni$$_{3}$$Si) -like clusters were formed. In contrast, the results of our previous work on the cladding subjected to thermal aging showed the amplitude of the Cr fluctuation was significantly increased and G (Ni-Si-Mn) phase was formed in the ferrite phase. Moreover, no changes were observed in the austenite by the aging.

Oral presentation

Transmission electron microscopy crystal structure analysis of G-phase precipitates in an overlay-clad duplex stainless steel on a light-water pressure vessel steel

Matsukawa, Yoshitaka*; Kakubo, Yuta*; Nozawa, Yasuko*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Takeuchi, Tomoaki; Yamaguchi, Yoshihito; Katsuyama, Jinya; Nishiyama, Yutaka

no journal, , 

The $$delta$$/$$gamma$$ duplex stainless steel overlay-welded on the inner wall of light-water reactor pressure vessels for protection from corrosion is known to exhibit a complex phase transformation under thermal aging. Our atom-probe tomography analysis revealed that in the $$delta$$-ferrite $$alpha$$/$$alpha$$' spinodal decomposition and precipitation of nanometer-sized Ni-Mn-Si clusters occur together at 673 K after 2000 hrs. The Ni-Mn-Si clusters visualized in the 3-D atom map are most likely G-phase, which is an intermetallic compound commonly observed in this type of duplex stainless steels; however, since crystallographic information is largely lost in atom-probe data, it is unknown if they are certainly G-phase accompanied with a specific supper-lattice structure or simple agglomerates of solute atoms without any crystal structure change. In the present study crystal structure of the Ni-Mn-Si nano-clusters has been analyzed by transmission electron microscopy to determine the fraction of G-phase over all clusters.

Oral presentation

Thermal aging time dependence of nanostructural changes in an overlay-clad duplex stainless steel on a light-water pressure vessel steel

Kakubo, Yuta*; Nozawa, Yasuko*; Matsukawa, Yoshitaka*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Kameda, Jun; Takeuchi, Tomoaki; Yamaguchi, Yoshihito; Katsuyama, Jinya; Nishiyama, Yutaka; et al.

no journal, , 

no abstracts in English

Oral presentation

Microstructure analysis for reactor pressure vessel weld-overlay cladding

Katsuyama, Jinya; Takeuchi, Tomoaki; Kakubo, Yuta*; Nozawa, Yasuko*; Toyama, Takeshi*; Nishiyama, Yutaka; Nagai, Yasuyoshi*

no journal, , 

Cladding of stainless steel is overlay-welded on the inner surface of reactor pressure vessels as a means for corrosion protection. Cladding material is composed of two phases of ferrite ($$delta$$) and austenite ($$gamma$$) phases. For cladding, it is known that hardening occurs due to the thermal ageing resulting in the nanostructure change. From the view point of stress, complex distribution of $$delta$$ phase has a possibility to raise stress and strain locally. Therefore, we performed nanostructure analysis for thermal aged cladding material using 3D-AP, and stress and strain analysis based on the FEM. After thermal ageing at 400 $$^{circ}$$C, Cr spinodal decomposition and hardening are observed. Tensile stress corresponding to the weld residual stress is applied to FEM model which has a complex distribution of the $$delta$$ and $$gamma$$ phases based on microscopic observation. Large plastic strain are produced at some regions of $$gamma$$ phase between $$delta$$ and $$delta$$ phases.

Oral presentation

Microstructure investigation for reactor pressure vessel weld-overlay cladding

Katsuyama, Jinya; Takeuchi, Tomoaki; Kakubo, Yuta*; Nozawa, Yasuko*; Yamaguchi, Yoshihito; Toyama, Takeshi*; Nishiyama, Yutaka; Nagai, Yasuyoshi*

no journal, , 

Cladding of stainless steel, which is composed of two phases of $$delta$$ and $$gamma$$ phases, is overlay-welded on the inner surface of RPVs as a means for corrosion protection. For cladding, it is known that hardening occurs due to the thermal ageing. Complex distribution of $$delta$$ phase and its hardening has possibility to raise stress locally. Therefore, we performed nanostructure analysis for thermal aged cladding material, and stress-strain analysis based on the FEM. After thermal ageing at 400 $$^{circ}$$C, it is shown that hardening of $$delta$$ phase is due to Cr spinodal decomposition. Tensile stress corresponding to the weld residual stress is applied to FEM model which has a complex distribution of the $$delta$$ phase based on microscopic observation. Plastic strain are produced at some regions of $$gamma$$ phase. Hardening of $$delta$$ phase results in large tensile stress beyond 350MPa of residual stress within $$gamma$$ phase near the $$delta$$ and $$delta$$ phases boundaries.

Oral presentation

Transmission electron microscopy crystal-structure analysis of G-phase precipitates in a $$delta$$/$$gamma$$ duplex stainless steel weld overlay cladding of light-water reactor pressure vessel steels

Matsukawa, Yoshitaka*; Kakubo, Yuta*; Nozawa, Yasuko*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Takeuchi, Tomoaki; Yamaguchi, Yoshihito; Katsuyama, Jinya; Nishiyama, Yutaka

no journal, , 

Crystal structure change and composition change of G-phase (Ni$$_{16}$$Si$$_{7}$$Mn$$_{6}$$) precipitates in a duplex stainless steel welded on an RPV steel were analyzed by transmission electron microscopy combined with atom-probe tomography. Based on TEM crystallographic analysis data and atom-probe composition analysis data, together with simulation results of electron diffraction patterns, it was found that those precipitates turned G-phase in structure-wise before they turned stoichiometric composition.

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