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Journal Articles

Hydrogen-accelerated spontaneous microcracking in high-strength aluminium alloys

Tsuru, Tomohito; Shimizu, Kazuyuki*; Yamaguchi, Masatake; Itakura, Mitsuhiro; Ebihara, Kenichi; Bendo, A.*; Matsuda, Kenji*; Toda, Hiroyuki*

Scientific Reports (Internet), 10, p.1998_1 - 1998_8, 2020/04

Age-hardening has been one and only process to achieve high strength aluminum alloys since unlike iron and titanium, pure aluminum does not have other solid phases during heat treatment. Highly-concentrated precipitations play therefore dominant role in mechanical properties and hydrogen embrittlement of aluminium alloys. It has been considered that the coherent interface between matrix and precipitation does not contribute to the crack initiation and embrittlement due to its coherency. Here, we discovered the origin of unprecedented quasi-cleavage fracture mode. Hydrogen partitioning at various defect sites is investigated comprehensively combined with experiment, theory and first-principles calculations. We demonstrate that despite low excess free volume, the aluminum-precipitation interface is more preferable trap site than void and grain boundary. The cohesivity of the interface deteriorates significantly with increasing occupancy while hydrogen atoms are trapped stably up to extremely high occupancy equivalent to spontaneous cleavage.

Journal Articles

Molecular dynamics simulations of phosphorus migration in a grain boundary of $$alpha$$-iron

Ebihara, Kenichi; Suzudo, Tomoaki

TMS 2020; 149th Annual Meeting & Exhibition Supplemental Proceedings, p.995 - 1002, 2020/02

Phosphorus (P) is known as an element which causes grain boundary (GB) embrittlement in steels. In addition, GB P segregation is promoted by the increase of vacancies and self interstitial atoms due to irradiation. Thus, the diffusion rate theory model for estimating irradiation-induced GB P segregation has been developed based on the atomic processes. Since the present model does not include the trapping and de-trapping processes at GBs, however, it cannot calculate the value which is directly compared with experimental results. In this study, we simulated the migration of a P atom in the $$Sigma$$3(111) symmetrical tilt GB. In addition, by tracking the migration of the P atom, the diffusion barrier energy was evaluated. As a result, the diffusion barrier energy was almost the same as the P segregation energy of an interstitial site in the GB, and it was found that P atoms migrate via interstitial sites in the GB.

Journal Articles

Optimization of mechanical properties in aluminum alloys $$via$$ hydrogen partitioning control

Toda, Hiroyuki*; Yamaguchi, Masatake; Matsuda, Kenji*; Shimizu, Kazuyuki*; Hirayama, Kyosuke*; Su, H.*; Fujiwara, Hiro*; Ebihara, Kenichi; Itakura, Mitsuhiro; Tsuru, Tomohito; et al.

Tetsu To Hagane, 105(2), p.240 - 253, 2019/02

 Times Cited Count:0 Percentile:100(Metallurgy & Metallurgical Engineering)

no abstracts in English

Journal Articles

First-principles calculation of multiple hydrogen segregation along aluminum grain boundaries

Yamaguchi, Masatake; Ebihara, Kenichi; Itakura, Mitsuhiro; Tsuru, Tomohito; Matsuda, Kenji*; Toda, Hiroyuki*

Computational Materials Science, 156, p.368 - 375, 2019/01

 Times Cited Count:5 Percentile:25.11(Materials Science, Multidisciplinary)

The segregation of multiple hydrogen atoms along aluminum (Al) grain boundaries (GBs) and fracture surfaces (FSs) was investigated through first-principles calculations considering the characteristics of GBs. The results indicate that hydrogen segregation is difficult along low-energy GBs. The segregation energy of multiple hydrogen atoms along GBs and FSs and the cohesive energy was obtained for three types of high-energy Al GBs. With increasing hydrogen segregation along the GBs, the cohesive energy of the GB decreases and approaches zero with no decrease in GB segregation energy. The GB cohesive energy decreases in parallel with the volume expansion of the region of low electron density along the GB.

Journal Articles

Surface energy reduction by dissociative hydrogen adsorption on inner surface of pore in aluminum

Yamaguchi, Masatake; Tsuru, Tomohito; Ebihara, Kenichi; Itakura, Mitsuhiro

Keikinzoku, 68(11), p.588 - 595, 2018/11

no abstracts in English

Journal Articles

Interpretation of thermal desorption spectra of hydrogen from aluminum using numerical simulation

Ebihara, Kenichi; Yamaguchi, Masatake; Tsuru, Tomohito; Itakura, Mitsuhiro

Keikinzoku, 68(11), p.596 - 602, 2018/11

Hydrogen embrittlement (HE) is considered as one cause of stress corrosion cracking. HE is a serious problem in the development of high strength aluminum alloy as with steels. For understanding HE, it is inevitable to know hydrogen trapping states in the alloys and it can be identified using thermal desorption spectrometry of H. In this study, we numerically simulated thermal desorption spectra of hydrogen in aluminum for a cylindrical and a plate specimens and interpreted the desorption peaks included in them on the basis of the trap site concentration and the trap energy. As a result, we found that the peak at the lowest-temperature side can result from grain boundaries and confirmed that the reported interpretation for other peaks is reasonable. We also obtained the result showing the possibility that the trap site concentration of defects changes during heating the specimens. This result may give a suggestion for the interpretation of temperature desorption spectra of steels.

Journal Articles

Study on simulation of thermal desorption spectra for a tempered martensitic steel with vacancies induced by hydrogen and strain

Ebihara, Kenichi; Saito, Kei*; Takai, Kenichi*

"Suiso Zeika No Kihon Yoin To Tokusei Hyoka" Kenkyukai Hokokusho, p.57 - 61, 2018/09

no abstracts in English

Journal Articles

Atomistic simulation of phosphorus segregation to $$Sigma$$3(111) symmetrical tilt grain boundary in $$alpha$$-iron

Ebihara, Kenichi; Suzudo, Tomoaki

Modelling and Simulation in Materials Science and Engineering, 26(6), p.065005_1 - 065005_10, 2018/09

 Times Cited Count:1 Percentile:85.18(Materials Science, Multidisciplinary)

Irradiation-induced grain boundary phosphorus segregation is an important factor for estimating the embrittlement of nuclear reactor pressure vessel steels, but the physical process of phosphorus migration to grain boundaries is still unclear. We numerically studied phosphorus migration toward $$Sigma$$3(111) symmetrical tilt grain boundary in $$alpha$$-iron using molecular dynamics. We found that, in the vicinity of the grain boundary within $$sim$$1 nm distance, an iron-phosphorus mixed dumbbell and an octahedral interstitial phosphorus atom push a self-interstitial atom into the grain boundary, and the phosphorus atom becomes a substitutional atom. A phosphorus vacancy complex in the region also becomes dissociated, and the vacancy is absorbed in the grain boundary without dragging phosphorus. The results claim that a novel view of the segregation process is required.

Journal Articles

Journal Articles

First-principles study of hydrogen segregation at the MgZn$$_{2}$$ precipitate in Al-Mg-Zn alloys

Tsuru, Tomohito; Yamaguchi, Masatake; Ebihara, Kenichi; Itakura, Mitsuhiro; Shiihara, Yoshinori*; Matsuda, Kenji*; Toda, Hiroyuki*

Computational Materials Science, 148, p.301 - 306, 2018/06

 Times Cited Count:10 Percentile:28.09(Materials Science, Multidisciplinary)

Hydrogen embrittlement susceptibility of high strength 7xxx series Al alloys has been recognized as the critical issues in the practical use of Al alloys. Focusing on the interface between MgZn$$_{2}$$ precipitates and an Al matrix, which is considered as one of the important segregation sites in these alloys, we investigated the stable $$eta$$-MgZn$$_{2}$$-Al interface, and the possible hydrogen trap sites in MgZn$$_{2}$$ and at the $$eta$$-MgZn$$_{2}$$-Al interface via first-principles calculation. Most of the interstitial sites inside the MgZn$$_{2}$$ crystal were not possible trap sites because their energy is relatively higher than that of other trap sites. The trap energy of the most favorable site at the $$eta$$-MgZn$$_{2}$$-Al is approximately -0.3 eV/H, which is more stable that of the interstitial site at the grain boundary. The interface between MgZn$$_{2}$$ and Al is likely to be a possible trap site in Al alloys.

Journal Articles

Numerical simulation of hydrogen thermal desorption profile under assumption of two kinds of trap sites for tempered martensitic steel

Tsuchida, Yutaka*; Ebihara, Kenichi

Tetsu To Hagane, 103(11), p.653 - 659, 2017/11

 Times Cited Count:0 Percentile:100(Metallurgy & Metallurgical Engineering)

A single peak in thermal desorption profiles of hydrogen, which are measured in low-temperature thermal desorption spectrometry (L-TDS) for a very thin plate specimen of tempered martensitic steel, was reproduced successfully by the superposition of two Gaussian distributions. Then, the parameters concerning the detrapping rate constants for both peaks, which are trap energy and pre-exponential factor, were calculated using the Choo-Lee plot. We confirmed that Kissinger model incorporating the obtained parameters could simulate the two peaks. In addition, we reproduced the single peak well using the reaction-diffusion equation incorporating the obtained parameters and the appropriate trap site concentration. From the results, we interpreted that the one peak corresponds to dislocation and the other to grain-boundary.

Journal Articles

Modeling of Phosphorus Transport by Interstitial Dumbbell in $$alpha$$-Iron Using First-Principles-Based Kinetic Monte Carlo

Ebihara, Kenichi; Suzudo, Tomoaki; Yamaguchi, Masatake

Materials Transactions, 58(1), p.26 - 32, 2017/01

 Times Cited Count:3 Percentile:64.77(Materials Science, Multidisciplinary)

In order to evaluate grain boundary (GB) phosphorous (P) segregation in nuclear reactor pressure vessel steels under irradiation, the rate-theory model based on first-principles calculations is developed. In this study, we evaluated the diffusion coefficient of the mixed interstitial dumbbell of a P atom and an iron(Fe) atom using a kinetic Monte Carlo (kMC) simulation based on first-principles calculations. The evaluated diffusion coefficient was almost the same with the diffusion coefficient of P atoms which migrate via octahedral interstitial sites, and was much faster than that for P transport by vacancies. Furthermore, from the simulation of the irradiation induced GB P segregation using the model which was modified to include P atoms of octahedral interstitial sites, it was found that the boundary condition at GB is not valid for P atoms of octahedral interstitial sites

Journal Articles

Analysis of intergranular cracking in an alloy steel by hydrogen-enhanced decohesion

Yamaguchi, Masatake; Ebihara, Kenichi; Itakura, Mitsuhiro

Proceedings of 2016 International Hydrogen Conference (IHC 2016); Materials Performance in Hydrogen Environments, p.563 - 571, 2017/00

no abstracts in English

Journal Articles

Determination of detrapping and trapping rate constants for hydrogen based on experimental thermal desorption spectra

Ebihara, Kenichi; Saito, Kei*; Takai, Kenichi*

Proceedings of 2016 International Hydrogen Conference (IHC 2016); Materials Performance in Hydrogen Environments, p.470 - 477, 2017/00

For understanding hydrogen (H) embrittlement of steels, it is necessary to infer the state that defects trap H in the steels. Thermal desorption spectra of H obtained by the thermal desorption spectrometry (TDS) are used for inferring such a state. Because the thermal desorption spectra include the influence of experimental conditions and hydrogen diffusion as well as information of the defects trapping H, it is necessary to interpret the spectra using the numerical simulation. In the presentation, we determined the detrapping and the trapping rate constants which are necessary for the simulation from the experimental spectra obtained for plate specimens which is so small that H diffusion is ignorable. Then we confirmed that the model using the obtained rate constants can simulate the spectra of larger cylindrical specimens, so that it was found that the rate constant for small specimens can be used for the simulation of the spectra for specimens of different shape or size.

Journal Articles

Study on modeling of thermal desorption spectra of hydrogen including variation of vacancy-type trap sites

Ebihara, Kenichi; Saito, Kei*; Takai, Kenichi*

"Suiso Zeika No Kihon Yoin To Tokusei Hyoka Kenkyukai Chukan Hokokukai" Shimposium Yokoshu (USB Flash Drive), p.30 - 35, 2016/09

no abstracts in English

Journal Articles

Multiscale thermodynamic analysis on hydrogen-induced intergranular cracking in an alloy steel with segregated solutes

Yamaguchi, Masatake; Ebihara, Kenichi; Itakura, Mitsuhiro

Corrosion Reviews, 33(6), p.547 - 557, 2015/11

A multiscale analysis has been conducted on hydrogen-induced intergranular cracking at ambient temperature in medium strength (840 MPa) Ni-Cr steel with antimony, tin, and phosphorous segregation. Combining first-principles calculations and fracture mechanics experiments, a multiscale relationship between threshold stress intensity factor ($$K_{rm th}$$) and cohesive energy of grain boundary (the ideal work of interfacial separation, 2$$gamma$$$$_{int}$$) was revealed. The $$K_{rm th}$$ was found to decrease rapidly under a certain threshold of 2$$gamma$$$$_{int}$$, where the 2$$gamma$$$$_{int}$$ decreases mainly by mobile hydrogen segregation on fracture surfaces. This segregation is considered to arise during formation of the fracture surfaces under thermodynamic equilibrium in slow crack growth. The resulting strong decohesion probably makes it difficult to emit dislocations at microcrack tip region, leading to a large reduction of stress intensity factor. Our analysis based on this mobile hydrogen decohesion demonstrates that the $$K_{rm th}$$ decreases dramatically within a low and narrow range of hydrogen content in iron lattice in high-strength steels.

Journal Articles

Numerical reproduction of hydrogen thermal desorption spectra of tempered martensitic steel based on experimental data

Ebihara, Kenichi; Saito, Kei*; Takai, Kenichi*

"Suiso Zeika No Kihon Yoin, Kaiseki To Hyoka" Shimposium Yokoshu (USB Flash Drive), p.27 - 33, 2015/09

The thermal desorption spectra which reflect the H segregation state can be obtained by heating a specimen including H at a constant rate. However, the simulation of spectra needs to extract information of the H segregation state because spectra is affected by experimental conditions and H diffusion. The detrapping activation energy E$$_a$$ and the pre-exponential factor of detraping rate constant which are simulation parameters are fixed from the data previously reported or by fitting experimental spectra. Instead, we evaluated both of them from the experimental spectra of the specimen in which the H diffusion effect can be ignored, and simulated the spectra using them. As a result, in the case of iron, we could simulate spectra better than by the previous parameters. In the case of tempered martensitic steel, we could simulate spectra by the value obtained by adjusting the evaluated p$$_0$$ along with the evaluated E$$_a$$.

Journal Articles

Numerical study on influence of Ohnesorge number and Reynolds number on the jet breakup behavior using the lattice Boltzmann method

Iwasawa, Yuzuru*; Abe, Yutaka*; Kaneko, Akiko*; Kanagawa, Tetsuya*; Saito, Shimpei*; Matsuo, Eiji*; Ebihara, Kenichi; Sakaba, Hiroshi*; Koyama, Kazuya*; Nariai, Hideki*

Proceedings of 23rd International Conference on Nuclear Engineering (ICONE-23) (DVD-ROM), 7 Pages, 2015/05

For the safety design in which heat is properly removed from the molten fuel after the core disruptive accident in a sodium-cooled fast reactor, the estimation of the breakup behavior of molten fuel discharged into the coolant like a jet is desired. In order to investigate the influence of viscocity on the jet behavior, we simulated a jet discharged into a coolant using the three-dimensional lattice Boltzmann model for two-phase fluid, and examined the influence of Ohnesorge number and Reynolds number on the jet behavior. As a result, we made clear that it is necessary to consider viscosity of the coolant as well as that of the jet for the estimation of jet behavior.

Journal Articles

Numerical simulation of jet breakup behavior by the lattice Boltzmann method

Matsuo, Eiji*; Abe, Yutaka*; Iwasawa, Yuzuru*; Ebihara, Kenichi; Koyama, Kazuya*

Nippon Kikai Gakkai Rombunshu (Internet), 81(822), p.14-00409_1 - 14-00409_20, 2015/02

In order to understand the jet breakup behavior of the molten core material into coolant during a core disruptive accident (CDA) for a sodium-cooled fast reactor (SFR), we simulated the jet breakup due to the hydrodynamic interaction using the lattice Boltzmann method (LBM). The applicability of the LBM to the jet breakup simulation was validated by comparison with our experimental data. In addition, the influence of several dimensionless numbers such as Weber number and Froude number was examined using the LBM. As a result, we validated applicability of the LBM to the jet breakup simulation, and found that the jet breakup length is independent of Froude number and in good agreement with the Epstein's correlation when the jet interface becomes unstable.

Journal Articles

Atomistic and continuum comparative studies on the stress distribution around a nano-crack on the grain boundary for modeling hydrogen embrittlement of iron

Ebihara, Kenichi; Kaburaki, Hideo; Itakura, Mitsuhiro

"Hagane No Kikaiteki Tokusei Ni Oyobosu Suiso No Koka To Sono Hyoka" Shimpojium Yokoshu (USB Flash Drive), 6 Pages, 2014/09

Since hydrogen(H) embrittlement is one factor causing degradation and/or fracture of steel, understanding its mechanism is required. The grain-boundary(GB) decohesion due to segregation of H is considered to cause the delayed fracture of high strength steels and the cold cracking in welding. In the model based on GB decohesion, information of strength of GBs estimated in the atomic scale is used for the estimation of strength or crack propagation in the macroscopic scale. However the modeling between the atomic and the macroscopic scales is not clear. In particular, the validity of the model using the elastic continuum around nano-cracks for stress concentration at the crack tip is not clear. Thus, we examined the difference of the stress distribution around the nano-crack which was estimated by molecular dynamics and by a continuum calculation. As a result, the discrepancy became remarkable at high strain. The stress concentration was not simulated by the elastic continuum model.

135 (Records 1-20 displayed on this page)