Molecular dynamics study of phosphorus migration in 3(111) and 5(0-13) grain boundaries of -iron

Ebihara, Kenichi; Suzudo, Tomoaki

Metals, 12(4), p.662_1 - 662_10, 2022/04

https://doi.org/10.3390/met12040662

Phosphorus atoms in steels accumulate at grain boundaries via thermal and/or irradiation effects and induce grain boundary embrittlement. Quantitative prediction of phosphorus segregation at grain boundaries under various temperature and irradiation conditions is therefore essential for preventing embrittlement. To develop a model of grain boundary phosphorus segregation in -iron, we studied the migration of a phosphorus atom in two types of symmetrical tilt grain boundaries (3[1-10](111) and 5[100](0-13) grain boundaries) using molecular dynamics simulations with an embedded atom method potential. The results revealed that, in the 3 grain boundary, phosphorus atoms migrate three-dimensionally mainly in the form of interstitial atoms, whereas in the 5 grain boundary, these atoms migrate one-dimensionally mainly via vacancy-atom exchanges. Moreover, de-trapping of phosphorus atoms and vacancies was investigated.

Molecular dynamics study of phosphorus migration in 5 grain boundary of -iron

Ebihara, Kenichi; Suzudo, Tomoaki

Proceedings of Joint International Conference on Supercomputing in Nuclear Applications + Monte Carlo 2020 (SNA + MC 2020), p.65 - 69, 2020/10

Phosphorus (P) is known as one of the elements which cause the grain boundary (GB) embrittlement in steels and its GB segregation is promoted by the increase of vacancies and self-interstitial atoms due to irradiation. Thus we have been developing the rate-theory model for estimating GB P segregation under several temperatures and irradiation conditions. Because the model does not include the trapping and de-trapping processes properly, however, the model cannot calculate GB P coverage which is measured by experiments. As for the de-trapping process, so far, we have considered the migration of a P atom in the GB region of 3 symmetrical tilt GB using molecular dynamics (MD). In the current study, we also simulated the P migration in 5 GB using MD and compared the result with that of 3. As a result, at 800K, it was found that a P atom cannot migrate in 5 without vacancies while a P atom can migrate between iron atoms in 3.

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

Ebihara, Kenichi; Suzudo, Tomoaki

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

AA2018-0168.pdf:2.74MB

https://doi.org/10.1088/1361-651X/aace6a

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 3(111) symmetrical tilt grain boundary in -iron using molecular dynamics. We found that, in the vicinity of the grain boundary within 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.

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.

Response to comment on "Grain Boundary Decohesion by Impurity Segregation in a Nickel-Sulfur System"

Yamaguchi, Masatake; Shiga, Motoyuki; Kaburaki, Hideo

Science, 309(5741), P. 1677d, 2005/09

Geng et al. claim in their comment that the segregation energy of sulfur atoms in a nickel grain boundary should be calculated not on average as we did but incrementally. From their own calculations using the incremental binding energies, they concluded that the two-monolayer (GB0 4/4, GB2 4/4) segregation that bring about the reduction of the tensile strength by one order of magnitude has only 1% occupation possibility. However, they miss the point that there are many paths to the (GB0 4/4, GB2 4/4) configuration. In our re-examined calculations, one path has the 50% possibility of the occupation even if we use the incremental binding energies. Furthermore, there may be other paths that have larger possibility. To begin with, Geng et al.'s claims are not meaningful because the prediction of the occupation possibility using the simple McLean's model is not quantitative but qualitative. On the other hand, they claim that another two-monolayer (GB1 4/4, GB2 4/4) configuration brings about the reduction of the tensile strength by one order of magnitude. Their results may be miscalculations, because that is not the case in our re-examined calculations. Furthremore, they suggest a new mechanism of the strong decohesion due to the directional change of the Ni-S bonds. However, they do not show any clear evidence.

Grain boundary decohesion by impurity segregation in a nickel-sulfur system

Yamaguchi, Masatake; Shiga, Motoyuki; Kaburaki, Hideo

Science, 307(5708), p.393 - 397, 2005/01

https://doi.org/10.1126/science.1104624

Sulfur-induced embrittlement of nickel has long been wrapped in mysteries why and how sulfur weakens the grain boundaries of nickel and why a critical intergranular sulfur concentration is required. From first-principles calculations, we found that a large grain boundary expansion is caused by a short-range overlap repulsion among densely segregated and neighboring sulfur atoms. This expansion results in a drastic grain boundary decohesion that reduces the grain boundary tensile strength by one order of magnitude. This decohesion may directly causes the embrittlement since the critical sulfur concentration of this decohesion agrees well with experimental data of the embrittlement.

Report of Examination of the Samples from Primary Loop Recirculation Piping (O1-PLR) at Onagawa Nuclear Power Station Unit-1 (Contract research)

The Working Team for Examination Operation of Samples from Primary Loop Recirculation Piping at Onag

JAERI-Tech 2004-003, 74 Pages, 2004/02

JAERI-Tech-2004-003.pdf:30.08MB

The present examination has been performed with the objective to provide technical basis for identifying causes of cracking through the examination of the samples, which was conducted at the post irradiation examinations facilities of JAERI, taken from the cracked region of the recirculation pipe at the Onagawa Nuclear Power Station Unit-1. The following findings were obtained from this examination result. (1) Cracks were observed near the weld region of inner surface of the pipe and depth of the crack was about 5 to 7mm. (2) Intergranular crackings were observed in the almost whole fracture surface. Partially transgranular cracking was observed at the surface layer with the depth of about 100m. Microstructure formed by cold work and increase of hardness were observed in these surface layers. Cracks initiated near the region where hardness value was the highest. Based on the examination results described above concerning presence of tensile residual stress by welding and relatively high dissolved oxygen contents in core coolant and so on, it is concluded that these cracks were initiated in the cold work layer of inner surface by stress corrosion cracking (SCC) and propagated along the grain boundaries.

Structure and energetics of clean and hydrogenated Ni surfaces and symmetrical tilt grain boundaries using the embedded-atom method

Shiga, Motoyuki; Yamaguchi, Masatake; Kaburaki, Hideo

Physical Review B, 68(24), p.245402_1 - 245402_8, 2003/12

https://doi.org/10.1103/PhysRevB.68.245402

Multilayer relaxations, surface/interface energies, hydrogen binding sites and energies were systematically studied using the embedded atom method (EAM), for a series of the clean and hydrogenated Ni surfaces and symmetrical tilt Ni grain boundaries. The hydrogen binding energies were in the range of 2.7-2.9 eV at the surface sites while 2.1-2.6 eV at the grain boundary sites, both being larger than at the fcc crystal interior site, 2.1 eV. These data are consistent with the conclusions from ab initio calculations that intergranular embrittlement is seen by hydrogen segregation at the Ni grain boundaries. It was found that multilayer relaxation of Ni plays a key role in trapping hydrogen at the grain boundaries.

Inhibition of sensitization in reactor pipe materials by grain boundary structure control, JAERI's nuclear research promotion program, H11-023 (Contract research)

Kokawa, Hiroyuki*; Shimada, Masayuki*; Wang, Z.*; Sato, Yutaka*; Sato, Yoshihiro*; Kiuchi, Kiyoshi

JAERI-Tech 2003-014, 22 Pages, 2003/03

JAERI-Tech-2003-014.pdf:1.68MB

no abstracts in English

Long-term durability test of acid recovery evaporators made of Ti-5% Ta aIloy and zirconium

; Fujisaku, Kazuhiko*; *; ; Koyama, Tomozo

JNC TN8410 2001-013, 255 Pages, 2001/05

JNC-TN8410-2001-013.pdf:24.24MB

Mock-ups of acid recovery evaporators which are made of Ti-5% Ta alloy and Zr were tested under inactive condition for forty thousands hours to improve a corrosion resistance of acid recovery evaporator in Tokai reprocessing plant (TRP). The mock-up unit was designed and produced referring to the specification of acid recovery evaporator in TRP and the evaporation performance of the mock-up was 1/27 of TRP. A long-term durability of both evaporators was demonstrated by results of operation data, evaporation performance and corrosion resistance. The mock-up unit did not suffer from any trouble during the running test and the operation data such as temperature, flow, concentrations of nitric acid and metal ions were fairly stable within standard condition. As for the corrosion resistance, cracks and local corrosion such as intergranular attack were not observed on both evaporators after the running test, and a corrosion of weld was not selective. The average corrosion rates at measuring points were less than 0.1mm/yr, respectively, however, thickness of the Ti-5% Ta alloy evaporator was slightly reduced at all points of vapor phase region. In addition, from the result by test coupon, it is found that both materials have low susceptibility to stress corrosion cracking in this environment. The destructive inspection showed that the mechanical properties of both materials were not degraded during the running test. Finally, the total running time of the mock-up unit is much more than a maximum running time of acid recovery evaporator made of stainless steel in TRP (nearly 15,000 hours). On the basis of the test results, an excellent durability of Ti-5% Ta alloy and Zr evaporators under was successfully demonstrated throughout the mock-up test from an engineering perspective.