Atomic interactions at the interface between iron or iron fluoride, and sodium by the first-principles calculation

Namie, Masanari; Saito, Junichi

Computational Materials Science, 239, p.112963_1 - 112963_7, 2024/04

https://doi.org/10.1016/j.commatsci.2024.112963

Meso-timescale atomistic simulations on coalescence process of He bubbles in Fe by SEAKMC method

Yamamoto, Yojiro*; Hayakawa, Sho*; Okita, Taira*; Itakura, Mitsuhiro

Computational Materials Science, 229, p.112389_1 - 112389_9, 2023/10

https://doi.org/10.1016/j.commatsci.2023.112389

He bubbles are characteristic microstructures under fusion reactor conditions. They approach and coalesce through their own migration, which significantly impacts the microstructure and material properties. However, these processes, which involve multiple migrations of metal atoms, cannot be treated by molecular dynamics (MD) due to its timescale limitation. In this study, self-evolving atomistic kinetic Monte Carlo (SEAKMC) was used to expand the timescale and reproduce bubble coalescences in Fe. To enhance selections of events that led to the process by avoiding trivial events with an extremely low activation energy such as tiny vibrations of a He atom or short-range displacements of the Fe atom, we introduced two algorithms into SEAKMC, a two-step saddle point search for the former measure and setting a threshold for a displacement distance of the Fe atom for the latter. Furthermore, by adding another algorithm to set an upper bound for the activation energy to prevent selections of events with an impractically high activation energy, we succeeded to reproduce the change in the configuration from dumbbell to elliptical up to a simulated time of s, 8 orders longer than MD timescales. The developed method is effective for analyzing microstructures of metallic materials containing light elements and is the only method that can reach timescales comparable to those of experiments.

Molecular dynamics simulation to elucidate effects of spatial geometry on interactions between an edge dislocation and rigid, impenetrable precipitate in Cu

Tsugawa, Kiyoto*; Hayakawa, Sho*; Okita, Taira*; Aichi, Masaatsu*; Itakura, Mitsuhiro; Suzuki, Katsuyuki*

Computational Materials Science, 215, p.111806_1 - 111806_8, 2022/12

https://doi.org/10.1016/j.commatsci.2022.111806

Dislocation core structure and motion in pure titanium and titanium alloys; A First-principles study

Tsuru, Tomohito; Itakura, Mitsuhiro; Yamaguchi, Masatake; Watanabe, Chihiro*; Miura, Hiromi*

Computational Materials Science, 203, p.111081_1 - 111081_9, 2022/02

https://doi.org/10.1016/j.commatsci.2021.111081

The deformation mode of some titanium (Ti) alloys differs from that of pure Ti due to the presence of alloying elements in -phase. Herein, we investigated all possible slip modes in pure Ti and the effects of Al and V solutes as typical additive elements on the dislocation motion in -Ti alloys using density functional theory (DFT) calculations. The stacking fault (SF) energies in possible slip planes indicated that both Al and V solutes reduce the SF energy in the basal plane and, in contrast, the Al solute increases the SF energy particularly in the prismatic plane. DFT calculations were subsequently performed to simulate dislocation core structures. The energy landscape of the transition between all possible dislocation core structures and the barriers for dislocation glide in various slip planes clarified the nature of dislocation motion in pure Ti. (i) the energy of prismatic core is higher than most stable pyramidal core, and thereby dislocations need to overcome the energy barrier of the cross-slip (22.8 meV/b) when they move in the prismatic plane, (ii) the energy difference between the prismatic and basal cores is larger (127 meV/b), that indicates the basal slip does not activate, (iii) however, the Peierls barrier for motion in the basal plane is not as high (16 meV/b). Direct calculations for the dislocation core around solutes revealed that both Al and V solutes facilitate dislocation motion in the basal plane by reducing the energy difference between the prismatic and basal cores. The effect of solutes characterizes the difference in the deformation mode of pure Ti and -Ti alloys.

Molecular dynamic simulations evaluating the effect of the stacking fault energy on defect formations in face-centered cubic metals subjected to high-energy particle irradiation

Terayama, Satoshi*; Iwase, Yuki*; Hayakawa, Sho*; Okita, Taira*; Itakura, Mitsuhiro; Suzuki, Katsuyuki*

Computational Materials Science, 195, p.110479_1 - 110479_12, 2021/07

https://doi.org/10.1016/j.commatsci.2021.110479

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

https://doi.org/10.1016/j.commatsci.2018.10.015

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.

Density functional theory study on the geometric and electronic structures of FeO and the reaction of Fe + O

Nakazawa, Tetsuya

Computational Materials Science, 146, p.334 - 345, 2018/04

https://doi.org/10.1016/j.commatsci.2017.11.027

Density functional theory calculations are performed on all possible structures of FeO using the hybrid B3LYP functional and the B3LYP functional combined with the broken-symmetry (BS) approach to obtain the most stable isomers. Based on the obtained stable isomers, the reaction mechanism of Fe + O toward rhombic Fe(-O) is considered. The BS-singlet state of the rhombic Fe(-O) 2.1 is found to be the ground state of all FeO isomers. The A" state of the open-cycle (-O) Fe(-O) 2.11 and A state of the near-linear OFeOFe 1.4 are found to have the second and third lowest energy states. The lowest-lying energy states of the bare FeO clusters do not favor three-dimensional structures, but favor the linear and planar structures. The singlet and nonet energy surfaces calculated for the reaction of Fe + O toward rhombic Fe(-O) suggest that the reaction adiabatically proceeds with spin inversion from the nonet to singlet state.

Application of hyper-molecular dynamics to self-interstitial diffusion in -iron

Abe, Yosuke

Computational Materials Science, 74, p.23 - 26, 2013/06

https://doi.org/10.1016/j.commatsci.2013.03.013

The diffusion process of a single self-interstitial atom (SIA) in -iron was studied by a hyper-molecular dynamics (hyper-MD) method that has been demonstrated to efficiently accelerate slow diffusive conformational transitions that occur in various materials. By adding a local bias potential, the acceleration of the dynamics becomes significantly higher at lower temperatures, e.g., on the order of 5 at 300 K, without changing the dynamics of the diffusion process as calculated using a conventional MD simulation. The validity of the results demonstrates that the hyper-MD method with an appropriate local bias potential can be applicable for determination of the diffusion process of SIAs in bulk materials.

Structural, bonding, and magnetic properties of small FeMo (, 6) clusters

Nakazawa, Tetsuya; Kaji, Yoshiyuki

Computational Materials Science, 55, p.365 - 375, 2012/04

https://doi.org/10.1016/j.commatsci.2011.11.007

Fast calculation approach to semi-empirical molecular orbital method using real space division method

Igarashi, Takahiro; Nakazawa, Tetsuya; Suzuki, Chikashi; Tsuru, Tomohito; Kaji, Yoshiyuki

Computational Materials Science, 50(12), p.3346 - 3349, 2011/12

https://doi.org/10.1016/j.commatsci.2011.06.027

The authors have developed a new approach for large-scale systems including over 100,000 atoms to obtain physical strength from the viewpoint of atom-atom bonding energy. In this study, the authors applied this method to SiH molecule and crystalline silica system, and carried out bond order and bonding energy analyses. In this analysis, the developed method offered almost the same analytical accuracy as the first principle method, while its calculation speed was much faster than that of the latter.

Ab initio calculations of Fe-Ni clusters

Nakazawa, Tetsuya; Igarashi, Takahiro; Tsuru, Tomohito; Kaji, Yoshiyuki

Computational Materials Science, 46(2), p.367 - 375, 2009/08

https://doi.org/10.1016/j.commatsci.2009.03.012

The clusters of Fe, Ni, and Fe-Ni are investigated computationally using a density functional approach. The geometries of clusters are optimized under the constraint of well-defined point group symmetries at the UB3LYP/LanL2DZ level. The equilibrium geometries and binding energies are presented and discussed, together with natural populations and natural electron configurations. In addition, the binding energies of FeNi clusters are found to generally decrease by successive substitutions of Ni atoms for Fe atoms. For Fe Ni clusters, the comparisons on total energies between isomers indicate that Ni atoms energetically prefer clustering in the mixed Fe-Ni clusters. The calculations for FeNi clusters show that the clustering leads to a segregation of Ni atoms from Fe atoms.