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Tsuru, Tomohito; Lobzenko, I.; Ogata, Shigenobu*; Han, W.-Z.*
Journal of Materials Research and Technology, 28, p.1013 - 1021, 2024/01
Times Cited Count：0 Percentile：0(Materials Science, Multidisciplinary)Some solute atoms induce hardening and embrittlement in body-centered-cubic refractory metals. Especially interstitial oxygen has a dramatic hardening effect in Nb, where the yield stress of oxygen-doped Nb alloys becomes more than twice as high as that of pure Nb. Conventional mechanisms cannot explain the oxygen-induced dramatic hardening since the interaction between dislocation and oxygen is relatively weak. Here, we focused on the three-body interaction of a screw dislocation with oxygen and vacancy. Our first-principles calculations revealed that the formation of vacancy-oxygen pair enhances the attractive interaction with a screw dislocation though the interaction between oxygen and dislocation is repulsive. Furthermore, this feature was found to be a unique nature of oxygen in Nb. The vacancy-oxygen pair increases the energy barrier for dislocation motion more significantly than an isolated vacancy and oxygen interstitial. We have discovered a new oxygen-induced mechanism: a unique octahedral-tetrahedral shuffling process of oxygen dominantly contributes to the dramatic hardening. Thus, the widely distributed vacancy-oxygen pairs behave as strong obstacles for dislocation motion that causes damage accumulation and successive hardening in oxygen-doped BCC alloys.
neutron diffraction and electron microscopy studyZheng, R.*; Gong, W.; Du, J.-P.*; Gao, S.*; Liu, M.*; Li, G.*; Kawasaki, Takuro; Harjo, S.; Ma, C.*; Ogata, Shigenobu*; et al.
Acta Materialia, 238, p.118243_1 - 118243_15, 2022/10
Times Cited Count：17 Percentile：93.62(Materials Science, Multidisciplinary)Kimizuka, Hajime*; Ogata, Shigenobu*; Shiga, Motoyuki
Nihon Butsuri Gakkai-Shi, 75(8), p.484 - 490, 2020/08
Hydrogen is an element having quantum nature, and exhibits a unique diffusion behavior not found in other impurity atoms. In this study, the diffusion behavior of hydrogen isotopes in palladium crystals was predicted at a wide range of temperatures using a first-principles approach that combines the path integral simulation and electronic structure calculation. It was shown that in the high temperature region, the influence of quantum fluctuations becomes apparent as the temperature decreases, causing the suppression of diffusion where the Arrhenius plot bends upward. On the other hand, in the low temperature region, the diffusion barrier decreases due to the manifestation of the quantum tunneling effect, and the Arrhenius plot bents downward. The competition of quantum effects in different temperature regions clearly explains the anomalous isotopic effect of hydrogen diffusion.
Ogata, Shigenobu*; Tsuru, Tomohito; Yuge, Koretaka*
Haientoropi Gokin; Kakuteru Koka Ga Umidasu Tasai Na Shinbussei, p.107 - 158, 2020/05
High-entropy alloys (HEAs) are alloys that are formed by mixing equal or relatively large proportions of five or more elements. These new alloys found to have excellent properties. In this chapter, we introduced how to express the mechanical properties using atomic models.
Tsuru, Tomohito; Wakeda, Masato*; Suzudo, Tomoaki; Itakura, Mitsuhiro; Ogata, Shigenobu*
Journal of Applied Physics, 127(2), p.025101_1 - 025101_9, 2020/01
Times Cited Count：6 Percentile：33.01(Physics, Applied)We explored softening/strengthening behavior in various solute in W matrix by density functional theory (DFT) calculations combined with solid solution model. As a result of DFT calculations for various solute, a clear trend was observed in the interaction energy between a solute and a screw dislocation, which has predominant influence on solid solution behavior. The predictions based on the solid solution model with DFT can reasonably reproduce the complicated softening/strengthening as a function of temperature and solute concentration. We conclude that this specific balance is the origin of macroscopic solid solution softening.
Tsuru, Tomohito; Itakura, Mitsuhiro; Yuge, Koretaka*; Aoyagi, Yoshiteru*; Shimokawa, Tomotsugu*; Kubo, Momoji*; Ogata, Shigenobu*
Proceedings of 4th International Symposium on Atomistic and Multiscale Modeling of Mechanics and Multiphysics (ISAM-4) (Internet), p.59 - 62, 2019/08
High entropy alloys (HEAs) are chemically complex single- or multi-phase alloys with crystal structures. There are no major components but five or more elements are included with near equiatomic fraction. In such a situation, deformation behavior can no longer be described by conventional solid solution strengthening model. Some HEAs, indeed, show higher strengthening behavior and anomalous slip. However, the mechanisms of these features have yet to be understood. In the present study, we investigate the core structure of dislocations in BCC-HEAs using density functional theory (DFT) calculations. We found that core structure of a screw dislocation is identified as is the case with common BCC metals. On the other hand, dislocation motion should be different from pure BCC metals because of chemical and configurational disorder around dislocation core. We confirmed the specific feature of dislocation motion in HEAs by two-dimensional Peierls potential surface.
Kimizuka, Hajime*; Ogata, Shigenobu*; Shiga, Motoyuki
Physical Review B, 100(2), p.024104_1 - 024104_9, 2019/07
Times Cited Count：16 Percentile：63.96(Materials Science, Multidisciplinary)The behavior of H isotopes in crystals is a fundamental and recurrent theme in materials physics. Especially, the information on H diffusion over a wide temperature range provides a critical insight into the quantum mechanical nature of the subject; however, this is not yet fully explored. From state-of-the-art ab initio calculations to treat both electrons and nuclei quantum mechanically, we found that the temperature dependence of H isotope diffusivities in face-centered-cubic (fcc) Pd has an unconventional "reversed S" shape on Arrhenius plots. Such irregular behavior is ascribed to the competition between different nuclear quantum effects with different temperature and mass dependencies, which leads to a peculiar situation, where the heavier tritium (
H) diffuses faster than the lighter protium (
H) in the limited temperature range of 80 - 400 K. This unveils the mechanism of anomalous crossovers between the normal and reversed isotope effects observed in the experiments at high and low temperatures.
Yang, P.-J.*; Li, Q.-J.*; Tsuru, Tomohito; Ogata, Shigenobu*; Zhang, J.-W.*; Sheng, H.-W.*; Shan, Z.-W.*; Sha, G.*; Han, W.-Z.*; Li, J.*; et al.
Acta Materialia, 168, p.331 - 342, 2019/04
Times Cited Count：57 Percentile：96.11(Materials Science, Multidisciplinary)Body-centred-cubic metallic materials, such as niobium (Nb) and other refractory metals, are prone to embrittlement due to low levels of oxygen solutes. The mechanisms responsible for the oxygen-induced rampant hardening and damage are unclear. Here we illustrate that screw dislocations moving through a random repulsive force field imposed by impurity oxygen interstitials readily form cross-kinks and emit excess vacancies in Nb. The vacancies bind strongly with oxygen and screw dislocation in a three-body fashion, rendering dislocation motion difficult and hence pronounced dislocation storage and hardening. This leads to unusually high strain hardening rates and fast breeding of nano-cavities that underlie damage and failure.
Kimizuka, Hajime*; Ogata, Shigenobu*; Shiga, Motoyuki
Physical Review B, 97(1), p.014102_1 - 014102_11, 2018/01
Times Cited Count：27 Percentile：76.07(Materials Science, Multidisciplinary)To understand the mechanism of high diffusivity of hydrogen in palladium, we have studied the process of fast diffusion of interstitial H in face-centered cubic Pd, based on a first principles simulation taking account of quantum mechanical nature of both electrons and nuclei under finite strains. The simulated results revealed that the activation barrier for hydrogen migration was drastically increased with decreasing temperature owing to nuclear quantum effects on the octahedral sites. However, under the lattice expansion, nuclear quantum effects became less important since the tetrahedral site is stabilized. This implies that the diffusion mechanism gradually changes from quantum-like to classical-like as the strain is increased.
Wakeda, Masato*; Tsuru, Tomohito; Koyama, Masanori*; Ozaki, Taisuke*; Sawada, Hideaki*; Itakura, Mitsuhiro; Ogata, Shigenobu*
Acta Materialia, 131, p.445 - 456, 2017/06
Times Cited Count：31 Percentile：79.73(Materials Science, Multidisciplinary)Most of the solute species show a significant interaction with the dislocation core, while only several solute species among them, such as Si, P, and Cu, significantly lower the Peierls potential of the screw dislocation motion. A first-principles interaction energy with the "Easy-core" structure excellently correlates with the change in the 
-surface caused by solute atoms (i.e., chemical misfit). We show the availability of the interaction energy to predict the effect of each species on macroscopic critical resolved shear stress (CRSS) of the dilute Fe alloy. The CRSS at low and high temperature for various alloys basically agree with experiment CRSS. These results provide a novel understanding of the interaction between a screw dislocation and solute species from the first-principles.
Shimizu, Futoshi; Ogata, Shigenobu*; Li, J.*
Materials Transactions, 48(11), p.2923 - 2927, 2007/11
Times Cited Count：862 Percentile：99.98(Materials Science, Multidisciplinary)Shear bands form in most bulk metallic glasses (BMGs) within a narrowrange of uniaxial strain 
. We propose this critical condition corresponds to embryonic shear band (ESB)propagation, not its nucleation. To propagate an embryonic shear band, the far-field shear stress 
must exceed the quasi steady-state glue traction glue of shear-alienated glass until the glass-transition temperature 
is approached internally due to frictional heating, at which point ESB matures asa runaway shear crack. The magnitude of 
is governed by recovery, which are extremely fast downhill dissipative processes of timescale comparable to atomic vibrations, which molecular dynamics (MD) simulations can well capture. We model 4 metallic glasses: a binary Lennard-Jones (LJ) system, twobinary embedded atom method (EAM) potential systems, and a quinternary EAM system. Despite vast differences in the structure and interatomic interactions, the glue values from the four MD calculations give
predictions in the range of 
. Large-scale MD simulations of a binary LJ system reveal ESB as shear transformation zones coalesced. Under the condition 
, ESB is seen to develop into mature shear bands with 
approaching , while if 
, ESB does not propagate, becomes diffuse, and eventually dies.
Shimizu, Futoshi; Ogata, Shigenobu*; Kimizuka, Hajime*; Kano, Takuma; Li, J.*; Kaburaki, Hideo
Journal of the Earth Simulator, 7, p.17 - 21, 2007/06
Shimizu, Futoshi; Ogata, Shigenobu*; Li, J.*
Acta Materialia, 54(16), p.4293 - 4298, 2006/09
Times Cited Count：225 Percentile：99.18(Materials Science, Multidisciplinary)Shear bands form in most bulk metallic glasses (BMGs) within a narrowrange of uniaxial strain 
. We proposethis critical condition corresponds to embryonic shear band (ESB)propagation, not its nucleation. To propagate an embryonic shearband, the far-field shear stress 
must exceed the quasi steady-state glue traction of shear-alienated glass until the glass-transition temperature is approached internally due to frictional heating, atwhich point ESB matures as a runaway shear crack. The incubationlengthscale 
necessary for this maturation is estimatedto be 
nm for Zr-based BMGs, below which size sample-scaleshear localization does not happen. In shear-alienated glass, the lastresistance against localized shearing comes from extremely fastdownhill dissipative dynamics (DDD) of timescale comparable to atomicvibrations, allowing molecular dynamics (MD) simulations to capturethis recovery process which governs . We model4 metallic glasses: a binary Lennard-Jones system, two binaryembedded-atom (EAM) potential systems, and a quinternary EAMsystem. Despite vast differences in the structure and interatomicinteractions, the four MD calculations give predictions of 
and 
respectively.
Ogata, Shigenobu*; Shimizu, Futoshi; Li, J.*; Wakeda, Masato*; Shibutani, Yoji*
Intermetallics, 14(8-9), p.1033 - 1037, 2006/08
Times Cited Count：118 Percentile：97.06(Chemistry, Physical)Shear deformations of Cu
Zr
bulk metallic glass (BMG) model systems are performed using molecular dynamics simulation. The results suggest that both the hydrostatic stress and the stress normal to the shear plane should affect the shear response. We see shear localization and shear band nucleation in both a small system of 2,000 atoms, and large systems of 524,288 atoms, and analyze local atomic structure evolution.
Shimizu, Futoshi; Ogata, Shigenobu*; Kaburaki, Hideo; Li, J.*
no journal, ,
Most bulk metallic glasses (BMG) yield at about 2% strain in uniaxialtension/compression test. A careful analysis of the elementary shearbehavior in contrast to crystalline concepts such the generalized stacking fault (GSF) energy reveal a particularly simple and direct explanation. We perform molecular dynamics (MD) simulations on 2-component model systems and a 5-component BMG system, observing and characterizing the nucleation and evolution of shear bands. Despite gross uncertainties in the interatomic interactions and the pre-deformation glass structure, our MD results give a reasonable account of the 2% universal yield point. The general concepts of glass rejuvenation and aging, which we call alienation and recovery process in the context of intense localized shear, and occurring mainly within a timescale of 1-100 atomic vibration periods, is postulated to play acritical role. This theory (various points has also been proposed byothers) can explain why the yield point is relatively insensitive to the interatomic potential and the structure of the pre-deformed glass.
Shimizu, Futoshi; Ogata, Shigenobu*; Kimizuka, Hajime*; Kano, Takuma; Kaburaki, Hideo
no journal, ,
no abstracts in English
Li, J.*; Shimizu, Futoshi; Ogata, Shigenobu*; Kaburaki, Hideo
no journal, ,
Most bulk metallic glasses yield at about 2% strain in uniaxial tension/compression tests. A careful analysis of the elementary shear behavior in contrast to crystalline concepts such as the generalized stacking fault energy reveals a simple explanation. We perform molecular dynamics simulations on 2-component model systems and a 5-component BMG system, observing and characterizing the nucleation and evolution of shear bands. Despite gross uncertainties in the interatomic interactions and the predeformation glass structure, our MD results give a reasonable account of the 2% universal yield point. The general concepts of glass rejuvenation and aging, which we call alienation and recovery processes in the context of intense localized shear, and occurring mainly within a timescale of 1-100 atomic vibration periods, is postulated to play a critical role. This mechanistic model can explain why the yield point is relatively insensitive to the interatomic potential and the structure of the pre-deformed glass.
Shimizu, Futoshi; Ogata, Shigenobu*; Li, J.*
no journal, ,
The aged-rejuvenation-glue-liquid (ARGL) shear band model has been proposed for bulk metallic glass, based on small-scale molecular dynamics simulations (up to 20,000 atoms) and thermomechanical analysis. The model predicts the existence of a critical lengthscale 
100 nm and timescale 100 ps, above which melting transition occurs in shear-alienated glass. Large-scale molecular dynamics simulations with up to 20 million atoms have directly verified this prediction. When the applied stress exceeds the glue traction (computed separately before), we indeed observe maturation of the shear band embryo into bona fide mode-II or III shear crack, accompanied by melting. In contrast, when the applied stress is below the glue traction, the shear band embryo does not propagate, becomes diffuse, and eventually dies. Thus this all-important quantity, the glue traction (a property of shear-alienated glass), controls the macroscopicyield point of well-aged glass.
Shimizu, Futoshi; Ogata, Shigenobu*; Li, J.*
no journal, ,
no abstracts in English
Li, J.*; Shimizu, Futoshi; Ogata, Shigenobu*
no journal, ,
Shear bands form in most bulk metallic glasses (BMGs) within a narrow range of uniaxial strain 
. We propose this critical condition corresponds to embryonic shear band (ESB) propagation, not its nucleation. To propagate an embryonic shear band, the far-field shear stress must exceed the quasi steady-state glue traction of shear-alienated glass until the glass-transition temperature is approached due to frictional heating, at which point ESB matures as a runaway shear crack. The incubation lengthscale necessary for this maturation is predicted to be nm for Zr-based BMGs, below which sample-scale shear localization does not happen. Molecular dynamics simulations with up to 20-million atoms have directly verified the aged-rejuvenation-glue-liquid (ARGL) shear band structural model and "march-to-melting" instability, which occurs with a characteristic incubation timescale of ps and lengthscale of nm.
