Dislocation modelling

Fan-Shun Meng, Shuhei Shinzato, Shihao Zhang, Kazuki Matsubara, Jun-Ping Du, Peijun Yu, Wen-Tong Geng, Shigenobu Ogata,“A highly transferable and efficient machine learning interatomic potentials study of α-Fe–C binary system”, Acta Materialia, 281 (2024) 120408.

Machine learning interatomic potentials(MLIPs) for α-iron and carbon binary system have been constructed aiming for understanding the mechanical behavior of Fe–C steel and carbides. The MLIPs were trained using an extensive reference database produced by spin polarized density functional theory(DFT) calculations. The MLIPs reach the DFT accuracies in many important properties which are frequently engaged in Fe and Fe–C studies, including kinetics and thermodynamics of C in α-Fe with vacancy, grain boundary, and screw dislocation, and basic properties of cementite and cementiteferrite interfaces. In conjunction with these MLIPs, the impact of C atoms on the mobility of screw dislocation at finite temperature, and the C-decorated core configuration of screw dislocation were investigated, and a uniaxial tensile test on a model with multiple types of defects was conducted.

Gaobing Wei, Hongxian Xie, Jun-Ping Du, Tingting He, Guanghong Lu, Shigenobu Ogata,“Disconnection units of twinning in body-centered-cubic metals”, Acta Materialia, 280, (2024) 120325-1-12.

Twin boundary (TB) migration, facilitated by the motion of disconnections, plays a pivotal role in the deformation of body-centered cubic (BCC) crystals. Comprehending the migration rules of twinning disconnections (TDs) under shear stress is significant in elucidating the role of twin migration in BCC plasticity. Nevertheless, our understanding of the atomic structure and migration mechanics of TDs remains incomplete. In this study, we employ the theory of interfacial defects and molecular dynamics (MD) simulations to thoroughly investigate potential {112}[111] TDs in BCC tantalum (Ta). These TDs are characterized by their Burgers vectors, which were predicted through related dichromatic pattern analysis. We reveal that single-layer and double-layer TDs represent the fundamental building blocks (units) of multi-layer TDs. Their migration directions are entirely opposite under the same shear loading, and they cannot annihilate each other when they migrate “face to face”. Furthermore, these migration rules governing single-layer and double-layer TDs offer a comprehensive explanation for the complex composition and decomposition patterns observed among various multi-layer TDs. What’s more, we demonstrated that TDs with zero Burgers vector can only be driven as a whole under coupled loading conditions. These findings significantly enhance our understanding of TB migrations in BCC metals.

Bo-Qing Li, Irene J. Beyerlein, Shuhei Shinzato, Shigenobu Ogata, Wei-Zhong Han,“Mechanism of solute hardening and dislocation debris-mediated ductilization in Nb-Si alloy”, Journal of Materials Science & Technology, 203 (2024) 167-179.

Niobium (Nb) is sensitive to even minute quantities of silicon (Si) solutes, which are known to induce pronounced hardening. However, the underlying mechanism for hardening remains elusive since the effect of Si solutes on dislocation behavior is unclear. Here, using tensile testing, in-situ microscopy and nanomechanical testing, the behavior of dislocations in dilute Nb-Si alloys, containing from 0 at.% to 0.8 at.% Si, is investigated. We show that the hardness, strength and strain hardening rate increase from two to four times, while the uniform elongation in tension only reduces 50% as the Si content increases. Dislocations evolve from complex entangled patterns in Nb to parallel long-straight screw dislocation-dominated structures in Nb-Si alloys. In-situ indentation reveals that the origins of the marked hardening in Nb-Si alloy are the reduction of dislocation mobility and cross-slip propensity. Large densities of dislocation debris-superjogs and loops introduced throughout the sample during warm rolling and annealing are found to provide active internal dislocation sources, which explain the minimal ductility loss seen in these Nb-Si alloys. These findings can help guide the alloy design of high-performance refractory materials for extreme temperature applications.

Bo-Qing Li, Irene J. Beyerlein, Shuhei Shinzato, Shigenobu Ogata, Wei-Zhong Han,“Mechanism of solute hardening and dislocation debris-mediated ductilization in Nb-Si alloy”, Journal of Materials Science & Technology,in press.

Akarsh Verma, Oliver K. Johnson, Gregory B. Thompson, Shigenobu Ogata and Eric R. Homer,“Solute influence in transitions from non-Arrhenius to stick-slip Arrhenius grain boundary migration”, Acta Materialia, 265 (2024) 119605-1-12.

Synthetic driving force based molecular dynamics simulations are used to evaluate the grain boundary velocities for an incoherent Σ3 [111] 60° {11 8 5} GB in elemental nickel and its copper-based alloys in the dilute limit. We examine the effects of temperature, solute content, and magnitude of the driving force on grain boundary velocity trends and their associated mechanisms. We observe that, for pure nickel and its copper alloys in the dilute limit at high driving forces, these special grain boundaries exhibit non-Arrhenius or anti-thermal migration behavior, where temperature and grain boundary velocity are inversely related. For lower driving forces, the increased copper content leads to stick-slip migration behavior and a likely transition from non-Arrhenius to Arrhenius temperature dependence. Interestingly, the ordered atomic motions are frustrated but unchanged by the solute content and stick-slip migration. While the results are generally consistent with the Cahn-Lücke-Stüwe (CLS) model, no solute drag is observed; rather, the solute effects are likely the result of solute pinning.

Tomohito Tsuru, Ivan Lobzenko, Shigenobu Ogata, Wei-Zhong Han,“First-principles analysis of the effects of oxygen, vacancies, and their complexes on the screw dislocation motion in body-centered cubic Nbl”, Journal of Materials Research and Technology, 28 (2024) 1013-1021.

Some solute atoms induce strengthening and embrittlement in body-centered cubic refractory metals. Especially, interstitial oxygen produces remarkable strengthening effects in Nb, wherein the yield stress of oxygen-doped Nb alloys is more than twice that of pure Nb. Conventional mechanisms cannot explain this oxygen-induced dramatic strengthening because the interaction between dislocations and oxygen atoms is not so significant. In a previous study, we found that the formation of vacancy–oxygen pairs enhances the attractive interaction with a screw dislocation and increases the energy barrier for dislocation motion associated with cross-kink nucleation in Nb–O alloys. However, the strengthening effect could not be described by the pinning model for dislocation motion. Herein, we focused on the atomic-level analysis of the fundamental process related to the dislocation motion around a vacancy, an oxygen atom, and a vacancy–oxygen pair. First-principles calculations revealed that the vacancy–oxygen pairs increase the energy barrier with respect to the dislocation motion more substantially than vacancies and oxygen interstitials owing to a unique oxygen-induced mechanism; an octahedral–tetrahedral shuffling process of oxygen is necessary for dislocation passing through vacancy–oxygen obstacles. Such event almost never happens in the real metallic materials. Instead, cross-kink nucleation occurs frequently to overcome the widely distributed vacancy–oxygen obstacles, which contributes to the dramatic strengthening.

Akarsh Verma, Oliver K. Johnson, Gregory B. Thompson, Ian Chesser, Shigenobu Ogata, Eric R. Homer,“Insights into factors that affect non-Arrhenius migration of a simulated incoherent Σ3 grain boundary”, Acta Materialia, 258 (2023) 119210-1-12.

Non-Arrhenius grain boundary migration, sometimes referred to as antithermal migration where temperature and GB velocity values are inversely related to each other, is examined in an incoherent twin Σ3 [111] 60° (11 8 5) nickel grain boundary. Molecular dynamics is used to simulate migration and examine the effect of various factors on the migration, including interatomic potential, system size, driving force, and variation of atomic grain boundary structure. A classical model for grain boundary migration, in its unsimplified form, is used to analyze the results. The grain boundaries exhibit migration mechanisms with very low apparent barrier heights to migration. As a result, the boundaries migrate quickly but exhibit a velocity saturation similar to that of dislocations. The various interatomic potentials exhibit different migration velocities, but their similarities suggest they all predict similar overall behaviors of migration. The variation of atomic structure in the same incoherent twin grain boundary leads to diverse behaviors with barrier heights that vary from non-Arrhenius to Arrhenius migration. Facet nucleation is confirmed not to be a requirement for this boundary based on an examination of simulation cell size; however, the presence and/or interaction between numerous facets does suggest a slowing and increased barrier height to migration for larger boundaries.

Rana Hossain, Hajime Kimizuka, and Shigenobu Ogata,“Asymmetry in core structure and mobility of basal dislocations in a Ti₃ SiC₂ MAX phase: An atomistic study with machine-learned force fields”, Physical Review Materials, 7-5, (2023) 053608-1-12.

MAX phases are a unique class of atomically layered ternary ceramics that deform plastically at room temperature owing to highly mobile basal dislocations (BDs). To understand and control the ductility of these materials, it is crucial to study the core structures and mobilities of BDs. In this study we developed a machine-learning-based spectral neighbor analysis potential (SNAP) to perform atomistic simulations of edge, screw, and mixed BDs in a Ti₃ SiC₂ MAX phase. The SNAP was trained on density functional theory (DFT)-calculated data. The SNAP calculations demonstrate that the BD core structure exhibits significant asymmetry that is dependent on the position of the weakly bonded Si layer relative to the Ti(4f)–Si slip plane. The dislocation core either splits into Shockley partials or remains compact, depending on whether compressive or tensile stresses act on the Si layer. This asymmetry in the BD core structure agrees well with DFT results. Differential displacement and Nye tensor distribution analyses reveal that undissociated BD cores are centered on Si layers and spread over adjacent parallel basal planes. Additionally, they are three orders of magnitude less mobile than partial BDs. The Peierls stresses of partial edge BDs (∼71 MPa) are closer to those of metals than those of ceramics, suggesting that edge BDs play a key role in the incipient plasticity of Ti₃ SiC₂ MAX phases at low temperatures. The findings of this study contribute to a deeper understanding of the complex behavior of BDs at the atomic level and provide theoretical support for elucidating the unique deformation modes of crystals with atomically layered structures owing to the motion of BDs.

Yan Li, Xufei Fang, Eita Tochigi, Yu Oshima, Sena Hoshino, Takazumi Tanaka, Hiroto Oguri, Shigenobu Ogata, Yuichi Ikuhara, Katsuyuki Matsunaga, Atsutomo Nakamura,“Shedding new light on the dislocation-mediated plasticity in wurtzite ZnO single crystals by photoindentation”, Journal of Materials Science & Technology, (2023) 206-216.

Dislocation-mediated plasticity in inorganic semiconductors and oxides has attracted increasing research interest because of the promising mechanical and functional properties tuned by dislocations. In this study, we investigated the effects of light illumination on the dislocation-mediated plasticity in hexagonal wurtzite ZnO, a representative third-generation semiconductor material. A (0001) 45^o off sample was specially designed to preferentially activate the basal slip on (0001) plane. Three types of nanoindentation tests were performed under four different light conditions (550 nm, 334 nm, 405 nm, and darkness), including low-load (60 μN) pop-in tests, high-load (500 μN) nanoindentation tests, and nanoindentation creep tests. The maximum shear stresses at pop-in were found to approximate the theoretical shear strength regardless of the light conditions. The activation volume at pop-ins was calculated to be larger in light than in darkness. Cross-sectional transmission electron microscope images taken from beneath the indentation imprints showed that all indentation-induced dislocations were located beneath the indentation imprint in a thin-plate shape along one basal slip plane. These indentation-induced dislocations could spread much deeper in darkness than in light, revealing the suppressive effect of light on dislocation behavior. An analytical model was adopted to estimate the elastoplastic stress field beneath the indenter. It was found that dislocation glide ceased at a higher stress level in light, indicating the increase in the Peierls barrier under light illumination. Furthermore, nanoindentation creep tests showed the suppression of both indentation depth and creep rate by light. Nanoindentation creep also yielded a larger activation volume in light than in darkness.

Rana Hossain, Hajime Kimizuka, Yoshinori Shiihara, Shigenobu Ogata,“Core structure and Peierls barrier of basal edge dislocations in Ti3AlC2 MAX phase”, Computational Materials Science, 209 (2022) 111366-1-9.

In this study, a chemical order related concept “history-dependent multi-layer generalized stacking fault energy” (HDML-GSFE) was proposed, and it was then demonstrated by employing the recent, very in- teresting multi-principal element alloy (CoCrNi medium-entropy alloys; MEA) with different chemical short-range order (CSRO) levels using a density functional theory (DFT)-based neural network interatomic potential. To demonstrate the impacts of the history dependency and interlayer (atomic interlayers of the slip system) coupling effect on the GSFE of CSRO MEAs, HDML-GSFEs were computed for different shear deformation pathways of the MEAs with different CSRO levels, such as interlayer multiple-time slipping, twin growth, and γ−ε(FCC-HCP) phase transformation. It was demonstrated that multiple-time slip- ping induces CSRO collapse, leading to local shear softening due to the history dependency of GSFE. In addition, it was found that the slipping of neighboring atomic interlayers is affected by the slipping re- sulting from the induced CSRO collapse of present interlayers because of the interlayer coupling effect of GSFE. Eventually, by employing a novel kinetic Monte Carlo (kMC) simulation method based on disloca- tion/disconnection loop nucleation events and using the HDML-GSFE with the history dependency and interlayer coupling effect, we proposed a laminated micro-substructure evolution that involves twinning and γ−εphase transformations subject to a finite shear strain rate and finite temperature.

Fan-Shun Meng, Jun-Ping Du, Shuhei Shinzato, Hideki Mori, Peijun Yu, Kazuki Matsubara, Nobuyuki Ishikawa, and Shigenobu Ogata,“General-purpose neural network interatomic potential for the α-iron and hydrogen binary system: Toward atomic-scale understanding of hydrogen embrittlement”, Physical Review Materials, 5, 11 (2021) 113606-1-16.

To understand the physics of hydrogen embrittlement at the atomic scale, a general-purpose neural network interatomic potential (NNIP) for the α-iron and hydrogen binary system is presented. It is trained using an extensive reference database produced by density functional theory (DFT) calculations. The NNIP can properly describe the interactions of hydrogen with various defects in α-iron, such as vacancies, surfaces, grain boundaries, and dislocations; in addition to the basic properties of α-iron itself, the NNIP also handles the defect properties in α-iron, hydrogen behavior in α-iron, and hydrogen-hydrogen interactions in α-iron and in vacuum, including the hydrogen molecule formation and dissociation at the α-iron surface. These are superb challenges for the existing empirical interatomic potentials, like the embedded-atom method based potentials, for the α-iron and hydrogen binary system. In this study, the NNIP was applied to several key phenomena necessary for understanding hydrogen embrittlement, such as hydrogen charging and discharging to α-iron, hydrogen transportation in defective α-iron, hydrogen trapping and desorption at the defects, and hydrogen-assisted cracking at the grain boundary. Unlike the existing interatomic potentials, the NNIP simulations quantitatively described the atomistic details of hydrogen behavior in the defective α-iron system with DFT accuracy. (Potential files download)

Hongxian Xie, Gaobing Wei, Yuanfang Lua, Junping Du, Fuxing Yin, Guang-Hong Lu and Shigenobu Ogata,“Driving force of zero-macroscopic-strain deformation twinning in face-centred-cubic metals”, Philosophical Magazine, 101, 21 (2021) 2318-2330.

Zero-macroscopic-strain deformation twinning (ZMS-DT) is widely observed in many face-centred-cubic (FCC) metals and alloys. However, the driving force of ZMS-DT is a controversial issue and has not been fully clarified for a long time. Based on molecular dynamics simulations to various FCC metals, we found that ZMS-DT, i.e. Σ3{112} incoherent twin boundary migration can be driven by simultaneously applying both normal and shear strains/stresses to the twin boundary (TB), and changing the sign of the normal or the shear strain/stress can change the direction of the incoherent TB migration. With analysing the results of atomistic strain energy calculation and anisotropic elasticity theory, we revealed the strain energy imbalance, which originates from elastic anisotropic response of materials, between the two sides of the twin boundary under normal–shear strain (or stress) coupling condition essentially drives the TB migration and twin growth. Eventually, we deduce that the elastic anisotropy ratio can be one of the key material constants which affect the twinnability of FCC metals.

Xiang Wang, Sixue Zheng, Shuhei Shinzato, Zhengwu Fang, Yang He, Li Zhong, Chongmin Wang, Shigenobu Ogata, Scott X. Mao,“Atomistic processes of surface-diffusion-induced abnormal softening in nanoscale metallic crystals”, Nature Communications, 12 (2021) 5237-1-9.

Ultrahigh surface-to-volume ratio in nanoscale materials, could dramatically facilitate mass transport, leading to surface-mediated diffusion similar to Coble-type creep in polycrystalline materials. Unfortunately, the Coble creep is just a conceptual model, and the associated physical mechanisms of mass transport have never been revealed at atomic scale. Akin to the ambiguities in Coble creep, atomic surface diffusion in nanoscale crystals remains largely unclear, especially when mediating yielding and plastic flow. Here, by using in situ nanomechanical testing under high-resolution transmission electron microscope, we find that the diffusion-assisted dislocation nucleation induces the transition from a normal to an inverse Hall-Petch-like relation of the strength-size dependence and the surface-creep leads to the abnormal softening in flow stress with the reduction in size of nanoscale silver, contrary to the classical “alternating dislocation starvation” behavior in nanoscale platinum. This work provides insights into the atomic-scale mechanisms of diffusion-mediated deformation in nanoscale materials, and impact on the design for ultrasmall-sized nanomechanical devices.

Yuji Sato , Thomas Swinburne , Shigenobu Ogata & David Rodney, “Anharmonic effect on the thermally activated migration of {101̄2} twin interfaces in magnesium”, Materials Research Letters, 9, 5 (2021) 231-238.

Using a recent linear scaling method that fully accounts for anharmonic thermal vibrations, we calculated the activation free energy for {101¯2} twin boundary migration in magnesium up to 450 K, under both resolved shear stresses and non-glide stresses resulting from c-axis tension. Comparing to direct molecular dynamics data, we show that the harmonic transition state theory unexpectedly overestimates the activation entropy above temperatures as low as 100 K, leading to underestimates of the nucleation time by many orders of magnitude. Whilst a specific interface is studied, anharmonic and non-glide effects are expected to be generally significant in thermally activated interface migration.

Tomohito Tsuru, Masato Wakeda, Tomoaki Suzudo, Mitsuhiro Itakura, and Shigenobu Ogata, “Anomalous solution softening by unique energy balance mediated by kink mechanism in tungsten-rhenium alloys”, Journal of Applied Physics, 127 (2020) 025101.

Nucleation of transmutation products such as rhenium (Re) and osmium (Os) is a central issue contributing to changes in mechanical properties under neutron irradiation in fusion reactors. In particular, Re solutes in tungsten (W) not only affect hardening via radiation-induced precipitation but also have a notable softening effect. We explored the softening/strengthening behaviors of various solutes in a W matrix by density functional theory (DFT) calculations combined with a solid solution model. Our DFT calculations of the solutes show a clear trend in the interaction energy between different solutes and screw dislocations, which also influences the solid solution behavior. These predictions, based on a solid solution model and DFT calculations, reasonably reproduce the complex softening/strengthening behavior as a function of temperature and solute concentration. Notably, solutes such as Re have relatively weak attractive interactions and do not markedly influence the pinning effect; however, such solutes can reduce the energy barrier for kink pair nucleation. We conclude that this specific balance is the origin of macroscopic solid solution softening in dilute body-centered cubic alloys.

Shuhei Shinzato, Masato Wakeda, Shigenobu Ogata, "An atomistically informed kinetic Monte Carlo model for predicting solid solution strengthening of body-centered cubic alloys", International Journal of Plasticity, 122 (2019) 319-337.

In order to predict solid solution strengthening in body-centered cubic dilute substitutional alloys, we developed an atomistically informed kinetic Monte Carlo (kMC) model for screw dislocation motion, which is a major determinant of the yield strength of the BCC alloys. The kMC model only requires parameters obtainable using atomic simulations. The parameters in the developed kMC model were actually determined for Fe–Si dilute alloys using atomistically derived activation energies of kink-pair nucleation and kink migration as well as their stress dependencies. The activation energies were computed using the nudged elastic band method with developed interatomic potentials based on first-principles density functional theory. Eventually, the critical resolved shear stress (CRSS), activation volume of the dislocation glide, and their temperature and solute concentration dependencies were directly obtained by two dimensional kMC dislocation glide simulations. Our kMC model qualitatively reproduced the trends of experimentally observed temperature and concentration dependencies of CRSS, and thus it can naturally describe solid solution strengthening, and softening without any empirical information. In addition, the limitations of the two dimensional model, including single slip and lack of non-Schmid effect, are discussed, which results in quantitative difference with the experimental CRSS.

Masato Wakeda, Tomohito Tsuru, Masanori Kohyama, Taisuke Ozaki, Hideaki Sawada, Mitsuhiro Itakura, and Shigenobu Ogata, “Chemical misfit origin of solute strengthening in iron alloys”,Acta Materialia, 131 (2017) 445-456.

In this first-principles study, we investigate the effect of many kinds of substitutional solute species on screw dislocation motion in bcc-Fe, dominating the strength of dilute Fe alloys. 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 firstprinciples interaction energy with the “Easy-core” structure excellently correlates with the change in the g-surface caused by solute atoms (i.e., chemical misfit). Based on the interaction energy, we predicted 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.

Jun-Ping Du, Yun-Jiang Wang, Yu-Chieh Lo, Liang Wan, and Shigenobu Ogata, “Mechanism transition and strong temperature dependence of dislocation nucleation from grain boundaries: An accelerated molecular dynamics study”,Physical Review B, 94 (2016) 104110-1-8.

Accelerated molecular dynamics reveals a mechanism transition and strong temperature dependence of dislocation nucleation from grain boundaries (GBs) in Cu. At stress levels up to ~ 90% of the ideal dislocation nucleation stress, atomic shuffling at the E structural unit in a GB acts as a precursor to dislocation nucleation, and eventually a single dislocation is nucleated. At very high stress levels near the ideal dislocation nucleation stress, a multiple dislocation is collectively nucleated. In these processes, the activation free energy and activation volume depend strongly on temperature. The strain-rate dependence of the critical nucleation stress is studied and the result shows that the mechanism transition from the shuffling-assisted dislocation nucleation mechanism to the collective dislocation nucleation mechanism occurs during the strain rate increasing from 10^-4 s-1 to 10^10 s-1. The findings of the mechanism transition and strong temperature dependence of GB dislocation nucleation have been missed in the previous conventional molecular dynamics studies.

Kazuki Matsubara, Hajime Kimizuka, Shigenobu Ogata, “Formation of {1121} twins from I1-type stacking faults in Mg: A molecular dynamics study”,Computational Materials Science, 122 (2016) 314-321.

The nature of the reaction and transient behavior of I1-type stacking faults (SFs) under shear stress was investigated to understand the role of I1 SFs in non-basal plastic deformation of Mg. This was because it has been suggested that I1 SFs formed via solute (such as Y) additions to Mg may act as the nucleation source of non-basal dislocations, which serve as important deformation modes to improve the workability. Crystal models of pure Mg containing an I1 SF were deformed by simple shear along the basal plane using molecular dynamics simulations with many-body interatomic potentials. The characteristic dissociation reaction was observed at the boundaries of the I1 SF, where < c+a > dislocations on the pyramidal plane, Shockley partial dislocations on the basal plane, and stair-rod dislocations were nucleated. Interestingly, instead of activation of the non-basal dislocations, {1121} twins were nucleated in theearly stage of the reaction and grew steadily as the applied stress was increased. It was suggested that the I1 SF was likely to act as a ‘‘reactive” defect to assist and accommodate the c-axis deformation. Further, the deformation-induced twin boundaries were found to act as moderate obstacles to dislocation movement on the basal planes in the Mg matrix, without significantly sacrificing its plasticity.

Ming-Shuai Ding, Jun-Ping Du, Liang Wan, Shigenobu Ogata, Lin Tian, Evan Ma, Wei-Zhong Han, Ju Li, and Zhi-Wei Shan, “Radiation-Induced Helium Nanobubbles Enhance Ductility in Submicron-Sized Single-Crystalline Copper”, Nano Letters, 16-7 (2016) 4118-4124.

The workability and ductility of metals usually degrade with exposure to irradiation, hence the phrase “radiation damage”. Here, we found that helium (He) radiation can actually enhance the room-temperature deformability of submicron-sized copper. In particular, Cu single crystals with diameter of 100–300 nm and containing numerous pressurized sub-10 nm He bubbles become stronger, more stable in plastic flow and ductile in tension, compared to fully dense samples of the same dimensions that tend to display plastic instability (strain bursts). The sub-10 nm He bubbles are seen to be dislocation sources as well as shearable obstacles, which promote dislocation storage and reduce dislocation mean free path, thus contributing to more homogeneous and stable plasticity. Failure happens abruptly only after significant bubble coalescence. The current findings can be explained in light of Weibull statistics of failure and the beneficial effects of bubbles on plasticity. These results shed light on plasticity and damage developments in metals and could open new avenues for making mechanically robust nano- and microstructures by ion beam processing and He bubble engineering.

Wei-Zhong Han, Ling Huang, Shigenobu Ogata, Hajime Kimizuka, Zhao-Chun Yang, Christopher Weinberger, Qing-Jie Li, Bo-Yu Liu, Xi-Xiang Zhang, Ju Li, Evan Ma, Zhi-Wei Shan,　“From “smaller is stronger” to “size-independent strength plateau”: towards measuring the ideal strength of iron”, Advanced Materials, 27 (2015) 3385-3390.

The trend from “smaller is stronger” to “size-independent strength plateau” is observed in the compression of spherical iron nanoparticles. When the diameter of iron nanospheres is less than a critical value, the maximum contact pressure saturates at 10.7 GPa, corresponding to a local shear stress of ≈9.4 GPa, which is comparable to the theoretical shear strength of iron.

Shigeto Takebayashi, Kohsaku Ushioda, Naoki Yoshinaga, and Shigenobu Ogata, “Impact toughness of 0.3 mass % carbon tempered martensitic steels evaluated by instrumented Charpy test”, Mater. Sci. Forum, 783-786 (2014) 1033-1038.

The effect of tempering temperature on the impact toughness of 0.3 mass% carbon martensitic steels with prior austenite grain (PAG) size of about 6 μm and 30 μm were investigated. Instrumented Charpy impact test (ICIT) method was used to evaluate the impact toughness. The tempering temperature of 723K gives the largest difference in the Charpy impact energy at room temperature (RT) between the specimens with two different PAG sizes. Investigation of the test temperature dependence of Charpy impact energy in the 723K tempered steels shows a steep transition at around 200 K for the 6 μm PAG specimen, while it shows a continuous slow transition in a wide range of temperature for the 60 μm PAG specimen. ICIT waveform analysis shows that the fracture propagation energy in stead of the fracture initiation energy mainly controls the temperature dependence of the impact energy. The carbide size distribution in these two specimens was investigated by SEM and TEM. The 60 μm PAG specimen shows the distribution of coarser carbides than does the 6 μm PAG specimen, which seems to be the main reason for the observed difference between them in the Charpy impact energy and the other properties of impact fracture.

Masato Wakeda, Hajime Kimizuka and Shigenobu Ogata, "Atomistic Study of Interaction between Screw Dislocation and Si Atom in Fe-Si Alloy", Journal of the Japan Institute of Metals, 77-10 (2013) 409-414.

Atomistic details of the interaction between screw dislocation and substitutional solute Si in Fe-Si alloy was investigated by atomistic modeling method. We developed an embedded atom method (EAM) potential for Fe-Si interaction based on the density functional theory calculations and then evaluated the interaction energy between Si and screw dislocation using the developed potential. The interaction energy is found to become larger as the Si atom approaches the screw dislocation. This indicates that attractive driving force acting between the Si atom and screw dislocation can lead to the athermal resistance. Using Nudged Elastic Band (NEB) method, we computed the energy barrier for screw dislocation glide associated with double kink formation, and found that the energy barrier is reduced by nearby existing Si atom. These results suggest that solute Si causes both solid-solution hardening and softening in Fe-Si alloys.

Shin Yamamoto, Yun-Jiang Wang, Akio Ishii, and Shigenobu Ogata, "Atomistic Design of High Strength Crystalline-Amorphous Nanocomposites", Materials Transactions, 54-9 (2013) 1592-1596.

There is a long-standing demand for materials which could simultaneously demonstrate multiple promising properties like high strength, good ductility and toughness. In this study, a three-dimensional bulk nanocomposite material which is composed of nanoscale crystalline metal and metallic glass is revealed to present high strength and potentially good ductility by molecular dynamics. A critical high strength is achieved by varying the ratio between crystalline and amorphous phase. The critical strength is revealed to be higher than that expected from the rule of mixture. The mechanism underlying the occurrence of critical strength in the nanocomposite is elucidated by the interaction between dislocation and matrix of amorphous phase. Our concept could guide the engineers to design more advanced bulk nanostructured materials.

Akio Ishii, Ju Li, and Shigenobu Ogata, ""Conjugate Channeling" Effect in Dislocation Core Diffusion: Carbon Transport in Dislocated BCC Iron", PloS one, 8-4 (2013) e60586-1-7.

Dislocation pipe diffusion seems to be a well-established phenomenon. Here we demonstrate an unexpected effect, that the migration of interstitials such as carbon in iron may be accelerated not in the dislocation line direction ξ, but in a conjugate diffusion direction. This accelerated random walk arises from a simple crystallographic channeling effect. c is a function of the Burgers vector b, but not ξ, thus a dislocation loop possesses the same everywhere. Using molecular dynamics and accelerated dynamics simulations, we further show that such dislocation-core-coupled carbon diffusion in iron has temperature-dependent activation enthalpy like a fragile glass. The 71° mixed dislocation is the only case in which we see straightforward pipe diffusion that does not depend on dislocation mobility.

Yun-Jiang Wang, Guo-Jie Jason Gao, and Shigenobu Ogata, "Size-dependent transition of deformation mechanism, and nonlinear elasticity in Ni3Al nanowires", Applied Physics Letters, 102 (2013) 041902-1-5.

A size-dependent transition of deformation mechanism is revealed in Ni3Al nanowire under atomistic uniaxial tension. Deformation twinning is replaced by phase transformation when the diameter of Ni3Al nanowire reduces to a critical value near 4?nm. Enhanced size-dependent nonlinear elasticity is observed in the nanowires, in comparison to their bulk counterpart which is benchmarked by combined density functional and atomistic study. This study provide fundamental understanding on the size-dependent deformation mechanisms of nanostructured alloys.

Yun-jiang Wang, Akio Ishii, and Shigenobu Ogata, "Grain Size Dependence of Creep in Nanocrystalline Copper by Molecular Dynamics", Materials Transactions, 53-1 (2012) 156-160.

The grain size dependence of creep is critical to understand the plastic deformation mechanism of nanoscale metals. Here we used molecular dynamics to study the stress-induced grain size exponent transition in creep of nanocrystalline copper. The grain size exponent was found to initially increase with increasing stress, then decrease after some critical stress. The derived grain size exponents are in agreement with experimental results for diffusional and grain boundary sliding creep at low stress. While, the founded decreasing grain size exponent with increasing stress for dislocation nucleation creep in nanocrystal is in contrast with conventional materials. We propose a constitutive equation for dislocation nucleation governed creep in nanocrystal to explain its grain size dependence transition with stress.

Yun-Jiang Wang, Akio Ishii, and Shigenobu Ogata, "Transition of creep mechanism in nanocrystalline metals", Physical Review B, 84-22 (2011) 224102-1-7.

Understanding creep mechanisms with atomistic details is of great importance to achieve the mechanical and thermodynamical stabilities of nanocrystalline (NC) metals over a wide temperature range. Here we report a molecular dynamics analysis of creep in NC copper dominated by competing deformation mechanisms. We found the dominating creep mechanism transits from grain boundary (GB) diffusion to GB sliding, and then dislocation nucleation with increasing stress. The derived stress exponent, small activation volume of 0.1?10b3, and grain size exponent all agree quantitatively with experimental values. We proposed a stress-temperature deformation map in NC metals accommodated by the competition among different stress-driven, thermally activated processes. The model is general to answer the question why deformation mechanism transits with stress in NC metals.

Hideki Mori, Hajime Kimizuka, and Shigenobu Ogata, "Microscopic Phase-Field Modeling of Edge and Screw Dislocation Core Structures and Peierls Stresses of BCC Iron", Journal of the Japan Institute of Metals, 75-2 (2011) 104-109. [in Japanese].

We investigate edge and screw dislocation core structures and Peierls stresses of BCC iron using microscopic phase-field (MPF) modeling. Parameters needed for the MPF modeling, such as the generalized-stacking-fault (GSF) energy, are determined based on first principles density functional theory (DFT) calculations. Screw dislocation core has six-fold symmetric structure and 0.05 nm width. The edge dislocation core is three times wider than the screw dislocation core. The Peierls stresses of edge and screw dislocations are estimated as 0.07 GPa and 2.8 GPa, respectively.

Hideki Mori, Hajime Kimizuka, and Shigenobu Ogata, "Dislocation Properties and Peierls Stress of BCC Iron Based on Generalized-Stacking-Fault Energy Surface by Using First Principles Calculations", Journal of the Japan Institute of Metals, 73-8 (2009) 595-600. [in Japanese]

We calculate the generalized-stacking-fault (GSF) energy surface of BCC iron by using first principles density functional theory (DFT). We employ the semidiscrete variational Peierls-Nabarro (SVPN) model to investigate the edge dislocation properties of BCC iron. The dislocation core width and Peierls stress are estimated as 0.20 nm and 80 MPa, respectively. We also estimate the GSF energy surface and dislocation properties using two different embedded atom method (EAM) potentials and compare with the DFT results.

Futoshi Shimizu, Shigenobu Ogata, Hajime Kimizuka, Takuma Kano, Ju Li, and Hideo Kaburaki, "First-Principles Calculation on Screw Dislocation Core Properties in BCC Molybdenum", Journal of Earth Simulator, 7 (2007) 17-21.

Predicting atomistic properties of a dislocation is a first step toward an understanding of plastic behavior of materials, in particular BCC metals. The core structure and Peierls stress of a screw dislocation in BCC metals have been studied over the years using the first-principles and empirical methods, however, their conclusions vary due to the inefficiency of the methods. We have executed first-principles calculations based on the density functional method, employing the most accurate 1×1×20 k-point samplings, to determine thecore structure and Peierls stress of the a0/2[111] screw dislocation of molybdenum. We have concluded that the core has a 6-fold structure, and determined the Peierls stress of 1.8 GPa for the simple shear strain along the (-110)<111> direction.