Rong Fu, Zhiyuan Rui, Junping Du, Shihao Zhang, Fanshun Meng, Shigenobu Ogata,“Temperature and loading-rate dependent critical stress intensity factor of dislocation nucleation from crack tip: Atomistic insights into cracking at slant twin boundaries in Nano-twinned TiAl alloys”, Journal of Materials Science & Technology, In Press (2024) .

This paper investigates the temperature and loading rate dependencies of the critical stress intensity factor (KIC) for dislocation nucleation at crack tips. We develop a new KIC formula with a generalized form by incorporating the atomistic reaction pathway analysis into Transition State Theory (TST), which captures the KIC of the first dislocation nucleation event at crack tips and its sensitivity to temperature and loading rates. We use this formula and atomistic modeling information to specifically calculate the KIC for quasi-two-dimensional crack tips located at 3 various slant twin boundaries in nano-twinned TiAl alloys across a wide range of temperatures and strain rates. Our findings reveal that twinning dislocation nucleation at the crack tip dominates crack propagation when twin boundaries (TBs) are tilted at 15.79° and 29.5°. Conversely, when TBs tilt at 45.29°, 54.74°, and 70.53°, dislocation slip becomes the preferred mode. Additionally, at TB tilts of 29.5° and 70.53°, at higher temperatures above 800 K and typical experimental loading rates, both dislocation nucleation modes can be activated with nearly equal probability. This observation is particularly significant as it highlights scenarios that molecular dynamics simulations, due to their time scale limitations, cannot adequately explore. This insight underscores the importance of analyzing temperature and loading rate dependencies of the KIC to fully understand the competing mechanisms of dislocation nucleation and their impact on material behavior.

Akarsh Verma, Sandeep Kumar Singh, Shigenobu Ogata,“Transitioning of deformation mode in bicrystalline Al-Mg alloy with Σ3 [111] 60°{1185} faceted grain boundary”, Materials Letters,375, (2024) 137232-1-5.

In this letter, the authors have elucidated the effect of alloying biocompatible and lighter magnesium (Mg) solute atoms in a bicrystalline aluminium (Al) solvent incorporated with a faceted coherent-incoherent grain boundary (GB) to analyze the tensile deformation mechanisms. Cryogenic temperature and high strain rate conditions were imposed through molecular dynamics simulations to report the detrimental effect of Mg addition on the tensile strength of Al-Mg alloys. Evidence of transitioning in deformation modes for pure Al and Al-Mg alloys was seen. On elaborating, for pure Al, GBs acted as the dislocation nucleation site; whereas for Al-Mg alloy, it was both the GBs and grains. It was observed that stacking faults (SFs) originated from GBs and propagated toward grains in Al. In contrast, in Al-Mg alloys, SFs were generated simultaneously from both GBs and grains. This simultaneous nucleation of dislocations and SFs from both GBs and grains renders bicrystalline alloys lower the elastic limit than pure metal. Conversely, beyond this elastic limit, the alloys exhibit higher flow stress than the pure metal due to the Mg-dislocation interactions. These findings offer valuable insights for tailoring the mechanical properties of Al-Mg alloys to meet aerospace, automotive and marine engineering-related application requirements.

Hongxian Xie, Tingting He, Gaobing Wei, Wei Fang, Shigenobu Ogata, Guang-Hong Lu,“A new criterion for formation capability of annealing twin in face-centered cubic metals/alloys”, Acta Materialia 278, (2024) 120245-1-10.

Annealing twins (ATs) are prevalent in face-centered cubic (FCC) metals/alloys and significantly influence their mechanical properties. Therefore, understanding the formation capability of ATs is crucial for controlling the mechanical properties of FCC metals/alloys. Although the "Stacking Fault Energy" is a widely used factor to characterize AT formation capability, it doesn’t comprehensively explain the observed experimental rankings of AT formation capability. In this study, through a combination of experiments and molecular dynamics simulations, we demonstrated that AT growth is primarily driven by the migration of the Σ3 incoherent twin boundary. This migration arises from the elastic strain energy imbalance between grains on either side of the twin boundary caused by the elastic anisotropy of crystal. Based on these findings, we propose a new criterion, defined as a ratio of the coherent twin boundary energy to the anisotropy factor, to represent the AT formation capability (inverse relationship). Such new criterion well explains the experimental observation, and can be generalized to the zero macroscopic strain deformation twin.

Akio Ishii, Keitaro Horikawa, Masato Hoshino, Kentaro Uesugi,“Strain-field calculation based on X-ray CT images via point matching of impurities using Voronoi diagram”, Materialia, 36 (2024) 102175.

We propose an approach for calculating the strain field inside materials using X-ray computed tomography (CT) images obtained during deformation tests. Our approach is based on the point matching of the detected impurities from the CT images, and the points in the reference and deformed CT images are used to construct a Voronoi diagram. In addition to the change in distance between the reference and deformed points, the volume and shape of the Voronoi polyhdra at each point are used for point matching. By applying the proposed method to a notched Al alloy sample, we calculated the change in the strain field around the notch during deformation in a tensile test using X-ray CT images, and our method was validated.

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.

Hongxian Xie, Gaobing Wei, Jun-Ping Du, Akio Ishii, Guanghong Lu, Peijun Yu, Shigenobu Ogata,“Shuffling pathway of anti-twinning in body-centered-cubic metals”, Scripta Materialia,246 (2024) 116083.

In this study, anti-twinning and deformation twinning of body-centered-cubic (BCC) metals under nanoindentation were examined. Molecular dynamics (MD) simulations of BCC tantalum (Ta) showed both the existence of anti-twinning and a novel two-layer-by-two-layer anti-twinning pathway consisting of a staggered shuffle sliding of {112} atomic planes in the [111] and [ 1 11] directions, generating finite shear displacement along the anti-twin direction. The density functional theory (DFT) multi-dimensional interlayer slipping energy landscape of the newly observed shuffling anti-twinning process has a lower energy barrier for both twin nucleation and growth than the recently proposed layer-by-layer anti-twinning process.

Le Li, Jun-Ping Du, Shigenobu Ogata, Haruyuki Inui,“Variation of first pop-in loads in nanoindentation to detect chemical short-range ordering in the equiatomic Cr-Co-Ni medium entropy alloy”, Acta Materialia, 269 (2024) 119775.

The variations of the first pop-in load in nanoindentation with annealing temperature and time have been investigated both by experiment and theoretical molecular dynamics (MD) simulations for the equiatomic Cr-Co-Ni medium-entropy alloy (MEA). Regardless of the loading rates, the first pop-in load tends to increase with the increase in electrical resistivity consistently by experiment and MD simulations. The trend in the variation of pop-in load with annealing temperature/time coincides with what is observed in the variation of electrical resistivity in our previous study, indicating that the first pop-in load in nanoindentation can thus be used to qualitatively detect the evolution of short-range ordering (SRO) in the Cr-Co-Ni MEA, and that the peak temperature for SRO for the equiatomic Cr-Co-Ni MEA is 673 K. MD simulations indicate that the first pop-in corresponds to the nucleation of partial dislocation loops beneath the indenter for both annealing conditions and that the increased first pop-in load in nanoindentation is attributed to the higher energy barrier for the nucleation of dislocation loops due to the higher degree of SRO.

Shihao Zhang, Fanshun Meng, Rong Fu, Shigenobu Ogata,“Highly efficient and transferable interatomic potentials for α-iron and α-iron/hydrogen binary systems using deep neural networks”, Computational Materials Science, 235 (2024) 112843-1-6.

Artificial neural network potentials (NNPs) have emerged as effective tools for understanding atomic interactions at the atomic scale in various phenomena. Recently, we developed highly transferable NNPs for 𝛼-iron and 𝛼-iron/hydrogen binary systems (Physical Review Materials 5 (11), 113606, 2021). These potentials allowed us to investigate deformation and fracture in 𝛼-iron under the influence of hydrogen. However, the computational cost of the NNP remains relatively high compared to empirical potentials, limiting their applicability in addressing practical issues related to hydrogen embrittlement. In this work, building upon our prior research on iron-hydrogen NNP, we developed a new NNP that not only maintains the excellent transferability but also significantly improves computational efficiency (more than 40 times faster). We applied this new NNP to study the impact of hydrogen on the cracking of iron and the deformation of polycrystalline iron. We employed large-scale through-thickness {110}⟨110⟩ crack models and large-scale polycrystalline 𝛼-iron models. The results clearly show that hydrogen atoms segregated at crack tips promote brittle-cleavage failure followed by crack growth. Additionally, hydrogen atoms at grain boundaries facilitate the nucleation of intergranular nanovoids and subsequent intergranular fracture. We anticipate that this high-efficiency NNP will serve as a valuable tool for gaining atomic-scale insights into hydrogen embrittlement.

Keitaro Horikawa, Akio Ishii,“Effect of purity on the internal morphology of blisters on aluminum surfaces”, Results in Materials, 21 (2024) 100522-1-10.

This study investigates the effect of purity of aluminum on the morphology of blisters formed on aluminum plates. To this end, aluminum samples with 99 % and 99.99 % purities are prepared, and blisters are generated on the sample surfaces through atmospheric heat treatment. Subsequently, the morphology of blisters on the surface and within the samples are examined using an ion-milling method and field-emission scanning electron microscopy (FE-SEM) with energy-dispersive X-ray spectroscopy. The FE-SEM images reveal that the shape and number density of the blisters vary with the purity of aluminum. The voids under the blisters are spherical when the purity of aluminum is greater than 99.99 % compared to flatter voids when the purity is 99 %. This morphological change is attributed to the local stress caused by neighboring impurity precipitates, and this is theoretically confirmed through Eshelby's ellipsoidal inclusion analysis.

T.L. Dora, Sandeep Kumar Singh, Radha Raman Mishra, Eric R. Homer, Shigenobu Ogata, Akarsh Verma,“Deformation and boundary motion analysis of a faceted twin grain boundary”, International Journal of Mechanical Sciences, 269 (2024) 109044-1-11.

In this article, molecular dynamics simulations are used to understand how a nickel bicrystal with faceted incoherent Σ3 grain boundaries respond to uniaxial tensile loading. The deformation response is studied over a wide range of temperatures (100 – 900 K) and strain rates (10⁷ – 10¹⁰ s⁻¹). The dislocation extraction algorithm and common neighbor analysis are employed to identify the deformation mechanisms. Our results reveal that the yield stress decreases with temperature and increases with strain rate; whereas the elastic modulus decreases with temperature and is independent of strain rate. Furthermore, incipient plasticity is detected ahead of the yield point at lower temperatures and lower strain rates. Interestingly, the incoherent twin grain boundaries are quite mobile under the uniaxial tensile loading at lower temperatures and lower strain rates. But this mobility decreased at higher temperatures and higher strain rates, thereby, confirming this faceted grain boundary's non-Arrhenius (anti-thermal) migration behavior even under mechanical loading. From a deformation perspective, the incoherent twin facet of the grain boundary served as the major source for stacking fault formation at lower temperatures and higher strain rates. However, with the increase in temperature, the stacking faults became shorter and originated from both the incoherent twin facet and the tips of coherent twin facet. These results are in qualitative agreement with the experimental results documented in the literature.

Jin-Yu Zhang, Gaël Huynh, Fu-Zhi Dai, Tristan Albaret, Shi-Hao Zhang, Shigenobu Ogata, David Rodney,“A deep-neural network potential to study transformation-induced plasticity in zirconia”, Journal of the European Ceramic Society, 44 (2024) 4243-4254.

Severe shear localization in metallic glasses (MGs) significantly limits their mechanical performance. Nanoglass (NG), composed of heterogeneous glassy domains created by introducing interfaces into MGs at the nanoscale, could be a promising strategy against severe shear localization, as demonstrated by numerous atomistic simulations. This study introduces a novel mesoscale kinetic Monte Carlo (kMC) model with a variable characteristic strain (VCS) to investigate the grain size effect in NGs. This model captures the complex evolution of shear bands during deformation, revealing a surprising transition from inhomogeneous to homogeneous deformation as the NG grain size decreases to approximately 10 nm. This transition is attributed to the impediment of shear band formation by the small grain size, facilitated by softer interfaces guiding early shear transformation zone (STZ) activities across the entire sample. Furthermore, a progressive reduction of elastic constants simulates the failure response observed in experiments. Our model predicts a critical grain size for the transition in agreement with molecular dynamics simulations and experiments, highlighting its potential for designing NGs with enhanced shear resistance. This mesoscale model enables the investigation of NG deformation with microstructural features on an experimentally-relevant spatial-temporal scale. This paves the way for tailoring NG microstructures to achieve enhanced mechanical performance, and opens new avenues for exploring the influence of interfaces in controlling shear localization.

Chih-Jen Yeh, Chang-Wei Huang, Yu-Chieh Lo, Shigenobu Ogata, Ding Yuan Li, Hsuan-Teh Hu, Jason Shian-Ching Jang,“Effect of nanoglass grain size investigated by a mesoscale variable characteristic strain model”, International Journal of Mechanical Sciences, 266 (2024) 108981-1-15.

Severe shear localization in metallic glasses (MGs) significantly limits their mechanical performance. Nanoglass (NG), composed of heterogeneous glassy domains created by introducing interfaces into MGs at the nanoscale, could be a promising strategy against severe shear localization, as demonstrated by numerous atomistic simulations. This study introduces a novel mesoscale kinetic Monte Carlo (kMC) model with a variable characteristic strain (VCS) to investigate the grain size effect in NGs. This model captures the complex evolution of shear bands during deformation, revealing a surprising transition from inhomogeneous to homogeneous deformation as the NG grain size decreases to approximately 10 nm. This transition is attributed to the impediment of shear band formation by the small grain size, facilitated by softer interfaces guiding early shear transformation zone (STZ) activities across the entire sample. Furthermore, a progressive reduction of elastic constants simulates the failure response observed in experiments. Our model predicts a critical grain size for the transition in agreement with molecular dynamics simulations and experiments, highlighting its potential for designing NGs with enhanced shear resistance. This mesoscale model enables the investigation of NG deformation with microstructural features on an experimentally-relevant spatial-temporal scale. This paves the way for tailoring NG microstructures to achieve enhanced mechanical performance, and opens new avenues for exploring the influence of interfaces in controlling shear localization.

Rana Hossain, Hajime Kimizuka, and Shigenobu Ogata,“Asymmetry in core structure and mobility of basal dislocations in a Ti ₃ SiC ₂ 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 Ti ₃ SiC ₂ 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 Ti ₃ SiC ₂ 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.

Akio Ishii,“Elastic investigation for the existence of B33 phase in TiNi shape memory alloys using atomistically informed Eshelby’s ellipsoidal inclusion”, Computational Materials Science, 218 (2023) 111954.

The existence of the B33 phase in TiNi alloys, which was reported to be a stable phase using density functional theory calculations but not confirmed experimentally, is controversial. Using Eshelby’s ellipsoidal inclusion, which was atomistically informed by density functional theory calculations, we investigated the existence of the B33 phase in the TiNi shape memory alloy. The calculated total strains of the heterogeneously nucleated B33 phase were similar to the eigenstrains of the B19’ phase, which were also calculated using density functional theory calculations. Considering the similarity of the atomic structures of B33 and B19’, this indicates that the B33 phase was elastically suppressed and changed to the B19’ phase by the original B2 matrix. We confirmed that the elastic inhomogeneity between the B2 matrix and B33 phase plays a role in this change.

Akio Ishii,“Activation energy of homogeneous nucleation of Zr hydride: Density functional theory calculation”, Computational Materials Science, 215 (2022) 111769.

Considering the nucleation process of Zr hydrides as phase transformation from hexagonal closed-packed (HCP) to face-centered tetragonal (FCT) structure, we calculated the activation energy of the homogeneous nucleation process of Zr hydrides and atomic rearrangement during nucleation for Zr₄H, Zr₂H, ZrH and ZrH₂ using density functional theory calculations and minimum energy path detection. At 0 K limit, although ZrH and ZrH₂ have lower chemical potentials and are more energetically stable than Zr₄H and Zr₂H, the latter have lower activation energies for nucleation. At finite temperatures, the crossover of activation energies occurs around 300 K, where ZrH becomes the most possible candidate with the lowest activation energy. This was explained by the difference in the atomic rearrangement and change in phonon frequency during phase transformation.

Ruixiao Zheng, Wu Gong, Jun-ping Du, Si Gao, Maowen Liu, Guodong Li, Takuro Kawasaki, Stefanus Harjo, Chaoli Ma, Shigenobu Ogata, Nobuhiro Tsuji,“Rediscovery of Hall-Petch strengthening in bulk ultrafine grained pure Mg at cryogenic temperature: A combined in-situ neutron diffraction and electron microscopy study”, Acta Materialia, 238 (2022.8) 118243-1-15.

Grain refinement can lead to the strengthening of metallic materials according to the Hall-Petch rela- tionship. However, our recent results suggested that grain boundary sliding is the dominant deformation mode in bulk ultrafine grained (UFG) pure Mg at room temperature, leading to softening. Here, for the first time, we report that the Hall-Petch strengthening can be regained in bulk UFG pure Mg at cryogenic temperature. At 77K, the UFG pure Mg with a mean grain size of 0.6 μm exhibited ultrahigh tensile yield strength and ultimate tensile strength of 309 MPa and 380 MPa, respectively. Combined in-situ neutron diffraction and electron microscopy investigation indicated that residual dislocation structures and deformation twins hardly formed in the UFG specimen during tensile test at 298K. In contrast, fast accumulation of lattice defects and remarkable reorientation were evident at 77K, suggesting that the grain-boundary-mediated process was suppressed and the plastic deformation was dominated by dislo- cation slip and deformation twinning. In addition, all the pure Mg specimens exhibited pronounced strain hardening at 77 K, which was mainly attributed to the suppressed grain boundary sliding and dynamic recovery. The mean dislocation density and relative fractions of dislocations with various Burgers vectors of the UFG specimen deformed at 77K were determined quantitatively from neutron diffraction data. De- spite the strong strain hardening capacity, the UFG specimen fractured prematurely at an elongation of 5% at 77K. The significantly reduced ductility at 77K was possibly caused by its higher tensile strength, which generated high intergranular stress and resulted in intergranular fracture before reaching the point of plastic instability. Potential ways for improving the strength and ductility synergy of the UFG Mg at cryogenic temperature were proposed.

Vei Wang, Jun-Ping Du, Hidetoshi Somekawa, Shigenobu Ogata and Wen Tong Geng,“Spin Polarization of Mn Could Enhance Grain Boundary Sliding in Mg”, Materials, 15 (2022) 3483-1-9.

Segregation of rare earth alloying elements are known to segregate to grain boundaries in Mg and suppress grain boundary sliding via strong chemical bonds. Segregation of Mn, however, has recently been found to enhance grain boundary sliding in Mg, thereby boosting its ductility. Taking the Mg (‾2114) twin boundary as an example, we performed a first-principles comparative study on the segregation and chemical bonding of Y, Zn, and Mn at this boundary. We found that both Y-4d and Mn-3d states hybridized with the Mg-3sp states, while Zn–Mg bonding was characterized by charge transfer only. Strong spin-polarization of Mn pushed the up-spin 3d states down, leading to less anisotropic Mn–Mg bonds with more delocalized charge distribution at the twin boundary, and thus promotes grain boundary plasticity, e.g., grain boundary sliding.

Zhi-Yu Nie, Yuji Sato, Shigenobu Ogata, Maria Jazmin Duarte, Gerhard Dehm, Ju Li, Evan Ma, De-Gang Xie, Zhi-Wei Shan,“Ultralong one-dimensional plastic zone created in aluminum underneath a nanoscale indent”, Acta Materialia, 232, (2022) 117944-1-10.

Nanoindentation on crystalline materials is generally believed to generate a three-dimensional plastic zone, which has a semi-spherical shape with a diameter no larger than a few times the indentation depth. Here, by observing nanoindentation on aluminum in situ inside a transmission electron micro- scope, we demonstrate that three-dimensional plasticity dominated by regular dislocations triumph as the contact size upon yielding increases above ∼100 nm. However, when the contact diameter is less than ∼50 nm, a narrow and long (hereafter referred to as “one dimensional”) plastic zone can be created in front of the tip, as the indenter successively injects prismatic dislocation loops/helices into the crys- tal. Interestingly, this one-dimensional plastic zone can penetrate up to 150 times the indentation depth, far beyond the prediction given by the Nix-Gao model. Our findings shed new light on understanding the dislocation behavior during nanoscale contact. The experimental method also provides a potentially novel way to interrogate loop-defect interactions, and to create periodic loop arrays at precise positions for the modification of properties ( e.g. , strengthening).

Si-Mian Liu, Akio Ishii, Shao-Bo Mi, Shigenobu Ogata, Ju Li, and Wei-Zhong Han,“Dislocation-Mediated Hydride Precipitation in Zirconium”, Small, (2022) 2105881-1-8.

The formation of hydrides challenges the integrity of zirconium (Zr) fuel cladding in nuclear reactors. The dynamics of hydride precipitation are complex. Especially, the formation of the butterfly or bird-nest configurations of dislocation structures around hydride is rather intriguing. By in-situ transmission electron microscopy experiments and density functional theory simulations, it is discovered that hydride growth is a hybrid displacive-diffusive process, which is regulated by intermittent dislocation emissions. A strong tensile stress field around the hydride tip increases the solubility of hydrogen in Zr matrix, which prevents hydride growth. Punching-out dislocations reduces the tensile stress surrounding the hydride, decreases hydrogen solubility, reboots the hydride precipitation and accelerates the growth of the hydride. The emission of dislocations mediates hydride growth, and finally, the consecutively emitted dislocations evolve into a butterfly or bird-nest configuration around the hydride.

Peijun Yu, Jun-Ping Du, Shuhei Shinzato, Fan-Shun Meng, Shigenobu Ogata,“Theory of history-dependent multi-layer generalized stacking fault energy— A modeling of the micro-substructure evolution kinetics in chemically ordered medium-entropy-alloys”, Acta Materialia, 224 (2022) 117504-1-12.

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)

Sixue Zheng, Shuhei Shinzato, Shigenobu Ogata, Scott Mao,“Experimental molecular dynamics for individual atomic-scale plastic events in nanoscale crystals”, Journal of the Mechanics and Physics of Solids, 158 (2021) 104687-1-16.

The experimental determination of critical stresses for yielding and plastic flow is of upmost importance for the understanding of the atomic-scale mechanical behaviors of nanoscale metals, which is limited in computational molecular dynamics due to their inherent high strain rates and empirical interatomic potentials. Here, we propose an in situ atomic-scale experimental mechanics, the so-called experimental molecular dynamics, which is capable of studying the stress-strain relations with respect to the individual atomic-scale plastic events, including full dislocation slip, deformation twinning and shear band in nanoscale metals with different crystal structures. The local stress, strain and their relationships were obtained based on the analyses of the change in lattice strain gauge, interplanar spacing and gauge length. Using this method, drastic stress drops and strain bursts, characteristics of individual plastic events, are investigated. The critical stresses for activating the nucleation and growth of atomic-scale defects are obtained. The newly developed experimental molecular dynamics with in situ mechanics approach has the advantage of quasi-static strain rate and no requirement of interatomic potentials over the computational one, which may provide new clues to establish the stress-based criteria for atomic-scale yielding and plastic flow.

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.

Yong-Jie Hu, Aditya Sundar, Shigenobu Ogata, and Liang Qi, “Screening of generalized stacking fault energies, surface energies and intrinsic ductile potency of refractory multicomponent alloys”, Acta Materialia, 210 (2021) 116800.

Body-centered cubic (bcc) refractory multicomponent alloys are of great interest due to their remarkable strength at high temperatures. Optimizing the chemical compositions of these alloys to achieve a combination of high strength and room-temperature ductility remains challenging. Systematic predictions of these correlated properties across a vast compositional space would speed the alloy discover process. In the present work, we performed first-principles calculations with the special quasi-random structure (SQS) method to predict the unstable stacking fault energy (γ_usf) of the (11-0)[111］slip system and the (11-0) -plane surface energy (γ_surf) for 106 individual binary, ternary and quaternary bcc solid-solution alloys with constituent elements among Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re and Ru. Moreover, with the first-principles data and a set of physics-informed descriptors, we developed surrogate models based on statistical regression to accurately and efficiently predict γ_usf and γ_surf for refractory multicomponent alloys in the 10-element compositional space. Building upon binary and ternary data, the surrogate models show outstanding predictive capability in the high-order multicomponent systems. The ratio between γ_usf and γ_surf can be used to populate a model of intrinsic ductility based on the Rice model of crack-tip deformation. Therefore, using the surrogate models, we performed a systematic screening of γ_usf, γ_surf and their ratio over 112,378 alloy compositions to search for alloy candidates that may have enhanced strength-ductility synergies. Search results were also validated by additional first-principles calculations.

Weitong Lin, Da Chen, Chaoqun Dang, Peijun Yu, Fanling Meng, Tao Yang, Yilu Zhao, Shaofei Liu, Junping Du, Guma Yeli, Chain Tsuan Liu, Yang Lu, Shigenobu Ogata, Ji-jung Kai, “Highly pressurized helium nanobubbles promote stacking-fault-mediated deformation in FeNiCoCr high entropy alloy”, Acta Materialia, 210 (2021) 116843.

Tailoring nanoscale defect structures for desirable deformation behaviors is crucial to designing and optimizing the mechanical properties of alloys. Distinguishing from the predominant toughening mechanisms (e.g., mechanical twinning and deformation-induced phase transformation), here we report an unusual stacking-fault-mediated deformation in equiatomic FeNiCoCr high-entropy alloy (HEA) by controllably introducing helium nanobubbles with high pressures of ~2.5-4.7 gigapascals. Using in situ transmission electron microscopy nanomechanical testing, we demonstrate that highly pressurized helium nanobubbles can not only increase the strength by serving as dislocation obstacles but also enhance the strain hardening capacity and accommodate considerable plasticity via facilitating the multiplication and interaction of interwoven stacking faults. Through atomistic simulations, we reveal that high helium pressures contribute to reducing the nucleation energy of partial dislocations at the nanobubbles surface, which enhances dislocation nucleation rates and offers sustainable stacking fault sources for retaining ductility. Our results provide a novel design strategy for tuning deformation mechanisms of HEAs via introducing highly pressurized helium nanobubbles, which may open up avenues towards the facile tailoring of mechanical responses in micro/nanoscale HEA components.

Kazuma Ito, Hideaki Sawada, Shingo Tanaka, Shigenobu Ogata and Masanori Kohyama,“Electronic origin of grain boundary segregation of Al, Si, P, and S in bcc-Fe: Combined analysis of ab initio local energy and crystal orbital Hamilton population”, Modelling and Simulation in Materials Science and Engineering, 29 (2020) 015001-1-23.

In steel, P and S cause serious grain boundary (GB) embrittlement, which is associated with high segregation energies. To investigate the origins of such high segregation energies of P and S, we applied the combination of ab initio local energy analysis and crystal orbital Hamiltonian population (COHP) analysis for the GB segregation of Al, Si, P, and S in bcc-Fe, which can provide local energetic and bonding views of segregation behavior of each solute, associated with the replacement between solute-Fe and Fe-Fe bonding at GB and bulk sites. The local energy analysis revealed that GB segregation of such solutes is mainly caused by the difference between local energy changes of Fe atoms adjacent to a solute atom in the GB and bulk sites, and that the local energy change of each Fe atom depends on the solute–Fe interatomic distance with a unique functional form for each solute species. The COHP analysis showed that such distance dependency of the Fe-atom local energy change is caused by that of solute–Fe bonding interactions, relative to the Fe-Fe ones, governed by the valence atomic-orbital characters of each solute species. P and S have smaller extents of atomic orbitals and larger numbers of valence electrons; thus, they greatly lower the local energies of Fe atoms at interatomic distances shorter than the bulk first-neighbor one, and they greatly increase those of Fe atoms at longer interatomic distances around the bulk second-neighbor one. Thus, high segregation energies of P and S occur at GB sites with short first-neighbor distances and reduced coordination numbers within the bulk second-neighbor distance. The GB embrittlement by P and S was also discussed by this local-bonding viewpoint. The combination of local energy and COHP analyses can provide novel insights into the behavior of solute elements in various materials.

Yuji Sato, Shuhei Shinzato, Takahito Ohmura, Takahiro Hatano, and Shigenobu Ogata,“Unique universal scaling in nanoindentation pop-ins”, Nature Communications, 11 (2020) 4177-1-9.

Power laws are omnipresent and actively studied in many scientific fields, including plasticity of materials. Here, we report the power-law statistics in the second and subsequent pop-in magnitudes during load-controlled nanoindentation testing, whereas the first pop-in is characterized by Gaussian-like statistics with a well-defined average value. The transition from Gaussian-like to power-law is due to the change in the deformation mechanism from dislocation nucleation to dislocation network evolution in the sharp-indenter induced abruptly decaying stress and dislocation density fields. Based on nanoindentation testing on the (100) and (111) surfaces of body-centered cubic (BCC) iron and the (100) surface of face-centered cubic (FCC) copper, the scaling exponents of the power laws were determined to be 5.6, 3.9, and 6.4, respectively. These power-law exponents are much higher than those typically observed in micro-pillar plasticity (1.0–1.8), suggesting that the nanoindentation plasticity belongs to a different universality class than the micro-pillar plasticity.

Akio Ishii, “Energetics of heterogeneous Mg {101-2} deformation twinning migration using an atomistically informed phase-field model”, Computational Materials Science, 183 (2020) 109907-1-14.

We have constructed an atomistically informed phase-field model for the quantitative energetic analysis of phase transformations. In our model, to describe the general phase transformation with a non-linear correlation between displacive and diffusive modes, we have defined two order parameters, ϒ and Φ, which describe the lattice distortion (displacive mode) and shuffling (diffusive mode), respectively. Our method provides a way to introduce the energetics from atomistic simulations to the phase-field model, describes ϒ and Φ in an atomic model, and derives phase-field parameters from the free energy calculated by atomistic simulation. As an application of our model, we used the energetics obtained from atomistic simulations using a density functional theory potential, and we calculated the free energy change during the heterogeneous {101-2} twin migration of hexagonally close-packed (HCP) Mg, which can be considered as a lattice distortion and shuffling mixed phase transformation, by combining our phase-field model with the nudged elastic band method. The activation energy, and the critical nucleus size of the heterogeneous {101-2} twin migration under a set stress were derived. The critical c-axis tensile stress (athermal stress), at which the activation energy becomes zero, is consistent with the experimental yield stress of {101-2} for the twinning deformation of HCP Mg nanopillars in tensile tests. The critical nucleus size of the heterogeneous {101-2} twin migration is on the range of nanometers under several hundred megapascals stress, which is consistent with the experimental observation of nanotwins.

Md. Lokman Ali, Shuhei Shinzato, Vei Wang, Zeqi Shen, Jun-ping Du, Shigenobu Ogata, “An Atomistic Modeling Study of the Relationship between Critical Resolved Shear Stress and Atomic Structure Distortion in FCC High Entropy Alloys - Relationship in Random Solid Solution and Chemical-Short-Range-Order Alloys -”, Materials Transactions, 61, 4 (2020) 605-609.

The relationship between the critical resolved shear stress (CRSS) at T = 0 K and the atomic structure distortion was studied using molecular dynamics (MD) simulation with atomic distortion (root-mean-square-atomic-displacement (RMSAD)) controlled Lennard-Jones (LJ) interatomic potentials for different face-centered-cubic (FCC) high entropy alloy (HEA) systems, such as ternary, quaternary, and quinary alloy systems. We demonstrated that an almost universal linear relationship exists between CRSS and RMSAD for the random solid solution (RSS) of these alloy systems. The universality was also confirmed by a more realistic embedded atom method (EAM) potential. However, alloy systems that have a chemical-short-range-order (CSRO) do not follow this universal linear relationship.

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.

Tomoaki Niiyama, Masato Wakeda, Tomotsugu Shimokawa, Shigenobu Ogata, “Structural relaxation affecting shear-transformation avalanches in metallic glasses”, Physical Review E, 100 (2019) 043002-1-10.

Structural relaxation by thermal treatments can dramatically alter the mechanical properties of metallic amorphous solids (metallic glasses), especially brittleness related to shear band nucleation. Using molecular dynamics simulations of shear deformation in two thermally processed metallic glass models, we investigate the morphology of the shear transformation avalanches, which is characterized by the power-law statistics. The two typical cases that are based on a less-relaxed (as-quenched) glass and a well-relaxed (well-aged) glass are investigated. During deformation, a shear-band like heterogeneous pattern is observed in the well-relaxed glass model, whereas the less-relaxed model exhibits homogeneous deformation patterns. By evaluating the spatial correlation functions of the non-affine least square displacements of atoms during each elemental avalanche event, we reveal that the shapes of avalanche regions in well-relaxed glasses tend to be anisotropic whereas those in less-relaxed glasses are isotropic. Further, we demonstrate that a temporal clustering of the direction of avalanche propagations and a considerable correlation between the anisotropy and size of an avalanche emerge in the well-relaxed glass model.

Yuji Sato, Shuhei Shinzato, Takahito Ohmura, Shigenobu Ogata, "Atomistic prediction of the temperature- and loading-rate-dependent first pop-in load in nanoindentation", International Journal of Plasticity, 121 (2019) 280-292.

Nanoindentation has been used for a long time to investigate the mechanical properties of materials. A well-known displacement burst, the so-called “pop-in,” is usually observed during nanoindentation tests. In particular for the first pop-in has been well studied because it should be directly related to the fundamental mechanical strength of the local volume beneath the indenter. The first pop-in load measured for a defect-free local volume is corresponding to the onset load of homogeneous dislocation nucleation, while measured for a local volume with immobile defects is that of heterogeneous dislocation nucleation. The first pop-in is a thermally activated event under loading; thus, the pop-in load exhibits both temperature and loading-rate dependencies. Although theoretical studies have successfully explained the temperature and loading-rate dependencies, to the best of our knowledge, an atomistic prediction for specific materials which takes into account the stress state beneath the indenter has not yet been reported. In this study, we propose an atomistically informed multiscale (two-scale) modeling to predict the temperature and loading-rate dependencies of the first pop-in load for the homogeneous dislocation nucleation considering an atomistically estimated stress state beneath the indenter and stress-dependent activation energies of dislocation nucleation. Body-centered-cubic Fe and Ta are chosen as target metals, although the method is generally applicable to any ductile material. In addition to the homogeneous dislocation nucleation, we also developed a heterogeneous nucleation theory to explain the difference between the experimentally and atomistically obtained homogeneous nucleation pop-in loads.

Ping-Jiong Yang, Qing-Jie Li, Tomohito Tsuru, Shigenobu Ogata, Jie-Wen Zhang, Hong-Wei Sheng, Zhi-Wei Shan, Gang Sha, Wei-Zhong Han, Ju Li, Evan Ma, "Mechanism of Hardening and Damage Initiation in Oxygen Embrittlement of Body-Centred-Cubic Niobium", Acta Materialia, 168 (2019) 331-342.

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. While self-interstitials anneal out fast during plastic flow, the vacancy-oxygen complexes are stable against passing dislocations. The debris in fact amplify the random force field, facilitating the generation of even more defects in a self-reinforcing loop. This leads to unusually high strain hardening rates and fast breeding of nano-cavities that underlie damage and failure.

Kazuma Ito, Hideki Sawada, and Shigenobu Ogata, "First-Principles Study on the Grain Boundary Embrittlement of bcc-Fe by Mn Segregation", Physical Review Materials, 3 (2019) 013609.

Developing steels with high strength and ductility is needed in order to improve the mechanical reliability and environmental performance of engineering products. The addition of Mn is a key technology for developing next-generation high-strength steels. However, the addition of Mn leads to a serious side effect, grain boundary (GB) embrittlement, which decreases the mechanical toughness of steels. Understanding the mechanism of GB embrittlement due to Mn is an essential process for improving the toughness of steels containing Mn. In this work, in order to reveal the fundamental mechanism of GB embrittlement by Mn, the effect of Mn on the cleavage fracture of bcc-Fe GBs, especially the influence of the difference in the magnetic coupling state between Mn and Fe, is investigated using uniaxial tensile simulations of the bcc-Fe Σ3(111) GB with and without Mn segregation using the first-principles density functional theory (DFT). The uniaxial tensile simulations demonstrate that Mn decreases the cleavage-fracture energy of the GB. In particular, the ferromagnetically coupled Mn substantially decreases the cleavage-fracture energy of the GB, promoting cleavage fracture. When ferromagnetically coupled Mn is present in the bcc-Fe GBs, the electrons contributing to the bonds between Mn and the surrounding Fe atoms easily localize to the Mn atom with increasing stress, and the bonding between Mn and the surrounding Fe atoms rapidly weakens, leading to a cleavage fracture of the GBs at a lower stress and strain. This unusual behavior is derived from the stability of the nonbonding Mn as a result of its half-filled d shell. These results show that the local magnetic state in GBs is one of the factors determining the macroscopic mechanical properties of steels containing Mn.

Guo-Jie Jason Gao, Yun-Jiang,Wang, and Shigenobu Ogata, "Incorporating a soft ordered phase into an amorphous configuration enhances its uniform plastic deformation under shear", AIP Advances 9, (2019) 15329.

Amorphous materials of homogeneous structures usually suffer from nonuniform deformation under shear, which can develop into shear localization and eventually destructive shear band. One approach to tackle this issue is to introduce an inhomogeneous structure containing more than one phase, which can reduce the local nonuniform shear deformation and hinder its percolation throughout the system. Using thermostated molecular dynamics (MD) simulations, we compare the deformation behavior between a homogeneous amorphous mixture of bidisperse disc particles, interacting via an n-6 Lennard-Jones potential of tunable softness, with an inhomogeneous one containing an evenly-distributed ordered phase. We change the population ratio of large to small particles to create a homogeneous or an inhomogeneous mixture, where the softness of a chosen phase can be manually adjusted by specifying n of the interparticle potential. Results of applying extensive quasistatic shear on the prepared mixtures reveal that the inhomogeneous amorphous mixture containing a soft ordered phase overall deforms more uniformly than the homogeneous one, which indicates that both the structure inhomogeneity and the inter-phase softness variance play important roles in enhancing the uniformity of the plastic deformation under shear.

Xie Xie, Yu-Chieh Lo, Yang Tong, Junwei Qiao, Gongyao Wang, Shigenobu Ogata, Hairong Qi, Karin A. Dahmen, Yanfei Gao, and Peter K. Liawa, "Origin of serrated flow in bulk metallic glasses", Journal of the Mechanics and Physics of Solids, 124 (2019) 634-642.

Bulk metallic glasses (BMGs) possess amorphous structure and show unique mechanical properties, such as extremely high strength and excellent damage tolerance, entitling them as potential structural materials. So far a great amount of work has been conducted to study BMGs’ macroscopic mechanical properties and examine corresponding microscopic deformation defects. However, the connection between macroscopic inhomogeneous deformation at room temperature and microscopic deformation carriers is still poorly understood, due to the lack of an appropriate experimental technique to directly probe the inhomogeneous deformation process on the proper spatial and temporal scales. Here we present the deformation details via in situ thermal imaging about the evolution of heat bands associated with successive serration behavior. For the first time, our experimental work clarifies the coupling of serrated flows with shear band activities, especially the often omitted fine serrations induced by shear band nucleation or the early stage of propagation. Meanwhile, serration behavior of BMGs is simulated through the kinetic Monte Carlo (kMC) method by integrating local heating (thermal softening and β-relaxation) effects, which exhibits good agreement with experimental results. These findings will advance our fundamental understanding of the shear band operation down to microscopic level, which may shed light on the control of shear banding for the application of BMGs.

Liang Wan, Wen Tong Geng, Akio Ishii, Jun-Ping Du, Qingsong Mei, Nobuyuki Ishikawa, Hajime Kimizuka and Shigenobu Ogata, "Hydrogen embrittlement controlled by reaction of dislocation with grain boundary in alpha-iron", International Journal of Plasticity, 112 (2019) 206-219.

Hydrogen atoms absorbed by metals in hydrogen-containing environments can lead to the premature fracture of the metal components used in load-bearing conditions. Since metals used in practice are mostly polycrystalline, grain boundaries (GBs) can play an important role in hydrogen embrittlement of metals. Here we show that the reaction of GBs with lattice dislocations is a key component in hydrogen embrittlement mechanism for polycrystalline metals. We use atomistic modeling methods to investigate the mechanical response of GBs in alpha-iron with various hydrogen concentrations. Analysis indicates that dislocations impingement and emission on the GB can provoke it to locally transform into an activated state with a more disordered atomistic structure, and introduce a local stress concentration. The activation of the GB segregated with hydrogen atoms can greatly facilitate decohesion of the GB. We propose a hydrogen embrittlement model that can give better explanation of many experimental observations.

Gurcan Aral, Md Mahbubul Islam, Yun-Jiang Wang, Shigenobu Ogata and Adri C. T. van Duin, "Oxyhydroxide of metallic nanowires in a molecular H2O and H2O2 environment and their effects on mechanical properties", Phys.Chem.Chem.Phys.20 (2018), 17289.

To avoid unexpected environmental mechanical failure, there is a strong need to fully understand the details of the oxidation process and intrinsic mechanical properties of reactive metallic iron (Fe) nanowires (NWs) under various aqueous reactive environmental conditions. Herein, we employed ReaxFF reactive molecular dynamics (MD) simulations to elucidate the oxidation of Fe NWs exposed to molecular water (H2O) and hydrogen peroxide (H2O2) environment, and the influence of the oxide shell layer on the tensile mechanical deformation properties of Fe NWs. Our structural analysis shows that oxidation of Fe NWs occurs with the formation of different iron oxide and hydroxide phases in the aqueous molecular H2O and H2O2 oxidizing environments. We observe that the resulting microstructure due to pre-oxide shell layer formation reduces the mechanical stress via increasing the initial defect sites in the vicinity of the oxide region to facilitate the onset of plastic deformation during tensile loading. Specifically, the oxide layer of Fe NWs formed in the H2O2 environment has a relatively significant effect on the deterioration of the mechanical properties of Fe NWs. The weakening of the yield stress and Young modulus of H2O2 oxidized Fe NWs indicates the important role of local oxide microstructures on mechanical deformation properties of individual Fe NWs. Notably, deformation twinning is found as the primary mechanical plastic deformation mechanism of all Fe NWs, but it is initially observed at low strain and stress level for the oxidized Fe NWs.

W.T. Geng, Vei Wang, Jin-Xu Li, Nobuyuki Ishikawa, Hajime Kimizuka, Kaneaki Tsuzaki, and Shigenobu Ogata, "Hydrogen trapping in carbon supersaturated α iron and its decohesion effect in martensitic steel", Scripta Materialia, 149 (2018) 79-83.

Our first-principles calculations demonstrate that hydrogen is more stable in carbon supersaturated martensite than in α‑iron, due to the carbon-induced tetragonality in martensite lattice. The trapped hydrogen leads to remarkable decohesion between (110) planes both inside the martensite and along the martensite/ferrite interface, with the former being more significant than the latter. This decohesion can explain recent precise observations that in martensite/ferrite dual-phase steels the hydrogen-promoted crack was initiated in the martensite region and that in lath martensite steel it propagated not on lath boundaries but showed quasi-cleavage feature along (110) planes at very high hydrogen concentration.

Masakazu Tane, Shogo Suzuki, Michiaki Yamasaki, Yoshihito Kawamura, Koji Hagihara, and Hajime Kimizuka, “Insignificant elastic-modulus mismatch and stress partitioning in two-phase Mg-Zn-Y alloys comprising of α-Mg and long-period stacking ordered phases”, Materials Science and Engineering: A, 710 (2018) 227-239.

Elastic-modulus mismatch and the resultant stress partitioning in two-phase Mg–Zn–Y alloys comprised of α-Mg and long-period stacking ordered (LPSO) phases were studied. Two-phase polycrystals containing anisotropically oriented 18R- or 14 H-type LPSO phase and single-phase polycrystals consisting of α-Mg or 18R-type LPSO phase were prepared by extrusion and directional solidification processes and their complete sets of anisotropic elastic properties were measured using resonant ultrasound spectroscopy. Elastic properties of the single and two-phase alloys were analyzed using Eshelby's inclusion theory, effective-medium approximation, and inverse Voigt-Reuss-Hill approximation, in which the crystallographic textures and microstructures formed by the preparation processes were taken into account. The analyses revealed that the elastic properties of 18R-LPSO phase were not unique and they depended on the solute Zn and Y atom concentrations. Additionally, the elastic modulus of 18R-LPSO phase embedded in the two-phase alloy was lower than that of the alloy consisting of single-phase 18R-LPSO phase. The analysis using first-principles calculations based on density functional theory indicated that the low elastic modulus was caused by low density and low stability of short-range ordered solute atom clusters embedded in the LPSO phase of the two-phase alloy. Because of low elastic modulus in the LPSO phase, the elastic mismatch and resultant elastic interaction between the α-Mg and LPSO phases were very small. As a result, the formation of LPSO phase had little effect on the stress partitioning to the LPSO phase, which was independent of the LPSO-phase morphology.

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.

Wen-Tong Geng, Liang Wan, Jun-Ping Du, Akio Ishii, Nobuyuki Ishikawa, Hajime Kimizuka, and Shigenobu Ogata, “Hydrogen bubble nucleation in α-iron”,Scripta Materialia, 134 (2017) 105-109.

Wereport a first-principles study on how H₂ molecules emerge in a nanovoid inα-iron. In a 9-vacancy void, after the walls are decorated with 24 H atoms, only H-dimers are allowed to be adsorbed on the walls; whereas in a spherical 27-vacancy void, H₂ molecules start to appear in the center of the void after the walls are saturated by 54 H atoms. The bubble pressure can reach 3.5 GPa, comparable to the measured H₂ pressure in blisters at the micrometer scale. The H-saturated nanovoid attracts vacancy more strongly than does the pristine nanovoid through strong H-vacancy interaction.

Liang Wan, Akio Ishii, Jun-Ping Du, Wei-Zhong Han, Qingsong Mei, and Shigenobu Ogata, “Atomistic modeling study of a strain-free stress driven grain boundary migration mechanism”, Scripta Materialia, 134 (2017) 52-56.

A recent experiment (Scripta Mater., 65:990, 2011) shows that the Σ7 {132}/{132} grain boundary in Al can migrate under external stress but produces no strain. Here, based on a bi-crystallographic analysis, an atomic shuffling path was identified as the feasible mechanism for this grain boundary migration. By a density functional theory calculation, it reveals that the enthalpy barrier of this atomic shuffling path increases by external shear stress applied with shear of the grain boundary along the tilt axis〈111〉, which is in good agreement with experimentally measured shear-direction-dependence of activation enthalpy for this grain boundary migration.

Tetsuo Mohri, Ying Chen, Masanori Kohyama, Shigenobu Ogata, Arkapol Saengdeejing, Somesh Kumar Bhattacharya, Masato Wakeda, Shuhei Shinzato, and Hajime Kimizuka, “Mechanical properties of Fe-rich Si alloy from Hamiltonian”,npj Computational Materials, 3 (2017) 1-14.

The physical origins of the mechanical properties of Fe-rich Si alloys are investigated by combining electronic structure calculations with statistical mechanics means such as the cluster variation method, molecular dynamics simulation, etc, applied to homogeneous and heterogeneous systems. Firstly, we examined the elastic properties based on electronic structure calculations in a homogeneous system and attributed the physical origin of the loss of ductility with increasing Si content to the combined effects of magneto-volume and D03 ordering. As a typical example of a heterogeneity forming a microstructure, we focus on grain boundaries, and segregation behavior of Si atoms is studied through high-precision electronic structure calculations. Two kinds of segregation sites are identified: looser and tighter sites. Depending on the site, different segregation mechanisms are revealed. Finally, the dislocation behavior in the Fe–Si alloy is investigated mainly by molecular dynamics simulations combined with electronic structure calculations. The solid-solution hardening and softening are interpreted in terms of two kinds of energy barriers for kink nucleation and migration on a screw dislocation line. Furthermore, the clue to the peculiar work hardening behavior is discussed based on kinetic Monte Carlo simulations by focusing on the preferential selection of slip planes triggered by kink nucleation.

Ichiro Kawarada, Ruixiao Zheng, Akinobu Shibata, Hidetoshi Somekawa, Shigenobu Ogata, and Nobuhiro Tsuji, “Mechanical Properties and Deformation Mechanism of Mg–Y Alloy with Various Grain Sizes”,Magnesium Technology 2017, (2017) 283-287.

In the present study, a Mg–Y dilute alloy was provided for a severe plastic deformation by high pressure torsion (HPT) and subsequent annealing. After the HPT by 5 rotations, nanocrystalline (NC) structures with an average grain size of 240 nm having deformed characteristics were obtained. Subsequent annealing at various temperatures for 2–60 min resulted in fully recrystallized structures with different average grain sizes ranging from 0.66 to 8.13 μm. Good balance of tensile strength and ductility could be realized in the fine grained specimens. For the specimen having a mean grain size of 2.13 μm, the yield strength and total tensile elongation were 180 MPa and 37%, respectively, which were much higher than those of pure Mg with a similar grain size. The significant contribution of Y on the microstructure and mechanical properties is discussed.

Gurcan Aral, Yun-Jiang Wang, Shigenobu Ogata, and Adri C. T. van Duin, “Effects of oxidation on tensile deformation of iron nanowires: Insights from reactive molecular dynamics simulations”,Journal of Applied Physics, 120 (2016) 135104-1-15.

The influence of oxidation on the mechanical properties of nanostructured metals is rarely explored and remains poorly understood. To address this knowledge gap, in this work, we systematically investigate the mechanical properties and changes in the metallic iron (Fe) nanowires (NWs) under various atmospheric conditions of ambient dry O2 and in a vacuum. More specifically, we focus on the effect of oxide shell layer thickness over Fe NW surfaces at room temperature. We use molecular dynamics (MD) simulations with the variable charge ReaxFF force field potential model that dynamically handles charge variation among atoms as well as breaking and forming of the chemical bonds associated with the oxidation reaction. The ReaxFF potential model allows us to study large length scale mechanical atomistic deformation processes under the tensile strain deformation process, coupled with quantum mechanically accurate descriptions of chemical reactions. To study the influence of an oxide layer, three oxide shell layer thicknesses of ∼4.81 Å, ∼5.33 Å, and ∼6.57 Å are formed on the pure Fe NW free surfaces. It is observed that the increase in the oxide layer thickness on the Fe NW surface reduces both the yield stress and the critical strain. We further note that the tensile mechanical deformation behaviors of Fe NWs are dependent on the presence of surface oxidation, which lowers the onset of plastic deformation. Our MD simulations show that twinning is of significant importance in the mechanical behavior of the pure and oxide-coated Fe NWs; however, twin nucleation occurs at a lower strain level when Fe NWs are coated with thicker oxide layers. The increase in the oxide shell layer thickness also reduces the external stress required to initiate plastic deformation.

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.

Narumasa Miyazaki, Masato Wakeda, Yun-Jiang Wang and Shigenobu Ogata, “Prediction of pressure-promoted thermal rejuvenation in metallic glasses”, npj Computational Materials, 2 (2016) 16013.

Rejuvenation is the structural excitation of glassy materials, and is a promising approach for improving the macroscopic deformability of metallic glasses. This atomistic study proposes the application of compressive hydrostatic pressure during the glass-forming quenching process and demonstrates highly rejuvenated glass states that have not been attainable without the application of pressure. Surprisingly, the pressure-promoted rejuvenation process increases the characteristic short- and medium-range order, even though it leads to a higher-energy glassy state. This ‘local order’–‘energy’ relation is completely opposite to conventional thinking regarding the relation, suggesting the presence of a well-ordered high-pressure glass/high-energy glass phase. We also demonstrate that the rejuvenated glass made by the pressure-promoted rejuvenation exhibits greater plastic performance than as-quenched glass, and greater strength and stiffness than glass made without the application of pressure. It is thus possible to tune the mechanical properties of glass using the pressure-promoted rejuvenation technique.

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.

Xu-Sheng Yang, Yun-Jiang Wang, Hui-Ru Zhai, Guo-Yong Wang, Yan-Jing Su, L. H. Dai, Shigenobu Ogata, and Tong-Yi Zhang, “Time-, stress-, and temperature-dependent deformation in nanostructured copper: Creep tests and simulations”, Journal of the Mechanics and Physics of Solids, 94 (2016) 191-206.

In the present work, we performed experiments, atomistic simulations, and high-resolution electron microscopy (HREM) to study the creep behaviors of the nanotwinned (nt) and nanograined (ng) copper at temperatures of 22 °C (RT), 40 °C, 50 °C, 60 °C, and 70 °C. The experimental data at various temperatures and different sustained stress levels provide sufficient information, which allows one to extract the deformation parameters reliably. The determined activation parameters and microscopic observations indicate transition of creep mechanisms with variation in stress level in the nt-Cu, i.e., from the Coble creep to the twin boundary (TB) migration and eventually to the perfect dislocation nucleation and activities. The experimental and simulation results imply that nanotwinning could be an effective approach to enhance the creep resistance of twin-free ng-Cu. The experimental creep results further verify the newly developed formula (Yang et al., 2016) that describes the time-, stress-, and temperature-dependent plastic deformation in polycrystalline copper.

Kang Pyo Soa, Xiaohui Liua, Hideki Mori, Akihiro Kushima, Jong Gil Park, Hyoung Seop Kim, Shigenobu Ogata, Young Hee Lee, Ju Li, “Ton-scale metal–carbon nanotube composite: The mechanism of strengthening while retaining tensile ductility”, Extream Mechanics Letters, 8 (2016) 245-250.

One-dimensional carbon nanotubes (CNT), which are mechanically strong and flexible, enhance strength of the host metal matrix. However, the reduction of ductility is often a serious drawback. Here, we report significantly enhanced plastic flow strength, while preventing tensile ductility reduction, by uniformly dispersing CNTs in Al matrix. Nanoscale plasticity and rupturing processes near CNTs were observed by in-situ mechanical tests inside Transmission Electron Microscope (TEM). CNTs act like forest dislocations and have comparable density (∼10^14/m^2), and such 1D nano-dispersion hardening is studied in detail by in situ TEM and molecular dynamics simulations. Rupture-front blunting and branching are seen with in situ TEM, which corroborates the result from macro-scale tension tests that our Al+CNT nanocomposite is quite damage- and fault-tolerant. We propose a modified shear-lag model called “Taylor-dispersion” hardening model to highlight the dual roles of CNTs as load-bearing fillers and “forest dislocations” equivalent that harden the metal matrix, for the plastic strength of metal+CNT nanocomposite.

Akio Ishii, Ju Li and Shigenobu Ogata, “Shuffling-controlled versus strain-controlled deformation twinning: The case for HCP Mg twin nucleation”,International Journal of Plasticity, 82 (2016) 32-43.

The atomistic pathways of deformation twinning can be computed ab initio , and quantified by two variables: strain which describes shape change of a periodic supercell, and shuffling which describes non-affine displacements of the internal degrees of freedom. The minimum energy path involves juxta-position of both. But if one can obtain the same saddle point by continuously increasing the strain and relaxing the internal degrees of freedom by steepest descent, we call the path strain-controlled, and vice versa. Surprisingly, we find the View the MathML source{1012}〈1011〉 twinning of Mg is shuffling-controlled at the smallest lengthscale of the irreducible lattice correspondence pattern, that is, the reaction coordinate at the level of 4 atoms is dominated by non-affine displacements, instead of strain. Shuffling-controlled deformation twinning is expected to have different temperature and strain-rate sensitivities from strain-controlled deformation twinning due to relatively weaker strength of long-range elastic interactions, in particular at the twin nucleation stage. As the twin grows large enough, however, elastic interactions and displacive character of the transformation should always turn dominant.

Masakazu Tane, Hajime Kimizuka, Koji Hagihara, Shogo Suzuki, Tsuyoshi Mayama, Tohru Sekino, and Yu Nagai, “Effects of stacking sequence and short-range ordering of solute atoms on elastic properties of Mg-Zn-Y alloys with long-period stacking ordered structures”, Acta Materialia, 96 (2015) 170-188.

The effects of stacking sequence and short-range ordering of solute atoms on the elastic properties of Mg-Zn-Y alloy single crystals with an 18R- or 10H-type long-period stacking ordered (LPSO) structure were studied. Instead of single crystals, the growth of which can be quite difficult, two directionally solidified (DS) Mg-Zn-Y alloy polycrystals, mainly consisting of 18R- or 10H-type LPSO structure, were prepared. X-ray pole figure analyses revealed that fiber textures, which differed in the two prepared alloys, were formed in the DS polycrystals. For the DS polycrystals, a complete set of elastic constants was measured during cooling from 300 to 7.5 or 5.5 K. By analyzing the elastic stiffness of DS polycrystals on the basis of a newly developed inverse Voigt-Reuss-Hill approximation, in which the detailed crystallographic texture could be taken into account, the elastic stiffness components of the single-crystalline LPSO phases from 300 to 7.5 or 5.5 K were determined. The elastic properties of the 18R- and 10H-LPSO phases were also evaluated by first-principles calculations based on density functional theory. Comparison of the measured elastic properties at 5.5 or 7.5 K with the first-principles calculations revealed that the elastic properties of the LPSO phase were virtually dominated by the stacking sequence of the LPSO structures and the formation energy of short-range ordered solute atom clusters, formed at the four consecutive atomic stacking layers. Importantly, the effects of the formation energy and stacking sequence were significant in the elastic moduli related to the atomic bonding between the stacking layers.

Masato Wakeda, Junji Saida, Ju Li, Shigenobu Ogata, “Controlled Rejuvenation of Amorphous Metals with Thermal Processing”, Scientific Reports, 5, (2015) 10545-1-8.

Rejuvenation is the configurational excitation of amorphous materials and is one of the more promising approaches for improving the deformability of amorphous metals that usually exhibit macroscopic brittle fracture modes. Here, we propose a method to control the level of rejuvenation through systematic thermal processing and clarify the crucial feasibility conditions by means of molecular dynamics simulations of annealing and quenching. We also experimentally demonstrate rejuvenation level control in Zr55Al10Ni5Cu30 bulk metallic glass. Our local heat-treatment recipe (rising temperature above 1.1Tg, followed by a temperature quench rate exceeding the previous) opens avenue to modifying the glass properties after it has been cast and processed into near component shape, where a higher local cooling rate may be afforded by for example transient laser heating, adding spatial control and great flexibility to the processing.

Ryuichi Tarumi, Masato Wakeda, Yoji Shibutani, “Numerical Study on Shear Deformation of Cu-Zr Metallic Glass -Molecular Dynamics Simulation and Radial Basis Function Analysis-”, Journal of the Society of Mater ials Science, Japan, 64-3 (2015), pp. 163-168 [in Japanese].

We conducted molecular dynamics (MD) simulation on simple shear deformation of Cu-Zr metallic glass. A metallic glass model is prepared by rapid quench from an equilibrium melting state. Shear deformation process is simulated by applying stepwise affine-displacement which is followed by structural relaxation for a certain time interval. Present MD simulation demonstrated typical deformation behavior of metallic glasses including elastic response, yielding and nucleation and growth of shear bands in the atomistic scale. To obtain a course-grained picture of the deformation, we transformed the atomistic relative displacements into a continuously differentiable field using the Gauss-type radial basis function (RBF). This analysis revealed that local structural relaxation and their percolation play a dominant role on the formation of shear band. We also revealed that source and sink of divergence of the displacement velocity have a side-by-side configuration due to accommodative motion for relaxation. These results indicate that the continuous field transformation by RBF is effective to understand the plastic deformation mechanism of metallic glasses.

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.

Shigenobu Ogata, “Diffusion Dynamics of Hydrogen and Carbon in Iron”, Bulletin of the Iron and Steel Institute of Japan, 19-11(2014), 745-750 [in Japanese].

Masakazu Tane, Yasuki Nagai, Hajime Kimizuka, Koji Hagihara, and Yoshihito Kawamura, "Elastic properties of a Mg-Zn-Y alloy single crystal with a long-period stacking ordered structure", Acta Materialia, 61-17 (2013) 6338-6351.

The elastic properties of an Mg85Zn6Y9 (at.%) alloy single crystal with a long-period stacking-ordered (LPSO) structure, synchronized with periodic enrichment of Zn and Y atoms, were investigated, the properties having remained unclear because of the difficulty in growing large single crystals. Directionally solidified (DS) Mg85Zn6Y9 alloy polycrystals consisting of a single phase of the 18R-type LPSO structure were prepared using the Bridgman technique. For the DS polycrystals, a complete set of elastic constants was measured with resonant ultrasound spectroscopy combined with electromagnetic acoustic resonance, in which the texture formed by the directional solidification was taken into account. By analyzing the elastic stiffness of DS polycrystals on the basis of a newly developed inverse Voigt-Reuss-Hill approximation, the elastic stiffness components of the single-crystalline LPSO phase were determined. It was revealed that the Young's modulus of the LPSO phase along <0001> in the hexagonal expression was clearly higher than that along <11-20>, and the Young's modulus and shear modulus were clearly higher than those of pure magnesium. These findings were validated by first-principles calculations based on density functional theory. Analyses by first-principles calculations and micromechanics modeling indicated that the long periodicity of the 18R-type stacking structure hardly enhanced the elastic modulus, whereas the Zn/Y-enriched atomic layers, containing stable short-range ordered clusters, exhibited a high elastic modulus, which contributed to the enhancement of the elastic modulus of the LPSO phase in the Mg-Zn-Y alloy.

Yoji Shibutani, Masato Wakeda, and Takamasa Yoshikawa, " Mechanics of Amorphous Metals（Elastic-Plastic Finite Element Analyses Using Inhomogeneous Defects Theory）", Transactions of the Japan society of mechanical engineers series A, 79-808 (2013) 1807-1817 [in Japanese].

Plastic deformation of amorphous metals is dependent on a mean stress (hydrostatic pressure), that is, compressible due to the random atomic structure. This property leads their intrinsic anisotropy on deformation. In addition, the localized shear bands occurring just after an elastic region do not allow the sufficient elongation. This is the crucial drawback of that material which has been strongly tried to overcome. In the present paper, a constitutive law based on the inhomogeneous defects theory and an evolutional law of defects density (equivalent to free volume) were formulated with the mean stress-dependent yield function. Several parameters used in the constitutive and the defects evolution laws were fitted to the experimental results. Finite element analyses were first performed using one element model to obtain the perfectly uniform deformation. Yield curves under some multiaxial stress states were obtained at room temperature. Employing the elastic limit as a yield stress and the parameter κ of 0.09 in Drucker-Prager yield criterion, the prediction agrees well to the FEM solutions. The uniaxial deformation behavior with an initial fluctuation of defects density using a block model, then, exhibits the localized shear bands after the maximum point, and the anisotropic angles of such bands to the stress axis were coincident with the experimental and the other computational results.

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), pp.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.

Yun-Jinag Wang, Guo-Jie Jason Gao, and Shigenobu Ogata, "Atomistic understanding of diffusion kinetics in nanocrystals from molecular dynamics simulations", Physical Review B, 88-11 (2013) 115413-1-7.

Understanding the grain size effect on diffusion in nanocrystals has been hampered by the difficulty of measuring diffusion directly in experiments. Here large-scale atomistic modeling is applied to understand the diffusion kinetics in nanocrystals. Enhanced short-circuit diffusivity is revealed to be controlled by the rule of mixtures for grain-boundary diffusion and lattice diffusion, which can be accurately described by the Maxwell-Garnett equation instead of the commonly thought Hart equation, and the thermodynamics of pure grain-boundary self-diffusion is not remarkably affected by varying grain size. Experimentally comparable Arrhenius parameters with atomic detail validate our results. We also propose a free-volume diffusion mechanism considering negative activation entropy and small activation volume. These help provide a fundamental understanding of how the activation parameters depend on size and the structure-property relationship of nanostructured materials from a physical viewpoint.

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.

Shigeto Takebayashi, Kohsaku Ushioda, Naoki Yoshinaga, and Shigenobu Ogata, "Effect of Carbide Size Distribution on the Impact Toughness of Tempered Martensitic Steels with Two Different Prior Austenite Grain Sizes Evaluated by Instrumented Charpy Test", Material Transactions, 54-7 (2013) 1110-1119.

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 and 60 μm was investigated. Instrumented Charpy impact test (ICIT) was used to evaluate the impact toughness. The tempering temperature of 723 K gives the largest difference in the Charpy impact energy at room temperature between the specimens with two different PAG sizes, where the finer PAG specimen shows higher impact energy at room temperature (RT). The other tempering temperatures do not show a significant difference as compared with that shown among the 723 K tempered specimens. Investigation of the test temperature dependence of Charpy impact energy in the 723 K 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 temperatures for the 60 μm PAG specimen. ICIT waveform analysis of these steels shows that the fracture propagation energy mainly controls the temperature dependence of the impact energy, while the fracture initiation energy stays nearly constant against the variation of the test temperature. The carbide size distribution in these two specimens was investigated by secondary electron microscope (SEM) and transmission electron microscope (TEM). The 60 μm PAG specimen shows distribution of coarser carbide than does the 6 μm PAG specimen, which seems to give rise to the observed difference between them in the Charpy impact energy and the other properties of impact fracture.

Guo-Jie Jason Gao, Yun-Jiang Wang, Shigenobu Ogata, "Studying the elastic properties of nanocrystalline copper using a model of randomly packed uniform grains", Computational Materials Science, 79 (2013) 56-62.

We develop a new Voronoi protocol, which is a space tessellation method, to generate a fully dense (containing no voids) model of nanocrystalline copper with precise grain size control; we also perform uniaxial tensile tests using molecular dynamical (MD) simulations to measure the elastic moduli of the grain boundary and the grain interior components at 300 K. We find that the grain boundary deforms more locally compared with the grain core region under thermal vibrations and is elastically less stiff than the core component at finite temperature. The elastic modulus of the grain boundary is lower than 30% of that of the grain interior. Our results will aid in the development of more accurate continuum models of nanocrystalline metals.

Yun-jiang Wang, Akio Ishii, and Shigenobu Ogata, "Entropic effect on creep in nanocrystalline metals", Acta Materialia, 61-10 (2013) 3866-3871.

We report a significant entropic effect on creep of nanocrystalline metal using molecular dynamics. Our simulations reveal that the activation entropy may contribute a multiplicative factor of many orders of magnitude to the steady-state creep rate. The relationship between activation entropy and enthalpy obeys an empirical Meyer-Neldel compensation rule. The activation volume is found to decrease with increasing temperature for dislocation nucleation creep, which agrees well with experimental results. The study opens up an avenue for quantitatively discussing the entropic effects on various thermally activated deformations in nanocrystals.

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, 041902-1-5 (2013).

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.

Shigenobu Ogata, "Prediction of Mechanical Properties of Materials from First-Principles", Journal of the Japan Society of Mechanical Engineers A, 78-791 (2012) 934-944. [in Japanese]

Current ability and expecting future development of Frst-principles predictive modeling for studying mechanicalproperties of materials is comprehensively discussed by means of introducing recent publications and reports. Particulary,I pick up topics for the elastic properties (e.g., elastic constants, elastic strain engineering, ideal strength), the plastic properties, (e.g., dislocation core properties, stacking faults, twinning), and the interfacial properties, (e.g., strength of grain boundaies, strength of interfaces composed of dissimilar matereials), which are necessary to understand materials behavior under external loadings.

Takehiro Yoshikawa, Toshiyuki Takayanagi, Hajime Kimizuka, and Motoyuki Shiga, "Quantum-thermal crossover of hydrogen and tritium diffusion in alpha-iron", Journal of Physical Chemistry C, 116-43 (2012) 23113-23119.

The diffusion coefficients of hydrogen (H) and tritium (T) in α-Fe have been computed using two approximate quantum dynamical techniques, that is, centroid molecular dynamics (CMD) and ring polymer molecular dynamics (RPMD), in the temperature range of T = 100-1000 K using the embedded atom method (EAM) potential. It has been found that the RPMD and CMD methods give very similar results. From a further analysis based on quantum transition-state theory (centroid density QTST) combined with path integral molecular dynamics (PIMD), it has been clear that there is a crossover between thermal and quantum mechanisms at about T = 500 and 300 K for H and T diffusions, respectively. The importance of nuclear quantum effects at low temperatures has been illustrated in terms of the effective free-energy surface map.

Akio Ishii, Shigenobu Ogata, Hajime Kimizuka, and Ju Li, "Adaptive-boost molecular dynamics simulation of carbon diffusion in iron", Physical Review B, 85-6 (2012) 064303-1-7.

We have developed an accelerated molecular dynamics (MD) method to model atomic-scale rare events. In this method, a smooth histogram of collective variables is first estimated by canonical ensemble molecular dynamics calculations, and then a temperature-dependent boost potential is iteratively constructed to accelerate the MD simulation. This method not only allows us to observe the rare events but also to evaluate the profile of free energy and trial frequency along the reaction coordinate. We employed this method to study carbon diffusion in bcc iron and evaluated carbon's temperature-dependent diffusivity. The obtained diffusivities agree well with the experimental measurements. Even at low temperature for which, to the best of our knowledge, no experimental data are available, the diffusivity can be evaluated accurately. Additionally, we study carbon diffusion inside the edge dislocation core in bcc iron, and demonstrate the applicability of the method to rare events on a rugged free-energy surface.

Akio Ishii, Hajime Kimizuka, and Shigenobu Ogata, "Multi-replica Molecular Dynamics Modeling", Computational Materials Science, 54-1 (2012) 240-248.

We propose a new unified modeling concept, multi-replica molecular dynamics (MRMD), which is a collective designation for molecular dynamics (MD) or particle dynamics (PD) that simultaneously follows the time evolution of two or more replica systems. Numerous methods in various fields (physics, chemistry, biology, materials science, astronomy, etc.) can be classified in MRMD, such as parallel replica methods, the nudged elastic band method, path integral methods, replica exchange methods, and the multi-walker metadynamics method. However, the relations between these MRMD methods have not been clarified, and a global understanding or unified methodological framework remains to be presented. Therefore, herein, we discuss the relations between various MRMD methods to provide a comprehensive understanding of these methods. Then, with this understanding, we describe a concise implementation of MRMD into conventional domain- and particle-decomposition parallel MD program codes, instead of the somewhat intuitive replica-parallel implementation. Finally, we demonstrate examples of different MRMD calculations performed using our parallel MRMD program “ParaMrMD.”

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.

Fanyan Meng, Guisheng Wang, Sanqiang Shi, and Shigenobu Ogata, "A streched carbon nanotube with a high-density of topological defect", Advanced Materials Research, 236-238 (2011) 2225-2228.

We have developed a theoretical method to obtain a single-walled carbon nanotube (SWCNT) with a high density of topological defects. Carbon nanotubes (CNTs) sustain elastic elongation up to 15-30% at low temperature because of the sufficiently high barrier of bond rotations. A large number of topological defects are activated simultaneously and widely distributed over the entire tube wall after heating the stretched tube to an elevated temperature. This is driven by the internal energy of the strained carbon nanotubes. The manner in which topological defects are distributed is affected by the initial strain and the heating temperature. Nanotubes with a large number of topological defects achieve the elongation without breaking.

Ryusuke Nakamura, Manabu Ishimaru, Akihiko Hirata, Kazuhisa Sato, Masakazu Tane, Hajime Kimizuka, T. Shudo, Toyohiko Konno, Hideo Nakajima, "Enhancement of nanovoid formation in annealed amorphous Al2O3 including W", Journal of Applied Physics, 110-6 (2011) 064324-1-7.

The effect of W on the nanovoid formation in annealed amorphous Al2O3 was studied by transmission electron microscopy and molecular dynamics simulations. A comparison of the void formation behavior in electron-beam deposited Al2O3 (without W) and resistance-heating deposited Al2O3 (with 10 at. % W) revealed that W enhances the formation and growth of nanovoids. An analysis of the pair distribution function (PDF) in both types of amorphous Al2O3 showed that the introduction of W into amorphous Al2O3 brings about a significant change in the amorphous structure. Furthermore, it was found by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) that sub-nm sized W clusters exist in as-deposited Al2O3 prepared by resistance-heating and then dissolve in the amorphous matrix with annealing. The combination of PDF analysis and HAADF-STEM observation provides evidence that the enhancement of void formation originates in the heterogeneous short-range atomic configurations induced by the addition of W.

Hajime Kimizuka and Shigenobu Ogata, "Slow diffusion of hydrogen at a screw dislocation core in alpha-iron", Physical Review B, 84-2 (2011) 024116-1-6.

Here we demonstrate and characterize the H-diffusion behavior around a screw dislocation in body-centered cubic (bcc) α-Fe by performing path-integral molecular dynamics modeling and adopting an ab initio?based potential. Counterintuitively, our results indicate that the H diffusivity along the dislocation line is significantly lower than lattice diffusion. Thus, the “fast” pipe diffusion does not occur for H in α-Fe.

Hajime Kimizuka, Hideki Mori, and Shigenobu Ogata, "Effect of Temperature on Fast Hydrogen Diffusion in Iron: A Path-Integral Quantum Dynamics Approach", Physical Review B, 83-9 (2011) 094110-1-7.

Here we explicitly present the diffusion coefficients (D) and activation energies (Ea) of interstitial H in α-Fe over a temperature range of 100 to 1000 K. These values were predicted by applying path-integral molecular dynamics modeling based on first principles. The obtained D and Ea values exhibit clear non-Arrhenius temperature dependence and a transition from quantum to classical behavior at around 500 K. Our results show that the quantum effects not only significantly lower the diffusion barrier but also change the diffusion pathway even at room temperature; thus, fast diffusion becomes possible.

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.

Hiroki Ushida, Shigenobu Ogata, and Hajime Kimizuka, "Molecular Dynamics Stability Analysis of fcc Crystal Against Local Share Deformation", Journal of the Society of Materials Science, Japan, 60-1 (2011) 71-78. [in Japanese].

We propose a framework that can be used to study the local thermodynamic stability of materials at finite temperatures, by reconstructing free energy surface based on metadynamics, constraint molecular dynamics and local atomic deformation tensor analysis methods. We apply the proposed framework to fcc embedded atom copper models, and estimate the activation energies, volumes, and critical local deformation tensor for a stacking fault nucleation event in copper single crystal.

Shigeto Takebayashi, Tomonori Kunieda, Naoki Yoshinaga, Kohsaku Ushioda, and Shigenobu Ogata, "Comparison of the Dislocation Density in Martensitic Steels Evaluated by Some X-ray Diffraction Methods", ISIJ International, 50-6 (2010) 875-882.

X-ray diffraction (XRD)-based modified Warren?Averbach (MWA) analysis, in comparison with the Williamson?Hall (WH) analysis, was applied to 0.3 mass% carbon martensitic steels, as-quenched and subsequently tempered at various temperatures, to give their dislocation densities. For the as-quenched martensite, the WH method gives a value of around 2.0×1016 m?2, which could be overestimated. Meanwhile, the MWA method gives a value of around 6.3×1015 m?2, which is below the possible upper limit of dislocation density, 1016 m?2. The MWA-derived value for the as-quenched steel seems to be 1.6?4.8 times higher than those expected from the precedent results derived by transmission electron microscope (TEM) observations. However, considering that the TEM-derived value gives the microscopically local average while the XRD-derived value gives the macroscopic average, such discrepancy between the TEM-derived value and MWA-derived value is tolerable. For the steels tempered at 723 K and 923 K, the MWA and WH methods give comparable values ranging in 1014 m?2, where the rearrangement of dislocation structure is observed by TEM. However, in these steels where the XRD peaks are narrower and the instrumental width of the present XRD system could be significant, care should be taken over the peak width correction.

Naohiro Toda, Hajime Kimizuka, and Shigenobu Ogata, "DFT-Based FEM Analysis of Nonlinear Effects on Indentation Process in Diamond Crystal", International Journal of Mechanical Sciences, 52-2 (2010) 303-308.

We apply a new framework of a finite-element method (FEM) analysis with constitutive relations based on density functional theory (DFT), as an efficient method to characterize the nonlinear and anisotropic elastic deformation of single-crystal diamond. In our scheme, the stress?strain relations are obtained during FEM analysis on the fly based on the plane-wave-based DFT total-energy calculations and their numerical database is simultaneously constructed, which enables us to obtain high-precision stress without any empirical parameters even under finite strained conditions. To check its validity and accuracy, the shear deformation behavior of diamond crystal is analyzed under the strained condition. Then we examine the nonlinear effects on the indentation deformation of diamond single crystal, by comparing the results from the DFT-based constitutive relations with those from the linear elastic ones.

Shigenobu Ogata, "Modeling and Simulation for Solid Materials", Bulletin of the Japan Society for Industrial and Applied Mathematics, 20-1 (2010) 57-63. [in Japanese]

Shigenobu Ogata, and Ju Li, "Toughness scale from first principles", Journal of Applied Physics, 106-11 (2009) 113534.-1-5

We correlate the experimentally measured fracture toughness of 24 metals and ceramics to their quantum mechanically calculated brittleness parameter. The brittleness parameter is defined as the ratio of the elastic energy density needed to spontaneously break bonds in shear versus in tension, and is a primitive-cell property. Under 300 GPa hydrostatic pressure, the model predicts that diamond has smaller brittleness than molybdenum at zero pressure, and thus should deform plastically without cracking at room temperature.

Hajime Kimizuka, Hideki Mori, Hiroki Ushida, and Shigenobu Ogata, "Evaluation of Hydrogen Diffusivity and Its Temperature Dependence in BCC Metals: A Path-Integral Centroid Molecular Dynamics Study", Journal of the Japan Institute of Metals, 73-8 (2009) 571-576. [in Japanese]

We have analyzed the diffusion behavior of interstitial hydrogen in bcc iron and niobium using path-integral centroid molecular dynamics (CMD) method, which can describe the real-time evolution of particles based on quantum statistical mechanics. In this study, the embedded-atom-method (EAM) potential model for the iron-hydrogen interaction is developed to reproduce the ab initio minimum energy path of hydrogen migration based on the density functional theory (DFT) data in the literature, while the description of niobium-hydrogen interaction is based on an empirical potential model. Time evolutions of mean-square displacements of hydrogen atoms in the two bulk metals are calculated at various temperatures, and then diffusion coefficients and activation energies of hydrogen migration are evaluated. Especially in the case of iron, the results are in good agreement with experimental measurements over a wide temperature range. In order to characterize the quantum effects on the hydrogen diffusion process, the CMD results are compared with those obtained from classical molecular dynamics method. The obtained results indicate that the quantum effects can play a significant role in hydrogen diffusivity over a wide temperature range in these bcc metals.

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.

Naohiro Toda, Hajime Kimizuka, and Shigenobu Ogata, "Density-Functional-Theory-Based Finite-Element Analysis of Diamond Single Crystal", Journal of Solid Mechanics and Materials Engineering, 3-3 (2009) 541-551.

We apply a finite-element analysis method based on first-principles density functional theory, to evaluate the nonlinear large elastic deformation of single-crystal diamond. The stress-strain relations are obtained during finite-element analysis on the fly based on the first-principles calculations and their numerical database is simultaneously constructed, which enables us to obtain high-precision stress without any empirical parameters even under finite strained conditions. The shear strength and mechanical behavior of diamond crystal are analyzed under various stress conditions, and then the uniaxial deformation of a diamond-crystal pillar model is examined through the present analysis method.

Ting Zhu, Ju Li, Shigenobu Ogata, and Sidney Yip, "Mechanics of Ultra-Strength Materials", MRS Bulletin, 34-3 (2009) 167-172.

Recent experiments on nanostructured materials, such as nanoparticles, nanowires, nanotubes, nanopillars, thin films, and nanocrystals have revealed a host of “ultra-strength” phenomena, defined by stresses in a material component generally rising up to a significant fraction (> 1/10) of its ideal strength ? the highest achievable stress of a defect-free crystal at zero temperature. While conventional materials deform or fracture at sample-wide stresses far below the ideal strength, rapid development of nanotechnology has brought about a need to understand ultra-strength phenomena, as nanoscale materials apparently have a larger dynamic range of sustainable stress (“strength”) than conventional materials. Ultra-strength phenomena not only have to do with the shape stability and deformation kinetics of a component, but also the tuning of its physical and chemical properties by stress. Reaching ultra-strength enables “elastic strain engineering”, where by controlling the elastic strain field one achieves desired electronic, magnetic, optical, phononic, catalytic, etc. properties in the component, imparting a new meaning to Feynman’s statement “there’s plenty of room at the bottom”. This article presents an overview of the principal deformation mechanisms of ultra-strength materials. The fundamental defect processes that initiate and sustain plastic flow and fracture are described, and the mechanics and physics of both displacive and diffusive mechanisms are reviewed. The effects of temperature, strain rate and sample size are discussed. Important unresolved issues are identified.

Masahiro Yamamoto, Kohei Kunizawa, Akinori Fujinami, Shigenobu Ogata, and Yoji Shibutani, "Formation of Atomistic Island in Al Film Growth by Kinetic Monte Carlo", Journal of Computational Science and Technology, 3-1 (2009) 148-158.

Kinetic Monte Carlo (KMC) method realizes the millisecond or second order atomistic thin film growth. Twenty five kinds of events which may occur on Al(111) surface were classified. An attempt frequency and an activation energy of each event were defined using vibration analyses and nudged elastic band (NEB) method by which the minimum energy path (MEP) can be reasonably predicted. Temperature and deposition rate dependences of Al(111) film growth were intensively investigated in the present paper. The higher temperature and the lower rate drive the layer-by-layer film structural change. Two types of islands (fcc and hcp) were seen by modeling without considering the events of diffusion of dimer and trimer, while only fcc islands remain with considering such events. Thus, we find that the primitive events of diffusion of dimer and trimer take important roles in determination of surface morphology.

Shigenobu Ogata, Yoshitaka Umeno, and Masanori Kohyama, "First-Principles Approaches to Intrinsic Strength and Deformation of Materials: Perfect Crystals, Nano-Structures, Surfaces and Interfaces", Modelling and Simulation in Materials Science and Engineering, 17-1 (2009) 013001-1-33.

First-principles studies on the intrinsic mechanical properties of various materials and systems through ab initio tensile and shear testing simulations based on density-functional theory are reviewed. For various materials, ideal tensile and shear strength and features of the deformation of bulk crystals without any defects have been examined, and the relation with the bonding nature has been analyzed. The surfaces or low-dimensional nano-structures reveal peculiar strength and deformation behavior due to local different bonding nature. For grain boundaries and metal/ceramic interfaces, tensile and shear behaviors depend on the interface bonding, which impacts on the research of real engineering materials. Remaining problems and future directions in this research field are discussed.

Akinori Fujinami, Shigenobu Ogata, Hajime Kimizuka, and Yoji Shibutani, "The Energetics of Large Deformations of a Single Polyimide Molecular Chain: DFT and MO Calculations", Macromolecular Theory & Simulations, 17-9 (2008) 488-495.

The large-deformation energetics of a single molecular chain of the rod-like polyimide PMDA-PDA was investigated using DFT, ab initio MO and semi-empirical MO methods. The force/displacement curves were calculated from tensile testing simulations along the axis of the molecular chain, allowing a discussion of the distribution and change of local strain of the molecular chain. The deformation behavior of a single PMDA-PDA molecular chain under finite deformations as functions of bending angle and dihedral angle between PMDA and PDA groups are compared. It is found that the semi-empirical MO calculations provide sufficient accuracy to express the energetics of large deformations except for compressive deformation.

Kohei Kunizawa, Masahiro Yamamoto, Shigenobu Ogata, and Yoji Shibutani, "Step-Growth Anisotropy on Thin Film Epitaxial Process", Journal of the Society of Materials Science, Japan, 57-8 (2008) 780-785.

Anisotropic growth process of two kinds of steps on Al(111) substrates is performed using kinetic Monte Carlo (kMC) method. Employed kMC parameters of activation energy and attempt frequency are estimated by nudged elastic band (NEB) method and transition state theory. Obtained set of results suggest that degree of the anisotropic growth clearly depends on substrate temperature and deposition rate. We find microscopic origin of the anisotropic growth is difference of diffusion rates along {111} and {100} steps, and there is a particular growth condition in which strong anisotropy is observed. At high deposition rate and low temperature, new islands which are easily generated on terraces, hinder the growth anisotropy weaker.

Masato Wakeda, Yoji Shibutani, Shigenobu Ogata, and Junyoung Park, "Multiple Shear Banding in a Computational Amorphous Alloy Model", Applied Physics A, 91-2 (2008) 281-285.

The strain localized phenomenon, so called shear bands (SBs), in an amorphous alloy have received a lot of attention in recent years. In this study, we microscopically investigated the nature and dynamics of multiple SBs using molecular dynamics model. In the SB region, intense shear-induced structural change occurred, typified by the annihilation of pentagonal short-range order, and significant localized heating accompanied with the SB propagation was observed. Moreover, a large number of fine SBs operated simultaneously at a high strain rate, whereas, only a few SBs appeared and propagated abruptly at a low strain rate. These results were discussed with respect to brittle/ductile deformation of bulk metallic glasses.

Hajime Kimizuka, Shigenobu Ogata, and Ju Li, "Hydrostatic Compression and High-Pressure Elastic Constants of Coesite, SiO2", Jounal of Applied Physics, 103-5 (2008) 053506-1-4.

Using density-functional theory, we computed all the independent elastic constants of coesite, a high-pressure polymorph of silica, as functions of pressure up to 15 GPa. The results are in good agreement with experimental measurements under ambient conditions. Also, the predicted pressure-dependent elastic properties are consistent with x-ray data in the literature concerning lattice strains at high pressures. We find that coesite, like quartz, exhibits a gradual softening of a shear modulus B44 with increasing pressure, in contrast to the rising bulk modulus.

Naohiro Toda, Hajime Kimizuka, and Shigenobu Ogata, "Database-accelerated parallelized local quasicontinuum method based on first-principles density-functional theory for crystals possessing internal degrees of freedom", Transactions of the Japan Society for Computational Methods in Engineering, 7-2 (2008) 273-278. [in Japanese].

We proposed a new finite-element analysis method based on a numerical stress-strain database to calculate the non-linear large elastic deformation for single crystals possessing internal degrees of freedom. The database is constructed during finite-element analysis on-the-fly based on the first-principles density functional theory, which is able to obtain high-precision stress without any empirical parameters even under finite strained conditions. The database significantly improves the total computational efficiency without loss of accuracy. We also carried out parallel-computational method for the database construction process to realize further improvement of computational efficiency. The effectiveness of the proposed inverse analysis method is demonstrated through numerical simulation for several example problems

Masato Wakeda, Shibutani Yoji, and Shigenobu Ogata, "Atomistic Formation Mechanism of Multiple Shear Bands in Amorphous Metals", Journal of the Society of Materials Science, Japan, 57-2 (2008) 119-125. [in Japanese].

At room temperature, plastic deformation of amorphous metals is localized in narrow bands, so called shear bands (SBs), which is a key issue of their brittle/ductile deformations. The extremely disordered structure and the abrupt fracture obtained in the experiments hinder us from making clear the nature and dynamics of SBs. In this study, atomistic modeling to produce the multiple shear bands (MSBs), not single shear band, was proposed, and the evolution process of MSBs including their coalescence and stationary was investigated using molecular dynamics simulations. We prepared a plate of Cu-Zr binary amorphous alloy model by the melt-quench process, and then performed uniaxial tensile and compressive tests under plane stress condition. During the loading, the deformation was suddenly localized in narrow bands shortly after the onset of yielding. The propagation of SBs was accompanied with drastic stress drop and significant local heating caused by the friction of atoms. Also, critical stresses of SB nucleation considerably differed between under tension and compression. This result indicates that Tresca or von Mises criterion, commonly used as a yield condition in the crystalline metals, is not appropriate to describe the yielding of the amorphous metals. The SB angles to the loading axis are observed to be 45?57° under tension, while 40?46° under compression. These angles agree well with the fracture angles observed in the experiments with multi-component metallic glasses. It is concluded that the critical stress state of SB nucleation is dependent on not only the shear stress but also the stress normal to the SB, and it can be described by Mohr-Coulomb criterion.

Masahiro Yamamoto, Akinori Fujinami, and Shigenobu Ogata, Yoji Shibutani, "Hybridized Atomistic Modeling of Migration Observed on Thin Film Surface by Incident Particles", Journal of Computational Science and Technology, 1-1 (2008) 14-21.

Innovative thin film technology to realize the finer electric devices needs to understand the atomic level process of film growth and its relationship to the film characterization. In this paper, the long film growth phenomena for a few micro-second order with the short severe collisions by incident particles are analyzed by the proposed hybridized atomistic modeling. This method combined molecular dynamics (MD) with kinetic Monte Carlo (KMC) can directly treat two types of events of deposition and diffusion, which have quite different time scales. The solutions suggest that the large incident kinetic energy of deposited atoms compatible to the realistic physical vapor deposition (PVD) impels to fluctuate the equilibrium on Al (111) surface very drastically and affects the atomic level surface morphology. It is found that the faster incident atoms with 1.0×104 m/s can make the smoother surface than those with the velocity of 1.0×103 m/s. This is due to much activated atomic migration, which can be realized only by MD.

Futoshi Shimizu, Shigenobu Ogata, and Ju Li, "Theory of Yield Point in Metallic Glass and Molecular Dynamics Calculations", Materials Transactions, 48-11 (2007) 2923-2927.

The aged-rejuvenation-glue-liquid (ARGL) shear band model has been proposed for metallic glasses (Acta Mater. 54 (2006) 4293), based on small-scale molecular dynamics simulations up to 20,000 atoms and thermomechanical analysis. The model predicts the existence of a critical lengthscale ?10 nm, above which melting could occur in shear-alienated glass. Large-scale molecular dynamics simulations with up to 5 million atoms have directly verified this prediction. When the applied stress exceeds the glue traction (computed separately before in a shear cohesive zone, or an amorphous-amorphous ``generalized stacking fault energy'' calculation), we indeed observe maturation of the shear band embryo into bona fide 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 which is a property of shear-alienated glass, controls the macroscopic yield point of well-aged glass. We further suggest that the disruption of chemical short-range order (``chemical softening'') governs the glue traction microscopically. Catastrophic thermal softening occurs only after chemical alienation and softening in our simulation, after the shear band embryo has already run a critical length.

Hideki Mori, Shigenobu Ogata, Ju Li, Seiji Akita, and Yoshikazu Nakayama, "Plastic Bending and Shape Memory Effect of Double-Wall Carbon Nanotubes", Physical Review B, 76-16 (2007) 165405-1-7.

Plastic bending of (5,5)@(10,10) double-wall carbon nanotube is analyzed using nudged elastic band minimum energy path calculations. At lower applied bending curvature, only the outer tube deforms plastically. However, at higher bending curvature, both the inner and outer tubes deform plastically. We find that the plastic deformation of the outer tube is more difficult than that of isolated single-wall carbon nanotube of the same diameter due to tube-tube interactions. In contrast, the plastic deformation of the inner tube is not strongly affected by the presence of the outer tube. We also analyze the shape-memory effect (SME) discovered experimentally, which is a thermal recovery process from the plastically bent state to the straight defect-free state, which can be repeated multiple times. We analyze the physics behind SME of carbon nanotubes, which is quite different from that of traditional shape-memory alloys.

Hideo Kaburaki, Ju Li, Sidney Yip, and Hajime Kimizuka, "Dynamical Thermal Conductivity of Argon Crystal", Journal of Applied Physics, 102-4 (2007) 043514-1-6.

The thermal conductivity of a rare-gas crystal (Ar) is computed using equilibrium molecular dynamics in conjunction with the Green-Kubo linear response formalism, and the Lennard-Jones potential with an appropriately long cutoff (4σ). Besides predicting absolute values of the conductivity from low temperature up to the liquid, the approach allows heat conduction to be understood as a dynamical process through the temporal behavior of the heat current correlation function. At low temperatures the correlation function shows a characteristic two-stage decay, a short-time relaxation which we attribute to single-particle motions in a local environment, and a more extended component corresponding to collective atomic motions (phonons). As temperature increases the second correlation component diminishes much faster than the first component, indicating a transition from mainly phase-coherent phonon transport to mainly phase-incoherent interatomic energy transfer in solids

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

Junyoung Park, Yoji Shibutani, Masato Wakeda, and Shigenobu Ogata, "Influence of Size and Number of Nanocrystals on Shear Band Formation in Amorphous Alloys", Materials Transactions, 48-5 (2007) 1001-1006.

In this study, binary (copper and zirconium) amorphous metals with embedded nanosized crystal structures are subjected to uniaxial tension using molecular dynamics simulations to reveal the mechanism of shear band structure formation. The number and the size of the nanocrystals are chosen as the study parameters. The number of nanocrystals affects the stress-strain curve and shear band formation while the size of the nanocrystals does not significantly affect the results. As reported in the experimental work published so far, under tension coalescent voids are found in the shear bands or at the interface between crystalline and amorphous materials. The simulation results show that the number of shear bands under compressive loading is much larger than that under tensile loading. We also found that, even under compressive loading, the shear bands started from regions with enough free volume.

Fanyan Meng, Shigenobu Ogata, Dongsheng Xu, Yoji Shibutani, and S. Q. Shi, "Thermal conductivity of an ultrathin carbon nanotube with an X-shaped junction", Physical Review B, 75-20 (2007), 205403-1-6.

The thermal conductivity of the ultrathin carbon nanotube with and without an X-shaped junction was investigated using nonequilibrium molecular-dynamics simulations. The ultrathin carbon nanotube exhibits superhigh thermal conductivity. The thermal conductivity of the nanotube with junctions was 20?80% less than that of a straight nanotube depending on temperature. There is a jump in the temperature profile around the junction, contributing to a larger temperature gradient and reduction in the thermal conductivity. The thermal conductivity of armchair nanotube junctions is sensitive to the topological structures at the junction region.

Masahiro Yamamoto, Kohei Kunizawa, Akinori Fujinami, Shigenobu Ogata, and Yoji Shibutani, "Formation of Atomistic Island in Al Film Growth by Kinetic Monte Carlo", Transactions of the JSME A, 73-728 (2007), 490-497. [in Japanese]

Kinetic Monte Carlo (KMC) method realizes the millisecond or second order atomistic thin film growth. Twenty five kinds of events which may occur on Al (111) surface were classified. An attempt frequency and an activation energy of each event were defined using vibration analyses and nudged elastic band (NEB) method by which the minimum energy path (MEP) can be reasonably predicted. Temperature and deposition rate dependences of Al(lll) film growth were intensively investigated in the present paper. The higher temperature and the lower rate drive the layer-by-layer film structural change. Two types of islands (fee and hep) were seen by modeling without considering the events of diffusion of dimer and trimer, while only fee islands remain with considering such events. Thus, we find that the primitive events of diffusion of dimer and trimer take important roles in determination of surface morphology.