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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.

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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.

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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.

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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.

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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.

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Akio Ishii,“Influence of elastic anisotropy on the shapes of ellipsoidal blisters and stress field around the blisters in solid materials”, AIP advances, 13 (2023) 125024-1-11.

To address the embrittlement challenges posed by gas blisters in anisotropic materials, the stable shape of constant-pressure blisters in anisotropic materials (hexagonal, tetragonal, and rhombohedral) was energetically investigated based on continuum theory (micromechanics), considering the blister as Eshelby’s ellipsoidal inclusion. The non-negligible change in the blister shape was confirmed in terms of the anisotropic factor η ≡ C₃₃₃₃/C₁₁₁₁. Although the spherical shape of the blister is preferable for isotropic and cubic materials (η = 1), the x₃ normal penny and capsule shapes were theoretically confirmed to be the most stable ones for η > 1 and η < 1, respectively. The penny and capsule shape blisters generate larger stress fields around themselves than the sphere shape blisters, thus inducing crack formation. The embrittlement due to the gas (typically hydrogen or helium) inside the blister for the anisotropic materials was more significant than isotropic and cubic embrittlement.

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Akio Ishii,“Ab initio stability prediction of β titanium and α and ω precipitates in β titanium matrix for titanium alloys using density functional theory and micromechanics”, Materials Today Communications, 38 (2023) 107708-1-10.

β titanium alloys have a wide range of applications, including in aerospace and transport. The β phase is stable only at high temperatures, and the phase stability can be improved by adding stabilizers. However, the β-phase-stabilization effects of β stabilizers have not yet been clearly elucidated. Here, I report the ab initio prediction of the energetic phase stability of β titanium alloys with Mo, V, W, Nb, and Ta as additive β stabilizer elements. The stability is predicted using a combination of atomistic simulation via density functional theory and continuum micromechanics (Eshelby’s ellipsoidal inclusion analysis). In particular, I consider the heterogeneity of the secondary ω and α phases (precipitation) in the β matrix. All β stabilizer species led to a significant energetic and elastic stabilization of the β phase. Mo and W additives stabilized the β phase relatively stronger and secondary ω and α precipitates may hardly nucleate in β phase in high concentrate condition. Although V, Nb, and Ta additives stabilized the β phase significantly, the β phase remained metastable. Regarding the morphology of these secondary phases, V helped α precipitation and Ta helped ω precipitation. The possibility of a coexistence of ω and α in the β matrix was suggested for Nb addition. The strain fields around the precipitates were also investigated and the results suggested that α precipitates cause a large residual strain around it though ω precipitates do not.

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Akio Ishii,“Morphology prediction of elastically interacting Zr hydride precipitates and cracks in alpha-Zr using atomistically informed Eshelby's ellipsoidal inclusion”, Computational Materials Science, 231 (2024) 112568-1-8.

We propose an atomistically informed Eshelby's inclusion analysis to investigate the morphology of secondary phases, which elastically interacted with each other through their respective local strain fields. Using the proposed method, we predict the morphology of delta-hydride precipitates and cracks, which interacted in the alpha–Zr matrix. Planar cracks nucleate along the basal-normal delta-hydride disk. And at the crack tip, the prismatic-normal delta-hydride disk also nucleates depending on the stress condition around the crack, constructing the hydride-crack network. The findings contribute to the understanding of the fracture mechanism of Zr alloys, such as delayed hydride cracking, which is caused by Zr hydride.

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Si-Mian Liu, Shi-Hao Zhang, Shigenobu Ogata, Hui-Long Yang, Sho Kano, Hiroaki Abe, *Wei-Zhong Han,“Direct Observation of Vacancy-Cluster-Mediated Hydride Nucleation and the Anomalous Precipitation Memory Effect in Zirconium”, Small, (2023) 2300319-1-7.

Controlling the heterogeneous nucleation of new phases is of importance in tuning the microstructures and properties of materials. However, the role of vacancy—a popular defect in materials that is hard to be resolved under conventional electron microscopy—in the heterogeneous phase nucleation remains intriguing. Here, this work captures direct in situ experimental evidences that vacancy clusters promote the heterogeneous hydride nucleation and cause the anomalous precipitation memory effect in zirconium. Both interstitial and vacancy dislocation loops form after hydride dissolution. Interestingly, hydride reprecipitation only occurs on those vacancy loop decorated sites during cooling. Atomistic simulations reveal that hydrogen atoms are preferentially segregated at individual vacancy and vacancy clusters, which assist hydride nucleation, and stimulate the unusual memory effect during hydride reprecipitation. The finding breaks the traditional view on the sequence of heterogeneous nucleation sites and sheds light on the solid phase transformation related to vacancy-sensitive alloying elements..

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Akarsh Verma, Shigenobu Ogata,“Magnesium based alloys for reinforcing biopolymer composites and coatings: A critical overview on biomedical materials”, Advanced Industrial and Engineering Polymer Research, 6 (2023) 341-355.

Magnesium (Mg) & its alloys are favourable for orthopaedic & cardiovascular medical device fabrication applications, but holds a natural ability to degrade biologically when put with aqueous solution of the substances and/or water-saturated tissue in the context of a living organism. Mg alloys nature to corrode inside the living organism body is mainly attributed to the excessive rates of corrosion of Mg. Poor corrosion resistance possessed by Mg decreases the mechanical properties of the implants, and adds toxic effects on the bone metabolism. A potential method for increasing Mg alloy resistance to corrosion without changing its properties is by the protective polymeric deposit coatings. Moreover, to impart better mechanical and biocompatible aspects to Mg based materials biopolymers have been used as a composite constituent. This review is based on such composite materials constituting Mg and biopolymers. Their resulting favourable mechanical and osteopromotive properties in conjunction with biocompatibility may help the clinicians to fix the existing orthopaedic related issues.

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Akio Ishii,“Ab initio prediction of temperature-dependent stability of heterogeneous B19' phase in TiNi alloy using atomistically informed Eshelby's ellipsoidal inclusion”, Materials Today Communications, 35 (2023) 105861.

In this study, we energetically predicted the temperature-dependent stability of the heterogeneous B19′ phase in the B2 matrix of TiNi alloys using density functional theory (DFT) and phonon analysis informed Eshelby's ellipsoidal inclusion. The temperature-dependent eigenstrains and elastic constants for Eshelby's ellipsoidal inclusion were calculated using DFT and phonon analysis under a quasi-harmonic approximation. The stable orientation of the disk-shaped B19' phase in the B2 matrix and its total strains were evaluated with respect to temperature using Eshelby's ellipsoidal inclusion analysis. Using the total strains, the elastically deformed atomic structure of the heterogeneous B19' phase in the B2 matrix was determined, and its temperature-dependent free energy was calculated using DFT calculations and phonon analysis. Comparing the temperature-dependent free energies of the B2 and elastically deformed B19' structures, we successfully predicted that the transformation between the two phases occurs at 300 K, which agrees with experimental observations. Moreover, it is shown that the temperature-dependent difference in the elastic constants between the B19′ phase and B2 matrix influences the phase transformation, which is the origin of the shape-memory effect of TiNi alloy.

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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.

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Heting Liao, Hajime Kimizuka, Hiroshi Miyoshi, Shigenobu Ogata,“Origin of the nucleation preference of coherent and semicoherent nanoprecipitates in Al–Cu alloys based on atomistically informed classical nucleation theory”, Journal of Alloys and Compounds, 938 (2022) 168559-1-13.

The age-hardening response during the heat-treatment process of Al–Cu alloys is significantly and non-linearly influenced by the type and size of metastable precipitates formed. In Al–Cu alloys, a semicoherent θ' phase, usually observed after the formation of coherent Guinier–Preston (GP) zones during aging, is the key strengthening precipitate. Thus, identifying the energetics of preferential nucleation of these precipitates is essential for clarifying the optimal conditions for the formation of precipitates that effectively contribute to hardening. In this study, using classical nucleation theory (CNT) along with a recently developed machine-learning-based interatomic potential with near first-principles accuracy, we characterized the nucleation preference of coherent GP zones and semicoherent θ' nanoprecipitates in Al–Cu alloys at various temperatures and solute concentrations. Our atomistically informed CNT model revealed the overall temperature and solute-concentration dependencies of the nucleation barriers of the nanoprecipitates, which determine the crossover temperatures at which the ease of formation of each precipitate alternates at the solute concentration of interest. The predicted results were in good agreement with the previous experimental observations. The findings of this study contribute to furthering the understanding of the driving forces for nucleation of precipitates in Al–Cu alloys at an atomic level and provide theoretical guidance for identifying the optimal age-hardening response.

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Jun-Ping Du, Peijun Yu, Shuhei Shinzato, Fan-Shun Meng, Yuji Sato, Yangen Li, Yiwen Fana, Shigenobu Ogata,“Chemical domain structure and its formation kinetics in CrCoNi medium-entropy alloy”, Acta Materialia, 240 (2022) 118314-1-11.

The formation of local chemical order in medium-entropy alloys and high-entropy alloys (MEAs/HEAs) has been strongly suggested in recent experimental observations. Since chemical order can lead to changes in mechanical and functional properties, tailoring of chemical order is a promising approach for further improving those properties of MEAs and HEAs. However, details remain unclear regarding the atomic structure of the chemical order and the formation kinetics. Here, employing a large-scale Monte Carlo/molecular dynamics hybrid annealing simulation with a neural network potential, we find a chemical-domain structure (CDS) after annealing below 800 K in FCC CrCoNi MEA. In addition, the forma- tion kinetics, such as the formation time and process and time–temperature–chemical-order diagrams of the CDS, were successfully obtained using a kinetic Monte Carlo simulation with artificial neural network acceleration. The findings provide key information for controlling chemical order via thermal processing.

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Yangen Li, Jun-Ping Du, Peijun Yu, Rui Li, Shuhei Shinzato, Qing Peng, Shigenobu Ogata,“Chemical ordering effect on the radiation resistance of a CoNiCrFeMn high-entropy alloy”, Computational Materials Science, 214 (2022) 111764-1-12.

The chemical ordering of CoNiCrFeMn and its effect on radiation resistance is analyzed in this study via Monte Carlo (MC) annealing simulation and molecular dynamics (MD) radiation damage simulation. MC annealing at a lower temperature of 600 K forms an initial stage Cr-rich region in CoNiCrFeMn due to a strong chemical ordering-driven phase decomposition; whereas, annealing at a higher temperature of 1100 K forms a chemical short-range order (CSRO). MD radiation damage simulation shows that the Cr-rich region formed by 600 Kannealing accelerates the aggregation and the evolution of defects, facilitating more dislocation loops formation. On the other hand, the CSRO structure formed by 1100 K annealing effectively delays the growth of defect number and tends to reduce the dislocation density and defect diffusion, suggesting better radiation resistance. However, the CSRO structure is destroyed by radiation, thus these advantages of CSRO will disappear in due time if CSRO cannot heal. Fortunately, since the CSRO can recover at working temperature of 1100 K, we speculate that under the working temperature of 1100 K and with a modest radiation damage rate, the advantages of CSRO can maintain for a long time. We finally discuss the condition of the CSRO preservation by proposing a CSRO radiation damage – diffusion healing competition model.

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Akio Ishii,“Ab initio morphology prediction of Zr hydride precipitates using atomistically informed Eshelby’s ellipsoidal inclusion”, Computational Materials Science, 211, (2022) 111500-1-14.

We energetically predicted the morphology of Zr hydride precipitates in a hexagonal close-packed (HCP) Zr matrix. Considering Zr hydride precipitates as ellipsoids, we used Eshelby’s ellipsoidal inclusions to calculate the elastic energy increment due to the presence of Zr hydride precipitates in the Zr matrix, in which the elastic anisotropy and inhomogeneity of the elastic constants between Zr and Zr hydride were considered. We compared the difference in the elastic energy increment between the ellipsoidal inclusions with different shapes: plates (mimicked by penny-shape ellipsoids), needles (mimicked by longitudinal ellipsoids) and sphere, and orientations to detect the stable structure with the minimum elastic energy increment. Eigenstrains of each Zr hydride and elastic constants of Zr hydrides and HCP Zr for Eshelby’s ellipsoidal inclusion analysis were determined using atomistic simulations based on a density functional theory calculation, achieving a parameter free 𝑎𝑏 𝑖𝑛𝑖𝑡𝑖𝑜 morphology prediction. The morphology predictions were implemented for two cases: with and without shear components of eigenstrain (𝑤∕ and 𝑤∕𝑜 shear). The ⟨1̄210⟩ longitudinal needle for the 𝛾 hydride (𝑤∕𝑜 shear) and plate (or disk) on the plane, which is 20◦ to 30◦ tilted about ⟨1̄210⟩-axis from basal plane (0001), for 𝛿 and 𝜖 hydrides (𝑤∕ shear) were successfully predicted as stable shapes and orientations of the precipitates under zero external stress conditions, qualitatively consistent with experimental observations. The external circumferential tensile stress on the basal plane reduces the elastic energy of [0001] parallel Zr hydride plates, which is also qualitatively consistent with the reoriented 𝛿 hydride precipitates observed in the experiment. On the other hand, predicted external stress for the reorientation of Zr hydride is quite high, around 10 GPa. This is inconsistent with experimental observation and further investigation is necessary. Generally, our predictions based on elasticity theory appear qualitatively consistent with experimental observations, suggesting an elastic origin of the morphology of Zr hydride precipitates in the HCP Zr matrix.

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

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

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Heting Liao, Hajime Kimizuka, Akio Ishii, Jun-Ping Du, Shigenobu Ogata,“Nucleation kinetics of the β" precipitate in dilute Mg–Y alloys: A kinetic Monte Carlo study”,Scripta Materialia, 210,(2022) 114480-1-4.

The β′′ precipitate is a primary strengthening precipitate in Mg–Y alloys. It nucleates as localized zigzag- and hexagonal-shaped clusters. Studies on the nucleation kinetics of β" precipitate are scarce. In this study, we applied the kinetic Monte Carlo (KMC) approach to explore the nucleation kinetics of the β" precipitates in the Mg–3.0 at.%Y system using a density functional theory-based interatomic potential. The time evolution of nucleation of the β" precipitates was characterized based on the KMC results. Using these results, we predicted the existence of an optimum temperature for the formation of the β" precipitates to be 550 K, at which the time necessary for nucleation is the shortest. Moreover, an upper temperature limit, above which the β" precipitates cannot nucleate, was computed as 700 K. This study explains precipitate nucleation in Mg–Y alloys at an atomic level and provides the theory for obtaining an optimal age-hardening response.

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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.

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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)

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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.

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Rodrigo Pinheiro Campos, Shuhei Shinzato, Akio Ishii, Shuichi Nakamura, Shigenobu Ogata,“Database-driven semigrand canonical Monte Carlo method: Application to segregation isotherm on defects in alloys”, Physical Review E, 104 (2021) 025310-1-15.

The application of existing semigrand canonical ensemble Monte Carlo algorithms to alloys requires the chemical potential difference values between pairs of atomic species in the alloys as inputs. However, finding the appropriate values for a target system at a desired temperature and bulk composition is a time-consuming task consisting of multiple test runs to determine the chemical potential differences. This problem becomes more serious when dealing with systems containing three or more atomic species, such as medium- and high-entropy alloys, due to the increase of the number of chemical potential differences that need to be calculated. Here we propose a method for sampling from the semigrand canonical ensemble that relies on energy databases acting as an external atomic reservoir at the desired temperature and composition. Given these energy databases, the desired bulk composition and corresponding chemical potential differences can be satisfied in a “single” Monte Carlo simulation. Moreover, the energy databases shed light on the underlying energetics of alloys, reflecting their local chemical ordering.We demonstrate the validity of this method using analyses of segregation isotherms at grain boundaries and dislocations in two alloy systems: Fe–1-at.-%-Si and NiCoCr medium-entropy alloy.We also discuss the possibly relevant information contained in such energy databases.

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Ruixiao Zheng, Jun-Ping Du, Si Gao, Hidetoshi Somekawa, Shigenobu Ogata, Nobuhiro Tsuji, “Transition of dominant deformation mode in bulk polycrystalline pure Mg by ultra-grain refinement down to sub-micrometer”, Acta Materialia, 198 (2020) 35-46.

Magnesium (Mg) and its alloys usually show relatively low strength and poor ductility at room temperature due to their anisotropic hexagonal close-packed (HCP) crystal structure that provides a limited number of independent slip systems. Here we report that unique combinations of strength and ductility can be realized in bulk polycrystalline pure Mg by tuning the predominant deformation mode. We succeeded in obtaining the fully recrystallized specimens of pure Mg having a wide range of average grain sizes, of which minimum grain size was 650 nm, and clarified mechanical properties and deformation mechanisms at room temperature systematically as a function of the grain size. Deformation twinning and basal slip governed plastic deformation in the conventional coarse-grained region, but twinning was suppressed when the grain size was refined down to several micro-meters. Eventually, grain boundary mediated plasticity, i.e., grain boundary sliding became dominant in the ultrafine-grained (UFG) specimen having a mean grain size smaller than 1 μm. The transition of the deformation modes led to a significant increase of tensile elongation and breakdown of Hall-Petch relationship. It was quantitatively confirmed by detailed microstructural observation and theoretical calculation that the change in strength and ductility arose from the distinct grain size dependence of the critical shear stress for activating different deformation modes.

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.

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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Akio Ishii and 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.

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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.

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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.

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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.

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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.

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