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Journal Articles

A Predictive model for transferability of plastic deformation through grain boundaries

Tsuru, Tomohito; Shibutani, Yoji*; Hirouchi, Tomoyuki*

AIP Advances (Internet), 6(1), p.015004_1 - 015004_9, 2016/01

 Times Cited Count:8 Percentile:41.71(Nanoscience & Nanotechnology)

The material strengths of polycrystalline metals have been widely predicted according to the grain size, where yield stress is governed by slip transfer through the grain boundary (GB). The transferability of a dislocation across a GB is enormously important in the deformation process as well as the interaction between a dislocation and GB. This paper proposes a new criterion for the transferability of dislocations through a GB that considers both the intergranular crystallographic orientation of slip systems and the applied stress condition. Atomistic simulations were carried out to investigate the slip transfer event of simple bicrystals composed of $$Sigma 3 (bar{1}12)$$ GB than $$Sigma 3 (bar{1}11)$$ GBs under uniaxial deformation and to illustrate the availability of this criterion. As a result, in contrast to the predictions of conventional criteria such as the $$M$$-value, dislocations propagated more easily across the $$Sigma 3 (bar{1}11)$$ and $$Sigma 3 (bar{1}12)$$ GB under given stress states, reflecting a larger $$L'$$-value of $$Sigma 3$$ bicrystal associated with higher transferability.

Journal Articles

Transfer and incorporation of dislocations to $$Sigma$$3 tilt grain boundaries under uniaxial compression

Shibutani, Yoji*; Hirouchi, Tomoyuki*; Tsuru, Tomohito

Journal of Solid Mechanics and Materials Engineering (Internet), 7(6), p.571 - 584, 2013/06

Microscopic yielding can be realized by the transfer of a dislocation across a grain boundary (GB), or by incorporation between the residual GB dislocation and the dislocations nucleated in the near-field of a GB due to the applied stress. In the present paper, a new boundary interaction criterion of $$L$$- or $$L$$'-value is proposed, which considers both the contributions of the geometric relationship between two grains and a GB, and the stress state applied to the near-field of a GB. This value and the others so far proposed were calculated for $$langle$$110$$rangle$$, $$langle$$100$$rangle$$, and $$langle$$111$$rangle$$ symmetric tilt grain boundaries under uniaxial compression normal to the GB. The dynamic transfer and incorporation of the dislocations nucleated under uniaxial compression normal to the GB plane were then examined using 3-dimensional molecular dynamics simulations.

Journal Articles

Grain growth prediction with inclination dependence of $$langle 110 rangle$$ tilt grain boundary using multi-phase-field model with penalty for multiple junctions

Hirouchi, Tomoyuki*; Tsuru, Tomohito; Shibutani, Yoji*

Computational Materials Science, 53(1), p.474 - 482, 2012/02

 Times Cited Count:18 Percentile:54.16(Materials Science, Multidisciplinary)

Multi-phase-field (MPF) model with a higher-order term representing energetic penalty for multiple junctions was proposed to predict the grain growth accompanying the inclination dependence of grain boundary (GB) energy and mobility. The inclination effect was introduced on the basis of GB energy obtained from molecular dynamics simulations. The preliminary grain growth simulation of an isolated grain surrounded by $$Sigma 3$$ GB certified that the analytical equilibrium shape was well reproduced. The augmented higher-order term added to conventional MPF model could improve convergence and stability of numerical calculations around triple junction (TJ) region even if there exists the large GB energy gap at the TJ. For the polycrystalline grain growth simulations with the GB energy distribution according to the misorientation angle of Al $$langle 110 rangle$$ tilt GB, $$Sigma 3$$ GB inclination lead the weak anisotropy characterized by $$Sigma 3 {111}$$ twin boundary.

Journal Articles

Fundamental role of $$Sigma$$3($$overline{1}$$11) and $$Sigma$$3($$overline{1}$$12) grain boundaries in elastic response and slip transfer

Tsuru, Tomohito; Kaji, Yoshiyuki; Shibutani, Yoji*

Journal of Applied Physics, 110(7), p.073520_1 - 073520_7, 2011/10

 Times Cited Count:6 Percentile:30(Physics, Applied)

To understand individual grain boundary characteristics, the influence of grain boundaries on the elastic and plastic deformation behaviors of copper bicrystals with $$Sigma$$3($$overline{1}$$11) twin and $$Sigma$$3($$overline{1}$$12) grain boundaries were investigated by large scale molecular statics simulation. Our results showed that an incoherent boundary experiences a reduction in elastic resistance due to the increase in excess free volume and structure-dependent local indentation modulus, while a coherent boundary has little effect on the elastic deformation. Plastic deformation of the $$Sigma$$3($$overline{1}$$12) indentation demonstrates easy slip transfer across the interface.

Journal Articles

Grain growth predictions by multi-phase-field modeling with higher-order term

Hirouchi, Tomoyuki*; Tsuru, Tomohito; Shibutani, Yoji*

Nihon Kikai Gakkai Rombunshu, A, 77(782), p.1723 - 1734, 2011/10

Multi-phase-field (MPF) modeling with a higher-order term, which can stably analyze the triple junction (TJ) behaviors even with large difference between grain boundary (GB) energies, is proposed for the more realistic grain growth prediction. Grain growth simulations of systems including multiple junctions show that the proposed MPF model can represent the stable TJ behaviors with wider range of GB energies than the conventional model, and well perform the quadruple junction behaviors in agreement with the responding theory. Two kinds of GB energy distribution models are employed here, which are based on all of and only low cuspate energies of $$langle$$1 1 0$$rangle$$ symmetric tilt GB energy in pure Al by molecular dynamics simulations. Polycrystalline grain growth simulations combining with either GB energy distribution exhibited the large amount of nucleation of low-energy boundaries, which would be quantitatively compatible to the experiments.

Journal Articles

Atomistic simulations of stress concentration induced and dislocation nucleation at grain boundaries

Tsuru, Tomohito; Kaji, Yoshiyuki; Tsukada, Takashi; Shibutani, Yoji*

Progress in Nuclear Science and Technology (Internet), 2, p.20 - 23, 2011/10

Stress concentration and defect nucleation of polycrystalline copper thin film is investigated by parallel molecular statics simulations. Uniaxial tensile deformation is applied to the two-dimensional polycrystalline model. As a result, it is found that the stress concentration is observed at triple point of grain junctions in the elastic and initial stage of plastic deformation. Then partial dislocations are first generated from the small angle grain boundaries. Twin deformations occur at triple points and grain boundaries which result in both another site of stress concentration around grain boundaries and local displacement relevant to the Burgers vector at grain boundary. One distinguishing characteristic of deformation mode of polycrystal is that the stress distribution strongly correlated with the presence of the partial dislocations and twin boundaries.

Journal Articles

Grain boundary characteristics evaluation by atomistic investigation methods

Kaji, Yoshiyuki; Tsuru, Tomohito; Shibutani, Yoji*

Materials Research Society Symposium Proceedings, Vol.1215, 6 Pages, 2010/10

In the present study three types of grain boundaries of coincidence site lattice (CSL), small angle (SA), and random types are considered as the representative example of grain boundaries. The grain boundary energies and atomic configurations of CSL are first evaluated by first-principle density functional theory (DFT) and the embedded atom method (EAM) calculations. SA and random grain boundaries are subsequently constructed by the same EAM and the fundamental characteristics are investigated by the discrete dislocation mechanics models and the Voronoi polyhedral computational geometric method. As the result, it is found that stress distributions of CSL and SA grain boundaries are localized around the grain boundary core. The random grain boundary shows extremely heterogeneous core structures including a lot of pentagon-shaped Voronoi polyhedral resulting from the amorphous-like structure.

Journal Articles

Minimum energy motion and core structure of pure edge and screw dislocations in aluminum

Tsuru, Tomohito; Kaji, Yoshiyuki; Shibutani, Yoji*

Journal of Computational Science and Technology (Internet), 4(3), p.185 - 193, 2010/09

The minimum energy motions of pure edge and screw dislocations in aluminum were investigated by atomistic transition state analysis. While the Peierls-Nabarro model and its modifications duplicate the essential nature of a dislocation within a crystalline lattice, the atomic-level relaxation of the dislocation core should be considered to estimate the minimum energy barrier. In this study, the minimum energy barriers and core structures for the quasi-static motions of pure edge and screw dislocations were investigated by the parallelized nudged elastic band method with the embedded atom method potential. We found that the local potential energy is distributed asymmetrically around the dislocation line for the most stable state and that it is bilaterally symmetrical at the transition state of the dislocation motion. The short-ranged structural relaxation of the core rearrangement as well as the wide-ranging elastic stress field is of great importance in realistic dislocation motion.

Journal Articles

Incipient plasticity of twin and stable/unstable grain boundaries during nanoindentation in copper

Tsuru, Tomohito; Kaji, Yoshiyuki; Matsunaka, Daisuke*; Shibutani, Yoji*

Physical Review B, 82(2), p.024101_1 - 024101_6, 2010/07

 Times Cited Count:28 Percentile:74.11(Materials Science, Multidisciplinary)

An incipient plastic deformation of several types of grain boundaries subjected to nanoindentation was investigated by atomistic simulations. Crystal defects such as grain boundaries undermine the nucleation resistance. In this paper, we examined the dislocation nucleation mechanism at the twin and several coincidence site lattice grain boundaries and the resulting weakening of the dislocation nucleation resistance. We found that for the twin and the relatively stable $$Sigma 11(bar{1}13)[110]$$ grain boundary the primary slip deformation is activated on the grain boundary plane prior to the defect-free region because of the low fault energy of the grain boundaries during slip deformation. Subsequently, the secondary slip is activated from the grain boundary. On the other hand the dislocation is initially generated from the heterogeneous grain boundary plane for the unstable high-energy grain boundaries.

Journal Articles

Nanoscale contact plasticity of crystalline metal; Experiment and analytical investigation via atomistic and discrete dislocation models

Tsuru, Tomohito; Shibutani, Yoji*; Kaji, Yoshiyuki

Acta Materialia, 58(8), p.3096 - 3102, 2010/05

 Times Cited Count:20 Percentile:72.03(Materials Science, Multidisciplinary)

Nanoscale incipient plastic deformation in crystalline metals occurs as the result of the collective motion of dislocations. It is known as "nanoplasticity" and recognized as the elementary process of the macroscopic deformation. In the present study, experimental tests are first conducted to educe the unique nature of the nanoscale deformation. Subsequently large-scale atomistic simulations are performed to predict the incipient plastic deformation and a new discrete dislocation model combined with the boundary element analysis is constructed to capture the collective motion of the dislocations. Our results suggest that the incipient plastic deformation requires much higher critical shear stress than the theoretical shear strength due to high compressive stress distribution beneath the indenter, and that the displacement burst is induced by surface rearrangement corresponding to hundreds of dislocation dipoles.

Journal Articles

Fundamental interaction process between pure edge dislocation and energetically stable grain boundary

Tsuru, Tomohito; Shibutani, Yoji*; Kaji, Yoshiyuki

Physical Review B, 79(1), p.012104_1 - 012104_4, 2009/01

 Times Cited Count:25 Percentile:71.19(Materials Science, Multidisciplinary)

The interaction between dislocations and grain boundaries is the principal factor for determining the mechanical properties and the plastic deformation behavior of metals. In atomic scale, however, specific interaction characteristics such as the reaction energy and pathway have yet to be revealed. We have investigated the interaction process between a dislocation and an energetically stable grain boundary, and the quantitative characteristics were determined via atomistic transition state analysis. As a result, the interaction energy is found to be 1.16$$times$$10$$^{-1}$$ eV/${AA}$, which is $$10^{4}$$ times higher than the Peierls potential. The lattice dislocations subsequently experience anomalous dissociations on the grain boundary, which becomes a key factor for the previously-unexplained dislocation disappearance and grain boundary migration.

Journal Articles

Theoretical investigation of the displacement burst observed in nanoindentation by collective dislocation loops nucleation model

Tsuru, Tomohito; Shibutani, Yoji*

Journal of Computational Science and Technology (Internet), 2(4), p.559 - 567, 2008/11

Abrupt growth of displacement has been recognized as one of the representative examples of nanoscale plastic behavior (nanoplasticity). This phenomenon corresponds to the early stage of plastic deformation and is greatly influenced by the collective dislocation emission. In the present paper a simplified model is constructed for the first displacement burst with use of the elastic theory based on both the Hertzian contact theory and the classical dislocation theory to evaluate the displacement burst in nanoindentation. As a result, it is shown that more than one hundred high-density dislocations are generated simultaneously and surface step corresponding to the Burgers vector of dislocation dipole of each emitted dislocation causes significant displacement burst.

Journal Articles

Dislocation nucleation and interaction under nanoindentation in single crystalline Al and Cu; Molecular dynamics simulations

Tsuru, Tomohito; Shibutani, Yoji*

Journal of Computational Science and Technology (Internet), 2(4), p.459 - 467, 2008/10

Unstable displacement burst or the abrupt growth of indent displacement after homogeneous elastic deformation observed in crystalline materials is a unique plastic deformation characteristic (nanoplasticity). In the present paper, a series of atomistic simulations of nanoindentation in single crystalline aluminum and copper are performed in analyzing the critical state for dislocation nucleation and interaction between dislocations beneath the indenter. With reference to the Hertzian solution based on isotropic linear elastic theory, both the anisotropic effect and nonlinear behavior of nanoindentation are discussed in detail. The discovery was made that the incipient yield process is strongly related to the triaxial stress state created beneath the indenter, and that energetically unfavorable interactions accompanied with cross slip induce the formation of prismatic dislocations.

Journal Articles

Multiscale modeling of three dimensional discrete dislocation dynamics and boundary element method and its application to nanoindentation problem

Tsuru, Tomohito; Shibutani, Yoji*

Nihon Kikai Gakkai Rombunshu, A, 74(743), p.933 - 938, 2008/07

In the nano-plastic deformation, material properties cannot be described by the average rate of dislocation behavior, and it becomes increasingly necessary to trace individual motion of dislocations. Molecular dynamics (MD) is one of the most effective methodologies to obtain dislocation motion. However, MD simulation depends on the computer power so strongly that it is difficult to treat mesoscopic behavior including collective dislocation motion. Discrete dislocation mechanics (DD) has a unique ability to treat dislocation motion. In the present paper, we construct a combined approach including both DD and the boundary element method, and succeed in representing the stress field of dislocation in the vicinity of traction free surface. Finally, we apply this model to the nanoindentation problem and found the relationship between displacement burst and collective dislocation motion.

Journal Articles

Atomistic investigation of structural changes and reaction energy induced by dislocation-grain boundary interaction process in fcc aluminum and bcc iron

Tsuru, Tomohito; Shibutani, Yoji*; Kaji, Yoshiyuki

Proceedings of 3rd Asian-Pacific Congress on Computational Mechanics (APCOM '07) & 11th International Conference on Enhancement and Promotion of Computational Method in Engineering and Science (EPMESC-11) (CD-ROM), 8 Pages, 2007/12

Interaction process of grain boundary and dislocation is one of the most important factors in the materials. Dislocations are successively piled up beside the grain boundary planes and they produce stress concentration which becomes a dominant stress factor of stress corrosion cracking (SCC). However, detailed mechanisms of the interaction between these processes and their influence on plastic behavior have yet to be definitively determined. In the present study, fundamental properties of the CSL grain boundaries and grain boundary-dislocations process are investigated by atomistic simulations. Then nudged elastic band (NEB) are performed to evaluate activation energy and minimum energy path in the interaction process of dislocation and $$Sigma 3(111)<110>$$ CSL. As a result, it is found that there is a strong correlation between grain boundary energy and excess free volume, and that the stable grain boundary can be high energy barrier to grain boundary-dislocation interaction.

Journal Articles

Theoretical investigation of the displacement burst observed in nanoindentation by collective dislocation loops nucleation model

Tsuru, Tomohito; Shibutani, Yoji*

Nihon Kikai Gakkai Rombunshu, A, 73(732), p.877 - 882, 2007/08

Abrupt growth of displacement observed in the relationship between indent load and indent depth in nanoindentation of crystalline materials, so-called displacement burst, has been recognized as one of the representative examples for the nanoplastic behavior. This behavior corresponds to the early stage of the plastic deformation and has greatly been influenced by the collective dislocation emission. In the present paper, we construct the simplified model of the first displacement burst by using the elastic theory based on both the Hertzian contact theory and the classical theory of dislocations to evaluate displacement burst under nanoindentation. As the result, there is strong correlation between burst width and critical indent depth where dislocation emission occur. Finally, it is shown that more than one hundred high-density dislocations are emitted simultaneously and surface step corresponding to each dislocation causes significant displacement burst.

Journal Articles

Atomistic simulation of shear localization in Cu-Zr bulk metallic glass

Ogata, Shigenobu*; Shimizu, Futoshi; Li, J.*; Wakeda, Masato*; Shibutani, Yoji*

Intermetallics, 14(8-9), p.1033 - 1037, 2006/08

 Times Cited Count:103 Percentile:97.03(Chemistry, Physical)

Shear deformations of Cu$$_{57}$$Zr$$_{43}$$ bulk metallic glass (BMG) model systems are performed using molecular dynamics simulation. The results suggest that both the hydrostatic stress and the stress normal to the shear plane should affect the shear response. We see shear localization and shear band nucleation in both a small system of 2,000 atoms, and large systems of 524,288 atoms, and analyze local atomic structure evolution.

JAEA Reports

Test results of HTTR control system

Motegi, Toshihiro; Iigaki, Kazuhiko; Saito, Kenji; Sawahata, Hiroaki; Hirato, Yoji; Kondo, Makoto; Shibutani, Hideki; Ogawa, Satoru; Shinozaki, Masayuki; Mizushima, Toshihiko; et al.

JAEA-Technology 2006-029, 67 Pages, 2006/06

JAEA-Technology-2006-029.pdf:3.07MB

The plant control performance of the IHX helium flow rate control system, the PPWC helium flow rate control system, the secondary helium flow rate control system, the inlet temperature control system, the reactor power control system and the outlet temperature control system of the HTTR are obtained through function tests and power-up tests. As the test results, the control systems show stable control response under transient condition. Both of inlet temperature control system and reactor power control system shows stable operation from 30% to 100%, respectively. This report describes the outline of control systems and test results.

Oral presentation

An Evaluation of nanoindentation-induced displacement burst and collective dislocation motion based on discrete dislocation mechanics and elastic energy of dislocations

Tsuru, Tomohito; Shibutani, Yoji*

no journal, , 

Abrupt growth of displacement observed in the relationship between indent load and indent depth in nanoindentation of crystalline materials, so-called displacement burst, has been recognized as one of the representative examples for the nanoplastic behavior. This behavior corresponds to the early stage of the plastic deformation and has greatly been influenced by the collective dislocation emission. In the present paper, we construct two models; first one is the computational model of dislocation mechanics, and second one is simplified energetic model of the first displacement burst. As the result of these models, it is found that surface step corresponding to each dislocation causes significant displacement burst and that more than one hundred high-density dislocations are emitted simultaneously.

Oral presentation

Atomistic/electronic and discrete dislocation simulations for grain boundary characteristics

Tsuru, Tomohito; Kaji, Yoshiyuki; Tsukada, Takashi; Shibutani, Yoji*

no journal, , 

Fundamental properties of grain boundaries (GBs) are investigated via Atomistic/electronic simulations and discrete dislocation models. Grain boundary and dislocation is one of the most important factors in the crystalline materials which contribute significantly to plastic behavior. In this study, we focused on the various kinds of grain boundaries including the coincidence site lattice, small angle, and random GBs, and evaluated the structural characteristics and fundamental properties effectively.

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