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Journal Articles

Age-hardening mechanisms of heterogeneous-nanostructured SUS316LN stainless steel fabricated by heavy cold rolling

Miura, Hiromi*; Watanabe, Chihiro*; Aoyagi, Yoshiteru*; Oba, Yojiro; Kobayashi, Masakazu*; Yoshinaga, Naoki*

Materials Science & Engineering A, 833, p.142531_1 - 142531_12, 2022/01

Journal Articles

First-principles modeling for dislocation motion of HEA alloys

Tsuru, Tomohito; Itakura, Mitsuhiro; Yuge, Koretaka*; Aoyagi, Yoshiteru*; Shimokawa, Tomotsugu*; Kubo, Momoji*; Ogata, Shigenobu*

Proceedings of 4th International Symposium on Atomistic and Multiscale Modeling of Mechanics and Multiphysics (ISAM-4) (Internet), p.59 - 62, 2019/08

High entropy alloys (HEAs) are chemically complex single- or multi-phase alloys with crystal structures. There are no major components but five or more elements are included with near equiatomic fraction. In such a situation, deformation behavior can no longer be described by conventional solid solution strengthening model. Some HEAs, indeed, show higher strengthening behavior and anomalous slip. However, the mechanisms of these features have yet to be understood. In the present study, we investigate the core structure of dislocations in BCC-HEAs using density functional theory (DFT) calculations. We found that core structure of a screw dislocation is identified as is the case with common BCC metals. On the other hand, dislocation motion should be different from pure BCC metals because of chemical and configurational disorder around dislocation core. We confirmed the specific feature of dislocation motion in HEAs by two-dimensional Peierls potential surface.

Journal Articles

Status of rock dynamics study in Horonobe Underground Research Laboratory, Japan

Sato, Toshinori; Aoyagi, Kazuhei; Matsuzaki, Yoshiteru; Miyara, Nobukatsu; Miyakawa, Kazuya

Rock Dynamics; Experiments, Theories and Applications, p.575 - 580, 2018/06

Rock dynamics is one of key issue for research and development of techniques for safe geological disposal of high-level radioactive waste. Horonobe Underground Research Laboratory (URL) is off-site URL constructed in soft sedimentary rock to the depth of 350m with three shafts and three level experimental galleries. Earthquake-resistant design of underground openings, observation of seismic records and groundwater pressure change due to earthquakes, and excavation disturbed zone experiment have been performed relating to the study of rock dynamics in URL project. This paper shows current status of Horonobe URL project and results of earthquake-resistant design of shafts, observation of seismic records and groundwater pressure change due to the 2011 off the Pacific coast of Tohoku Earthquake.

Journal Articles

Temporal trend in excavation disturbed zone around a modeled disposal pit in Horonobe Underground Research Laboratory

Aoyagi, Kazuhei; Miyara, Nobukatsu; Ishii, Eiichi; Matsuzaki, Yoshiteru

Shigen, Sozai Koenshu (Internet), 5(1), 7 Pages, 2018/03

no abstracts in English

Journal Articles

Atomic scale simulations of relationship between macroscopic mechanical properties and microscopic defect evolution in ultrafine-grained metals

Tsuru, Tomohito; Aoyagi, Yoshiteru*; Shimokawa, Tomotsugu*

Materials Transactions, 57(9), p.1476 - 1481, 2016/09

 Times Cited Count:1 Percentile:8.89(Materials Science, Multidisciplinary)

The effects of grain size and intragranular dislocation on yield mechanism and subsequent plastic deformation in ultrafine-grained (UFG) Al and Cu were investigated by large-scale atomic simulations. Polycrystalline atomic models with and without intragranular dislocation sources were used to elucidate the relationship between mechanical properties and defect texture. It is found that the intragranular dislocation plays a significant role in both incipient yield and grain boundary mediated dislocation nucleation. In addition UFG Cu yields earlier than UFG Al because partial dislocations in Cu are more likely to activate from grain boundaries, where the partial dislocation leaves deformation twin and secondary dislocation tends to move on twin boundary accompanied by the shift of twin boundary plane.

Journal Articles

Heterogeneous plastic deformation and Bauschinger effect in ultrafine-grained metals; Atomistic simulations

Tsuru, Tomohito; Aoyagi, Yoshiteru*; Kaji, Yoshiyuki; Shimokawa, Tomotsugu*

Modelling and Simulation in Materials Science and Engineering, 24(3), p.035010_1 - 035010_10, 2016/03

 Times Cited Count:7 Percentile:41.71(Materials Science, Multidisciplinary)

The effect of the dislocation density on yield strength and subsequent plastic deformation of ultrafine-grained metals was investigated in large-scale atomistic simulations. Polycrystalline models were constructed and uniaxial tension and compression were applied to elucidate the heterogeneous plastic deformation and the Bauschinger effect. The initial yield becomes heterogeneous as the dislocation density decreases owing to a wide range of Schmid factors of activated slip systems in each grain. A different mechanism of the Bauschinger effect was proposed, where the Bauschinger effect of ultrafine-grained metals is caused by the change in dislocation density in the process of forward and backward loadings.

Journal Articles

Atomistic simulation of yield and plastic deformation in bulk nanostructured metals

Tsuru, Tomohito; Aoyagi, Yoshiteru*; Kaji, Yoshiyuki; Shimokawa, Tomotsugu*

Proceedings of 23rd International Conference on Nuclear Engineering (ICONE-23) (DVD-ROM), 4 Pages, 2015/05

The influence of dislocation density on yield strength, which is a key factor in the anomalous deformation behavior in ultrafine-grained (UFG) metals, was investigated by huge scale atomistic simulations. Polycrystalline models with intragulanular Frank-Read sources were constructed to elucidate the relationship between the inter- and intra-granular plastic deformation processes and the mechanical properties. Then the uniaxial tension and compression were applied to the polycrystalline copper. Consequently it was found that Frank-Read sources were activated prior to intergranular dislocation emission, and the yield event of the whole system seems to occur when some dislocation sources activated. The yield stress is strongly influenced by the number of intragranular dislocation sources, i.e., dislocation density. Additionally, the Bauschinger effect of UFG metals is caused by the change in dislocation density in the process of forward and backward deformation.

Journal Articles

Crystal plasticity modeling and simulation considering the behavior of the dislocation source of ultrafine-grained metal

Aoyagi, Yoshiteru*; Tsuru, Tomohito; Shimokawa, Tomotsugu*

International Journal of Plasticity, 55, p.43 - 57, 2014/04

 Times Cited Count:35 Percentile:89.26(Engineering, Mechanical)

UFGMs with a grain size less than 1 $$mu$$m exhibits remarkable material and mechanical properties, and a computational model predicting these properties is desired in the field of materials science and engineering. When grains are of the submicron order, dislocation loops are hardly generated from Frank-Read sources smaller than the grain size. In this study, we develop a crystal plasticity model considering the effect of the grain boundary and dislocation source. We thoroughly investigate the effect of dislocation behavior on the material properties of UFGMs.

Journal Articles

Influence of competition between intragranular dislocation nucleation and intergranular slip transfer on mechanical properties of ultrafine-grained metals

Tsuru, Tomohito; Aoyagi, Yoshiteru*; Kaji, Yoshiyuki; Shimokawa, Tomotsugu*

Nihon Kinzoku Gakkai-Shi, 78(1), p.45 - 51, 2014/01

 Times Cited Count:0 Percentile:0.01(Metallurgy & Metallurgical Engineering)

Huge-scale atomistic simulations of shear deformation tests to the aluminum polycrystalline thin film containing the Frank-Read source are performed to elucidate the relationship between the inter- and intragranular plastic deformation processes and the mechanical properties of ultrafine-grained metals. While the first plastic deformation occurs by the dislocation bow-out motion within the grain region for both models, the subsequent plastic deformation is strongly influenced by the resistance of the slip transfer by dislocation transmission through grain boundaries. The influence of the competition between the intragranular dislocation nucleation and intergranular slip transfer on the material strength is considered.

Journal Articles

Influence of competition between intragranular dislocation nucleation and intergranular slip transfer on mechanical properties of ultrafine-grained metals

Tsuru, Tomohito; Aoyagi, Yoshiteru*; Kaji, Yoshiyuki; Shimokawa, Tomotsugu*

Materials Transactions, 54(9), p.1580 - 1586, 2013/09

 Times Cited Count:12 Percentile:59.9(Materials Science, Multidisciplinary)

Mechanical properties of the bulk nanostructured metals, in which the volume fraction of the grain boundary is remarkably increased, cannot be explained by the average quantity of dislocation motion as predicted in conventional course-grained metals. Therefore, it is necessary to understand the effect of each grain and dislocation motion on macroscopic mechanical properties. In the present study, large scale atomistic simulations are performed to investigate the activation process of both intergranular and intragranular dislocations. A dipole of Frank-Read source was modeled as a dislocation source and bow-out motion was simulated to investigate the effect of grain size and orientation on the critical shear stress and macroscopic plastic deformation.

Journal Articles

Modeling and simulation on ultrafine-graining based on multiscale crystal plasticity considering dislocation patterning

Aoyagi, Yoshiteru*; Kobayashi, Ryotaro*; Kaji, Yoshiyuki; Shizawa, Kazuyuki*

International Journal of Plasticity, 47, p.13 - 28, 2013/08

 Times Cited Count:23 Percentile:75.98(Engineering, Mechanical)

In this study, we derive reaction-diffusion equations for dislocation patterning of dislocation cell structures and subgrains. In order to express the generation of dislocation pattern responding to deformation progress, information of slip rate and stress and effect of interactions between slip systems on formation of cell structures are introduced into the reaction rate coefficients of reaction-diffusion equations. Moreover, we propose a multiscale crystal plasticity model based on dislocation patterning. Then we carry out pseudo-three-dimensional FE-FD hybrid simulation for severe compression of FCC polycrystal using the present model. Some processes of ultra fine-graining, i.e., generation of dislocation cell structures, subgrains, dense dislocation walls and lamella subdivisions with high angle boundaries are numerically reproduced, and we investigate the effect of dislocation behavior on the processes of ultrafine-graining.

Journal Articles

A Phenomenological micromechanical model of FCC metals under radiation induced crystal defects

Aoyagi, Yoshiteru; Tsuru, Tomohito; Kaji, Yoshiyuki

Effects of Radiation on Nuclear Materials; 25th Volume (ASTM STP 1547), p.269 - 287, 2013/01

 Times Cited Count:0 Percentile:0.1

In this study, we construct a new crystal plasticity model considering densities of crystal defects induced by irradiation. Evolution equation of densities of imperfect dislocation loop is proposed by considering disappearance of imperfect dislocation loops originated in dislocation movement. Moreover, in order to predict increase of CRSS due to irradiation and decrease of work-hardening ratio, information of densities of imperfect dislocation loop and stacking fault tetrahedron are introduced into a hardening modulus of crystal. Value of controlling parameters operating effect of crystal defects on flow stress are decided by a MD simulation. We conduct a crystal plasticity simulation for simple tension of polycrystal using the presented model. The macroscopic stress-strain responses such as increase of yield stress and decrease of work-hardening ratio due to irradiation are numerically predicted. We investigate comprehensively the effect of behaviors of crystal defects on the mechanical property of irradiated materials and the generation of dislocation substructures depending on mechanical conditions.

Journal Articles

Atomistic simulations of activation ability of dislocation source within grain boundary

Tsuru, Tomohito; Aoyagi, Yoshiteru*; Shimokawa, Tomotsugu*; Kaji, Yoshiyuki

International Workshop on Bulk Nanostructured Metals; Proceedings Book, p.56_1 - 56_4, 2012/06

Bulk nanostructured metals composed of sub-micron diameter grains have a dramatic increase in the volume fraction located in the grain boundary region. Mechanical properties of the nano-structured metals cannot be predicted based on assumptions of the average quantity of collective motion of dislocations, and therefore it is increasingly necessary to understand the role of each grain and its effect on the plastic deformation. In the present study, a computational modelling based on large scale atomistic simulation is implemented to capture the effect of grain size and macroscopic mechanical properties.

Journal Articles

Simulation on nanostructured metals based on multiscale crystal plasticity considering effect of grain boundary

Aoyagi, Yoshiteru; Shimokawa, Tomotsugu*; Shizawa, Kazuyuki*; Kaji, Yoshiyuki

Materials Science Forum, 706-709, p.1751 - 1756, 2012/01

 Times Cited Count:2 Percentile:79.2

In this study, we develop a crystal plasticity model considering an effect of grain boundary. In order to predict increase of local critical resolved shear stress due to existence of grain boundaries, information of grain boundary as a role of dislocation sources is introduced into a hardening law of crystal plasticity. In addition, carrying out FE simulation for plastic deformation of FCC polycrystal, the stress-strain responses such as increase of yield stress due to existence of grain boundary are discussed. We investigate comprehensively the effect of dislocation behavior on the material property of nanostructured metal. The increase of yield stress and the decrease of hardening ratio with the reduction of grain size are caused by local differences on CRSS and dislocation behavior, respectively.

Journal Articles

Crystal plasticity simulation considering oxidation along grain boundary and effect of grain size on stress corrosion cracking

Aoyagi, Yoshiteru; Kaji, Yoshiyuki

Materials Transactions, 53(1), p.161 - 166, 2012/01

 Times Cited Count:5 Percentile:38.42(Materials Science, Multidisciplinary)

Journal Articles

Formulation of discrete balance laws of single phase in lattice scale for recrystallization

Muramatsu, Mayu*; Aoyagi, Yoshiteru; Shizawa, Kazuyuki*

Nihon Kikai Gakkai Rombunshu, A, 77(780), p.1304 - 1319, 2011/08

A crystal lattice in a metal during recrystallization process is modeled as an elastic bar element subject only to stretch and its kinematics is discussed. The balance laws of mass, momentum, angular momentum and energy of the lattice element are formulated. These laws are summed up over a phase in a representative volume element (RVE) and averaged in the RVE so as to prepare to develop macroscopic balance laws for a continuum mixture consisting of several phases. When the RVE converges on a material point at the final procedure of formulation, the present model can be regarded as a director model whose direction vector expressing the crystal orientation is attached to a material point of simple body. During the averaging process, two useful theorems are proposed for averaging terms associated with mass source and then these theorems are verified. Moreover, defining the representative lengths both in macroscopic and microscopic scales and performing an order-estimation for the balance law of angular momentum, this law can be separated into the bulk and lattice parts. The former results in the usual form, so that the Cauchy stress keeps symmetric even though the spin angular momentum of crystal lattice is taken into account. On the other hand, the latter corresponds to the evolution equation of crystal orientation of KWC type phase-field model.

Journal Articles

Stress corrosion cracking behavior of type 304 stainless steel irradiated under different neutron dose rates at JMTR

Kaji, Yoshiyuki; Kondo, Keietsu; Aoyagi, Yoshiteru; Kato, Yoshiaki; Taguchi, Taketoshi; Takada, Fumiki; Nakano, Junichi; Ugachi, Hirokazu; Tsukada, Takashi; Takakura, Kenichi*; et al.

Proceedings of 15th International Conference on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors (CD-ROM), p.1203 - 1216, 2011/08

In order to investigate the effect of neutron dose rate on tensile property and irradiation assisted stress corrosion cracking (IASCC) growth behavior, the crack growth rate (CGR) test, tensile test and microstructure observation have been conducted with type 304 stainless steel specimens. The specimens were irradiated in high temperature water simulating the temperature of boiling water reactor (BWR) up to about 1dpa with two different dose rates at the Japan Materials Testing Reactor (JMTR). The radiation hardening increased with the dose rate, but there was little effect on CGR. Increase of the yield strength of specimens irradiated with the low dose rate condition was caused by the increase of number density of frank loops. Little difference of radiation-induced segregation at grain boundaries was observed in specimens irradiated by different dose rates. Furthermore, there was little effect on local plastic deformation behavior near crack tip in the crystal plasticity simulation.

Journal Articles

Hierarchical computational approaches of the effects of interstitial and vacancy loops on plastic deformation

Tsuru, Tomohito; Aoyagi, Yoshiteru; Kaji, Yoshiyuki

Materials Research Society Symposium Proceedings, Vol.1298, 6 Pages, 2011/04

Dislocation channeling observed in irradiated metals is generated as the result of strain localization and it has been thought to be the dominant stress factor in irradiation assisted stress corrosion cracking. In the present study, a hierarchical computational modeling based on atomistic and continuum simulations were constructed to describe the fundamental characteristics of the plastic deformation in irradiated materials. Several anomalous defects are introduced under irradiation. At first individual influences of the irradiation defects on plastic deformation can be identified by atomistic simulations. Then local interaction events between glide dislocation and irradiation defects were introduced into the crystal plasticity finite element analysis. We found that softening after the first yield event observed in experimental study is caused by annihilation of irradiation defects resulting from unfaulting by dislocation sweeping.

Journal Articles

A Multiscale crystal plasticity simulation on ultrafine-graining based on dislocation patterning

Aoyagi, Yoshiteru; Kobayashi, Ryotaro*; Shizawa, Kazuyuki*

Nihon Kikai Gakkai Rombunshu, A, 77(775), p.448 - 461, 2011/03

Ultrafine-grained metals whose grain size is less than one micron have attracted interest as high strength materials. However, a mechanism of ultrafine-graining based on evolution of dislocation structures has not been clarified. In this study, we derive reaction-diffusion equations for dislocation patterning of dislocation cell structures and subgrains. In order to express the generation of dislocation pattern responding to deformation progress, information of slip rate and stress and effect of interactions between slip systems on formation of cell structures are introduced into the reaction rate coefficients of reaction-diffusion equations. Moreover, we propose a multiscale crystal plasticity model based on dislocation patterning. Then we carry out two-dimensional FE-FD simulation for severe compression of FCC polycrystal using the present model. Some processes of ultrafine-graining, i.e., generation of dislocation cell structures, subgrains, dense dislocation walls and lamella subdivisions with high angle boundary is numerically reproduced, and we investigate the effect of dislocation behaviors on the processes of ultrafine-graining.

Journal Articles

A Triple-scale dislocation-crystal plasticity simulation on yield point drop of annealed FCC ultrafine-grained metal

Kurosawa, Eisuke*; Aoyagi, Yoshiteru; Shizawa, Kazuyuki*

Nihon Kikai Gakkai Rombunshu, A, 76(772), p.1547 - 1556, 2010/12

In order to express the increase of critical resolved shear stress, the conventional Bailey-Hirsh's relationship is extended on the basis of physical consideration for grain boundary that plays a role of dislocation source. A triple-scale dislocation-crystal plasticity FE simulation based on the above model, geometrically necessary crystal defects and the homogenization method is carried out for annealed FCC polycrystals with different initial grain size and initial dislocation density. Yield point drop and propagation of L$"{u}$ders bands observed in macroscopic specimen with annealed FCC fine-grains are numerically reproduced. Moreover, macroscopic yielding of specimen and microscopic grain yielding are investigated in detail so as to clarify the initial yield behavior of annealed ultrafine-grained metals. It is also shown that plastic deformation is easy to be localized and the tensile ductility decreases as the grain size reduces.

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