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

https://doi.org/10.2320/matertrans.MH201515

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.

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.

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

https://doi.org/10.2320/jinstmet.J2013039

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.

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.

Multiscale computational approach of grain size effect on plastic deformation

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

no journal, , 

Recent progress in grain boundary engineering allows the effective control of grain scale and microstructure in metallic systems, improving bulk material properties. It has been possible to produce nano-crystalline and ultrafine-grained metals with submicron grain size. 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 multiscale computational modelling based on atomistic and crystal plasticity (CP) analysis is developed to capture the effect of grain size and macroscopic mechanical properties.

Large-scale atomistic simulations on deformation mechanism of ultrafine-grained metals

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

no journal, , 

In this study, huge-scale atomistic simulations of the polycrystalline thin film containing the Frank-Read source are performed to elucidate the fundamental deformation mechanism 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. Subsequently, the Bauschinger effect of the ultrafine-grain metals is investigated using three-dimensional polycrystalline model with dislocation sources within the grain region.

First-principles predictions of dislocation motion in high-entropy alloys

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

no journal, , 

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.

Strain rate sensitivity of dislocation nucleation from grain boundary

Tirtom, I.; Wang, Y.*; Shimokawa, Tomotsugu*; Ogata, Shigenobu*

no journal, , 

Tension/compression anisotropy in yield stress and Bauschinger effect in ultrafine-grained metals

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

no journal, , 

The plastic deformation characteristics of ultrafine-grained metals were investigated by huge scale atomistic simulations. Some polycrystalline models were constructed and the uniaxial tension and compression were applied to the polycrystalline aluminum and copper models. It is found that yield stress is strongly influenced by the number of intragranular dislocation sources, i.e., dislocation density. The Bauschinger effect of UFG metals is caused by the change in dislocation density in the process of forwarding and backwarding deformation. Additionally, the yield stresses of tensile and compressive deformation have some sort of plastic anisotropy. UFG aluminum shows more significant anisotropy than UFG copper.

First-principles study on dislocation core with short-range order in BCC high entropy alloys

Tsuru, Tomohito; Lobzenko, I.; Yuge, Koretaka*; Aoyagi, Yoshiteru*; Shimokawa, Tomotsugu*; Kubo, Momoji*; Ogata, Shigenobu*

no journal, , 

In the present study, we investigated the short-range order and the core structure of dislocations in body centered cubic (BCC) HEAs using density functional theory (DFT) calculations. Special quasirandom structures (SQS) scheme was employed to mimic randomly-distributed five-component BCC-HEAs with equiatomic fraction. According to the phase stability based on DFT calculations, MoNbTaVW and ZrNbTaTiHf HEAs are taken as the typical cases of energetically stable and unstable BCC-HEAs.