Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

In order to elucidate the origin of excellent mechanical properties of high-entropy alloys (HEA), it is essential to develop the atomic-level depiction of defect structures, taking into account the influence of composition. Especially in body-centered cubic (BCC) HEA an alteration of constituent elements may lead to a dramatic change in the behavior on the macroscopic level. To study that effect, we employed a machine learning technique and constructed highly accurate robust potentials for two BCC medium-entropy alloys: MoNbTa and ZrNbTa. Even though they have close composition, the mechanical properties of the two alloys differ not only quantitatively, but also qualitatively. We show that the group IV element Zr decreases values of bulk and elastic constants. The influence of short-range order on stacking fault and twin boundary energies is discussed. Also, we show the difference in the screw dislocation core shapes between the two alloys. The cores of 〈111〉 screw dislocations in the ZrNbTa case demonstrate a non-compact shape substantially extended on the (110) plane.

DOI： 10.1016/j.commatsci.2023.112010

Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.commatsci.2022.111366

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Frontiers Media SA  

The stress state at an atomic level and its governing physics inside a random alloy are essential elements in developing a model for solid solution strengthening in random alloys, which is one of the primary strengthening mechanisms of high-entropy alloys (HEAs). Through first-principles calculation, we investigated the atomic stress in fcc and bcc random alloys that were subsets of CrMnFeCoNi and VNbMoTaW HEAs, respectively. The results showed a correlation between the atomic pressure dispersion and the experimental yield stress for the bcc random alloys, as observed in a previous study on fcc alloys. By focusing on the charge transfer and volume change with respect to a bulk crystal, we examined whether the internal stress fields in the fcc and bcc alloys could be interpreted from a unified viewpoint in terms of these physical quantities. Regression analyses using the random forest method revealed that the charge transfer and volume change simultaneously govern the stress state inside an alloy, albeit with varying degrees of intensity.

File： Shiihara et al. - 2022(2).pdf

DOI： 10.3389/fmats.2022.895626

Authorship：Lead author   Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Authorship：Lead author   Publishing type：Research paper (scientific journal)  

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

This study reports grain boundary (GB) energy calculations for 46 symmetric-tilt GBs in alpha-iron using molecular mechanics based on an artificial neural network (ANN) potential and compares the results with calculations based on the density functional theory (DFT), the embedded atom method (EAM), and the modified EAM (MEAM). The results by the ANN potential are in excellent agreement with those of the DFT (5% on average), while the EAM and MEAM significantly differ from the DFT results (about 27% on average). In a uniaxial tensile calculation of Sigma 3(1 (1) over bar2) GB, the ANN potential reproduced the brittle fracture tendency of the GB observed in the DFT while the EAM and MEAM mistakenly showed ductile behaviors. These results demonstrate the effectiveness of the ANN potential in calculating grain boundaries of iron, which is in high demand in modern industry. (C) 2021 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc.

File： Shiihara et al. - 2022.pdf

DOI： 10.1016/j.scriptamat.2021.114268

Publishing type：Research paper (scientific journal)   Publisher：Iron and Steel Institute of Japan  

DOI： 10.2355/isijinternational.isijint-2021-512

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

In this study, the effect of grain boundary on the frictional behavior under the oil-lubricated condition was investigated by varying the grain size (crystallite size) of the pure Fe samples prepared via physical vapor deposition. A high fraction of grain boundary in the sample yielded a low friction coefficient mu in the case of a lubrication oil that formed a chemisorbed film on the surface of the material. Thick absorbed layer formed on the surface was observed only in the sample with high fraction of grain boundary. These results indicate that a disordered structure in the vicinity of grain boundary preferentially forms chemisorbed films and reduces mu by protecting the surface of the material.

DOI： 10.1016/j.triboint.2020.106781

Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

Nanocrystallization processes applied to metallic surfaces can enhance their tribological properties under boundary lubrication. To reveal the mechanism, the adsorbability of acetic acid as a fatty lubricant molecule on a nanostructured iron surface was investigated using first-principles calculations. The nanostructured surface was modeled as a surface slab including a grain boundary (GB) per unit cell. Comparing the adsorbability of the Fe (110) surface with symmetrical tilt Sigma 3 (111) grain boundaries to that of an Fe (110) surface without GBs, we found that the molecule tends to be adsorbed more strongly (0.76 eV in average) on sites close to the grain boundary than on the clean surface. A local energy analysis revealed that the adsorption was enhanced even at a relatively distant position (approximately 5 angstrom from the adsorbed molecule) and that the energy contributions of atoms of different types were qualitatively different for the adsorption on the surface with GBs.

DOI： 10.1016/j.apsusc.2020.148604

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN INST METALS & MATERIALS  

Revealing atomic-scale distributions of energy and stress in defective or complex systems, based on the behavior of electrons, should contribute much to materials science and engineering, while only few practical ab initio methods were developed for this purpose. Thus, we developed computational techniques of local-energy and local-stress calculations within the plane-wave PAW (projector augmented wave)-GGA (generalized gradient approximation) framework. This is natural extension of ab initio energy-density and stress-density schemes, while the inherent gauge dependency is removed by integrating these densities in each local region where the contained gauge-dependent terms are integrated to be zero. In this overview, we explain our scheme with some details and discuss the concepts or physical meanings of local energy and local stress via the comparison with related schemes using similar densities, LCAO (linear combination of atomic orbitals) methods, Green's function formulation implemented by multiple scattering or TB (tight-binding) methods, or EAM (embedded-atom method) potentials. We present recent applications to metallic surfaces, grain boundaries (GBs) with and without solute segregation, tensile tests of metallic GBs, local elastic constants of microstructures in alloys, and machine-learning based GB-energy prediction, where the local-energy and local-stress analyses provide novel aspects of phenomena, deep insights into the mechanism, and effective data for novel machine-learning based modelling. We discuss unsettled issues and future applications, especially for large-scale metallic systems.

DOI： 10.2320/matertrans.MT-M2020291

Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER/PLENUM PUBLISHERS  

Coarse-grained molecular dynamics simulations were carried out to investigate the frictional properties of lubricant molecules on nanostructured metal surfaces. The simulation cell consists of a lubricant film enclosed between two metal surfaces. The attractive potential of specific iron atoms on the surface was modified such that the lubricant molecule adsorb preferentially on these atoms. These particular iron atoms were arranged to reproduce a grain boundary surface nanostructure. It is found that below the critical normal stress, the strength of the interaction between Fe and the lubricant molecule has little effect on the friction coefficient. However, the behavior of the lubricant film on the metal surface is sensitive to the interaction strength. Large attraction forces increase the adherence of the lubricant film.

DOI： 10.1007/s11249-020-1276-2

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society ({APS})  

File： Lobzenko et al. - 2020.pdf

DOI： 10.1103/PhysRevLett.124.085503

Other Link： https://link.aps.org/article/10.1103/PhysRevLett.124.085503

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：日本機械学会  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Iron and Steel Institute of Japan  

The effect of lattice defects on the tribological behavior under tricresyl phosphate (TCP) added poly-alpha-olefin (PAO) lubrication was investigated in the nanostructured steels produced by heavy plastic deformation processes. In surface-nanostructured SUJ2-bearing steel, tribological behavior with high friction coefficient was observed in ball-on-disk tests when compared to non-deformed steel. In addition, a similar phenomenon was observed in ultra-low carbon (ULC) steel with a high density of lattice defects (grain boundary, dislocation and so on). By increasing the density of lattice defects, a higher friction coefficient was observed. The reason for the tribological behavior with high friction coefficient seems to be that the compound film of Fe-O-P system formed in the ball-on-disk test was worn down.

DOI： 10.2355/isijinternational.isijint-2019-707

Publishing type：Research paper (scientific journal)  

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：日本機械学会  

Authorship：Last author   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC MECHANICAL ENGINEERS  

The peridynamics is a particle methods considered superior to the finite element method in describing fracture phenomena since fracture can be simply expressed as bond breaking between the peridynamics particles. The Non-Ordinary State-Based (NOSB) peridynamics is a variant of the particle method which allows us to implement constitutive laws as in the finite element method. To maintain its computational accuracy, parameters used in the peridynamic simulations needs to be optimized through parameter searches, hence the peridynamics requires a fast quasi-implicit algorithm which allows to directly compare the results to the one obtained by the finite element method. The FIRE algorithm is such an algorithm well examined through molecular dynamics studies and is easy to implement existing explicit algorithms. The objective of this study is to describe the details of the implementation of the FIRE method to the peridynamic elastic and elasto-plastic computational codes and the effectiveness through the comparison with another quasi-static algorithm, the energy based relaxation method. The computational results show the effectiveness of the algorithm and some examples of the parameter search in simple problems such as elastic and elasto-plastic deformation analyses and a stress analysis near a crack.

DOI： 10.1299/mej.18-00363

Publishing type：Research paper (scientific journal)   Publisher：日本鉄鋼協会  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier B.V.  

Hydrogen embrittlement susceptibility of high strength 7xxx series Al alloys has been recognized as the critical issues in the practical use of Al alloys. In spite of the recent improvement of experimental technique, the hydrogen distribution in Al alloys is still unclear. Focusing on the interface between MgZn2 precipitates and an Al matrix, which is considered as one of the important segregation sites in these alloys, we investigated the stable η-MgZn2–Al interface, and the possible hydrogen trap sites in MgZn2 and at the η-MgZn2–Al interface via first-principles calculation. Most of the interstitial sites inside the MgZn2 crystal were not possible trap sites because their energy is relatively higher than that of other trap sites. The trap energy of the most favorable site at the η-MgZn2–Al is approximately −0.3 eV/H, which is more stable that of the interstitial site at the grain boundary. The interface between MgZn2 and Al is likely to be a possible trap site in Al alloys. Moreover, hydrogen atoms do not tend to be trapped around Zn, but a trap site around Mg is favorable
this observation is consistent with previous experimental observations.

DOI： 10.1016/j.commatsci.2018.03.009

Publishing type：Research paper (scientific journal)   Publisher：Elsevier  

Publishing type：Research paper (scientific journal)   Publisher：International federation of societies of cosmetic chemists  

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Fe-rich Fe-Si alloys show peculiar bulk-modulus changes depending on the Si concentration in the range of 0-15 at.% Si. In order to clarify the origin of this phenomenon, we have performed density-functional theory calculations of supercells of Fe-Si alloy models with various Si concentrations. We have applied our recent techniques of ab initio local energy and local stress, by which we can obtain a local bulk modulus of each atom or atomic group as a local constituent of the cell-averaged bulk modulus. A2-phase alloy models are constructed by introducing Si substitution into bcc Fe as uniformly as possible so as to prevent mutual neighboring, while higher Si concentrations over 6.25 at.% Si lead to contacts between SiFe8 cubic clusters via sharing corner Fe atoms. For 12.5 at.% Si, in addition to an A2 model, we deal with partial D03 models containing local D03-like layers consisting of edge-shared SiFe8 cubic clusters. For the cell-averaged bulk modulus, we have successfully reproduced the Si-concentration dependence as a monotonic decrease until 11.11 at.% Si and a recovery at 12.5 at.% Si. The analysis of local bulk moduli of SiFe8 cubic clusters and Fe regions is effective to understand the variations of the cell-averaged bulk modulus. The local bulk moduli of Fe regions become lower for increasing Si concentration, due to the suppression of bulk-like d-d bonding states in narrow Fe regions. For higher Si concentrations till 11.11 at.% Si, corner-shared contacts or 1D chains of SiFe8 clusters lead to remarkable reduction of local bulk moduli of the clusters. At 12 at.% Si, on the other hand, two-or three-dimensional arrangements of corner-or edge-shared SiFe8 cubic clusters show greatly enhanced local bulk moduli, due to quite different bonding nature with much stronger p-d hybridization. The relation among the local bulk moduli, local electronic and magnetic structures, and local configurations such as connectivity of SiFe8 clusters and Fe-region sizes has been analyzed. The ab initio local stress has opened the way for obtaining accurate local elastic properties reflecting local valence-electron behaviors.

DOI： 10.1088/2053-1591/aa97a4

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We propose a new scheme based on machine learning for the efficient screening in grain-boundary (GB) engineering. A set of results obtained from first-principles calculations based on density functional theory (DFT) for a small number of GB systems is used as a training data set. In our scheme, by partitioning the total energy into atomic energies using a local-energy analysis scheme, we can increase the training data set significantly. We use atomic radial distribution functions and additional structural features as atom descriptors to predict atomic energies and GB energies simultaneously using the least absolute shrinkage and selection operator, which is a recent standard regression technique in statistical machine learning. In the test study with fcc-Al [110] symmetric tilt GBs, we could achieve enough predictive accuracy to understand energy changes at and near GBs at a glance, even if we collected training data from only 10 GB systems. The present scheme can emulate time-consuming DFT calculations for large GB systems with negligible computational costs, and thus enable the fast screening of possible alternative GB systems.

DOI： 10.1088/1361-651X/aa8276

Publishing type：Research paper (scientific journal)   Publisher：IPO Publishing  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Tensile deformation and failure of Sigma 9 tilt grain boundaries (GBs) in Al and Cu have been examined by first-principles tensile tests (FPTTs). Local-energy and local-stress schemes were applied to clarify the variations of local energies and local hydrostatic stresses for all atoms during the deformation process. The GBs in Al and Cu exhibited quite different tensile behaviours in the FPTTs, despite their similar initial configurations. For the Al GB, there are two stages of deformation before failure. In the first stage, the back bonds of the interfacial bonds are mainly stretched, due to special high strength of the interfacial reconstructed bonds. In the second stage, the interfacial bonds begin to be significantly stretched due to high concentrated stresses, while stretching of the back bonds is suppressed. The atoms at the interfacial, back and bulk bonds have very different variations of local energies and local stresses during each stage, because the behaviour of each atom is significantly dependent on each local structural change due to the high sensitivity of sp electrons to the local environment in Al. The Cu GB has much higher tensile strength, and a natural introduction of stacking faults (SFs) occurs via the {111}< 112 > shear slip in the bulk regions between the interfaces before the maximum stress is reached. This is caused by the smaller SF energy and lower ideal shear strength of Cu than Al, and is triggered by highly accumulated local energies and stress at the interface atoms. The local-energy distribution around the SF is consistent with the previous theoretical estimation. After the introduction of the SF, the local energies and stresses of all the atoms in the Cu GB supercell tend to become similar to each other during the tensile process, in contrast to the inhomogeneity in the Al GB. The origins of the different tensile behaviours observed for Al and Cu GBs are discussed with respect to the different bonding natures of Al and Cu, which are dominated by three sp valence electrons per atom for Al and by fully occupied d bands and s electrons for Cu.

DOI： 10.1088/1361-651X/25/1/015005

Publishing type：Research paper (scientific journal)  

Authorship：Last author   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Surface stresses and their changes by layer relaxation are crucial to understand the chemical and mechanical properties of transition-metal surfaces. Following our previous study (Shiihara et al., Phys. Rev. B 87 (2013) 125430) on unrelaxed surfaces of fcc late transition metals, both relaxed and unrelaxed surfaces of a series of 4d transition metals, including bcc and hcp structures, have been investigated by using the ab initio local-stress scheme and the Friedel stress model based on the second-moment tight-binding approximation. In ab initio results, an in-plane tensile stress is observed only at the surface top layer in an unrelaxed surface, while the layer relaxation reduces the tensile stress on the top layer and generates a small stress on the subsurface layer, except for the Pd(111) surface showing outward relaxation. All these results are properly reproduced by the Friedel stress model, especially for close-packed surfaces. Thus the surface stresses and their changes by layer relaxation are dominated by the d-band width changes, because of the coordination reduction and the d-orbital overlap change due to layer relaxation, generally for the series of 4d transition metals. The role of the d-electrons in the correlation among the surface relaxation, surface stress, and surface chemical reactivity is also discussed. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.susc.2015.08.009

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本計算工学会  

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：公益社団法人 日本材料学会  

A first-principles local stress analysis has been applied in order to reveal stress distribution in the vicinity of semiconductor hetero-interfaces of GaAs/XAs (X = Al, As). The weighted Bader integration scheme based on the Yu-Trinkle algorithm is found to be so effective that stress density is accurately integrated to the local stress over the Bader atomic volume. Applying the developed local stress calculation to semiconductor interfaces, we found that interface stress is localized, in the range of a single atom, in the vicinity of the interface. We also found that the nanoscopic stress state reflects the electronic bonding of atoms in the vicinity of the interfaces.

DOI： 10.2472/jsms.65.113

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Segregation of Si and Mg at grain boundaries (GBs) in Al and Cu has been investigated using density-functional theory calculations combined with recently developed local-energy and local-stress schemes. The physics behind the impurity-segregation energy is effectively analyzed by the local-energy decomposition. For the 9 tilt and 5 twist GBs in Al and Cu, Si shows large segregation-energy gains only at tighter sites, where local configuration of remarkably short Si-Al or Si-Cu bonds with high charge densities of covalent-bonding features are formed, leading to the local-energy stabilization as the final-state effects. On the other hand, Mg shows large gains only at looser sites. For Mg in the Cu GBs, the formation of stable Mg-Cu bonds or Mg states at looser sites is the origin of the preferential segregation as the final-state effects. For Mg in the Al GBs, however, the local energies of Mg-Al bonds are not so stable at looser sites, while the instability of Al atoms at looser sites in pure GBs before substitution is the origin of the preferential segregation as the initial-state effects. The behaviors of Si and Mg in Al GBs are dominated by the difference in local sp bonding nature among Mg, Al and Si, while Si-Cu and Mg-Cu hybridization interactions dominate the behaviors of Si and Mg in Cu GBs.

DOI： 10.1007/s10853-015-9294-4

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

An empirical formula for the stress concentration factor is developed for an unequal-sized cavity pair in an arbitrary orientation. Three-dimensional finite element linear elastic analyses are performed to evaluate the stress concentration factors for different sizes, orientations, and separations of cavities. A suitable mathematical function is chosen to fit the numerical results of the finite element analyses. An application is given for evaluating the maximum stress concentration factor, which governs fatigue crack initiation in aluminum die cast test pieces from an engine block. From the X-ray CT image, the location and geometry of the gas pores are evaluated so as to develop the proposed empirical formula for this actual multi-pore system simplified to a dual spherical pore system. A proof of the formula is shown by comparison with voxel finite element analysis. The proposed empirical formula can be satisfactorily used as a scientific guideline for selecting a casting method for car engine blocks from a fatigue crack initiation perspective. (C) 2015 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.apm.2015.01.032

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

Background and purposeExtensive skin wrinkling during facial expressions is one of the considerable problems in aesthetic dermatology. Although a few in silico studies have been performed with the aim of revealing the mechanism of a wrinkled appearance, there have been few studies that take into account the influence of skin roughness (i.e. microrelief), which exists on human skin in vivo. In this study, finite element simulations were performed using multilayer skin models with microrelief to investigate how extensive wrinkling appears on human skin, especially focusing on the role of surface roughness in the wrinkling mechanism.
MethodsLinear and post-buckling analyses were performed on soft elastic laminate models using the finite element method. A simplified multilayer model of human skin was employed to examine the contribution of skin's multilayer structure to the large-wrinkle mechanism. Microrelief was included in the model to assess its effect on the mechanism.
ResultsA large wrinkle was observed as dermal buckling following a number of buckling events on the stratum corneum. The existence of microrelief had an effect on the suppression of dermal buckling.
ConclusionSkin's multilayer structure should play a major role in the appearance of large wrinkles on human skin via its post-buckling behavior. This study suggested that fine microrelief on the skin surface hampers large wrinkles. These findings should be valuable for the development of cosmetic or medical treatments to prevent unfavorable skin deformations.

DOI： 10.1111/srt.12175

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE INC  

An empirical method is developed for obtaining the stress concentration factor for a pair of equally sized spherical cavities embedded in a large continuum in three-dimensional space. For practical applications such as die-cast materials containing many pores, we construct a simple and robust closed-form equation to evaluate the stress concentration factor considering the interaction between two cavities. The stress concentration factor can be used to evaluate the effect of pores on the material strength and the probable location of pores that will initiate a fatigue crack. Three-dimensional finite element linear elastic analysis was carried out to evaluate the stress concentration factors for arbitrary locations of the two cavities. The effects of the inter-cavity distance and the orientation of the inter-cavity axis with respect to the loading direction on the stress concentration factor are numerically obtained by systematically changing each of these parameters. Two empirical equations are proposed to fit the stress concentration factor data calculated by finite element analysis after considering various boundary conditions from a mechanical standpoint, and the parameters of the empirical formula are obtained by non-linear curve fitting with regression analysis. (C) 2014 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.apm.2014.07.005

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Using density-functional theory calculations combined with recent local-energy and local-stress schemes, we studied the effects of Si segregation on the structural, mechanical and magnetic properties of the Sigma 3(111) and Sigma 11(332) Fe GBs formed by rotation around the [110] axis. The segregation mechanism was analyzed by the local-energy decomposition of the segregation energy, where the segregation energy is expressed as a sum of the following four terms: the local-energy change of Si atoms from the isolated state in bulk Fe to the GB segregated state, the stabilization of replaced Fe atoms from the GB to the bulk, the local-energy change of neighboring Fe atoms from the pure GB to the segregated GB and the local-energy change of neighboring Fe atoms from the system of an isolated Si atom in the bulk Fe to the pure bulk Fe. The segregation energy and value of each term greatly depends on the segregation site and Si concentration. The segregation at interface Fe sites with higher local energies in the original GB configurations naturally leads to higher segregation-energy gains, while interface sites with lower local energies can lead to larger energy gains if stronger Si-Fe interactions occur locally in the final segregated configurations. The high Si concentration reduces the segregation-energy gain per Si atom due to the local-energy increases of Si atoms neighboring to each other or through the reduction in the number of stabilized Fe atoms per Si atom as observed in a Si dimer in bulk Fe. In the Si-segregated GBs, Si-Fe bonds enhance local Young's moduli and tend to suppress the interface weakening, while the GB adhesion is slightly reduced. And Fe atoms contacting Si atoms have reduced magnetic moments, due to Si-Fe sp-d hybridization interactions.

DOI： 10.1088/0953-8984/26/35/355005

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The mechanical properties of polycrystalline metals are strongly dependent on the microscopic structure, stability, and elastic properties of grain boundaries (GBs). By using ab initio local energy, local stress, and local Young's modulus, we attempt to provide a comprehensive view on the stability and structural properties of a series of < 110 > symmetrical tilt GBs (STGBs) in bcc Fe. We deal with four representative STGBs, the Sigma 3 (112), Sigma 3 (111), Sigma 11 (332), and Sigma 9 (221) GBs with the rotation angles ranging from 109.47A degrees to 38.94A degrees. The Sigma 3 (112) GB shows special stability due to stable structural units of four-membered rings with negligible bond-length changes except for substantial bond-direction changes, like stacking faults or twins. The other GBs are constructed by periodic arrangement of 5-3 and bulk structural units as usual coincidence GBs, while the 5-3 unit in the Sigma 9 (221) GB has an aspect of an edge-dislocation array in a small-angle tilt GB such as alternate compressive and tensile stresses at both the edges and relatively wide spread of local energy and local stress on both sides. In the GBs other than the Sigma 3 (112) GB, there exist commonly two kinds of interface atoms; atoms with larger atomic volumes reveal higher local energies, tensile stresses, and enhanced magnetic moments, while the other kind of atoms forming compressed bonds reveal lower local energies, compressed stresses, and reduced magnetic moments. For the four GBs, the local Young's modulus averaged in the structural units ranges from 60 to 90 % of the bulk Young's modulus in accordance with the degree of structural disorder or GB energies. There exists clear correlation among the local Young's modulus, local energy, local stress, local magnetic moment, and local bonding nature at the structural units in the Fe GBs.

DOI： 10.1007/s10853-014-8038-1

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Institute of Metals and Materials  

DOI： 10.2320/materia.53.405

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The energy-density and stress-density schemes (Shiihara et al 2010 Phys. Rev. B 81 075441) within the projector augmented wave (PAW) method based on the generalized gradient approximation (GGA) have been applied to tilt and twist grain boundaries (GBs) and single vacancies in Cu and Al. Local energy and local stress at GBs and defects are obtained by integrating the energy and stress densities in each local region by the Bader integration using a recent algorithm (Yu et al 2011 J. Chem. Phys. 134 064111) as well as by the layer-by-layer integration so as to settle the gauge-dependent problem in the kinetic terms. Results are compared with those by the fuzzy-Voronoi integration and by the embedded atom method (EAM). The features of local energy and local stress at GBs and vacancies depend on the bonding nature of each material. Valence electrons in Al mainly located in the interatomic regions show remarkable response to structural disorder as significant valence charge redistribution or bond reconstruction, often leading to long-range variations of charges, energies and stresses, quite differently from d electrons in Cu mainly located near nuclei. All these features can be well represented by our local energy and local stress. The EAM potential for Al does not reproduce correct local energy or local stress, while the EAM potential for Cu provides satisfactory results.

DOI： 10.1088/0953-8984/25/30/305006

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We present first-principle calculations on symmetric tilt grain boundaries (GBs) in bcc Fe. Using density functional theory (DFT), we studied the structural, electronic and magnetic properties of Sigma 3(111) and Sigma 11(332) GBs formed by rotation around the [110] axis. The optimized structures, GB energies and GB excess free volumes are consistent with previous DFT and classical simulation studies. The GB configurations can be interpreted by the structural unit model as given by Nakashima and Takeuchi (2000 ISIJ 86 357). Both the GBs are composed of similar structural units of three-and five-membered rings with different densities at the interface according to the rotation angle. The interface atoms with larger atomic volumes reveal higher magnetic moments than the bulk value, while the interface atoms with shorter bond lengths have reduced magnetic moments in each GB. The charge density and local density of states reveal that the interface bonds with short bond lengths have more covalent nature, where minority-spin electrons play a dominant role as the typical nature of ferromagnetic Fe. In order to understand the structural stability of these GBs, we calculated the local energy and local stress for each atomic region using the scheme of Shiihara et al (2010 Phys. Rev. B 81 075441). In each GB, the interface atoms with larger atomic volumes and enhanced magnetic moments reveal larger local energy increase and tensile stress. The interface atoms constituting more covalent-like bonds with reduced magnetic moments have lower local energy increase, contributing to the stabilization, while compressive stress is generated at these atoms. The relative stability between the two GBs can be understood by the local energies at the structural units. The local energy and local stress analysis is a powerful tool to investigate the structural properties of GBs based on the behavior of valence electrons.

DOI： 10.1088/0953-8984/25/13/135004

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We present a stress model, "the Friedel stress" model, based on the second-moment tight-binding approach, as a quantitative model of in-plane surface stress on transition metal surfaces. By comparing the results obtained with the stress model and ab initio local stress calculation on fcc (111) surfaces, we demonstrate that the in-plane surface stress originates from a surface d-band width change due to the reduced coordination number. This indicates that surface stress can occur without the direct effects of charge redistribution, which is widely accepted as the origin of surface stress. The variation in in-plane stress induced by the strain parallel to the surface is also reproduced correctly by the Friedel stress, which further proves the dependence of surface stress on the surface d-band width. We demonstrate that surface stress has a strong correlation with the surface chemical reactivity mediated by the surface d-band behavior. DOI: 10.1103/PhysRevB.87.125430

DOI： 10.1103/PhysRevB.87.125430

Language：English   Publishing type：Research paper (international conference proceedings)  

Theoretical strength is one of important and fundamental mechanical properties of solid materials. Although a lot of studies, mainly employing ab initio methods, have been devoded, many questions still remain open. For example, ideal strength under multi-axis loading and the effect of temperature are important issues to provide more insight to experimental observation. In this study, the effect of normal stress on ideal shear strength in covalent crystals (Si, C, Ge and SiC) is examined by ab initio density functional theory calculations. Unlike metals, the response of shear strength to normal stress in the covalent crystals depends on the balance between bond-order short-range bonding and pairwise repulsion of atoms, resulting in different responses among the crystals. We also investigate the effect of finite temperature on ideal shear strength in metals (Cu and Al). Although classical molecular dynamics is employed, obtained results seem reasonable thanks to careful choice of the interatomic potential. Our results indicate that the critical shear stress observed in a nano-pillar punch experiment, which is 25% of theoretical strength of perfect crystal, is not only due to temperature effect but also due to inhomogeneous structure triggering instability. Methods of local and global instability analysis, as the effort to link between the stability of crystal lattices and the instability of an entire system containing defects, are also reviewed.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：一般社団法人 日本機械学会  

Defects in crystals such as surfaces, interfaces, and grain boundaries significantly affect mechanical responses of materials under deformation in microscopic scale. While locally-distributed energy and stress obtained by &lt;I&gt;ab-initio&lt;/I&gt; method are considered to be useful tools for capturing the local effects of defects inside crystals, topological analysis of electron density proposed by Bader is another way to approach such local effects. Estimating the changes of electron density or its curvatures on some specific points such as bond midpoints, we can obtain the characteristics of the local electronic structure as well as by the partial density of states or the bond order. In this study, we have performed the topological analysis and the PDOS analysis on shear deformation of diamond crystals, carbon, Si, and Ge. Using the results, we conclude that the stress responses depend on the balance between the bonding strength and the angular dependence of sp3 configuration.

DOI： 10.1299/jmmp.6.81

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Ideal shear strength under superimposed normal stress of cubic covalent crystals (C, Si, Ge, and SiC) is evaluated by ab initio density functional theory calculation. Shear directions in [11(2) over bar] and [1(1) over bar 0] on the (111) plane are examined. The critical shear stress along the former direction is lower than that along the latter in all the crystals unless the hydrostatic tension is extremely high. In both the [11(2) over bar]-shear and [1(1) over bar 0]-shear, critical shear stress is increased by compression in C but is decreased in the other crystals. The different response of the critical shear stress to normal stress is due to the strength of the bond-order term, i.e. dependence of the short-range interatomic attraction on the bond-angle.

DOI： 10.1088/0953-8984/23/38/385401

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本機械学会  

An empirical prediction method for the fatigue strength considering the effect of porosity was proposed. We prepared six types of specimen of ADC12 high pressure die cast aluminum alloy, where the casting condition was different to each other, and obtained the S-N curve for each specimen type from the fatigue test. The porosity volume fraction for all specimens was calculated using the X-ray CT images. The image-based finite element analysis was also carried out for all specimens to evaluate the maximum first principal stress corresponding to the nominal stress amplitude in the fatigue test. The maximum stress was referred to as the local stress amplitude, and its ratio to the nominal stress amplitude was defined as the local stress concentration factor. We modified the S-N data by using the local stress amplitude instead of the nominal stress amplitude, and obtained a single master S-N curve. From the statistical investigation, the local stress concentration factor was found to be determined empirically by the porosity volume fraction. The relationship was identified by the least-square approximation. Consequently, we found that the fatigue strength was predicted from the porosity volume fraction via the local stress amplitude and the master S-N curve.

DOI： 10.1299/kikaia.77.48

Publishing type：Research paper (scientific journal)   Publisher：日本機械学会  

Effect of casting defects on the fatigue strength of die cast aluminum alloy was addressed through the tension-compression fatigue test and the X-ray CT inspection, which revealed the amount of porosity included in the specimen. Six types of casting condition were employed to examine the effect of different porosity. Most of fatigue crack sources were the pores. The porosity volume fraction was calculated for all specimens from the three-dimensional X-ray CT images. The fatigue limit of each specimen type was estimated from the fatigue test results. The correlations among fatigue limit, porosity volume fraction, Vickers hardness, and dendrite arm spacing were evaluated, and the result showed that the porosity volume fraction most dominantly affected the fatigue limit in the averaged sense.

DOI： 10.1299/kikaia.77.39

Language：English   Publishing type：Research paper (scientific journal)  

A practical way to calculate local quantum stress within the framework of stress density developed by Filippetti and Fiorentini [Phys. Rev. B 61, 8433 (2000)] is proposed and applied to Al (111) surfaces. Through detailed analysis of the gauge-dependent term in the kinetic stress density derived from the kinetic-energy density, it has been shown that the local stress components can be uniquely obtained by defining appropriate local regions where the gauge-dependent term integrates to zero. In Al (111) surface-slab calculations implemented by the projector-augmented-wave method, we have observed Friedel-type oscillation of the layer-by-layer stress, which reveals clear correlation with charge redistribution at the surface. © 2010 The American Physical Society.

DOI： 10.1103/PhysRevB.81.075441

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We have proposed the real-space Kerker method for fast self-consistent-field calculations in real-space approaches using non-orthogonal basis functions. In large-scale systems with many atoms, the Kerker method is a very efficient way to prevent charge sloshing, which induces numerical instability during the self-consistent iterations. We construct the Kerker preconditioning matrix with non-orthogonal basis functions and the preconditioning is performed by solving linear equations. The proposed real-space Kerker method is identical to the method in reciprocal space, with the following two advantages: (i) the method is suitable for massively parallel computation since it does not use the fast Fourier transform. (ii) The preconditioning is performed in an acceptable computational time since time-consuming integration, including the exponential kernel, need not be performed, unlike the method used by Manninen et al (1975 Phys. Rev. B 12 4012).

DOI： 10.1088/0965-0393/16/3/035004

Language：English   Publishing type：Research paper (scientific journal)   Publisher：一般社団法人 日本機械学会  

In order to estimate the material property with sufficient accuracy and acceptable computational time using &lt;i&gt;ab-initio&lt;/i&gt; calculation, the reduction of degrees of freedom required to approximate highly oscillated wave function around the nuclei is necessary. The curving-grid mesh which allocates a fine mesh around the nuclei seems effective for the reduction. We investigate the convergence property for &lt;i&gt;ab-initio&lt;/i&gt; finite-element calculation with curving-grid mesh in terms of total energy. Through the calculation of an oxygen molecule, we found that the favorable convergence property, &lt;i&gt;i.e.&lt;/i&gt; the variational convergence with the uniform rate, of the finite element method with uniform mesh is conserved. The application to bulk rock-salt magnesium oxide including 216 atoms shows the effectiveness of the curving-grid mesh in large-scale &lt;i&gt;ab-initio&lt;/i&gt; calculation.

DOI： 10.1299/jmmp.2.95

Language：English   Publishing type：Research paper (scientific journal)   Publisher：一般社団法人日本機械学会  

We have investigated the usability of the spectral element method in &lt;i&gt;ab-initio&lt;/i&gt; electron structure calculations on the basis of norm-conserving pseudopotential method. The spectral element method as a variant of the finite element method employs a Gauss-Lobatto-Legendre polynomial as a shape function. Using higher-order shape function, we obtain rapid convergence of the error in the total energy with respect to the number of degrees of freedom. This property significantly reduces the computational cost when one performs the highly accurate calculation. In the spectral element method positions of quadrature points are chosen so that some matrices turn to be diagonal matrix and sparse matrix. This also reduces the computational task required in the density functional calculation.

DOI： 10.1299/jmmp.2.1288

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER CERAMIC SOC  

The Finite Element Method (FEM) is implemented for atomic-scale analyses of metal/ceramic interface problems based on the Density Functional Theory (DFT), which is indispensable to prediction of ultimate bond strength in various combinations of metals and ceramics. The DFT simulation contributes much for the design of the new breed of functional materials without laborious trial and error experiments. For interface problems, an inconveniently large number of atoms should be included in a unit cell. The traditional DFT scheme of the plane-wave basis is not effective for such a large-scale calculation, since its scheme requires quadratic-scaling operation for matrix-vector product. We propose to reduce inherently prolonged computational time by employing finite discretization of real space to perform the DFT scheme. The sparseness of the Hamiltonian matrix improves computational efficiency so effectively as to increase the matrix-vector product linearly in proportion to number of atoms. Additional merit of the DFT by the FEM is in parallel computation caused by real space discretization, which makes decomposition of the matrix systematic. This is not the case with the traditional DFT using Fourier transforms. The finite element formulation is summarized and a trial computation with a Si dimer is demonstrated to verify the applicability of the FEM.

Presentation type：Oral presentation (general)  

Non-ordinary state-basedペリダイナミクスとハミルトニアン粒子法の類似性について講演した

Presentation type：Oral presentation (invited, special)  

Presentation type：Oral presentation (invited, special)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (invited, special)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (invited, special)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (general)