材料
Online ISSN : 1880-7488
Print ISSN : 0514-5163
59 巻 , 8 号
選択された号の論文の12件中1~12を表示しています
特集 分子動力学
論文
  • 都留 智仁, 阿部 陽介, 加治 芳行, 塚田 隆, 實川 資朗
    2010 年 59 巻 8 号 p. 583-588
    発行日: 2010/08/15
    公開日: 2010/08/20
    ジャーナル フリー
    The size- and spacing- dependent obstacle strength due to the Cu precipitation in α-Fe is investigated by atomistic simulations, in which the effect on phase transformation of Cu precipitation is considered by a conventional self-guided molecular dynamics (SGMD) method that has an advantage to enhance the conformational sampling efficiency in MD simulations. A sequence of molecular statics simulations of the interaction between a pure edge dislocation and spherical Cu precipitation are performed to investigate the obstacle strength associated with phase transformation. It was shown that the SGMD method can accelerate calculating the bcc to 9R structure transformation of a small precipitate, enabling the transformation without introducing any excess vacancies. Such metallographic structures increase the obstacle strength through strong pinning effects as a result of the complicated atomic rearrangement within the Cu precipitation.
  • 陸 茉莉花, 松本 龍介, 武富 紳也, 宮崎 則幸
    2010 年 59 巻 8 号 p. 589-595
    発行日: 2010/08/15
    公開日: 2010/08/20
    ジャーナル フリー
    Highly accurate prediction of material strength under a practical hydrogen environment and development of materials with minimal hydrogen effect are essential for safe use of hydrogen energy. Here, we estimated the relationship between grain boundary (GB) properties and GB cohesive energy under gaseous hydrogen environment for <110> symmetrical tilt GBs and <001> twist GBs in alpha iron using atomistic simulations. We employed the embedded-atom-method (EAM) potential developed by Wen et al. and conjugate gradient (CG) method for structure relaxation. First, we estimated the relationship between GB energy and GB free volume for various misorientation angles. After the estimation of GB properties, we incorporated a hydrogen atom into various occupation sites and obtained the distribution of hydrogen trap energy around the GBs. We found a good correlation among GB energy, GB free volume, and the number of trapped hydrogen atoms: high energy GBs have large gaps, and many hydrogen atoms are trapped in those spaces. Finally, we estimated the GB cohesive energy and found that there is negligible hydrogen influence on the low energy GBs. The reduction of the GB cohesive energy of Σ3{112} boundary was estimated to be only 6.61% under high-pressure hydrogen environment (T = 300K, p = 70MPa).
  • 榎本 龍博, 松本 龍介, 武富 紳也, 宮崎 則幸
    2010 年 59 巻 8 号 p. 596-603
    発行日: 2010/08/15
    公開日: 2010/08/20
    ジャーナル フリー
    Estimations of the fundamental relationship between hydrogen and lattice defects are important for understanding hydrogen embrittlement mechanisms and for predicting the amount of penetrating hydrogen. In this paper, hydrogen-trap energy around the lattice defects (e.g., atomic vacancy, stacking fault, grain boundaries, and free surfaces) within the Al lattice are calculated using first-principles calculations based on Generalized Gradient Approximation (GGA) and Ultra-Soft (US) pseudo potential. The hydrogen-trap energies obtained are 0.30 eV at the atomic vacancy, 0.002 eV at the stacking fault, 0.02 eV at the Σ3{111} Symmetrical Tilt Grain Boundary (STGB), 0.19 eV at the Σ3{112} STGB, and 0.45-0.60 eV at the free surfaces. Moreover, the hydrogen occupancies at the hydrogen-trap sites around the lattice defects are evaluated for several hydrogen gaseous environments using the obtained hydrogen-trap energies. Hydrogen occupancies around the lattice defects are extremely low under the pure hydrogen gaseous conditions. However, according to experimental data, hydrogen concentration can become higher for Al structures due to the hydrogen atoms generated by the surface oxidation process under a moist environment or other reasons. In such cases, the vacancy has a strong interaction with the hydrogen atoms.
  • 木下 佑介, 大野 信忠
    2010 年 59 巻 8 号 p. 604-609
    発行日: 2010/08/15
    公開日: 2010/08/20
    ジャーナル フリー
    Electronic structures of (6,0), (8,0), and (10,0) single-walled boron nitride nanotubes (SWBNNTs) under tension, torsion and flattening are investigated using first-principles calculations. Energy bands and charge distributions of the SWBNNTs are calculated within the density-functional theory, and forces required to deform the SWBNNTs are estimated from the energy variation with deformation. Our calculations show that the tension, torsion and flattening decrease energy gaps of the SWBNNTs because of a decrease in the energy of the conduction band minimum (CBM). The decrease in the CBM energy is caused by an overlap of CBM charge densities between boron atoms. It is found that the flattening deformation leads to the larger decrease in energy gaps of the SWBNNTs with the smaller force than the tension and torsion.
  • 松中 大介, 土井 嘉治, 渋谷 陽二
    2010 年 59 巻 8 号 p. 610-615
    発行日: 2010/08/15
    公開日: 2010/08/20
    ジャーナル フリー
    Effects of substrate strain on epitaxial growth are studied, using the kinetic Monte Carlo (kMC) method. The strain dependences of the activation energy barrier and the attempt frequency are considered. The homo-epitaxial growth on Ag (111) surface with uniform tensile strain is simulated, and influences of substrate strain on the nucleation of island and the morphology are investigated. On the tensile-strained surface, the island density increases due to the suppression of the adatom diffusion on terrace. The averaged coordination number of atom constituting of islands decreases and the shape of island is less compact. The growth behavior on the strained substrate is same as at lower temperature.
  • 木下 惠介, 下川 智嗣, 喜成 年泰
    2010 年 59 巻 8 号 p. 616-623
    発行日: 2010/08/15
    公開日: 2010/08/20
    ジャーナル フリー
    Extrinsic grain boundary dislocations (EGBDs) are often observed in ultra fine-grained metals produced by severe plastic deformation processes. The mechanical properties of such metals are different from those of coarse-grained metals, so EGBDs could be important factor to elucidate the unique mechanical properties. In this paper, in order to clarify the influence of EGBDs on the emission of dislocations from the crack tip, we evaluate the mechanical field around the crack tip using J-integral in atomic simulations. It is found that quantitative evaluations of the crack-tip shielding effect due to EGBDs can be performed by J-integral taking grain boundary structures into account.
  • 米川 嘉明, 齋藤 賢一
    2010 年 59 巻 8 号 p. 624-630
    発行日: 2010/08/15
    公開日: 2010/08/20
    ジャーナル フリー
    Inelastic deformation of gold (Au) cluster is investigated by using molecular dynamics (MD) simulations. We performed compression and unloading test by computer simulation, where two silicon (Si) plates approach each other and push single Au cluster of 4 nm, 8 nm or 12 nm diameter in between. The potential function we utilized for Au-Au interaction is an embedded atom method (EAM) type proposed by Cai et al. for intermetallic alloys. On the other hand, the interactions between Si and Au atoms are developed on the Lennard-Jones framework by assessing the interaction force obtained by the AFM experiment. By using common neighbor analysis (CNA) suitably used in crystalline structures, initial f.c.c. structure once decreases in compression, but is then recovered in unloading. From stress-strain curves, it is understood that these nano-sized Au clusters possess tremendously large recovery strain in unloading after compression. The large recovery strain is estimated at 10% in average. Smaller cluster tends to show larger recovery strain depending on temperature. The large recovery strain may lead to the possibility of superelastic response of the Au cluster. The present paper also discusses the cause of large recovery appeared in whole shape and clarifies the mechanism of characteristic rearrangement of atoms. By compression, stacking faults are introduced inside the cluster, and then twin deformation occurs with crystalline rotation. In unloading, large recovery strain is obtained.
  • 西村 正臣, 屋代 如月, 荒井 政大
    2010 年 59 巻 8 号 p. 631-636
    発行日: 2010/08/15
    公開日: 2010/08/20
    ジャーナル フリー
    We have so far shown that amorphous metals have many “unstable” atoms even at the equilibrium state, by local lattice instability analysis (LLIA) which discusses the positive definiteness of atomic elastic stiffness coefficients, Bαij. In the present study, we put our focus on the stability switching by the “probabilistic” fluctuation and the “deterministic” mechanical load. We have performed molecular dynamics simulations on Ni amorphous and evaluated the stability switching under no-load equilibrium as well as uniaxial tension. It is definitely true that the ratio of unstable atoms decreases/increases according to the system energy ; however, it is revealed that one-way change of stabilization or destabilization never occur but both positive and negative stability-switching are activated from their ratio under the equilibrium state. That is, a straightforward image of “stabilization/destabilization of local configuration” is not correct for structural change in amorphous metal but “shuffle of atomic arrangement” which involves atomic stabilization and destabilization simultaneously. In fact, we have proved that (1) both switching drastically increase at the slow-down point just before the stress-strain peak, (2) many stabilization/destabilization atoms can be found in the locally deformed area, and (3) such switching atoms actually feel hydrostatic tension in the dilated local configuration on the way of “shuffle”.
論文
  • 篠塚 計, 田村 学, 江阪 久雄, 谷川 博康, 芝 清之
    2010 年 59 巻 8 号 p. 637-644
    発行日: 2010/08/15
    公開日: 2010/08/20
    ジャーナル フリー
    In international thermonuclear experimental reactor (ITER), reduced activation ferritic/martensitic steels will be used for plasma-facing materials. However, raising the operating temperature is planned in order to elevate efficiency of electric power generation in a prototype fusion reactor. Oxide dispersion strengthened (ODS) steels are promising candidate for high temperature and plasma-facing materials. In the 8Cr-W-V-Ta ODS steels, δ-ferrite grains which exist in martensite and elongate to the hot-rolling direction affects creep deformation behavior. The ratio of δ-ferrite is dependent on normalizing temperature. In this work, the relation between normalizing temperature and creep strength of the 8Cr-W-V-Ta ODS steel was investigated. Two kinds of as-received ODS steels, 8Cr-2W-V-Ta and 8Cr-1W, were normalized at 1050°C for 1 h. Specimens of different normalizing temperatures were prepared by re-normalizing the as-received specimens at 950°C and 1250°C for 1 h. Creep tests were performed at 700°C and 205 MPa in parallel to the hot-rolling direction. Microstructures of specimens were observed using OM, and Vickers hardness tests were performed. The area fraction of δ-ferrite decreased with the increase of normalizing temperature in 8Cr-2W-V-Ta, but the fraction of the specimen normalized at 1050°C was highest in 8Cr-1W. Minimum creep rates decreased as the area fraction and Vickers hardness of δ-ferrite increased. The time-to-rupture increased with increasing the area fraction of δ-ferrite. Although creep-cavities formed along δ-ferrite in martensite, the progress toward main-crack was obstructed by δ-ferrite. Therefore, the increase in the area fraction of δ-ferrite contributed not only to the decrease in the minimum creep rate but also to the delay of the main-crack propagation.
  • 金山 英幸, 張 聖徳, 坂根 政男, 長澤 忠, 小林 馨
    2010 年 59 巻 8 号 p. 645-652
    発行日: 2010/08/15
    公開日: 2010/08/20
    ジャーナル フリー
    This paper studies the creep characteristic of a liquid crystal polymer (LCP) thin film. Tensile creep tests were performed using three directional specimens at three testing temperatures. Creep curves consisted of mainly transient creep and almost no steady and accelerated creep stages were found. The minimum creep strain rate and rupture lifetime were significantly influenced by the specimen direction and testing temperature. This paper developed a new method of correlating the minimum creep strain rate independent of the specimen direction and testing temperature. This paper also developed a correlation method of creep rupture lifetime independent of the specimen direction and testing temperature.
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