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61.
Non-equilibrium grain boundary structure and inelastic deformation using atomistic simulations 总被引:1,自引:0,他引:1
Garritt J. TuckerDavid L. McDowell 《International Journal of Plasticity》2011,27(6):841-857
Grain boundary influence on material properties becomes increasingly significant as grain size is reduced to the nanoscale. Nanostructured materials produced by severe plastic deformation techniques often contain a higher percentage of high-angle grain boundaries in a non-equilibrium or energetically metastable state. Differences in the mechanical behavior and observed deformation mechanisms are common due to deviations in grain boundary structure. Fundamental interfacial attributes such as atomic mobility and energy are affected due to a higher non-equilibrium state, which in turn affects deformation response. In this research, atomistic simulations employing a biased Monte Carlo method are used to approximate representative non-equilibrium bicrystalline grain boundaries based on an embedded atom method potential, leveraging the concept of excess free volume. An advantage of this approach is that non-equilibrium boundaries can be instantiated without the need of simulating numerous defect/grain boundary interactions. Differences in grain boundary structure and deformation response are investigated as a function of non-equilibrium state using Molecular Dynamics. A detailed comparison between copper and aluminum bicrystals is provided with regard to boundary strength, observed deformation mechanisms, and stress-assisted free volume evolution during both tensile and shear simulations. 相似文献
62.
提出了一种可由脉冲功率驱动的新型二次电子倍增阴极构型,并对其进行了动力学过程的初步理论研究。首先,针对该二次电子倍增阴极,建立了动力学模型,获得了二次电子的位移和速度方程,讨论了电子初始出射速度对其轨迹、渡越时间和碰撞能量的影响,理论给出了渡越时间和碰撞能量的近似解析表达式。其次,通过动力学方程与Vaughan二次电子产额经验公式的耦合求解,获得了该二次电子倍增阴极的工作区间,并对其进行了细致讨论。结果表明:该新型二次电子倍增阴极二极管概念上是可行的,在涂敷高二次电子产额系数材料的圆柱形介质上施加合适的轴向和径向静电场(MV/m量级)以及轴向静磁场(T量级),可以达到电子沿阴极表面螺旋行进过程中实现二次电子倍增并最终获得电流沿轴向放大的设计目标。另外,讨论了正电荷沉积引发的二次电子倍增饱和现象,并对阴极发射电流密度进行了理论粗估,结果表明:阴极发射电流密度可达kA/cm2水平,具备强流发射特性;增加外加径向场强幅值可有效提升阴极发射电流密度。 相似文献
63.
《Annales de l'Institut Henri Poincaré (C) Analyse Non Linéaire》2021,38(5):1337-1371
In this paper we consider minimizers of the functional where is a bounded open set and where are the first k eigenvalues on Ω of an operator in divergence form with Dirichlet boundary condition and with Hölder continuous coefficients. We prove that the optimal sets have finite perimeter and that their free boundary is composed of a regular part, which is locally the graph of a -regular function, and a singular part, which is empty if , discrete if and of Hausdorff dimension at most if , for some . 相似文献
64.
T. Verkholyak A. Honecker W. Brenig 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,49(3):283-287
The Jordan-Wigner transformation is applied to study the ground
state properties and dimerization transition in the J1-J2
XXZ chain. We consider different solutions of the mean-field
approximation for the transformed Hamiltonian. Ground state
energy and the static structure factor are compared with complementary
exact diagonalization and good agreement is found
near the limit of the Majumdar-Ghosh model.
Furthermore, the ground state phase diagram is discussed
within the mean-field theory. In particular, we show that an
incommensurate ground state is absent for large J2 in a fully
self-consistent mean-field analysis. 相似文献
65.
Mechanical spectra (complex Young's modulus Y*=Y′+iY″ versus frequency and temperature) of polycrystalline cupric oxide CuO were measured by vibrating reed method from liquid nitrogen temperature to room temperature at the kilohertz frequency. The abnormal behavior of internal friction and the change of slope of the real part of the complex Young's modulus versus temperature are observed around 213 and 230 K, where two successive magnetic transitions were established. At low temperatures, below 130 K, a higher internal friction platform with the decrease of temperature was clearly observed which might show a phase transition. Our study indicates that mechanical spectrum is an effective tool to detect subtle phase transitions and there is a strong magnetolattice coupling in CuO. 相似文献
66.
Yue Zhao HongLi Suo Min Liu DanMin Liu YingXiao Zhang MeiLing Zhou 《Physica C: Superconductivity and its Applications》2006,440(1-2):10-16
Biaxially textured Ni–5 at.%W substrates have been prepared by cold rolling, followed by three different annealing routes. In this paper, the processes of melting Ni and W metals, flat rolling, various annealing methods are described in detail. The Ni–5 at.%W tapes annealed under either high vacuum or flowing Ar (7% H2) gas were characterized by X-ray pole figures, ODF, EBSD as well as AFM analysis. The texture analysis indicated that as fabricated tapes have a sharp cube texture formed after annealing at a wide temperature range of 800–1100 °C. The high quality of cube orientation on tapes was obtained after a two-step annealing (TSA), where the percentage of the cube texture component was as high as 93.5% within a misorientation angle smaller than 8° from EBSD analysis. Furthermore, it was also observed that the number of twin boundaries in this tape decreased with respect to that of tapes annealed both in vacuum and one-step gas annealing. From AFM on 1 μm2 areas, it was concluded that the roughness (RMS) on the tape surface reached 0.98 nm. 相似文献
67.
J.H. Harding 《Interface Science》2003,11(1):81-90
This article discusses grain boundary diffusion in ceramics. It gives a brief review of the experimental data available for ionic oxides and the problems of interpretation associated with it. The fundamental differences between grain boundary diffusion in metals and ceramics are noted. Calculations of the segregation of defects and impurities to grain boundaries are discussed together with methods of calculating diffusion coefficients in these boundaries. New results for alumina and chromia are presented. The problem of defining a grain boundary width is discussed with respect to new calculations on nickel oxide. 相似文献
68.
69.
U.D. Kulkarni 《Progress in Crystal Growth and Characterization of Materials》1997,34(1-4):165-173
Attention is focussed on the fact that the β—Mn, the σ—phase and the Al5Ti3 and the Al11Ti7 superlattice derivatives of the δ—AlTi phase all belong to a category of structures which comprises an identical periodic arrangement of square and rhombic tiles — the shapes of the rhombic tiles being different in each of these cases. While the first and the second structures constitute periodic approximants of the octagonal and the dodecagonal quasicrystals respectively, the last two (superlattice) structures are the approximants of a hypothetical quasiperiodic superlattice structure. The strip projection method has been used here to model the intermediate structures which arise during the continuous periodic to quasiperiodic transitions between the relevant structures and was found to be quite successful in explaining many of the related experimental findings. 相似文献
70.
A new type of boundary condition, named Möbius or antiperiodic boundary conditions, is proposed and tested, both analytically and within the context of numerical simulations. It is shown that these boundary conditions are very useful for twist grain boundary atomistic simulations. By contrast to the use of the ordinary Born von Kármán periodic boundary conditions, they allow only one grain boundary per box instead of two. The risk of migration and overinteraction of two grain boundaries at high temperature is thus avoided while more complex grain boundaries can also be tackled at the same computer price. Such examples are presented and discussed. 相似文献