Lattice instability at a fast moving crack tip

A molecular dynamics method is used to analyze the dynamic propagation of an atomistic crack tip. The simulation shows that the crack propagates at a relatively constant global velocity which is well below the Rayleigh wave velocity. However the local propagation velocity oscillates violently, and it is limited by the longitudinal wave velocity. The crack velocity oscillation is caused by a repeated process of crack tip blunting and sharpening. When the crack tip opening displacement exceeds a certain critical value, a lattice instability takes place and results in dislocation emissions from the crack tip. Based on this concept, a criterion for dislocation emission from a moving crack tip is proposed. The simulation also identifies the emitted dislocation as a source for microcrack nucleation. A simple method is used to examine this nucleation process. (C) 1996 American Institute of Physics.

On the unstable stacking criterion for ideal and cracked copper-crystals

The unstable stacking criteria for an ideal copper crystal under homogeneous shearing and for a cracked copper crystal under pure mode II loading are analysed. For the ideal crystal under homogeneous shearing, the unstable stacking energy gamma(us) defined by Rice in 1992 results from shear with no relaxation in the direction normal to the slip plane. For the relaxed shear configuration, the critical condition for unstable stacking does not correspond to the relative displacement Delta = b(p)/2, where b(p) is the Burgers vector magnitude of the Shockley partial dislocation, but to the maximum shear stress. Based on this result, the unstable stacking energy Gamma(us) is defined for the relaxed lattice. For the cracked crystal under pure mode II loading, the dislocation configuration corresponding to Delta = b(p)/2 is a stable state and no instability occurs during the process of dislocation nucleation. The instability takes place at approximately Delta = 3b(p)/4. An unstable stacking energy Pi(us) is defined which corresponds to the unstable stacking state at which the dislocation emission takes place. A molecular dynamics method is applied to study this in an atomistic model and the results verify the analysis above.

The effect of thermal-activation on dislocation processes at an atomistic crack-tip

The effects of thermal activation on the dislocation emission from an atomistic crack tip are discussed, Molecular dynamics simulations at different constant temperatures are carried out to investigate the thermal effects. The simulated results show that the processes of the partial dislocation generation and emission are temperature dependent. As the temperature increases, the incipient duration of the partial dislocation nucleation becomes longer, the critical stress intensity factor for partial dislocation emission is reduced and, at the same loading level, more dislocations are emitted. The dislocation velocity moving away from the crack tip and the separations of partial dislocations are apparently not temperature dependent. The simulated results also show that, as the temperature increases, the stress distribution along the crack increases slightly. Therefore stress softening at the crack tip induced by thermal activation does not exist in the present simulation. A simple model is proposed to evaluate the relation of the critical stress intensity factor versus temperature. The obtained relation is in good agreement with our molecular dynamics results.

Deformation Twinning in Nanocrystalline NI

Deformation twinning is observed upon large plastic deformation in nanocrystalline (nc) Ni by transmission electron microscopy examinations. New and compelling evidence has been obtained for several twinning mechanisms that operate in nc grains, with the gain boundary emission of partial dislocations determined as the most proficient. Deformation twinning in nc Ni is discussed in comparison with molecular dynamics simulation results, based on generalized planar fault energy curves.

From transistor to trapped-ion computers for quantum chemistry

Over the last few decades, quantum chemistry has progressed through the development of computational methods based on modern digital computers. However, these methods can hardly fulfill the exponentially-growing resource requirements when applied to large quantum systems. As pointed out by Feynman, this restriction is intrinsic to all computational models based on classical physics. Recently, the rapid advancement of trapped-ion technologies has opened new possibilities for quantum control and quantum simulations. Here, we present an efficient toolkit that exploits both the internal and motional degrees of freedom of trapped ions for solving problems in quantum chemistry, including molecular electronic structure, molecular dynamics, and vibronic coupling. We focus on applications that go beyond the capacity of classical computers, but may be realizable on state-of-the-art trapped-ion systems. These results allow us to envision a new paradigm of quantum chemistry that shifts from the current transistor to a near-future trapped-ion-based technology.

纳米晶体微观结构与力学性质的分子动力学模拟

纳米晶体材料是由尺度在1-100nm的微小颗粒组成的体系.该文应用分子动力学结合Finnis-Sinclair多体势函数模拟了纳米晶铜的微观结构和单向拉伸变形,以及纳米铜晶粒的结构与扩散性质,并采用了局部晶序分析、晶向分布函数等多种手段,对它们的结构进行了分析.研究了晶粒尺寸的变化对它们的影响.对纳米晶铜的微观结构的模拟表明：随着晶粒尺寸的减小,纳米晶体的晶界结构变化并不明显,而晶粒内部的晶格畸变加剧,导致其结构的无序度明显增加,并且晶粒内部结构和晶界结构的差别也越来越小;晶粒内部的原子的平均能量明显升高,但界面原子的平均能量变化很小.由于受晶格畸变和晶界比例增加的影响,纳米晶体的密度小于单晶的密度.对纳米晶粒的结构与扩散性质进行了分子动力学模拟的结果显示：随着晶粒尺寸的减小,晶粒表面层包含的原子比例迅速增加,表面层的原子平均能量上升,而晶粒内部的保持不变,但不仍然要高于相应单晶体的值,而表面层的厚度基本为一常数.纳米晶粒的扩散系数随着它的尺寸的增加而迅速减小,虽现指数衰减关系.这种减小主要是由于晶粒的表面层原子比例的减小和这些原子的能量降低有关.表面原子的扩散在晶粒的扩散中占主导的地位.

双晶晶界在高温下的热稳定性的分子动力学研究

晶界结构在高温下的热稳定性问题是一个长期争论而又未能解决的问题，其争论的焦点是：在远低于熔点的温度下，晶界结构是否发生了可观察到的无序化，即是否存在一个远低于熔点的结构转化温度。为了能澄清这一争论，本文系统地研究了晶界结构的热稳定性。为了消除相互作用势的影响和系统误差，本文首先采用Morse势和经验多体势分别对铝、铜单晶的熔化过程进行了分子动力学模拟。在平衡态下，通过计算表征结构无序化的静态结构因子、径向分布函数和单晶原子位形图，获得了铝、铜单晶的熔点，结果表明：多体势计算的铝和铜的单晶熔点更接近实验值。因此，采用经验多体势应用分子动力学方法分别模拟了铝、铜Σ3、Σ5、Σ9、Σ11、Σ19、Σ33六种对称倾侧双晶晶界晶界结构由有序向无序转化的过程，计算了平衡态下的表征结构无序化的静态结构因子、径向分布函数和晶界原子位形图并将多体势获得的铝、铜单晶熔点作为晶界结构转化温度的约化熔点，获得了铝、铜Σ3、Σ5、Σ9、Σ11、Σ19、Σ33六种对称倾侧双晶晶界结构的转化温度和熔点，结果表明：1．Σ5、Σ9、Σ11、Σ19、Σ33五种对称倾侧双晶晶界均在远低于单晶熔点温度时，晶界结构发生了可观察到的无序化，而且双晶晶界结构的转变温度相差不大，双晶晶界熔点也低于单晶熔点。2．Σ3晶界在温度远低于熔点时，其晶界结构没有发生可观察到的无序化；Σ3晶界的转化温度与单晶熔点接近。所以，可以认为Σ3晶界不存在转化温度。这是由于Σ3晶界为共格孪晶，具有较低的能量。综上所述，除Σ3共格孪晶外，在远低于熔点温度下，晶界结构发生了可观察到的无序化，即：存在一个远低于熔点的转化温度，此时其静态结构因子约为0.5左右；晶界结构的熔点均低于单晶熔点，此时其静态结构因子约为0.15左右。从全文模拟结果可以看出，静态结构因子、径向分布函数、晶界原子位形图三种方法在确定晶界的结构转化温度和熔点时，静态结构因子是最有效、最准确的定量方法。

Atomistic Simulation On Size-Dependent Yield Strength And Defects Evolution Of Metal Nanowires

A simple derivation based on continuum mechanics is given, which shows the surface stress is critical for yield strength at ultra-small scales. Molecular dynamics (MD) simulations with modified embedded atom method (MEAM) are employed to investigate the mechanical behaviors of single-crystalline metal nanowires under tensile loading. The calculated yield strengths increasing with the decrease of the cross-sectional area of the nanowires are in accordance with the theoretical prediction. Reorientation induced by stacking faults is observed at the nanowire edge. In addition. the mechanism of yielding is discussed in details based on the snapshots of defects evolution. The nanowires in different crystallographic orientations behave differently in stretching deformation. This study on the plastic properties of metal nanowires will be helpful to further understanding of the mechanical properties of nanomaterials. (C) 2009 Elsevier B.V. All rights reserved.

Plastic Deformation Of Nanocrystalline Nickel

A high-resolution electron microscopy study has uncovered the plastic behavior of accommodating large strains in nanocrystalline (NC) Ni subject to cold rolling at liquid nitrogen temperature. The activation of grain-boundary-mediated-plasticity is evidenced in NC-Ni, including twinning and formation of stacking fault via partial dislocation slips from the grain boundary. The formation and storage of 60A degrees full dislocations are observed inside NC-grains. The grain/twin boundaries act as the barriers of dislocation slips, leading to dislocation pile-up, severe lattice distortion, and formation of sub-grain boundary. The vicinity of grain/twin boundary is where defects preferentially accumulate and likely the favorable place for onset of plastic deformation. The present results indicate the heterogeneous and multiple natures of accommodating plastic strains in NC-grains.

Hydroelectric Voltage Generation Based On Water-Filled Single-Walled Carbon Nanotubes

A DFT/MD mutual iterative method was employed to give insights into the mechanism of voltage generation based on water-fitted single-walled carbon nanotubes (SWCNTs). Our calculations showed that a constant voltage difference of several mV would generate between the two ends of a carbon nanotube, due to interactions between the water dipole chains and charge carriers in the tube. Our work validates this structure of a water-fitted SWCNT as a promising candidate for a synthetic nanoscale power cell, as well as a practical nanopower harvesting device at the atomic level. 

Inherent Shear-Dilatation Coexistence In Metallic Glass

Shear deformation can induce normal stress or hydrostatic stress in metallic glasses [ Nature Mater. 2 ( 2003) 449, Intermetallics 14 ( 2006) 1033]. We perform the bulk deformation of three-dimensional Cu46Zr54 metallic glass (MG) and Cu single crystal model systems using molecular dynamics simulation. The results indicate that hydrostatic stress can incur shear stress in MG, but not in crystal. The resultant pronounced asymmetry between tension and compression originates from this inherent shear-dilatation coexistence in MG.

Hybrid Qm/Mm Simulation Of The Hydration Phenomena Of Dipalmitoylphosphatidylcholine Headgroup

The polar headgroup of dipalmitoylphosphatidylcholine (DPPC) molecule both in gas phase and aqueous Solution is investigated by the hybrid quantum mechanical/molecular mechanical (QM/MM) method, in which the polar head of DPPC molecule and the bound water molecules are treated with density functional theory (DFT), while the apolar hydrocarbon chain of DPPC molecule is treated with MM method. It is demonstrated that the hybrid QM/MM method is both accurate and efficient to describe the conformations of DPPC headgroup. Folded structures of headgroup are found in gas phase calculations. In this work, both monohydration and polyhydration phenomena are investigated. In monohydration, different water association sites are studied. Both the hydration energy and the quantum properties of DPPC and water molecules are calculated at the DFT level of theory after geometry optimization. The binding force of monohydration is estimated by using the scan method. In polyhydration, more extended conformations are found and hydration energies in different polyhydration styles are estimated. (C) 2008 Elsevier Inc. All rights reserved.

An Atomistic-Continuum Hybrid Simulation Of Fluid Flows Over Superhydrophobic Surfaces

Recent experiments have found that slip length could be as large as on the order of 1 mu m for fluid flows over superhydrophobic surfaces. Superhydrophobic surfaces can be achieved by patterning roughness on hydrophobic surfaces. In the present paper, an atomistic-continuum hybrid approach is developed to simulate the Couette flows over superhydrophobic surfaces, in which a molecular dynamics simulation is used in a small region near the superhydrophobic surface where the continuum assumption is not valid and the Navier-Stokes equations are used in a large region for bulk flows where the continuum assumption does hold. These two descriptions are coupled using the dynamic coupling model in the overlap region to ensure momentum continuity. The hybrid simulation predicts a superhydrophobic state with large slip lengths, which cannot be obtained by molecular dynamics simulation alone.

Transport Properties And Induced Voltage In The Structure Of Water-Filled Single-Walled Boron-Nitrogen Nanotubes

Density functional theory/molecular dynamics simulations were employed to give insights into the mechanism of voltage generation based on a water-filled single-walled boron-nitrogen nanotube (SWBNNT). Our calculations showed that (1) the transport properties of confined water in a SWBNNT are different from those of bulk water in view of configuration, the diffusion coefficient, the dipole orientation, and the density distribution, and (2) a voltage difference of several millivolts would generate between the two ends of a SWBNNT due to interactions between the water dipole chains and charge carriers in the tube. Therefore, this structure of a water-filled SWBNNT can be a promising candidate for a synthetic nanoscale power cell as well as a practical nanopower harvesting device.

Smaller Deborah number inducing more serrated plastic flow of metallic glass

Spherical nano-indentations of Cu46Zr54 bulk metallic glass (BMG) model systems were performed using molecular dynamics (MD) computer simulations, focusing specifically on the physical origin of serrated plastic flow. The results demonstrate that there is a direct correlation between macroscopic flow serration and underlying irreversible rearrangement of atoms, which is strongly dependent on the loading (strain) rate and the temperature. The serrated plastic flow is, therefore, determined by the magnitude of such irreversible rearrangement that is inhomogeneous temporally. A dimensionless Deborah number is introduced to characterize the effects of strain rate and temperature on serrations. Our simulations are shown to compare favorably with the available experimental observations.