界面结构对Cu/Ni多层膜纳米压痕特性影响的分子动力学模拟

金属多层膜调制周期下降到纳米级时,其力学性质会发生显著改变. Cu-Ni晶格失配度约为2.7%,可以形成共格界面和半共格界面,实验中实现沿[111]方向生长的调制周期为几纳米且具有异孪晶界面结构的Cu/Ni多层膜,其力学性质发生显著改变.本文采用分子动力学方法对共格界面、共格孪晶界面、半共格界面、半共格孪晶界面等四种不同界面结构的Cu/Ni多层膜进行纳米压痕模拟,研究压痕过程中不同界面结构类型的形变演化规律以及位错与界面的相互作用,获取Cu/Ni多层膜不同界面结构对其力学性能的影响特征.计算结果表明,不同界面结构的样品在不同压痕深度时表现出的强化或软化作用机理不同,软化机制主要是由于形成了平行于界面的分位错以及孪晶界面的迁移,强化机制主要是由于界面对位错的限定作用以及失配位错网状结构与孪晶界面迁移时所形成的弓形位错之间的相互作用.     

Effect of material stiffness on hardness: A computational study based on model potentials

We investigate the dependence of the hardness of materials on their elastic stiffness. This is possible by constructing a series of model potentials of Morse type; starting with modelling natural Cu, the model potentials exhibit an increased elastic modulus, while keeping all other potential parameters (lattice constant, bond energy) unchanged. Using molecular-dynamics simulation, we perform nanoindentation experiments on these model crystals. We find that the crystal hardness scales with the elastic stiffness. Also the load drop, which is experienced when plasticity sets in, increases in proportion to the elastic stiffness, while the yield point, i.e. the indentation at which plasticity sets in, is independent of the elastic stiffness.     

考虑相界效应的Ni基单晶合金纳米压痕模拟

利用分子动力学方法分别模拟金刚石压头压入Ni模型和Ni基单晶合金γ/γ′模型的纳米压痕过程,通过计算得到两种模型[001]晶向的弹性模量及硬度.采用中心对称参数分析不同压入深度时两种模型内部位错形核、长大过程以及Ni基单晶合金γ/γ′(001)相界面错配位错对纳米压痕过程的影响.结果显示:压入深度0.641 nm之前,两种模型的压入载荷-压入深度曲线相似,说明此时相界面处的错配位错对纳米压痕过程的影响很小;压入深度0.995 nm时,在错配位错处发生位错形核,晶体在γ相中沿着{111}面滑移,随即导致Ni基单晶合金γ/γ′模型压入载荷的下降,并在压入深度达到1.487 nm之前低于Ni模型相同压入深度时的压入载荷;压入深度从1.307 nm开始,由于相界面错配位错的阻碍作用,Ni基单晶合金γ/γ′模型压入载荷上升速度较快.     

初始压入位置对Ni基单晶合金纳米压痕影响研究

针对Ni基单晶合金建立初始压入γ 相的γ /γ' 模型和初始压入γ'相的γ'/γ 模型, 采用分子动力学方法模拟金刚石压头压入两种模型的纳米压痕过程, 计算两种模型[001]晶向硬度. 采用中心对称参数分析两种模型(001)相界面错配位错对纳米压痕过程的影响. 结果显示: 弛豫后, 两种模型(001)相界面错配位错形式不同, 其中γ'/γ 模型(001)相界面错配位错以面角位错形式存在; 压入深度在0.930 nm 之前, 两种模型(001)相界面错配位错变化不大, 压入载荷-压入深度及硬度-压入深度曲线较符合; 压入深度在0.930 nm之后, γ'/γ 模型(001)相界面错配位错长大很多, 导致相同压入深度时γ'/γ 模型比γ /γ'模型压入载荷和硬度计算结果小; 压入深度在2.055 nm之后, γ /γ'模型(001)相界面错配位错对γ 相中位错进入γ'相有阻碍作用, 但仍有部分位错越过(001) 相界面进入γ' 相中, γ'/γ 模型(001)相界面处面角位错对γ' 相中位错进入γ 相有更明显的阻碍作用, 几乎无位错越过(001) 相界面进入γ 相中, 面角位错的强化作用更明显, 所以γ'/γ 模型比γ /γ'模型压入载荷上升速度快.     

SiC/C界面辐照性能的分子动力学研究

本文通过分子动力学模拟的方法,研究了5种含不同空间结构的SiC/C界面的材料受辐照后的缺陷分布随时间以及PKA位置的变化关系,并与单质SiC中缺陷分布情况进行对比.利用径向分布函数分析了辐照对界面原子排列情况的影响.研究结果表明,SiC/C界面的抗辐照能力明显低于SiC内部,不同的空间结构对界面缺陷数量存在一定影响.由径向分布函数推得界面区域石墨原子密度高则界面原子排列情况受辐照影响越大.     

石墨烯/铝基复合材料在纳米压痕过程中位错与石墨烯相互作用机制的模拟研究

石墨烯因其优异的力学性能已成为增强金属基复合材料的理想增强体.然而,目前对石墨烯/金属基复合材料在纳米压痕过程中嵌入石墨烯与位错之间的相互作用仍不清晰.本文采用分子动力学模拟方法,对90°,45°和0°位向的石墨烯/铝基复合材料进行了纳米压痕模拟,研究了压痕加载和卸载过程中石墨烯/铝基复合材料的位错形核及演化,以获取不同位向的石墨烯与位错的相互作用机制,并分析其对塑性区的影响.研究发现,石墨烯可以有效阻碍位错运动,并且石墨烯会沿着位错滑移方向发生弹性变形.在纳米压痕过程中,位错与不同位向石墨烯之间的相互作用差异导致塑性区的变化趋势不同.研究结果表明,在石墨烯/铝基复合材料中,位向不同的石墨烯对位错阻碍强度和方式不同,且石墨烯位向为45°的复合材料的硬度高于其他模型.此外,石墨烯/铝基复合材料的位错线总长度的演化规律与石墨烯位向紧密相关.本文研究可为设计和制备高性能石墨烯/金属基复合材料提供一定的理论指导.     

Molecular dynamics study of the mechanical characteristics of Ni/Cu bilayer using nanoindentation

<正>In the present work,a three-dimensional molecular dynamics simulation is carried out to perform the nanoindentation experiment on Ni single crystal.The substrate indenter system is modeled using hybrid interatomic potentials including the many-body potential embedded atom method(EAM),and two-body morse potential.To simulate the indentation process,a spherical indenter(diameter = 80 A,1 A=0.1 nm) is chosen.The results show that the mechanical behaviour of a monolithic Ni is not affected by crystalline orientation.To elucidate the effect of a heterogeneous interface, three bilayer interface systems are constructed,namely Ni(100)/Cu(111),Ni(110)/Cu(111),and Ni(111)/Cu(111).The simulations along these systems clearly describe that mechanical behaviour directly depends on the lattice mismatch. The interface with the smaller mismatch between the specified crystal planes is proved to be harder and vice versa.To describe the relationship between film thickness and interface effect,we choose various values of film thickness ranging from 20 A to 50 A to perform the nanoindentation experiment.It is observed that the interface is significant only for the relatively small thickness of film and the separation between interface and the indenter tip.It is shown that with the increase in film thickness,the mechanical behaviour of the film shifts more toward that of monolithic material.     

Low energy ion assisted atomic assembly of metallic superlattices

Metallic superlattices with planar, unalloyed (unmixed) interfacial structures are difficult to fabricate by all conventional vapor deposition methods. Molecular dynamics simulations have been used to explore the ways in which inert gas ions can be used to control the atomic assembly of a model Cu/Co metallic super lattice system. High energy, high atomic weight ions are shown to smooth rough interfaces but introduce undesirable intermixing at interfaces. Light ions with very low energies fail to flatten the rough surfaces that are naturally created during deposition at ambient temperature where surface atom mobility is kinetically constrained. The optimum energies for achieving the lowest combination of interfacial roughness and interlayer mixing have been found for each inert gas ion species and the key mechanisms of surface structure reorganization activated by ion impacts have been identified over the range of ion masses and energies studied. Optimum ion energies that maximize the interface structural perfection have been identified.     

纳米尺度下Si/Ge界面应力释放机制的分子动力学研究

采用分子动力学方法研究了纳米尺度下硅(Si)基锗(Ge)结构的Si/Ge界面应力分布特征,以及点缺陷层在应力释放过程中的作用机制.结果表明:在纳米尺度下, Si/Ge界面应力分布曲线与Ge尺寸密切相关,界面应力下降速度与Ge尺寸存在近似的线性递减关系;同时,在Si/Ge界面处增加一个富含空位缺陷的缓冲层,可显著改变Si/Ge界面应力分布,在此基础上对比分析了点缺陷在纯Ge结构内部引起应力变化与缺陷密度的关系,缺陷层的引入和缺陷密度的增加可加速界面应力的释放.参考对Si/Ge界面结构的研究结果,可在Si基纯Ge薄膜生长过程中引入缺陷层,并对其结构进行设计,降低界面应力水平,进而降低界面处产生位错缺陷的概率,提高Si基Ge薄膜质量,这一思想在研究报道的Si基Ge膜低温缓冲层生长方法中初步得到了证实.     

Fe/Si多层膜的层间耦合与界面扩散

对以本征Si及重掺杂ｐ型和ｎ型Si作为中间层的Fe/Si多层膜的层间耦合进行研究,并通过退火,增大Fe,Si之间的扩散,分析界面扩散对层间耦合的影响. 实验结果表明,层状结构良好的制备态的多层膜,Fe,Si之间也存在一定程度的扩散,它是影响层间耦合的 主要因素,远远超过了半导体意义上的重掺杂,使不同种类的Si作为中间层的层间耦合基本 一致.进一步还发现,在一定范围内增大Fe,Si之间的扩散,即使多层膜的层状结构已经有了相当的退化,Fe/Si多层膜的反铁磁耦合强度基本保持不变. 关键词：Fe/Si多层膜层间耦合界面扩散     

银纳米团簇负热容现象的分子动力学模拟

采用分子动力学方法和嵌入原子法(EAM)多体势函数,模拟研究了银纳米团簇在不同温度直到熔化过程中的结构变化,得到了体系能量和热容量随温度的变化关系.结果显示:银纳米团簇在临近熔点附近出现了负热容现象.研究了弛豫后银纳米团簇的稳态结构变化及其在不同时刻结构的演变过程.结果表明:产生负热容现象的主要原因是纳米团簇在熔点附近,结构发生了巨大的变化,形成由{111}和{100}面围成的结构十分稳定和能量更低的多面体.     

Effect of vacancies on incipient plasticity during contact loading

Atomistic simulations and experimental nanoindentation tests were used to examine the effect of vacancies on the inception of plastic deformation in Ni. Molecular dynamics have shown the effect of vacancy position on the yield load and demonstrate a variety of mechanisms which are responsible for the inception of plastic deformation during indentation. In cases where the vacancy position is close to regions of high shear stresses the nucleation of dislocations is related to the location of a vacancy; however, homogeneous nucleation of dislocations is also observed for vacancy-containing crystals. Complementary experiments have been used to demonstrate the effect of indenter size on the onset of yielding in the presence of vacancies. Both the simulations and experiments show that larger indenter tips increase the chance of weakening the material in the presence of vacancies.     

含孔洞CoCrNiFeCu高熵合金纳米压入的分子动力学模拟

高熵合金具有优异的力学性能、耐磨耐蚀耐高温等性能,成为未来最有发展潜力的新型材料之一.因此,本文通过分子动力学模拟的方法探究了孔洞对CoCrNiFeCu高熵合金模型纳米压痕力学性能和位错演化的影响.结果表明,屈服点、载荷、杨氏模量和硬度等力学参数随着孔洞深度D的增加呈现先增加后稳定的趋势.孔洞的存在显著影响了位错形核的位置,随着孔洞深度的增加,初始位错优先在孔洞与表面之间形成,随后集中在模型表面形成,且位错环由平面圆环状转变为“翼状”.在孔洞深度较小时(D <40?),位错环沿(110)面水平扩展,随着孔洞深度的增加,位错环开始向下扩展.此外,在压痕深度为30?时,位错密度随着孔洞深度的增加逐渐增加.     

Molecular dynamics simulation of nanoscale mechanical behaviour of ZnO under nanoscratching and nanoindentation

Molecular dynamics is used to simulate the mechanical behaviour of zinc oxide under nanoscratching and nanoindentation. The effects of indenter speed and substrate temperature on the structure-phase formation, slip vector, radial distribution function, and residual stresses are investigated. Simulation results show that the dislocation loops nucleate and propagate, forming a body-centred tetragonal lattice structure along the slip direction due to high local stress. Furthermore, the dislocation loops nucleate and propagate due to the resolved shear stress along the 45° slip direction under nanoscratching. The average mean biaxial stress and the normal stress of the O layers are –9.35 and –4.36 GPa, respectively, and those of the Zn layers are –0.80 and –0.30 GPa, respectively. This may be attributed to the energetic O atoms, with which unstable atoms have high activation.     

铜原子纳米团簇热力学性质的分子动力学模拟研究

利用分子动力学模拟方法,研究了CuN(N=80,140,216,312,408,500,628和736)纳米团簇在热化和冷凝过程中结构和热力学性质的变化,模型采用的是Johnson的EAM作用势.模拟结果表明:铜团簇的熔点和凝固点随其尺寸线性增加,并逐渐向大块晶体靠拢;所有团簇的凝固点都低于熔点,出现凝固过程中的滞后现象;在熔点和凝固点附近,团簇都具有负热容特性,负热容是由相变前后团簇内部结构突变引起的.     

Impact of intermolecular vibrational coupling effects on the sum-frequency generation spectra of the water/air interface

ABSTRACT

We have examined the impact of intermolecular vibrational coupling effects of the O-H stretch modes, as obtained by the surface-specific velocity-velocity correlation function approach, on the simulated sum-frequency generation spectra of the water/air interface. Our study shows that the inclusion of intermolecular coupling effects within the first three water layers, i.e. from the water/air interface up to a distance of 6?Å towards the bulk, is essential to reproduce the experimental SFG spectra. In particular, we find that these intermolecular vibrational contributions to the SFG spectra of the water/air interface are dominated by the coupling between the SFG active interfacial and SFG inactive bulk water molecules. Moreover, we find that most of the intermolecular vibrational contributions to the spectra originate from the coupling between double-donor water molecules only, whereas the remaining contributions originate mainly from the coupling between single-donor and double-donor water molecules.     

纳米铜团簇凝结规律的分子动力学研究

采用分子动力学方法对包含147,309和561个原子数的液态纳米铜团簇凝结过程进行模拟研 究,结果表明降温速率及团簇原子数对凝结得到常温下的固态团簇结构有重要影响：在模拟 时间内,降温速度越慢,团簇原子数越少,凝结得到铜团簇越倾向生成二十面体结构,反之 则倾向生成面心立方结构.同时探讨了该现象的物理机理.关键词：铜团簇凝结结构分子动力学