纳米多层膜“软"相结构参量对硬度的Hall-Petch表征

采用直流磁控溅射方法制备了不同调制比的Ni/Al纳米多层膜,利用X射线衍射技术和纳米压入连续刚度法分析了薄膜微结构及塑性变形的尺度依赖性.实验结果表明,尽管调制比有所不同,多层膜的硬度与“软"相的微结构特征参量随调制波长减小具有相似变化规律,说明多层膜的变形机制对“软"相的微结构约束存在敏感性.随着薄膜特征尺度的减小,为统一多层膜中晶界和膜界两种强化机制,提出一个与“软”相相关的表征参量r(r＝L_sub/d,L关键词： 纳米多层膜 塑性变形 调制波长 Hall-Petch关系     

Size and Interface Control of Novel Nanocrystalline Materials Using Pulsed Laser Deposition

We have developed a novel method based upon pulsed laser deposition to produce nanocrystalline materials with an accurate grain size and interface control. Using this method, the grain size in the case of Cu thin films was controlled by introducing a few monolayers of insoluble elements having high surface energy such as W, which increases interfacial energy and provides more nucleation sites. The grain size is determined by the thickness of Cu layer and the substrate temperature at which it transforms into islands (nanocrystalline grains) of fairly uniform size which we desgnate as self-assembling approach. Using this approach, the grain size was reduced from 160nm (Cu or Si (100) substrate) to 70–80nm for a simple W layer (Cu/W/Si (100)) to 4nm for a multilayer (Cu/W/Cu/W/Si (100)) thin film. The hardness of these films was evaluated using a nanoindentation technique, a significant increase in hardness from 2.0GPa for coarse-grained 180nm to 12.5GPa for 7nm films was observed. However, there is decrease in hardness below 7nm for copper nanocrystals. The increase in hardness with the decrease in grain size can be rationalized by Hall–Petch model. However, the decrease in slope and eventually the decrease in hardness below a certain grain size can be explained by a new model based upon grain-boundary deformation (sliding). We also used a similar materials processing approach to produce quantum dots in semiconductor heterostructures consisting of Ge and ZnO dots or nanocrystals in AlN or Al₂O₃ matrix. The latter composites exhibit novel optoelectronic properties with quantum confinement of phonons, electrons, holes and excitons. Similarly, we incorporated metal nanocrystals in ceramics to produce improved mechanical and optical properties.     

Evolution of triple junction defect structure at plastic deformation of metallic polycrystals

A method for the determination of the components of the Frank vector of disclinations in triple junctions (TJ) of real polycrystalline aggregates has been proposed. Using this method we found that real polycrystals contain junction disclinations (JD), the nature of which is related to their thermo-mechanical history. One of the elements of the evolution of the network of initial junction disclinations is the formation of disclination dipole configurations in the adjacent TJ. The influence of the types of boundaries forming the junctions on JD Frank vector power is discussed.     

晶体相场法模拟纳米晶材料反霍尔-佩奇效应的微观变形机理

采用晶体相场模型模拟获得了平均晶粒尺寸从11.61–31.32 nm的纳米晶组织, 研究了单向拉伸过程纳米晶组织的强化规律的微观变形机理. 模拟结果表明: 晶粒转动、晶界迁移等晶间变形行为是纳米晶材料的主要微观变形方式, 纳米晶尺寸减小, 有利于晶粒转动, 使屈服强度降低, 显示出反霍尔-佩奇效应.当纳米晶较小时, 变形量超过屈服点达到4%, 位错运动开启, 其对变形的直接贡献有限, 主要通过改变晶界结构而影响变形行为, 位错运动破坏三叉晶界, 引发晶界弯曲, 促进晶界迁移. 随纳米晶增大, 晶粒转动困难, 出现晶界锯齿化并发射位错的现象. 关键词： 晶体相场 纳米晶 反霍尔-佩奇效应 微观变形     

Nanocrystalline gold with small size: inverse Hall–Petch between mixed regime and super-soft regime

ABSTRACT

Molecular dynamics simulations were used to study the atomic mechanisms of deformation of nanocrystalline gold with 2.65–18?nm in grain size to explore the inverse Hall–Petch effect. Based on the mechanical responses, particularly the flow stress and the elastic-to-plastic transition, one can delineate three regimes: mixed (10–18?nm, dislocation activities and grain boundary sliding), inverse Hall-Petch (5–10?nm, grain boundary sliding), and super-soft (below 5?nm). As the grain size decreases, more grain boundaries present in the nanocrystalline solids, which block dislocation activities and facilitate grain boundary sliding. The transition from dislocation activities to grain boundary sliding leads to strengthening-then-softening due to grain size reduction, shown by the flow stress. It was further found that, samples with large grain exhibit pronounced yield, with the stress overshoot decrease as the grain size decreases. Samples with grain sizes smaller than 5?nm exhibit elastic-perfect plastic deformation without any stress overshoot, leading to the super-soft regime. Our simulations show that, during deformation, smaller grains rotate more and grow in size, while larger grains rotate less and shrink in size.     

纳米ZnO胶体可见发射机制的研究

关于ZnO可见发射机制的讨论目前尚无定论。本文研究了不同颗粒尺寸的纳米晶ZnO胶体的发射性质,观测到两个发射带,其中之一是激子发射,另一个是可见发射;发现两个发射带的峰值能量之间存在线性关系,并由此提出了可见发射机制是来自导带的电子到深陷阱的跃迁。     

爆炸载荷下纳米晶铜晶粒度分布及影响因素研究

 采用爆炸动态加载使粗晶铜发生高应变率塑性大变形的方法制备了纳米晶铜。利用X射线衍射法对其晶粒度进行了检测,借助于LS-DYNA3D非线性有限元程序对试样变形过程进行了数值模拟,在此基础上对应变和应变率进行了统计,分析了宏、细观应变对晶粒细化程度的影响。结果表明：采用爆炸加载法可制备出纳米晶铜,平均晶粒度范围可有效控制在100 nm以内;爆炸加载过程中应变率高达10⁴ s^-1,应变的提高有利于晶粒细化;在爆炸加载方向晶粒度成不均匀分布。     

Texture evolution and operative mechanisms during large-strain deformation of nanocrystalline nickel

The large-strain deformation of nanocrystalline nickel was investigated at room temperature and cryogenic (liquid N₂) temperature. Deformation mechanisms ranging from grain boundary sliding to slip, operate due to a wide distribution of grain sizes. These mechanisms leave their finger print in the deformation texture evolution during rolling of nanocrystalline nickel. The occurrence and severance of different mechanisms is understood by a thorough characterization of the deformed samples using X-ray diffraction, X-ray texture measurements, electron back-scattered diffraction and transmission electron microscopy. Crystal plasticity-based viscoplastic self-consistent simulations were used to further substantiate the experimental observations. Thus, a comprehensive understanding of deformation behavior of nanocrystalline nickel, which is characterized by simultaneous operation of dislocation-dominated and grain boundary-mediated mechanisms, has been developed.     

Time-Division Observation of Plastic Deformation Process Using Digital Speckle Pattern Interferometry

Time-division digital speckle pattern interferometry to observe temporal variation of in-plane deformation fields of a whole process of plastic deformation until fracture was proposed. Experiment of tensile test of an aluminum plate exhibited dynamic movements of successive fringe patterns which were very complicated. They suggest that plastic deformation propagates nonlinearly in the specimen by repetition of energy relaxation processes and concentration processes.Presented at 1996 International Workshop on Interferometry (IWI ‘96), August 27-29, Wako, Saitama, Japan.     

Grain size reduction of copper subjected to repetitive uniaxial compression combined with accumulative fold

This paper reports a novel method of repetitive uniaxial compression combined with accumulative fold for preparing bulk submicron- to nanocrystalline copper starting with a coarse grained counterpart. Grain size reduction and microstrain variations of the high purity copper samples after different passes of compression and fold are investigated by scanning electron microscope and x-ray diffraction (XRD), respectively. Our results show that the average grain size of samples decreases from about 830nm to 127nm as the number of compression passes increases to 30. Microstrain in the compressed sample is found to increase for the first 20 passes, but to decrease at the last 10 passes. The variations of compressive yield strength and the shift of XRD peaks to larger diffraction angles are observed in the squeezed sample. Our experimental results demonstrate that the repetitive uniaxial compression combined with accumulative fold is an effective method to prepare bulk nanocrystalline metallic materials, in particular for soft metals such as Cu, Al and Pb.     

氧化锌可见区发光机制

探究与缺陷相关的氧化可见区发光机制对获得高效激子发光和实现紫外激光有重要的意义，也是该领域研究的基本问题之一，本文用X射线衍射、X射线光电子能谱，电子顺磁共振和光致发光谱研究了ZnO:Mn纳米薄膜的结构和发光性质，证明了氧空位或缺陷分布于纳米晶表面，提出了可见发光中心是Vo^**和[Vo^*,electron]或[Vo^**,two electrons]复合体的发光模型。     

Plastic analysis of crack problems in three-dimensional icosahedral quasicrystalline material

Due to their complexity, the basic plastic properties of all quasicrystalline materials are essentially unknown [M. Feuerbacher, C. Thomas, K. Urban, Plastic behaviour of quasicrystalline materials, in Quasicrystals: Structure and Physical Properties, H.R. Trebin, ed., Wiley/VCH, Berlin, 2003]. Plastic deformation of cracks in icosahedral quasicrystals have been analyzed in a strict and systematic form and the crack tip opening displacement (CTOD) and size of the plastic zone around the crack tip was determined exactly. CTOD is suggested as a parameter of plastic fracture for quasicrystalline materials. The present work may also provide a novel methodology for plastic analysis of quasicrystals.     

Fe基纳米晶合金晶粒尺寸反常变化的物理机制

晶粒尺寸在很大程度上决定了Fe基纳米晶合金的磁学性能,其随退火温度变化的物理机理是纳米晶领域重要的研究内容.研究了初始晶化温度与二次晶化温度之间退火1 h Fe基纳米晶合金晶粒尺寸随退火温度的变化,并建立了相应的模型.利用提出的模型分析了该温度范围内Fe基纳米晶合金晶粒尺寸随退火温度升高先减小后增大的物理机制. 研究发现,在初始晶化温度与二次晶化温度之间等时退火,当退火温度约为Fe基纳米晶合金熔点的0.6倍时其晶粒尺寸最小.在研究的温度区间内,理论研究结果与实验符合得较好. 本研究提供了一种快速获得小晶粒尺寸纳米晶合金的方法.     

Plastic analysis of the crack problem in two-dimensional decagonal Al-Ni-Co quasicrystalline materials of point group 10,10

The fundamental plastic nature of the quasicrystalline materials remains an open problem due to its essential complicacy. By developing the proposed generalized cohesive force model, the plastic deformation of crack in point group 10, 10 decagonal quasicrystals is analysed strictly and systematically. The crack tip opening displacement （CTOD） and the size of the plastic zone around the crack tip are determined exactly. The quantity of the crack tip opening displacement can be used as a parameter of nonlinear fracture mechanics of quasicrystalline material. In addition, the present work may provide a way for the plastic analysis of quasicrystals.     

纳米多晶铜中冲击波阵面的分子动力学研究

冲击波阵面反映材料在冲击压缩下的弹塑性变形行为以及屈服强度、应变率条件等宏观量, 还与冲击压缩后的强度变化联系. 本文使用分子动力学方法, 模拟研究了冲击压缩下纳米多晶铜中的动态塑性变形过程, 考察了冲击波阵面和弹塑性机理对晶界存在的依赖, 并与纳米多晶铝的冲击压缩进行了比较. 研究发现: 相比晶界对纳米多晶铝的贡献而言, 纳米多晶铜中晶界对冲击波阵面宽度的影响较小; 并且其塑性变形机理主要以不全位错的发射和传播为主, 很少观察到全位错和形变孪晶的出现. 模拟还发现纳米多晶铜的冲击波阵面宽度随着冲击应力的增加而减小, 并得到了冲击波阵面宽度与冲击应力之间的定量反比关系, 该定量关系与他人纳米多晶铜模拟结果相近, 而与粗晶铜的冲击压缩实验结果相差较大.     

Research on factors influencing on the microwave permeability of nanocrystalline FeB alloys

Nanocrystalline FeB alloys have been prepared with optimized grain size and internal stress. Samples prepared under different annealing conditions are analysed by x-ray diffraction, and the permeability $\mu(\omega)$ is measured by HP8510B Vector Network Analyser in the frequency range 2--18GHz. The results show that annealing leads to the growth of the grain size and reduces the internal stress, and smaller grain size and larger internal stress favours the magnetic dissipation.     

Thermal stability of nanotwinned and nanocrystalline microstructures produced by cryogenic shear deformation

Nanotwinned microstructures are of significant interest due to their high strength and enhanced thermal stability, attributed to the presence of a dense network of coherent twin interfaces. Propensity for twinning during deformation is known to increase at low temperature and/or high-strain-rate. In this study, we use high-strain-rate (~10^3?s^?1) shear deformation in cutting over a range of strains (γ ~1–5) and temperatures (cryogenic to ambient) to engineer a variety of microstructures in three face-centred cubic (FCC) metals – copper, brass and aluminium. The microstructures include nanocrystalline-equiaxed and densely (nano) twinned types of controllable domain size. The effects of low-temperature deformation and stacking fault energy on the resulting microstructure, hardening, stored energy and associated recrystallization kinetics are established. For copper, the nanotwinned microstructures are found to be thermally more stable and stronger than the equiaxed counterparts comprised of random high-angle grain boundaries. This enhanced effect of nanotwins on microstructure stability is, however, not observed in brass, while aluminium did not show any indications of twinning over the investigated range of deformation conditions.     

Investigation of activities of grain boundaries in nanocrystalline Al under an indenter by a multiscale method

Activities of grain boundaries in nanocrystalline Al under an indenter are studied by a multiscale method. It is found that grain boundaries and twin boundaries can be transformed into each other by emitting and absorbing dislocations. The transition processes might result in grain coarsening and refinement events. Dislocation reflection generated by a piece of stable grain boundary is also observed, because of the complex local atomic structure within the nanocrystalline Al. This implies that nanocrystalline metals might improve their internal structural stability with the help of some special local grain boundaries.     

Brittle-ductile behavior of a nanocrack in nanocrystalline Ni: A quasicontinuum study

The effects of stacking fault energy, unstable stacking fault energy, and unstable twinning fault energy on the fracture behavior of nanocrystalline Ni are studied via quasicontinuum simulations. Two semi-empirical potentials for Ni are used to vary the values of these generalized planar fault energies. When the above three energies are reduced, a brittle-to-ductile transition of the fracture behavior is observed. In the model with higher generalized planar fault energies, a nanocrack proceeds along a grain boundary, while in the model with lower energies, the tip of the nanocrack becomes blunt. A greater twinning tendency is also observed in the more ductile model. These results indicate that the fracture toughness of nanocrystalline face-centered-cubic metals and alloys might be efficiently improved by controlling the generalized planar fault energies.     

Investigation of grain boundary activity in nanocrystalline Al under an indenter by using a multiscale method

Grain boundary activity in nanocrystalline Al under an indenter is studied by using a multiscale method.It is found that grain boundaries and twin boundaries can be transformed into each other by emitting and absorbing dislocations.The transition processes might result in grain coarsening and refinement events.Dislocation reflection generated by a piece of stable grain boundary is also observed,because of the complex local atomic structure within the nanocrystalline Al.This implies that nanocrystalline metals might improve their internal structural stability with the help of some special local grain boundaries.