纳米颗粒及其在润滑油脂中的应用

纳米材料科学与技术的发展有力地推动了新型先进润滑材料与技术的发展，与之相适应，无机纳米颗粒作为润滑油脂抗磨损添加剂的研究受到了广泛关注．业已发现，某些纳米硫化物、纳米氧化物、纳米稀土化合物及纳米软金属均具有良好抗磨损作用，这同纳米颗粒在磨损表面的沉积及其在一定程度上对磨损表面的“修复”作用密切相关；利用有机-无机复合技术在纳米颗粒表面引入有机修饰分子，可有效地抑制纳米颗粒的团聚，从而提高纳米颗粒在润滑油中的分散稳定性；而纳米润滑添加剂实现工业化应用的关键在于规模化制备技术开发成功及生产成本的大幅度降低．     

纳米金刚石对电沉积镍基复合镀层微观结构及抗磨性能的影响

利用直流电沉积技术制备了纳米和微米镍镀层,以及纳米金刚石增强镍基复合镀层,考察了纳米金刚石颗粒对纳米及微米镍基质的表面形貌、微结构、硬度及磨损性能的影响．结果表明：引入纳米金刚石使得微米镍镀层的硬度和抗磨性能显著提高;但引入纳米金刚石对纳米镍镀层硬度的影响不大,对镀层的抗磨性能则反而产生不利影响．     

纳米铜添加剂改善钢-铝摩擦副摩擦磨损性能的研究

采用环-块摩擦磨损试验机对比考察了钢-铝摩擦副在液体石蜡与含纳米铜颗粒液体石蜡润滑下的摩擦磨损特性,研究了对纳米铜添加剂添加量与载荷对其摩擦磨损性能的影响,通过对磨损表面粗糙度、形貌及其主要元素的能谱分析,探讨了纳米铜颗粒作为添加剂时钢-铝摩擦副的润滑机制.结果表明:含0.25%纳米铜颗粒液体石蜡时,钢-铝摩擦副的摩擦磨损性能最优;在不同载荷下纳米铜颗粒可以改善铝的摩擦磨损性能,特别在中等载荷(50～125 N)下,其抗磨减摩作用更明显;纳米铜颗粒能够在磨损表面形成一层低剪切强度的铜保护膜,有效地避免粘着磨损,同时阻止铝元素向钢表面的转移,从而显著改善钢-铝摩擦副的摩擦磨损性能.     

纳米复合材料激光熔覆层组织及抗磨性能

利用5kWCO2激光器，在Ni基高温合金表面制备了纳米Al2O3／钴基合金熔覆层，分析了熔覆层的组织结构及其抗磨性能．结果表明，当纳米Al2O3颗粒含量较低时，Al2O3颗粒能均匀分布于熔覆层中，从而形成纳米氧化物弥散强化的复合材料涂层；Al2O3颗粒在熔池中长大，尺寸为250-450nm；复合材料熔覆层的硬度随纳米Al2O3含量的增加而提高；当纳米Al2O3颗粒含适中时，熔覆层的抗磨性能较好；而当纳米Al2O3颗粒含量过高(3．0％)时，复合材料熔覆层的抗磨性能反而降低。     

考虑表面效应的形状记忆合金纳米梁弯曲特性研究

形状记忆合金(Shape Memory Alloys, SMAs)因其具有形状记忆效应和超弹性,在航空航天、生物医疗、微机电系统领域中得到了广泛的应用．当微结构尺度达到微纳米，表面效应对微结构力学性能的影响是十分显著的．本文基于梁弯曲变形理论以及Gurtin-Murdoch表面弹性理论，考虑拉压不对称、温度对于SMA纳米梁的影响，建立了考虑表面效应的SMA纳米梁相变力学模型．分析了弯曲载荷、温度、表面残余应力以及表面弹性模量对SMA纳米梁力学性能的影响规律．研究表明在SMA纳米梁相变阶段，忽略和考虑表面效应所得的截面应力及应变相对误差较为明显；在相同弯矩下，随温度的增加SMA纳米梁的截面应力随之增加，并且表面效应对其影响有减小趋势；表面残余应力对SMA纳米梁的影响显著．该文研究结果为SMA纳米梁在微机电领域的设计以及应用提供了一定基础与依据．     

纳米Al2O3填充环氧树脂复合材料的摩擦学性能

研究了干摩擦条件下纳米Al2O3微粒含量及其表面改性处理对环氧树脂基复合材料滑动摩擦磨损性能的影响.结果表明，经过表面化学接枝处理后，少量的纳米Al2O3(体积分数约0．24％)即可大幅度提高环氧树脂的摩擦磨损性能，起到显著的减摩和耐磨作用．复合材料的热变形稳定性、显微硬度及磨损表面形貌分析结果表明，对纳米Al2O3微粒进行适当的表面处理有利于加强纳米微粒同基体树脂的结合，从而改善复合材料的摩擦学性能．     

局部表面纳米化双层嵌套式金属薄壁吸能结构的数值模拟和设计

基于局部表面纳米化技术，设计了一种双层嵌套式金属薄壁吸能结构。在表面纳米化技术对金属力学性能影响的研究基础上，优化了环向交错式和连续式条纹局部表面纳米化布局，得到了双层嵌套式方管吸能结构设计方案和吸能参数。结果表明，表面纳米化对材料的屈服极限提升显著，所设计的局部纳米化双层嵌套式方管吸能结构的比吸能可提高57.1%。同时也证实局部表面纳米化是一种有效的吸能提升技术。     

纳米Si3N4填充聚双马亚酰亚胺摩擦磨损性能研究

采用烧铸成型法制备纳米Si3N4颗粒填充聚双马来酰亚胺复合材料,考察了纳米Si3N4质量分分别为0．5％、1．0％、1．5％及2．0％的复合材料的摩擦学性能,并且扫描电子显微镜对磨损表面形貌和磨屑进行了分析。结果表明,纳米Si3N4颗粒对聚双马亚酰亚胺的摩擦磨损性能具有明显的改性作用,尤其是当纳米Si3N4的质量分数为1．5％时,复合材料的磨擦磨损性能最佳,摩擦系数降为0．25,磨损率降低72％。     

具有表面效应的球形纳米电极颗粒中的反应-扩散-力学全耦合分析

表面效应在纳米电极颗粒中占有主导地位，论文首先建立了锂离子电池充放电过程中考虑表面效应的反应-扩散-力学全耦合模型；然后对比了有无表面效应对锂离子浓度、径向应力和环向应力的影响；最后探讨了反应系数和尺寸效应对浓度和扩散诱导应力的影响.数值结果表明表面效应随着充电时间的增加逐渐减小直至充电结束趋于稳定；表面压效应对本模型的浓度无影响但抑制了扩散诱导应力的增加；较慢的化学反应和较小的纳米电极颗粒尺寸可抑制电极应力的增加.     

Au纳米颗粒织构化表面的黏着和摩擦学行为研究

利用自组装技术在单晶硅(100)面制备了Au纳米颗粒织构化表面(nanoparticle-textured surfaces,NPTS),采用原子力显微镜(AFM)和UMT-2MT摩擦磨损试验机考察了Au纳米颗粒织构化对表面微/纳尺度黏着与摩擦性能的影响机理.结果表明:在颗粒堆积密度较低的表面,接触力学符合连续接触力学模式;在颗粒堆积密度较高的表面,形成多峰接触,有效地减少了接触面积,降低了黏着和摩擦.与光滑硅表面相比,组装时间为3.0 h的Au纳米颗粒织构化表面的黏着力降低了77%,在试验载荷为7 nN时,其摩擦力降低了42%.     

A nonlocal beam model for out-of-plane static analysis of circular nanobeams

In this paper, out-of-plane static behavior of circular nanobeams with point loads is investigated. Inclusion of small length scales such as lattice spacing between atoms, surface properties, grain size etc. are considered in the analysis by employing Eringen’s nonlocal elasticity theory in the formulations. The nonlocal equations are arranged in cylindrical coordinates and applied to the beam theory. The effect of shear deformation is considered. The governing differential equations are solved exactly by using the initial value method. The displacements, rotation angle about the normal and tangential axes and the force resultants are established and the analytical expressions are presented. The predicted trends of the size effect at the nano scale agree with those given in the experiments. The results can be used for designing nanoelectromechanical systems (NEMS) where the curved nanobeams are used as a basic component.     

Effects of lattice misorientations on strain heterogeneities in FCC polycrystals

It is well documented that the highly heterogeneous deformation behaviour and lattice rotation typically observed within grains in a polycrystal are attributed to microstructural features such as grain structure, topology, size, etc. In this work, the effects of low- and high-angle grain boundaries on the mechanical behaviour of FCC polycrystals are investigated using a micro-mechanical model based on crystal plasticity theory. The constitutive framework relies on dislocation mechanics concepts to describe the plastic deformation behaviour of FCC metallic crystals and is validated by comparing the measured and predicted local and macroscopic deformation behaviour in a thin Al-0.5% Mg polycrystal tensile specimen containing a relatively small number of surface grains. Comparisons at the microscopic (e.g. local slip distribution) and macroscopic (e.g. average stress-strain response) levels elucidate the role of low-angle grain boundaries, which are found to have a profound effect on both the local and average deformation behaviour of FCC polycrystals with a small number of grains. However, this effect diminishes when the number of grains increases and becomes negligible in bulk polycrystals. In light of the widely accepted view that high-angle grain boundaries strongly influence the mechanical behaviour of very fine-grained metals, this work has shown that low-angle grain boundaries can also play an equally important role in the deformation behaviour of polycrystals with a relatively small number of grains.     

Grain size effects in multiphase steels assisted by transformation-induced plasticity

The influence of the austenitic grain size on the overall stress–strain behavior in a multiphase carbon steel is analyzed through three-dimensional finite element simulations. A recently developed multiscale martensitic transformation model is combined with a plasticity model to simulate the transformation-induced plasticity effects of a grain of retained austenite embedded in a ferrite-based matrix. Grain size effects are included via a surface energy term in the Helmholtz energy. Tensile simulations for representative orientations of the grain of retained austenite show that the initial stability of the austenite increases as the grain size decreases. Consequently, the effective strength is initially higher for smaller grains. The influence of the grain size on the evolution of the transformation process strongly depends on the grain orientation. For “hard” orientations, the transformation rate is higher for larger grains. In addition, the phase transformation is partially suppressed as the grain size decreases. In contrast, for “soft” orientations, the transformation rate is lower for larger grains. The phase transformation is more homogeneous for smaller grains and, consequently, the effective transformation strain is larger. Nevertheless, in multiphase carbon steels with a relatively low percentage of retained austenite, the influence of the austenitic grain size on the overall constitutive response is smaller than the influence of the austenitic grain orientation.     

Intra-granular plastic slip heterogeneities: Discrete vs. Mean Field approaches

In this paper, we derive the mechanical fields (internal stresses, elastic energy) arising from the presence of an inelastic distortion field representing a typical intra-granular “microstructure” as the one observed during the plastification of metallic polycrystals. This “microstructure” is due to the formation of discrete intra-granular plastic slip heterogeneities characterized by at least two internal lengths: the first one is the individual grain size which represents a stochastic parameter inherent to the processing route (prior working, annealing), and, the second one is the spatial distance between active slip lines or slip bands associated with inhomogeneous plastic slip in the interior of grains. These internal lengths can be observed and measured using conventional experimental techniques (EBSD, TEM, AFM). The micro-mechanical modeling of the mechanical fields associated with plastic slip events inside grains is performed with two different assumptions. The first one is based on the well-known Eshelby’s problem of plastic inclusion where only the grain diameter is considered as internal length scale. This classical method considers homogeneous plastic distortion in the grain and leads to a uniform and grain size independent total strain field in the grain. The second method accounts for a non-uniform plastic distortion in the grain characterized by its discrete nature and the two aforementioned internal lengths. Both methods consider grains as spherical inclusions with a given diameter embedded in a homogeneous medium. For the second method, plastic slip is constrained by grain boundaries seen as impenetrable obstacles to dislocations. Thus, plastic strain is embodied by distributions of discrete circular glide loops. After writing the field equations and the free energy of the medium, a micro-mechanical formulation based on the Fourier transform method is developed. It is then found that in contrast with the mean-field approach, the internal stress fields as well as the elastic energy corresponding to different dislocation configurations depend on internal lengths associated to the deformed medium. Different possible configurations associated with intra-granular plastic flow due to circular glide dislocation loops are analyzed. Finally, the results are discussed with respect to the grain size dependence of the flow strength and the Bauschinger effect for plastically deforming polycrystals and perspectives to develop new micro–macro transition schemes accounting for internal length scales are sketched out.     

纳米晶铜单向拉伸变形的分子动力学模拟

纳米材料是由尺度在1－100nm的微小颗粒组成的体系，由于它具有独特的性能而备受关注。本文简要地回顾了分子动力学在纳米材料研究中的应用，并运用它模拟了平均晶粒尺寸从1．79－5．38nm的纳米晶体的力学性质。模拟结果显示：随着晶粒尺寸的减小，系统与晶粒内部的原子平均能量升高，而晶界上则有所下降；纳米晶体的弹性模量要小于普通多晶体，并随着晶粒尺寸的减小而减小；纳米晶铜的强度随着晶粒的减小而减小，显示了反常的Hall-Petch效应；纳米晶体的塑性变形主要是通过晶界滑移与运动，以及晶粒的转动来实现的；位错运动起着次要的、有限的作用；在较大的应变下（约大于5％），位错运动开始起作用；这种作用随着晶粒尺寸的增加而愈加明显。     

A multiscale thermomechanical model for cubic to tetragonal martensitic phase transformations

We develop a multiscale thermomechanical model to analyze martensitic phase transformations from a cubic crystalline lattice to a tetragonal crystalline lattice. The model is intended for simulating the thermomechanical response of single-crystal grains of austenite. Based on the geometrically nonlinear theory of martensitic transformations, we incorporate microstructural effects from several subgrain length scales. The effective stiffness tensor at the grain level is obtained through an averaging scheme, and preserves crystallographic information from the lattice scale as well as the influence of volumetric changes due to the transformation. The model further incorporates a transformation criterion that includes a surface energy term, which takes into account the creation of interfaces between martensite and austenite. These effects, which are often neglected in martensitic transformation models, thus appear explicitly in the expression of the transformation driving force that controls the onset and evolution of the transformation. In the derivation of the transformation driving force, we clarify the relations between different combinations of thermodynamic potentials and state variables. The predictions of the model are illustrated by analyzing the response of a phase-changing material subjected to various types of deformations. Although the model is developed for cubic to tetragonal transformations, it can be adapted to simulate martensitic transformations for other crystalline structures.     

THE DISLOC ATION EQU ATIONS OF A SIMPLE CUBIC CRYSTAL IN THE ISOTROPIC APPROXIMATION-A SOLVABLE MODEL

The dislocation equations of a simple cubic lattice have been obtained by using Green's function method based on the discrete lattice theory with the coefficients of the secondorder differential terms ...     

Crystal plasticity model with enhanced hardening by geometrically necessary dislocation accumulation

A strain gradient dependent crystal plasticity approach is used to model the constitutive behaviour of polycrystal FCC metals under large plastic deformation. Material points are considered as aggregates of grains, subdivided into several fictitious grain fractions: a single crystal volume element stands for the grain interior whereas grain boundaries are represented by bi-crystal volume elements, each having the crystallographic lattice orientations of its adjacent crystals. A relaxed Taylor-like interaction law is used for the transition from the local to the global scale. It is relaxed with respect to the bi-crystals, providing compatibility and stress equilibrium at their internal interface. During loading, the bi-crystal boundaries deform dissimilar to the associated grain interior. Arising from this heterogeneity, a geometrically necessary dislocation (GND) density can be computed, which is required to restore compatibility of the crystallographic lattice. This effect provides a physically based method to account for the additional hardening as introduced by the GNDs, the magnitude of which is related to the grain size. Hence, a scale-dependent response is obtained, for which the numerical simulations predict a mechanical behaviour corresponding to the Hall-Petch effect. Compared to a full-scale finite element model reported in the literature, the present polycrystalline crystal plasticity model is of equal quality yet much more efficient from a computational point of view for simulating uniaxial tension experiments with various grain sizes.     

考虑尺寸效应的多晶金属循环塑性分析

本文采用多晶塑性分析方法，设材料点包含一定数量的各向异性单晶晶粒并考虑晶粒尺寸的影响，计算材料点的应力和应变时利用了Taylor假设。模型引入考虑尺寸效应的晶体滑移硬化函数，同时针对晶体滑移引入背应力及其方向性硬化的描述，以反映不同晶粒尺寸材料在循环加载条件下的力学行为。利用该模型，本文第一作者采用显式格式编制了与ABAQUS商用有限元软件接口的用户材料子程序（VUMAT），实例计算证实该模型可以反映和描述多晶金属材料在材料反复加载条件下的循环塑性行为与尺寸效应。     

Microstructural size effects on mechanical properties of high purity nickel

The aim of this article is to provide experimental results in order to understand the microstructural size effects which occur with a decrease in the thickness of polycrystalline nickel samples from 3.2 mm to 12.5 μm. The influence of the thickness, grain size and ratio thickness to grain size on the mechanical properties and strain hardening were investigated by mechanical tests and TEM observations. The results show the presence of three different domains of mechanical behaviour: polycrystalline, multicrystalline and quasi-single crystalline depending on the thickness and on the number of grains across the thickness. The transition between the three domains is due to the occurrence of surface effects involving a decrease in the long-range internal backstress revealed by the TEM observations.