基于分子动力学单晶硅的纳米压痕过程研究

基于分子动力学的基本理论,在微纳米尺度下建立了单晶硅的纳米压痕分子动力学模型。研究了在纳米压痕过程中单晶硅基体的变形机理、势能变化和温度变化。研究发现:在纳米压痕过程中基体上出现了位错、空位及滑移带,基体两侧有凸起现象。当压头撤离时,基体与压头间存在颈缩现象。在系统达到平衡时系统的势能出现不同,这是因为原子位错运动使得系统增加的势能小于压头原子所做的功。温度的变化与位错变形的程度相关,位错变形越剧烈系统温度升高的越快。     

基于分子动力学单晶硅的纳米压痕过程研究（英文）

基于分子动力学的基本理论,在微纳米尺度下建立了单晶硅的纳米压痕分子动力学模型。研究了在纳米压痕过程中单晶硅基体的变形机理、势能变化和温度变化。研究发现:在纳米压痕过程中基体上出现了位错、空位及滑移带,基体两侧有凸起现象。当压头撤离时,基体与压头间存在颈缩现象。在系统达到平衡时系统的势能出现不同,这是因为原子位错运动使得系统增加的势能小于压头原子所做的功。温度的变化与位错变形的程度相关,位错变形越剧烈系统温度升高的越快。     

一种单晶镍基合金蠕变初期的位错组态

利用ＴＥＭ研究了单晶镍基合金平行于应力轴的（１００）晶面拉伸蠕变初期的位错组态,表明：形变特征是错在γ相八面体滑移系中运动,在基体通道中,１／２〈１１０〉型位错运动相遇,发生反应而增殖;由于（１００）晶面的基体通道受压应力,位错运动阻力大,密度小,位错运动多以交滑移和Ｏｒｏｗａｎ弓入方式进行;当受拉应力通道中弓出的位错环经交滑移进入压应力通道后,入口两侧被钉扎,位错可定向弓入成为形貌类似于双端Ｆ－     

晶粒尺寸对γ-TiAl合金力学性能影响的纳米压痕研究

为研究纳米压痕过程中晶粒尺寸对γ-Ti Al合金力学性能及变形行为的影响,利用Voronoi方法建立多晶γ-Ti Al模型,采用分子动力学方法模拟压头压入不同晶粒尺寸模型的压痕过程,得到相应尺寸下的载荷-深度曲线,并计算了7种晶粒尺寸下γ-Ti Al的硬度。结果表明:当晶粒尺寸小于9.9 nm时,晶粒尺寸与硬度表现出反Hall-Petch关系,位错和晶界活动共同促使材料发生塑性变形,晶界活动起主导作用。当晶粒尺寸大于9.9 nm时,晶粒尺寸与硬度符合Hall-Petch关系,晶界对材料变形影响较小,位错主导基体发生塑性变形。另外,分析了γ-Ti Al在压痕过程中的应力传递和形变恢复过程,发现致密晶界网格结构能够有效抑制压痕缺陷及内应力向材料内部传递;晶粒尺寸越小,压头下方的内应力分布越均匀,沿压痕方向的弹性恢复比越小。     

纳米多晶Cu-Ni合金三轴拉伸变形模拟

采用分子动力学对纳米多晶Cu-Ni合金三轴拉伸性能进行了模拟研究，从微观尺度探究了Ni含量及温度对合金拉伸性能的影响规律。结果表明，当Ni含量为10%时，最大拉应力达21.70 GPa,位错之间的相互作用是造成拉伸性能提升的重要原因。从能量、位错和堆垛层错方面对Ni含量为10%合金的性能进行了验证，表明Cu-Ni合金符合固溶弱化规律。当温度为100 K时，存在最大拉应力为22.21 GPa;当温度为900 K时，最大拉应变为0.1045。晶界原子百分比随加载温度的升高而增加，晶内原子百分比随加载温度的升高而减少；温度升高导致晶界熔化，原子无序度增加，对位错的阻碍作用下降。拉伸过程中位错密度不断降低，由位错缠结而引起的强化作用不断减少，导致高温时合金的拉伸性能下降。     

SiCp/Al复合材料磨削加工去除机理的有限元分析

本文建立了SiCp/Al复合材料的二维实体模型,基于压痕断裂力学的方法,研究了压痕深度的变化对SiCp/Al复合材料磨削加工去除机理的影响。结果表明:随着压痕深度的增加,压头下方SiC颗粒的第一主应力逐渐变大,Al基体的von Mises等效应力也逐渐变大。当压痕深度大于等于0.15μm时,压头下方会形成塑形变形区;压痕深度大于等于0.292μm时,SiC颗粒会由于拉应力的作用而产生径向裂纹;当压痕深度超过0.34μm时,Al基体由于局部被压溃而影响SiCp/Al复合材料延性去除机理。     

一种镍基单晶高温合金870℃高周疲劳变形微观结构研究（英文）

研究了一种镍基单晶高温合金在870℃时的高周疲劳性能及其变形组织结构。结果表明:该合金的疲劳寿命随着应力水平的升高而减小,870℃时光滑试样的疲劳强度为443 MPa;利用透射电镜(TEM)观察疲劳循环试样的位错组态,发现在疲劳变形的初始和中期阶段,位错组态主要为界面位错,位错在基体通道中{111}面运动,并交互反应形成三维位错网络结构。当应力水平提高到550 MPa以上时,在变形的末期,观察到高密度位错集中于位错滑移带及位错切入γ'相现象。在循环应力和高温叠加作用下,基体通道中诱发析出大量圆形细小二次γ'相。二次γ'相的析出有益于阻止基体位错的滑动,抑制位错切入γ'相,有利于提高合金的疲劳强度。     

晶粒尺寸对γ-TiAl力学性能影响的纳米压痕研究

为研究纳米压痕过程中晶粒尺寸对γ-TiAl合金力学性能及变形行为的影响,利用Voronoi方法建立多晶γ-TiAl模型,采用分子动力学方法模拟压头压入不同晶粒尺寸模型的压痕过程,得到相应尺寸下的载荷-深度曲线,并计算了7种晶粒尺寸下γ-TiAl的硬度。结果表明：当晶粒尺寸小于9.9nm时,晶粒尺寸与硬度表现出反Hall-Petch关系,位错和晶界活动共同促使材料发生塑性变形,晶界活动起主导作用。当晶粒尺寸大于9.9nm时,晶粒尺寸与硬度符合Hall-Petch关系,晶界对材料变形影响较小,位错主导基体发生塑性变形。另外,分析了γ-TiAl在压痕过程中的应力传递和形变恢复过程,发现致密晶界网格结构能够有效抑制压痕缺陷及内应力向材料内部传递;晶粒尺寸越小,压头下方的内应力分布越均匀,沿压痕方向的弹性恢复比越小。     

TiC颗粒增强钛基复合材料的形变

研究了TiC颗粒增强钛基复合材料增强颗料和基体之间的不均匀形变,得出颗粒和基体之间的形变过程中产生形变摩擦阻力,通过弹塑性力学计算出的TiC/Ti界面上颗粒阻碍基体变形力为TiC强度的0．6倍,阻碍基体中滑移的进行,使基体得到强化。另一方面,当界面上的形变应力同基体的某个晶面形成位错源时,可放出位错环,使应力得到释放,同时增加基体的位错密度,位错互相缠绕形成胞状亚结构,强化基体。     

利用纳米压入加载曲线确定金属薄膜的屈服强度和硬化指数 Ⅱ.实验及验证

通过比较实测与计算所得整体材料纳米压入加载曲线及Berkovich压头的面积函数,对已建立的Berkovich压头绝对钝化量的确定方法进行了实验验证在此基础上,测定了蒸发恢制在Si单晶上三种不同膜厚Al膜的屈服强度和硬化指数,并与拉伸分离法获得的结果进行了比较结果表明,利用纳米压入加载曲线确定陶瓷基体上金属薄膜基本力学性能的方法是可行的．     

Indentation size effect and shear transformation zone size in a bulk metallic glass in two different structural states

The existence of an indentation size effect (ISE) in the onset of yield in a Zr-based bulk metallic glass (BMG) is investigated by employing spherical-tip nanoindentation experiments. Statistically significant data on the load at which the first pop-in in the displacement occurs were obtained for three different tip radii and in two different structural states (as-cast and structurally relaxed) of the BMG. Hertzian contact mechanics were employed to convert the pop-in loads to the maximum shear stress underneath the indenter. Results establish the existence of an ISE in the BMG of both structural states, with shear yield stress increasing with decreasing tip radius. Structural relaxation was found to increase the yield stress and decrease the variability in the data, indicating “structural homogenization” with annealing. Statistical analysis of the data was employed to estimate the shear transformation zone (STZ) size. Results of this analysis indicate an STZ size of ～25 atoms, which increases to ～34 atoms upon annealing. These observations are discussed in terms of internal structure changes that occur during structural relaxation and their interaction with the stressed volumes in spherical indentation of a metallic glass.     

Discrete dislocation analyses of circular nanoindentation and its size dependence in polycrystals

Nanoindentation has been extensively used to measure the mechanical behavior of materials at the micro- and nanoscale. However, the material response of nano/microindentation and its intrinsic mechanisms are very complicated, especially when considering heterogeneous polycrystalline materials. In this contribution, nanoindentation in polycrystals, performed with a circular indenter, is studied by numerical modeling based on the two-dimensional discrete dislocation plasticity of Van der Giessen and Needleman [Van der Giessen E, Needleman A. Model Simul Mater Sci Eng 1995;3:689]. The dependence of indentation hardness is investigated with respect to four typical characteristic dimensions: indenter radius, grain size, indentation depth, and the distance between the grain boundary and the indenter. Results show that these characteristic dimensions have considerable influence on the nanoindentation hardness. Further investigation shows that their influence takes effect mainly in two ways, i.e. via strain hardening and the indentation size effect. Although both effects are size dependent, their underlying mechanisms are clearly different. For the present polycrystal case, the strain hardening effect is mainly associated with the constraints of grain boundaries and dislocation obstacles to the dislocation glide, while the indentation size effect is related to the average strain gradient beneath the circular indenter.     

Size dependent nanoindentation of a soft film on a hard substrate

Nanoindentation experiments on Al/glass systems show that, as the indentation depth increases, the hardness decreases during a shallow indentation, and increases when the indenter tip approaches the film–substrate interface. We associate the rise in hardness during two stages with the strong strain gradient effects, the first stage is related with the small scale effects and the second stage with the strain gradient between the indenter and the hard substrate. Using the strain gradient theory proposed by Chen and Wang and the classical plasticity theory, the observed nanoindentation behavior is modeled and analyzed by means of the finite element method, and it is found that the classical plasticity cannot explain the experiment results but the strain gradient theory can describe the experiment data at both shallow and deep indentation depths very well. The results prove that both the strain gradient effects and substrate effects exist in the nanoindentation of the film–substrate system.     

The role of probe shape on the initiation of metal plasticity in nanoindentation

The dislocation nucleation stress of crystalline materials is frequently estimated from the maximum shear stress, assuming Hertzian contact up to the first “pop-in” event, which is a sudden displacement burst during load-controlled nanoindentation. However, an irregular indenter tip shape will significantly change the stress distribution, and therefore the maximum shear stress from a Hertzian estimation. Here, we assess possible errors and pitfalls of the Hertzian estimation of initial plastic yield at the nanoscale. The near-apex shapes of two Berkovich indenters, one sharp and one worn, were measured by atomic force microscopy and directly input into finite element analysis (FEA) models for “virtual” nanoindentation experiments on single-crystal tungsten. Experiments were also carried out with those indenters. Excellent agreement is found between experimental and FEA force–displacement relationships, but the discrepancies between Hertzian and FEA estimates of the shear stresses are over 25% for the sharp indenter and over 50% for the blunt indenter. This demonstrates that small irregularities in the shape of indenter tips can cause significant deviations from the Hertzian estimation of dislocation nucleation stress.     

纳米压痕过程的多尺度模拟与Rice-Thomson位错模型分析

采用多尺度准连续介质法（简称QC方法）对单晶Ag薄膜纳米压痕过程进行模拟,研究压头宽度对纳米压痕过程中接触应力分布、位错形核临界载荷以及纳米硬度的影响,并用Rice-Thomson位错模型（简称R-T位错模型）进行分析。结果表明,纳米压痕获得的载荷-位移曲线呈现出的不连续性与位错之间的协同作用密切相关;压头尺寸对纳米压痕过程中接触应力分布、位错形核临界载荷以及纳米硬度具有明显的影响：随着压头宽度的增加,法向和切向接触应力以及纳米硬度值递减,呈现出明显的压头尺寸效应;而压头下方薄膜内位错形核临界载荷却递增,且与压头半宽度的平方根成正比。模拟结果与相应实验结果以及R-T位错模型计算结果吻合     

Discrete and continuous deformation during nanoindentation of thin films

This paper describes nanoindentation experiments on thin films of polycrystalline Al of known texture and different thicknesses, and of single crystal Al of different crystallographic orientations. Both single-crystalline and polycrystalline films, 400–1000 nm in thickness, are found to exhibit multiple bursts of indenter penetration displacement, h, at approximately constant indentation loads, P. Recent results from the nanoindentation studies of Suresh et al. (Suresh, S., Nieh T.-G. and Choi, B.W., Scripta mater., 1999, 41, 951) along with new microscopy observations of thin films of polycrystalline Cu on Si substrates are also examined in an attempt to extract some general trends on the discrete and continuous deformation processes. The onset of the first displacement burst, which is essentially independent of film thickness, appears to occur when the computed maximum shear stress at the indenter tip approaches the theoretical shear strength of the metal films for all the cases examined. It is reasoned that these displacement bursts are triggered by the nucleation of dislocations in the thin films. A simple model to estimate the size of the prismatic dislocation loops is presented along with observations of deformation using transmission electron microscopy and atomic force microscopy. It is demonstrated that the response of the nanoindented film is composed of purely elastic behavior with intermittent microplasticity. The overall plastic response of the metal films, as determined from nanoindentation, is shown to scale with film thickness, in qualitative agreement with the trends seen in wafer curvature or X-ray diffraction measurements.     

纳米压痕技术及其在薄膜/涂层体系中的应用

综述了纳米压痕技术的发展历程及其在薄膜领域的应用。介绍了当前实验室条件下主要采用的电磁驱动式纳米压痕仪的构造和工作过程。为了保证测试结果的准确性,要在合适的温度、湿度下进行压入实验,借助保载来消除一些可以避免的误差。阐述了压头的分类和选择原则,玻氏压头相比于维氏压头具有更小的中心线与棱面夹角,避免了尖端横刃对于压入结果准确性的影响,因此最常用的压头为玻氏压头;表征断裂韧性最合适的压头为立方角压头;表征微机电系统的弯曲采用楔形压头。总结了通过最大载荷和压入面积得到涂层力学参量的分析流程。归纳了将纳米压痕法应用于表征薄膜涂层的硬度和弹性模量、室温下蠕变性能、断裂韧性、残余应力、塑性性能等力学量的研究,如表征硬度和弹性模量的Oliver-Pharr法的应用,识别蠕变柔量的Lee-Radok模型的应用,分析断裂韧性的Lawn-Evans-Marshall模型的应用。在涂层制备过程中,制备参数的改变可以使得涂层具有不同的力学性能,涂层厚度远小于表面尺寸,硬度和弹性模量仍然存在各向异性,非晶态结构涂层具有更高的硬度和弹性模量。采用碳纳米管强化可以提高涂层的断裂韧性,涂层内存在适量的残余应力数值和合...     

Structure of nanoindentations in heavily n- and p-doped (0 0 1) GaAs

We have studied the nanoindentation structures achieved at room temperature (RT) on (0 0 1) GaAs with either n or p doping. Elastic–plastic nanoindentations were made over a wide range of loads (between 0.2 and 50 mN) at RT with a Berkovich indenter using two different orientations. Transmission electron microscopy was used to observe systematically the nanoindentation structures (central zone and rosette arms) and to investigate changes in dislocation activity. The mechanical response of both types of samples is relatively similar in terms of hardness, critical shear stress or pop-in load amplitude. In contrast, the indentation rosette structure appears to be sensitive to both doping and indenter orientation. Perfect dislocations show long screw segments only in n-doped specimens, a finding that is attributed to mobility effects. Moreover, p-doped specimens show no partial dislocations while n-doped specimens show partial dislocations in both rosette arms.