基于微纳米压入法的高熵合金柏氏矢量研究

采用微纳米压入法对CoCrFeNiMn高熵合金进行多种应变率下的压入测试研究,实验获得了材料硬度与压深之间的关系并通过计算分析得到了其不同工况下的柏氏矢量值,探究了压入深度和应变率对柏氏矢量的影响．实验结果表明,所测材料柏氏矢量值在一定范围内呈现出一定的波动性,随着压深的增大,柏氏矢量表现出尺寸效应,即柏氏矢量随压深呈增加趋势;并且在同等压深下,柏氏矢量存在率效应,随着应变率的增加,柏氏矢量值先减小后增加,柏氏矢量从滑移主导向原子失配主导的转变是其率效应转变的主要原因．     

准脆性单晶材料动态压痕试验研究

本研究设计了由钛合金压杆与金刚石压头组成的动态压痕试验装置,研究准脆性单晶材料(冰糖和RDX含能晶体)等的高应变率响应。试验装置引入了动量缓冲块,可以避免压杆造成的多次压痕加载,对样品施加产生80μs脉冲宽度的动态压痕载荷。对比了多次与单次加载压痕加载的冰糖单晶压痕坑,其动态硬度值约为5.18MPa。RDX单晶在压痕载荷作用下非常容易发生断裂破碎,测得动态硬度值为1.304MPa。相比RDX单晶,冰糖单晶的塑性变形能力更好。用光学显微镜观察到了压痕坑的径向裂纹和侧边裂纹,试验研究给出了准脆性晶体在动态压痕载荷下的裂纹扩展模式。     

不同形状压头作用下的压痕蠕变数值研究

采用有限元方法分析了圆锥压头、球形压头和圆柱形平压头作用下铝合金(2A12)的压痕蠕变行为,给出了三种压头作用下的等效蠕变应变、压痕深度-时间曲线和硬度-时间曲线.对于给定尺寸的三种压头,在相同载荷作用下,圆锥压头作用下的最大等效蠕变应变最大,圆柱形平压头作用下的最大等效蠕变应变最小;不同形状压头的压痕深度-时间曲线相似,由"瞬态蠕变"和"稳态蠕变"两段组成;而硬度随时间和压痕深度均呈现递减的趋势,不存在稳态数值.结果表明,在研究高温压痕蠕变现象、确定材料蠕变参数方面,平压头具有相当的优势:压头下方的应力场相对稳定,可不考虑摩擦和堆积的影响,计算简便准确.     

FCC铝纳米压痕的多尺度模拟

采用准连续介质方法模拟面心立方(FCC)铝单晶薄膜在纳米压痕下产生的变形过程.分别用四种不同的压头宽度,得出载荷-位移响应曲线和应变能变化曲线,发现压头宽度越大,晶体产生塑性变形的临界载荷越大;临界载荷的大小和采用能量理论预测的大小基本一致;模拟过程中,观察到位错成核现象,了解到载荷-位移响应曲线的突降是由位错成核现象所引起,四种情况中压头载荷的降幅大致相同;最后分析了模型在原子层次下的变形机理.     

血红细胞力学性能的纳米压痕实验和有限元模拟

采用纳米压痕技术和有限元方法研究了血红细胞的生物力学性能. 进行了血红细胞的纳米压痕实验, 得到了血红细胞的材料参数和变形形貌; 在实验基础上, 建立了血红细胞的三维有限元模型, 模拟了血红细胞的压痕载荷-位移曲线, 并考虑了参数效应. 数值模拟结果和实验数据符合很好. 通过改变压头与材料之间的摩擦系数和压头曲率半径等参数, 比较了载荷-位移曲线的变化情况. 研究表明摩擦系数对压痕载荷-位移曲线和应力分布影响很小, 而压头曲率对载荷-位移曲线的影响明显.     

利用维氏和玻氏压头表征半导体材料断裂韧性

压痕法是测量材料断裂韧性 ($K_{\rm IC})$ 的常用方法之一, 如何根据不同的材料、不同的压头选择适合的公式, 是当前面临的一大问题. 因此,在不同载荷下对单晶硅 (111) 和碳化硅 (4H-SiC, 0001面) 这两种半导体材料进行了维氏微米硬度和玻氏纳米压痕实验, 对实验产生的裂纹长度$c$进行了统计分析, 并采用13个压痕公式计算材料的$K_{\rm IC}$, 开展了微米划痕实验, 验证压痕法评估半导体材料$K_{\rm IC}$的适用性. 研究结果表明: 为了消除维氏压痕实验产生的$c$的固有离散性, 需要多次测量取平均值; 裂纹长度与压痕尺寸的比值随压痕载荷的增大而增大; 材料的裂纹类型与载荷相关且低载荷下表现为巴氏裂纹, 高载荷下表现为中位裂纹; 与微米划痕实验得到的单晶硅和碳化硅材料的$K_{\rm IC}$平均值 (分别为0.96 MPa,$\cdot$,$\sqrt{\rm m}$和2.89 MPa,$\cdot$,$\sqrt{\rm m}$) 相比, 在同一压头下无法从13个公式中获得同时适用于单晶硅和碳化硅材料的压痕公式,但在同一材料下可以获得同时适用于维氏和玻氏压头的$K_{\rm IC}$计算公式; 基于中位裂纹系统发展而来的压痕公式更适合用于评估半导体材料的$K_{\rm IC}$, 且维氏压头下的$K_{\rm IC}$与玻氏压头下$K_{\rm IC}$的关系不是理论上的1.073倍, 应为1.13$\pm 压痕法是测量材料断裂韧性(K_(IC))的常用方法之一,如何根据不同的材料、不同的压头选择适合的公式,是当前面临的一大问题.因此,在不同载荷下对单晶硅(111)和碳化硅(4H-Si C, 0001面)这两种半导体材料进行了维氏微米硬度和玻氏纳米压痕实验,对实验产生的裂纹长度c进行了统计分析,并采用13个压痕公式计算材料的K_(IC),开展了微米划痕实验,验证压痕法评估半导体材料K_(IC)的适用性.研究结果表明:为了消除维氏压痕实验产生的c的固有离散性,需要多次测量取平均值;裂纹长度与压痕尺寸的比值随压痕载荷的增大而增大;材料的裂纹类型与载荷相关且低载荷下表现为巴氏裂纹,高载荷下表现为中位裂纹;与微米划痕实验得到的单晶硅和碳化硅材料的K_(IC)平均值(分别为0.96 MPa·m~(1/2)和2.89 MPa·m~(1/2))相比,在同一压头下无法从13个公式中获得同时适用于单晶硅和碳化硅材料的压痕公式,但在同一材料下可以获得同时适用于维氏和玻氏压头的K_(IC)计算公式;基于中位裂纹系统发展而来的压痕公式更适合用于评估半导体材料的K_(IC),且维氏压头下的K_(IC)与玻氏压头下K_(IC)的关系不是理论上的1.073倍,应为1.13±0.01.     

灵芝孢子孢壁的弹性模量和硬度研究

采用纳米压痕技术和数值模拟研究灵芝孢子孢壁的弹性模量和硬度.利用原位纳米力学测试与分析系统,测试灵芝孢子孢壁的弹性模量和硬度.得到了载荷--位移曲线图和硬度、弹性模量随压痕深度变化的值.并用有限元方法模拟压痕过程,利用ANSYS软件,按照灵芝孢子孢壁和Berkovich压头的结构,建立了二维计算模型,得到纳米压痕的等效应力分布以及压痕过程中加载和卸载时的载荷--位移曲线.考察了摩擦、压头尖端半径对模拟结果的影响.结果显示:灵芝孢子孢壁的平均弹性模量为2.0GPa,硬度为0.13GPa.模拟结果在趋势上与实验结果有较好的吻合,与理论分析的载荷--位移关系基本一致.摩擦、压头尖端半径小于100nm时对模拟结果不会造成明显影响.研究结果为分析孢子的破壁机理提供必要参数.     

含能单晶微纳米力学性能试验研究及数值表征

利用微纳米压痕实验测定β-HMX 单晶(010) 晶面和α-RDX 单晶(210) 晶面的力学性能参数和微观破坏特征,并利用数值拟合确定了含能单晶的部分本构参数. 通过微纳米压痕实验连续刚度法(CSM) 得到HMX 单晶和RDX 单晶的弹性模量和硬度,RDX 单晶的硬度和模量都大于HMX 单晶,其硬度值均表现出一定的尺寸效应. 利用原子力显微镜(AFM) 分析了HMX 单晶和RDX 单晶的微观破坏机理,裂纹随着载荷的增大生成并扩展,裂纹面产生方向为晶体的最易解理破坏方向. 利用ABAQUS 有限元软件进行了纳米压痕数值模拟,结合微纳米压痕实验加卸载曲线,选取了合适的含能单晶塑性损伤本构模型的损伤本构参数.      

考虑压头曲率半径和应变梯度的微压痕分析

在压头尖端曲率半径取100nm的前提下，采用Chen和Wang的应变梯度理论，对微压痕实验进行了系统的数值分析. 首先通过拟合载荷-位移实验曲线的后半段来确定材料的屈服应力和幂硬化指数值,然后用有限元方法数值模拟压痕实验，并将计算得到的整段载荷-位移曲线及硬度-位移曲线和实验结果进行了比较. 结果表明应变梯度理论所预测的计算结果和实验结果很好地符合,包括压痕深度在亚微米和微米范围内的整段曲线.     

基于应变率阶跃测试的晶体铜压入应变率敏感性研究

纳米压入测试可以原位获取材料的诸多力学性能,包括弹性模量,硬度,屈服应力,应变率敏感指数等。本文利用应变率阶跃测试技术对多晶铜试样的应变率敏感性进行测试分析,硬度-位移曲线表明压头下方所存在的变形梯度对各阶跃应变率下的硬度值存在明显影响;采用基于晶体细观机制的塑性应变梯度理论对压入变形梯度效应予以修正,比较了修正与未修正数据所得的应变率敏感指数,在有效剔除压入变形梯度影响的基础上,应变率阶跃测试可实现单次压入下材料应变率敏感性的测试表征。     

压头不同晶向下纳米压痕的多尺度研究

为了研究压头晶体各向异性对纳米压痕的影响,采用多尺度准连续介质(QC)法模拟了不同晶向Ni压头与Ag薄膜的纳米压痕过程。通过对比不同晶向下压头在薄膜上触发的原子滑移,发现压头的晶向引起的界面失配位错在很大程度上决定薄膜开启初始原子滑移系的难易。然后对比了压头在不同晶向下测得的薄膜纳米硬度,发现其计算值是一样的。最后研究压头表面和压痕表面的正应力和切应力的分布,分析了应力分布与原子滑移系的关系。     

Quasicontinuum multiscale modeling of the effect of rough surface on nanoindentation behavior

Roughness of surface has as an important influence on identifying the mechanical behavior and performance of crystalline metals. In this study, nanoindentation simulations are conducted by the two dimensional quasicontinuum method to determine the load–penetration response and the critical load associated with the onset of plasticity in rough surfaces of a face-centered cubic single crystal copper. The arithmetic roughness index, ranging between 2 and 13 Å, is used to specify the roughness of surface. Results of indentation with different roughnesses are in good agreement with previous studies for the indenter size of 10–140 Å. The resultant load–penetration scattering, which stems from the roughness, indicates different dislocation nucleation steps, different subsequent dislocations intervals and varying stiffness values of samples. It can be concluded that the surface roughness has a significant effect on the first dislocation emission because of the indenter position and surface interactions beneath it. Moreover, the critical penetration depth for the first dislocation emission increases by the increase of the contact area between the indenter and surface.

     

Predictive modeling of nanoindentation-induced homogeneous dislocation nucleation in copper

Nanoscale contact of material surfaces provides an opportunity to explore and better understand the elastic limit and incipient plasticity in crystals. Homogeneous nucleation of a dislocation beneath a nanoindenter is a strain localization event triggered by elastic instability of the perfect crystal at finite strain. The finite element calculation, with a hyperelastic constitutive relation based on an interatomic potential, is employed as an efficient method to characterize such instability. This implementation facilitates the study of dislocation nucleation at length scales that are large compared to atomic dimensions, while remaining faithful to the nonlinear interatomic interactions. An instability criterion based on bifurcation analysis is incorporated into the finite element calculation to predict homogeneous dislocation nucleation. This criterion is superior to that based on the critical resolved shear stress in terms of its accuracy of prediction for both the nucleation site and the slip character of the defect. Finite element calculations of nanoindentation of single crystal copper by a cylindrical indenter and predictions of dislocation nucleation are validated by comparing with direct molecular dynamics simulations governed by the same interatomic potential. Analytic 2D and 3D linear elasticity solutions based on the Stroh formalism are used to benchmark the finite element results. The critical configuration of homogeneous dislocation nucleation under a spherical indenter is quantified with full 3D finite element calculations. The prediction of the nucleation site and slip character is verified by direct molecular dynamics simulations. The critical stress state at the nucleation site obtained from the interatomic potential is in quantitative agreement with ab initio density functional theory calculation.     

纳米压痕过程的三维有限元数值试验研究

采用有限元方法模拟了纳米压痕仪的加、卸载过程，三维有限元模型考虑了纳米压痕仪的标准Berkovich压头．介绍了有限元模型的几何参数、边界条件、材料特性与加载方式，讨论了摩擦、滑动机制、试件模型的大小对计算结果的影响，进行了计算结果与标准试样实验结果的比较，证实了模拟的可靠性．在此基础上，重点研究了压头尖端曲率半径对纳米压痕实验数据的影响．对比分析了尖端曲率半径r＝0与r＝100nm两种压头的材料压痕载荷—位移曲线．结果表明，当压头尖端曲率半径r≠0时，基于经典的均匀连续介质力学本构理论、传统的实验手段与数据处理方法，压痕硬度值会随着压痕深度的减小而升高．     

Size,geometry and nonuniformity effects of surface-nanocrystalline aluminum in nanoindentation test

In the present research, the mechanical behavior of the surface-nanocrystalline aluminum (SNCA) is investigated through nanoindentation experiment and theoretical modeling. Firstly, through microscopical observation and measurement for the SNCA material, a microstructure cell model is developed. Secondly, based on the microstructure cell model and the strain gradient plasticity theory, and based on introducing a parameter accounting for the grain size nonuniformity effect, the discrete features of the hardness–depth relations of the SNCA material are described. The “U-type” feature of the hardness–depth experimental curves is modeled and simulated. Thirdly, in the SNCA material the mechanical property of the grain boundary, i.e., the strength of plastic zone penetrating the grain boundary is characterized by introducing a criterion parameter, the critical effective plastic strain. The “waterfall-type” feature of the hardness–depth curves is modeled and simulated. It is worth pointing out that, in the present study, the length scale parameter in the strain gradient plasticity theory is taken as a universal material parameter instead of a simulation parameter, and it is determined through applying the strain gradient plasticity theory to the modeling of the corresponding single crystal aluminum.     

Combined numerical simulation and nanoindentation for determining mechanical properties of single crystal copper at mesoscale

Constitutive laws are critical in the investigation of mechanical behavior of single crystal or polycrystalline materials in applications spanning from microscale to macroscale. In this investigation, a combined FEM simulation and experimental nanoindentation approach was taken to determine the mechanical behavior of single crystal copper incorporating the mesoplastic constitutive model. This model was implemented in a user-defined subroutine in 3D ABAQUS/Explicit code. Nanoindentation was modeled using the multiscale modeling technique involving mesoplasticity and elasticity, i.e., mesoplastic constitutive model was used near the local nanoindentation region (where the dislocations are generated) while elastic constitutive model was used in rest of the region in the workmaterial. The meso-mechanical behavior of the crystalline structure and the effect of the mesoplastic parameters on the nanoindentation load-displacement relationships were investigated in the FEM analysis. Nanoindentation tests were conducted on single crystal copper to determine load-displacement relationships. Appropriate mesoplastic parameters were determined by fitting the simulated load-displacement curves to the experimental data. The mesoplastic model, with appropriate parameters, was then used to determine the stress-strain relationship of a single crystal copper at meso-scale. The effect of indenter radius (3.4-) on material hardness under nanoindentation was simulated and found to match the experimental data for several indenter radii (3.4, 10 and ). A comparison of the topographies of nanoindentation impressions in the experiments with FEM results showed a reasonably good agreement.     

The indenter tip radius effect in micro- and nanoindentation hardness experiments.

Nix and Gao established an important relation between the microindentation hardness and indentation depth. Such a relation has been verified by many microindentation experiments (indentation depths in the micrometer range), but it does not always hold in nanoindentation experiments (indentation depths approaching the nanometer range). Indenter tip radius effect has been proposed by Qu et al. and others as possibly the main factor that causes the deviation from Nix and Gao's relationship. We have developed an indentation model for micro- and nanoindentation, which accounts for two indenter shapes, a sharp, conical indenter and a conical indenter with a spherical tip. The analysis is based on the conventional theory of mechanism-based strain gradient plasticity established from the Taylor dislocation model to account for the effect of geometrically necessary dislocations. The comparison between numerical result and Feng and Nix's experimental data shows that the indenter tip radius effect indeed causes the deviation from Nix-Gao relation, but it seems not be the main factor. The project supported by the National Natural Science Foundation of China (10121202) and the Ministry of Education of China (20020003023)     

Computer simulations of nanoindentation on Cu (1 1 1) with a void

This paper employs static atomistic simulations to investigate the effect of a void on the nanoindentation of Cu (1 1 1). The simulations minimize the potential energy of the complete system via finite element formulation to identify the equilibrium configuration of any deformed state. The size and depth of the void are treated as two variable parameters. The numerical results reveal that the void disappears when the indentation depth is sufficiently large. A stress concentration is observed at the internal surface of the void in all simulations cases. The results indicate that the presence of a void has a significant influence on the nanohardness extracted from the nanoindentation tests.     

Bending of single crystal microcantilever beams of cube orientation: Finite element model and experiments

The aim of this work is to investigate the microstructure evolution, stress-strain response and strain hardening behavior of microscale beams. For that purpose, two single crystal cantilever beams in the size dependent regime were manufactured by ion beam milling and beams were bent with an indenter device. A crystal plasticity material model for large deformations was implemented in a finite element framework to further investigate the effect of boundary constraints. Simulations were performed using bulk material properties of single crystal copper without any special treatment for the strain gradients. The difference between the slopes of the experimental and the simulated force displacement curves suggested negligible amount of strain gradient hardening compared to the statistical hardening mechanisms.