多层膜结构对硬度实验影响的数值分析

为了研究多层膜结构对纳米微硬度实验的影响,用FEPG对纳米微硬度实验中压头的压入过程进行数值模拟,分析了由TiN,Al,Ti组成的多层膜中软膜材料不同、软膜与硬膜厚度比不同以及多层膜层数不同对膜层体系产生的影响,得出梯度变化的膜层结构能够提高膜层的整体性能,为纳米微硬度实验提供了一定的参考依据．     

含界面效应纳米尺度圆环形涂层中螺型位错分析

研究了纳米尺度圆环形涂层(界面层)中螺型位错与圆形夹杂以及无限大基体材料的干涉效应.涂层与夹杂的界面和涂层与基体的界面均考虑界面应力效应.运用复势方法,获得了三个区域复势函数的解析解答.利用求得的应力场和Peach-Koehler公式,得到了作用在螺型位错上位错力的精确表达式.主要讨论了界面应力对涂层(界面层)中螺型位错运动和平衡稳定的影响规律.结果表明,界面应力对界面附近位错的运动有大的影响,由于界面应力的存在,可以改变涂层内位错与夹杂/基体干涉的引斥规律,并使位错在涂层内部产生三个稳定或非稳定的平衡点.考虑界面效应后,有一个额外的排斥力或吸引力作用在位错上,使原有的位错力增加或减小.     

晶体位错尺寸效应的多尺度分析

本文采用准连续介质多尺度方法,分析了面心立方（fcc）晶体铝阶梯孪晶界在不同尺寸情况下（试件尺寸长高比从1：1到8：1）受剪切作用的晶界变形。了解在不同尺寸下,晶界结构位错的成核过程,得到了大试件比值与小试件比值下作用力与应变的关系曲线及不同试件尺寸下应变能的变化曲线。其中随着试件比值的增加,作用力在应变比较小的时候变化情况相似,但当应变达到3％以后,呈现出明显的不同;应变能随试件长高比的增大而减小,各个试件在各自不同的加载阶段,应变能变化趋势同作用力变化趋势相一致。本计算揭示了不同尺寸下阶梯孪晶界在剪切作用下的微观机理,证实其尺寸效应性质。     

梯度纳米结构金属力学性能、变形机理和多尺度计算研究进展

近年来,梯度纳米结构金属因其优越的力学性能和独特的塑性变形机理受到广泛关注,已成为材料与力学学科的热点和前沿.论文首先介绍梯度纳米结构金属的强度、塑性、加工硬化和抗疲劳等核心力学性能,以及晶粒长大、塑性应变梯度和几何必需位错等塑性变形机理及其力学研究.其次介绍梯度纳米结构金属的多尺度计算与模拟研究.最后讨论梯度纳米结构金属研究领域存在的挑战.     

磁控溅射氮化钛/氮化铝纳米多层膜的抗磨性能及其在金属加工刀具中的应用研究

采用新型高速反应真空磁控溅射系统制备了氮化钛/氮化铝纳米多层膜,探讨了周期(λ)对多层膜力学性能及抗磨性能的影响;在微钻针及车刀表面分别制备了上述多层膜,并分别对电路板以及中碳钢棒进行实机加工测试,以验证其实用性.结果表明:通过控制溅射参数可以制备出周期范围为2.4～67.6nm的氮化钛/氮化铝多层膜;当周期λ≤3.6nm时,相应的多层膜具有较高的硬度、良好的粘附性能及优异的抗磨性能.实机测试结果显示,所制备的多层膜可以改善金属车削和微钻削加工条件,改善程度优于单层氮化钛镀膜.     

A study of conditions for dislocation nucleation in coarser-than-atomistic scale models

We perform atomistic simulations of dislocation nucleation in defect free crystals in 2 and 3 dimensions during indentation with circular (2D) or spherical (3D) indenters. The kinematic structure of the theory of Field Dislocation Mechanics (FDM) is shown to allow the identification of a local feature of the atomistic velocity field in these simulations as indicative of dislocation nucleation. It predicts the precise location of the incipient spatially distributed dislocation field, as shown for the cases of the Embedded Atom Method potential for Al and the Lennard–Jones pair potential. We demonstrate the accuracy of this analysis for two crystallographic orientations in 2D and one in 3D. Apart from the accuracy in predicting the location of dislocation nucleation, the FDM based analysis also demonstrates superior performance than existing nucleation criteria in not persisting in time beyond the nucleation event, as well as differentiating between phase boundary/shear band and dislocation nucleation. Our analysis is meant to facilitate the modeling of dislocation nucleation in coarser-than-atomistic scale models of the mechanics of materials.     

Indentation Schmid factor and orientation dependence of nanoindentation pop-in behavior of NiAl single crystals

Instrumented nanoindentation techniques have been widely used to characterize the small-scale mechanical behavior of materials. The elastic-plastic transition during nanoindentation is often indicated by a sudden displacement burst (pop-in) in the measured load-displacement curve. In defect-free single crystals, the pop-in is believed to be the result of homogeneous dislocation nucleation because the maximum shear stress corresponding to the pop-in load approaches the theoretical strength of the materials and because the statistical distribution of pop-in stresses is consistent with what is expected for a thermally activated process of homogeneous dislocation nucleation. This paper investigates whether this process is affected by crystallography and stress components other than the resolved shear stress. A Stroh formalism coupled with the two-dimensional Fourier transformation is used to derive the analytical stress fields in elastically anisotropic solids under Hertzian contact, which allows the determination of an indentation Schmid factor, namely, the ratio of maximum resolved shear stress to the maximum contact pressure. Nanoindentation tests were conducted on B2-structured NiAl single crystals with different surface normal directions. This material was chosen because it deforms at room temperature by {1 1 0}〈0 0 1〉 slip and thus avoids the complexity of partial dislocation nucleation. Good agreement is obtained between the experimental data and the theoretically predicted orientation dependence of pop-in loads based on the indentation Schmid factor. Pop-in load is lowest for indentation directions close to 〈1 1 1〉 and highest for those close to 〈0 0 1〉. In nanoindentation, since the stress component normal to the slip plane is typically comparable in magnitude to the resolved shear stress, we find that the pressure sensitivity of homogeneous dislocation nucleation cannot be determined from pop-in tests. Our statistical measurements generally confirm the thermal activation model of homogeneous dislocation nucleation. That is, the extracted dependence of activation energy on resolved shear stress is almost the same for all the indentation directions considered in this study, except for those close to 〈0 0 1〉. Because very high pop-in loads are measured for orientations close to 〈0 0 1〉, which implies a large contact area at pop-in, there is a higher probability of activating pre-existing dislocations in these orientations, which may explain the discrepancy near 〈0 0 1〉.     

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.     

On the nonlocal nature of dislocation nucleation during nanoindentation

Using static atomistic simulations, we study the full details of the mechanism by which dislocations homogeneously nucleate beneath the surface of a initially defect-free crystal during indentation. The mechanism involves the collective motion of a finite disk of atoms over two adjacent slip planes, the diameter of which depends on the indenter size. The nucleation mechanism highlights the need for nonlocal considerations in the development of a nucleation criterion. We review three nucleation criteria from the literature, each of which is based on purely local measures of the state of stress, and show that none are sufficiently general to predict nucleation in realistic atomic systems. We then propose a criterion based on an eigenmode analysis of the atomic-scale acoustic tensor. We demonstrate the accuracy of the criterion, which also works in the presence of existing topological defects like free surfaces or dislocation cores. The dependence of the size of the nucleated disk on the indenter radius leads to a self-similar nucleation process and virtually no indentation size effect (ISE), suggesting that homogeneous nucleation is only possible for very small indenters.     

Nucleation of partial dislocations at a crack and its implication on deformation mechanisms of nanostructured metals

Nucleation of partial dislocations at a crack is analyzed based a multiscale model that incorporates atomic information into continuum-mechanics approach. The crack and the slip plane as the extension of the crack are modeled as a surface of displacement discontinuities embedded in an elastic medium. The atomic potential between the adjacent atomic layers along the slip plane is assumed to be the generalized stacking fault energy, which is obtained based on atomic calculations. The relative displacements along the slip plane, corresponding to the configurations of partial dislocations and stacking faults, are solved through the variational boundary integral method. The energetics of partial dislocation nucleation at the crack in FCC metals Al and Cu are comparatively studied for their distinctive difference in the intrinsic stacking fault energy. Compared with nucleation of perfect dislocations in previous studies, several new features have emerged. Among them, the critical stress and activation energy for nucleation of partial dislocations are markedly lowered. Depending on the value of stacking fault energy and crack configuration, the saddle-point configurations of partial dislocations can be vastly different in terms of the nucleation sequence and the size of the stacking fault. These findings have significant implication for identifying the fundamental dislocation and grain-boundary-mediated deformation mechanisms, which may account for the limiting strength and the high strain rate sensitivity of nanostructured metals.     

Analysis of dislocation nucleation from a crystal surface based on the Peierls-Nabarro dislocation model

Dislocation nucleation from a stressed crystal surface is analyzed based on the Peierls-Nabarro dislocation model. The variational boundary integral approach is used to obtain the profiles of the embryonic dislocations in various three-dimensional nucleation configurations. The stress-dependent activation energies required to activate dislocations from their stable to unstable saddle point configurations are determined. Compared to previous analyses of this type of problem based on continuum elastic dislocation theory, the present analysis eliminates the uncertain core cutoff parameter by allowing for the existence of an extended dislocation core as the embryonic dislocation evolves. Moreover, atomic information can be incorporated to reveal the dependence of the nucleation process on the profile of the atomic interlayer potential as compared to continuum elastic dislocation theory in which only elastic constants and Burgers vector are relevant. Finally, the presented methodology can also be readily used to study dislocation nucleation from the surface heterogeneities such as cracks, steps, and quantum structures of electronic devices.     

A stress-gradient based criterion for dislocation nucleation in crystals

Dislocations are the main lattice defects responsible for the strength and ductility of crystalline solids. The generation of new dislocations is an essential aspect of crystal defect physics, but a fundamental understanding of the mechanical conditions which lead to dislocation nucleation has remained elusive. Here, we present a nucleation criterion motivated from continuum thermomechanical considerations of a crystalline solid undergoing deformation, and demonstrate the criterion's ability to correctly predict dislocation nucleation via direct atomistic simulations. We further demonstrate that the commonly held notion of a nucleation criterion based on the magnitude of local stress components is incorrect.     

Nanoindentation and Microindentation of CuAlNi Shape Memory Alloy

Nanoindentation and microindentation studies were conducted within individual grains of a CuAlNi shape memory alloy. Linear surface features were observed near the indentations after unloading, many of which were responsive to heating. Crystallographic orientation information was obtained from electron backscattering diffraction in order to compare the orientation of observed surface features to predicted austenite–martensite interfaces, slip planes, and possible fracture planes in this alloy. Most of the features observed can be attributed to austenite–martensite interfaces, which remain in the material after unloading due to the constraints of the plastic deformation created by indentation. Due to the temperature dependence of the transformation stress in shape memory alloys, these stress-induced martensites are observed to diminish with heating and to reappear with cooling. Plastic deformation is observed in the form of pile-up near the indentation.     

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.     

氢对金属中裂纹形核的影响

本文建立了金属中裂纹形核的一个理论模型，通过理论分析，讨论了氢对裂纹形核的影响。定义了与材料晶粒和脆性沉淀物尺寸有关的材料影响参数。结果表明，本文提出的模型有更明确的物理意义，环境中的氢参加裂纹形核和生长的三个阶段。