初始压入位置对Ni基单晶合金纳米压痕影响研究

针对Ni基单晶合金建立初始压入γ 相的γ /γ' 模型和初始压入γ'相的γ'/γ 模型, 采用分子动力学方法模拟金刚石压头压入两种模型的纳米压痕过程, 计算两种模型[001]晶向硬度. 采用中心对称参数分析两种模型(001)相界面错配位错对纳米压痕过程的影响. 结果显示: 弛豫后, 两种模型(001)相界面错配位错形式不同, 其中γ'/γ 模型(001)相界面错配位错以面角位错形式存在; 压入深度在0.930 nm 之前, 两种模型(001)相界面错配位错变化不大, 压入载荷-压入深度及硬度-压入深度曲线较符合; 压入深度在0.930 nm之后, γ'/γ 模型(001)相界面错配位错长大很多, 导致相同压入深度时γ'/γ 模型比γ /γ'模型压入载荷和硬度计算结果小; 压入深度在2.055 nm之后, γ /γ'模型(001)相界面错配位错对γ 相中位错进入γ'相有阻碍作用, 但仍有部分位错越过(001) 相界面进入γ' 相中, γ'/γ 模型(001)相界面处面角位错对γ' 相中位错进入γ 相有更明显的阻碍作用, 几乎无位错越过(001) 相界面进入γ 相中, 面角位错的强化作用更明显, 所以γ'/γ 模型比γ /γ'模型压入载荷上升速度快.     

Berkovich nanoindentation study of monocrystalline tungsten: a crystal plasticity study of surface pile-up deformation

Abstract

In this paper, it was investigated whether Berkovich indentation test with a triangular-based pyramidal imprint would exhibit the same surface pile-up deformation behaviour as in Vickers or spherical indentation tests. The characteristic correlation between the pile-up patterns of monocrystalline tungsten and the geometry of slip systems was examined both experimentally and computationally. Surface pile-up patterns for three different crystallographic orientations of specimens with corresponding rotational crystal symmetry were characterised. In addition, the effect of the varying azimuthal orientation of the indenter on the pile-up patterns was also discussed. Predictions from finite element simulation based on the crystal plasticity theory are also presented and compared with the measured results. It was found that the surface pile-up patterns of Berkovich indentation did not necessarily reflect the rotational crystal symmetry of tungsten single crystal specimens. The pile-up patterns were affected by the variation of the indenter’s azimuthal orientation. The height of the pile-up hillocks was often highly non-uniform even on the same surface plane indicating strong influence of slip geometry leading to the plastic anisotropy.     

MD simulation of the effect of contact area and tip radius on nanoindentation

Nanoindentation,namelydepth-sensingindentation(DSI),involvesforcingarigidindenterwithknowngeometryintothesurfaceofamaterialwhilecontinuouslymonitoringtheloadontheindenter,thedisplacementoftheindenterintothesurface,andthetimeoftheexperiment.Thedepthisthenusedtocalculatetheprojectedareaofcontactforthepurposeofcalculatingthehardnessandelasticmodulus.Infact,variouserrorsareassociatedwiththisprocedure.Oneofthemcomesfromthemeasurementofthepenetrationdepth.Ideally,thepenetrationdepthshouldbecalcula…     

A crystal plasticity model of a formation of a deformation band structure

The formation of deformation bands with the typically alternating sign of the misorientation across their boundaries is interpreted as spontaneous deformation instability caused by anisotropy of hardening. To analyse the nature of the fragmentation, a model of a rigid-plastic crystal domain deformed by symmetric double slip in a plane-strain compression is considered. The basic reason for the deformation band existence is that a local decrease in number of active slip systems in the bands is energetically less costly than a homogeneous deformation by multislip. However, such model of the bands predicts their extreme orientation and their width tends to zero. This trend is modified by hardening caused by a build up of the band boundaries and by a dislocation bowing (Orowan) stress. The model provides an explanation of observed orientation of the bands, their width and the significant change in the structural morphology seen as the band reorientation occurs at large strains. The predictions are in a favourable agreement with the available observations.     

Theoretical study on nanoindentation hardness measurement of a particle embedded in a matrix

The finite element method was used to simulate indentation tests on a particle embedded in a matrix, to investigate the influence of the properties of the particle and the matrix, and the indentation depth on the measured hardness. The particle’s work-hardening exponent and the mismatch in particle and matrix yield strength have a significant influence on the measured hardness. A particle-dominated indentation depth was identified, within which the measured nanoindentation hardness agrees very well with the true hardness of the particle material. Numerical results from the simulations of a wide range of material properties determined that the measured hardness is within 5% difference of the particle’s true hardness when the indentation depth is less than 13.5% of the particle’s radius. The results can be used in practice as a guideline to measure the hardness of a particle embedded in a matrix and provides the theoretical basis to develop a particle-embedded method to measure the hardness of individual particles.     

Investigation on dislocation-based mechanisms of void growth and coalescence in single crystal and nanotwinned nickels by molecular dynamics simulation

Abstract

Molecular dynamics simulations were conducted to elucidate dislocation mechanisms of the void growth and coalescence in single crystal and nanotwinned nickels subjected to uniaxial tension. The simulation results reveal that twin boundary is capable of decreasing the critical stress, suppressing the emission of dislocations and reducing the overall stiffness of the crystal. A size-scale dependence of critical stress is definitely illustrated through stress–strain response, where the larger void size leads to the lower critical stress and strain. It is the successive emissions of leading partials and the subsequent trailing partials that cause the atoms on the void surfaces to escape from the void surfaces continually, and consequently the voids grow to be larger and larger with increasing strain. The voids in the nanotwinned nickel coalesce earlier than those in the single crystal nickel even though the initiation of dislocations in the former is later than that in the latter. Void fraction remains a constant during elastic deformation, while it presents a linear increase with increasing strain during plastic deformation. Evolution of void fraction during void growth and coalescence is independent on void size.     

Deformation study of bicrystalline and nano-polycrystalline structures using phase field crystal method

Deformation behaviors of bicrystalline and nano-polycrystalline structures of various tilt angles and inclination angles in two dimensions are investigated in detail using a two-mode phase field crystal model.The interaction between grain boundary(GB)and dislocation is also examined in bicrystals and nano-polycrystals that both contain asymmetric and symmetric tilt GBs,with energy analysis being carried out to analyze these processes.During deformation simulations,we assume the volume of each simulation cell at every time step is coincident with that of the initial state just before deformation.Our simulation results show that the behaviors of symmetric and asymmetric GBs in bicrystals and nano-polycrystals differ from each other depending on tilt angle and inclination angle.A new dislocation emission mechanism of interest is observed in bicrystals which contain low angle symmetric tilt GBs.Low angle GB has a higher mobility relative to high angle GB in both bicrystalline and nano-polycrystalline structures,as does asymmetric GB to symmetric GB.The generation,motion,pileup and annihilation of dislocations,grain rotation and grain coalescence are observed,which is consistent with the simulation results obtained by molecular dynamics.These simulation results can provide strong guidelines for experimentation.     

镍基单晶高温合金界面位错网在剪切载荷作用下的演化

运用分子动力学方法,研究了镍基单晶高温合金γ/γ' 相界面错配位错网在剪切载荷作用下的演化特征.结果表明:(100),(110) 和 (111) 三种相界面形成的位错网在载荷作用下有不同形式和不同程度的损伤,其变形和损伤随温度的增加而增加.在相同的剪切载荷和温度作用下,(100) 相界面形成的正方形位错网最稳定. 关键词： 镍基单晶高温合金 界面位错网 分子动力学     

Explicit incorporation of cross-slip in a dislocation density-based crystal plasticity model

We present a crystal plasticity model that incorporates cross-slip of screw dislocations explicitly based on dislocation densities. The residence plane of screw dislocations is determined based on a probability function defined by activation energy and activation volume of cross-slip. This enables the redistribution of screw-dislocations and dislocation density patterning due to the effect of stacking fault energy. The formulation is employed for explaining the cross-slip phenomenon in aluminium during uniaxial tensile deformation of ?100? single crystal and a single slip orientation of single crystal, and compare the results with experimental observations. The effect of cross-slip on the stress–strain evolution is seen using this explicit treatment of cross-slip.     

Fracture of polycrystalline graphene membranes by in situ nanoindentation in a scanning electron microscope

Failure of polycrystalline graphene grown by chemical vapor deposition was investigated by nanoindentation in a scanning electron microscope. Circular graphene membranes were subject to central point loads using a nanomanipulator combined with an atomic force microscope cantilever as a force sensor. The grain boundaries of the polycrystalline graphene were visualized by Raman spectroscopy coupled with a carbon isotope labeling technique. Graphene membranes without any grain boundary had a failure strength of 45.4 ± 10.4 GPa, compared to 16.4 ± 5.1 GPa for those with grain boundaries when a Young's modulus was assumed to be 1 TPa. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Identification of elastic-plastic and phase transition characteristics for relaxor ferroelectric PMN-PT anisotropic single crystals using nanoindentation technique

As one kind of important ferroelectric ceramics, relaxor ferroelectric PMN-PT single crystals have triggered a revolution in electromechanical devices owing to their giant piezoelectric properties and ultra-high electromechanical coupling factors. The present study focused on the mechanical responses of [100]- and [110]-oriented poled PMN-PT ferroelectric single crystals under an indenter loading. The hardness and Young’s modulus with different crystallographic orientations of the crystals were measured by using the continuous stiffness measurement (CSM) with nanoindentation technique. Using a spherical indenter pressured at different indentation depths, the typical quasi-static nanoindentation tests with displacement-controlled mode were performed on the PMN-PT single crystal samples. Load–displacement curves of indentations were recorded to reveal the yielding or inelasticity behaviour in [100]- and [110]-oriented PMN-PT through a pop-in event. It was further verified by the stress–strain curves evaluated from the corresponding load–displacement curves, to show the similar characteristic on the elastic–inelastic transition. When a Berkovich indenter was employed for mechanical response testing, another pop-in event was observed at a smaller indentation depth compared to the one for elastic–inelastic transition, which may indicate a pressure-induced phase transition from rhombohedral (R) to tetragonal (T) of the PMN-PT single crystals.     

Direct observation of melted Mott state evidenced from Raman scattering in 1T-TaS_2 single crystal

The evolution of electron correlation and charge density wave(CDW)in 1T-TaS_2 single crystal has been investigated by temperature-dependent Raman scattering,which undergoes two obvious peaks of A_(1g) modes about 70.8 cm~(-1) and 78.7 cm~(-1) at 80 K,respectively.The former peak at 70.8 cm~(-1) is accordant with the lower Hubbard band,resulting in the electron-correlation-driven Mott transition.Strikingly,the latter peak at 78.7 cm~(-1) shifts toward low energy with increasing the temperature,demonstrating the occurrence of nearly commensurate CDW phase(melted Mott phase).In this case,phonon transmission could be strongly coupled to commensurate CDW lattice via Coulomb interaction,which likely induces appearance of hexagonal domains suspended in an interdomain phase,composing the melted Mott phase characterized by a shallow electron pocket.Combining electronic structure,atomic structure,transport properties with Raman scattering,these findings provide a novel dimension in understanding the relationship between electronic correlation,charge order,and phonon dynamics.     

诊断LiF单晶弹性变形的瞬态X射线衍射

利用瞬态X射线衍射技术对LiF单晶沿晶向［100］方向冲击加载的晶格变形进行了诊断研究。实验在神光Ⅱ装置的球形靶上进行,北四路激光驱动Cu靶获得的类He线作为X射线背光源,第九路为加载光源,对大小为7 mm7 mm、厚300 m的受激光加载的LiF单晶衍射,实验获得了LiF单晶晶面(200)压缩和未压缩状态的衍射信号。实验结果表明：LiF单晶在激光沿［100］方向冲击加载下,晶格发生了弹性变形,(200)晶面间距变小,衍射线上移,晶格压缩量为11%;该瞬态X射线衍射技术可用于冲击加载下的微观动态响应特性测量。     

Realistic problems in the physics of plasticity and hardness for single crystals and polycrystalline materials

The basic results from investigations of certain real problems in the physics of plasticity for single crystals and polycrystalline metal alloys carried out under the direction of the authors are given. The microdeformation patterns and formation of the flow limit in polycrystalline material are treated; the features of the mechanisms of deformation, deformational hardening, and the defect substructure in high-strength metal alloys are characterized. Analyses are carried out for phenomena involving activation of grain boundaries by grain boundary flows of impurity atoms, and experimentally based features of deformation on different structural levels under active extension, creep, and sign-alternating loading conditions. The main attention is given to the development of collective deformation modes. A discussion of some structural aspects of the realization of meso-level plastic flow with different deformation conditions is presented. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 5–15, August, 1998.     

Al纳米线不同晶向力学行为和变形机制的模拟

采用分子动力学模拟计算方法,考察具有较高层错能的Al纳米线沿不同晶向的力学行为和变形机制。在相同计算条件下与具有较低层错能的Ni、Cu、Au和Ag等FCC金属纳米线进行比较。结果表明：在力学行为方面,Al纳米线的弹性模量呈现明显的结构各向异性,满足E_[111] > E_[110] > E_[100]的关系,这一关系在FCC金属纳米线中普遍成立;Al纳米线的屈服应力随晶向呈现σ_y[100] > σ_y[111] > σ_y[110]的关系,这一关系在具有较低层错能的FCC金属纳米线中不具有普遍性,这与体系中位错形成机制密切相关。根据拉伸变形过程微观结构的演变规律,阐明Al纳米线不同晶向的变形机制,并与具有较低层错能的Ni、Cu、Au和Ag等FCC金属纳米线的变形机制进行比较。结果表明,对于尺度较小的高层错能Al纳米线,Schmid因子和广义层错能均难以准确预测其变形机制。     

单晶钨纳米线拉伸变形机理的分子动力学研究

利用分子动力学方法, 对本课题组率先采用金属催化的气相合成法制备出的高纯度单晶钨纳米线进行拉伸变形数值模拟, 通过分析拉伸应力-应变全曲线及其微观变形结构, 揭示出单晶钨纳米线的拉伸变形特征及微观破坏机理. 结果表明: 单晶钨纳米线的应力-应变全曲线可分为弹性阶段、损伤阶段、相变阶段、强化阶段、 破坏阶段等五个阶段, 其中相变是单晶钨纳米线材料强化的重要原因; 首次应力突降是由于局部原子产生了位错、孪生等不可逆变化所致; 第二次应力突降是发生相变的材料得到强化后, 当局部原子再次产生位错导致原子晶格结构彻底破坏而形成裂口、且裂口不断发展成颈缩区时, 材料最终失去承载能力而断裂. 计算模拟得到的单晶钨纳米线弹性模量值与实测值符合较好. 关键词： 分子动力学 应力应变曲线 微观机理 单晶钨纳米线     

Evidence of reduction in spin disorder by Ho3+ doping in La0.7Ca0.3MnO

We have shown in a recent study that substitution of Ho³⁺ ions (4f¹⁰; magnetic momen _μB) in La_0.7Ca_0.3MnO₃ causes significant reduction in electrical resistivity compared with Y³⁺ (4d⁰; non-magnetic) ion substitution. This reduction in resistivity was attributed to the reduced spin disorder scattering in La_0.7Ca_0.3MnO₃ samples containing magnetic Ho³⁺. We have estimated the Mn-spin canting angles in Ho³⁺ - and Y³⁺-doped La_0.7Ca_0.3MnO₃ compounds from the resistivity data using the magnetic localization model. We find that the canting angles of the Mn spins in the Ho³⁺ doped compounds are smaller than those obtained for the Y³⁺-doped compounds for all compositions and at all applied magnetic fields, showing clearly a reduction in the spin disorder in the former. The difference between the T _C values for Ho³⁺ - and Y³⁺-doped compounds for all compositions may be attributed to the presence of an internal field due to Ho³⁺ doping. This internal field may be responsible for the decrease in spin disorder in the Ho³⁺-doped compounds. The increase in the canting angles with increase in Ho³⁺ and Y³⁺ content could be attributed to the decrease in the strength of the ferromagnetic exchange interactions. A strong ferromagnetic coupling (as discussed recently by the present authors and co-workers) of Ho³⁺ moments with the Mn moments is responsible for the observed behaviour.     

Strengthening and softening in gradient nanotwinned FCC metallic multilayers

Plastic-deformation behaviors of gradient nanotwinned (GNT) metallic multilayers are investigated in nanoscale via molecular dynamics simulation. The evolution law of deformation behaviors of GNT metallic multilayers with different stacking fault energies (SFEs) during nanoindentation is revealed. The deformation behavior transforms from the dislocation dynamics to the twinning/detwinning in the GNT Ag, Cu, to Al with SFE increasing. In addition, it is found that the GNT Ag and GNT Cu strengthen in the case of a larger twin gradient based on more significant twin boundary (TB) strengthening and dislocation strengthening, while the GNT Al softens due to more TB migration and dislocation nucleation from TB at a larger twin gradient. The softening mechanism is further analyzed theoretically. These results not only provide an atomic insight into the plastic-deformation behaviors of certain GNT metallic multilayers with different SFEs, but also give a guideline to design the GNT metallic multilayers with required mechanical properties.