Contributions of different strengthening mechanisms to the shear strength of an extruded Mg–4Zn–0.5Ca alloy

The shear deformation behaviour of an extruded Mg–4Zn–0.5Ca alloy was studied using shear punch testing at room temperature. The extrusion process effectively refined the microstructure, leading to a grain size of 4.6 ± 1.4 μm. Contributions of different strengthening mechanisms to the room temperature shear yield stress, and overall flow stress of the material, were calculated. These mechanisms include dislocation strengthening, grain boundary strengthening, solid solution hardening and strengthening resulting from second-phase particles. Grain boundary strengthening and solid solution hardening made significant contributions to the overall strength of the material, while the contributions of second-phase particles and dislocations were trivial. The observed differences between calculated and experimental strength values were discussed based on the textural softening of the material.     

Magnetic studies of the dislocation structure of iron single crystals deformed at 295 K

The dislocation density in iron single crystals deformed at 295 K has been studied by measuring the coercive field, the initial susceptibility, the Rayleigh constant, and the reversible susceptibility in the approach to ferromagnetic saturation as functions of the resolved shear stress. The influence of different dislocation types on the saturation susceptibility has been calculated. In this way it is possible to distinguish dislocation structures composed of screw or edge dislocations and to reveal long-range internal stresses, which govern the work-hardening in the deformation stage II/III. The dislocation density increases in stage I linearly and in stage II/III quadraticaly with the resolved shear stress. In stage O mainly isolated screw dislocations are created.     

Edge dislocations in crystal structures considered as traveling waves in discrete models

The static stress needed to depin a 2D edge dislocation, the lower dynamic stress needed to keep it moving, its velocity, and displacement vector profile are calculated from first principles. We use a simplified discrete model whose far field distortion tensor decays algebraically with distance as in the usual elasticity. Dislocation depinning in the strongly overdamped case (including the effect of fluctuations) is analytically described. N parallel edge dislocations whose average interdislocation distance divided by the Burgers vector of a single dislocation is L>1 can depin a given one if N=O(L). Then a limiting dislocation density can be defined and calculated in simple cases.     

Thermal dislocation depinning in a deformed crystal

The temperature dependence of the dislocation mobility threshold is investigated on the basis of a dislocation model suggested by Frenkel-Kontorova. The critical value is obtained for the stress/temperature, corresponding to the dislocation depinning from its equilibrium position. The universal behaviour of a barrier height at finite stress and temperature is revealed and investigated.     

冲击加载下孔洞贯通的微观机理研究

利用分子动力学方法计算模拟了沿〈100〉晶向冲击加载下单晶铜中双孔洞的贯通过程.发现孔洞周围发射剪切型位错环是孔洞塌缩和增长的原因.在拉伸阶段,孔洞首先分别独立增长,随后其周围塑性变形区开始交叠和相互作用,最后两个孔洞开始直接贯通.这种贯通模式和实验对延性材料中孔洞贯通过程的显微观察结果一致.对四种不同θ值（θ为两个孔洞中心连线与冲击加载方向之间的夹角）的模型分别进行了计算模拟,发现在相同的冲击加载强度下,θ＝0°和θ＝30°的孔洞之间没有相互贯通; 关键词： 纳米孔洞 分子动力学 冲击加载 贯通     

Rubber friction on smooth surfaces

We study the sliding friction for viscoelastic solids, e.g., rubber, on hard flat substrate surfaces. We consider first the fluctuating shear stress inside a viscoelastic solid which results from the thermal motion of the atoms or molecules in the solid. At the nanoscale the thermal fluctuations are very strong and give rise to stress fluctuations in the MPa-range, which is similar to the depinning stresses which typically occur at solid-rubber interfaces, indicating the crucial importance of thermal fluctuations for rubber friction on smooth surfaces. We develop a detailed model which takes into account the influence of thermal fluctuations on the depinning of small contact patches (stress domains) at the rubber-substrate interface. The theory predicts that the velocity dependence of the macroscopic shear stress has a bell-shaped form, and that the low-velocity side exhibits the same temperature dependence as the bulk viscoelastic modulus, in qualitative agreement with experimental data. Finally, we discuss the influence of small-amplitude substrate roughness on rubber sliding friction.     

Void effect on mechanical properties of copper nanosheets under biaxial tension by molecular dynamics method

The relationship between void size/location and mechanical behavior under biaxial loading of copper nanosheets containing voids are investigated by molecular dynamics method. The void location and the void radius on the model are discussed in the paper. The main reason of break is discovered by the congruent relationship between the shear stress and its dislocations. Dislocations are nucleated at the corner of system and approached to the center of void with increased deformation. Here, a higher stress is required to fail the voided sheets when smaller voids are utilized. The void radius influences the time of destruction. The larger the void radius is, the lower the shear stress and the earlier the model breaks. The void location impacts the dislocation distribution.     

The evolution of persistent slip bands in copper single crystals

The transformation of the so-called matrix structure into persistent slip bands (PSBs) during the fatigue of copper single crystals has been investigated by transmission electron microscopy (TEM). By cyclic pre-deformation a saturated, hard matrix structure was established which is not capable of further hardening. A sudden increase of the applied amplitude of the resolved plastic shear strain initiated the transformation of the matrix structure into PSBs. The number of deformation cycles with enlarged amplitude of resolved plastic shear strain was increased from experiment to experiment in order to obtain crystals with PSBs in consecutive stages of evolution. Surface observations indicated strain localization well before first fragments of the typical ladder-like dislocation pattern of PSBs could be identified in the bulk. From our experiments, we conclude that the transformation from the matrix structure into PSBs very likely starts from the centers of the veins which exhibit small dislocation-poor, soft areas. These areas are enclosed by a harder shell, where a high dislocation density is maintained and which may develop into first dislocation walls. During the evolution of PSBs the frequency distribution of the wall spacings narrows. This indicates that a shift of dislocation walls (1–2 nm/cycle) plays an important rôle in establishing the typical regular ladder-like dislocation pattern of well-developed PSBs.     

The stress-velocity relationship of twinning partial dislocations and the phonon-based physical interpretation

The dependence of dislocation mobility on stress is the fundamental ingredient for the deformation in crystalline materials. Strength and ductility, the two most important properties characterizing mechanical behavior of crystalline metals, are in general governed by dislocation motion. Recording the position of a moving dislocation in a short time window is still challenging, and direct observations which enable us to deduce the speed-stress relationship of dislocations are still missing. Using large-scale molecular dynamics simulations, we obtain the motion of an obstacle-free twinning partial dislocation in face centred cubic crystals with spatial resolution at the angstrom scale and picosecond temporal information. The dislocation exhibits two limiting speeds: the first is subsonic and occurs when the resolved shear stress is on the order of hundreds of megapascal. While the stress is raised to gigapascal level, an abrupt jump of dislocation velocity occurs, from subsonic to supersonic regime. The two speed limits are governed respectively by the local transverse and longitudinal phonons associated with the stressed dislocation, as the two types of phonons facilitate dislocation gliding at different stress levels.     

Change in electrical resistivity of an Al-4% Cu alloy during plastic deformation

The paper reports the results of measuring the change in electrical resistivity of an Al-4% Cu alloy containing Guinier-Preston zones during plastic extension at various temperatures and strain rates. The deformation temperature has a marked influence on the nature of the observed effect. The results obtained are discussed on the assumption that there are two different mechanisms by which plastic deformation affects the resistivity of the alloy: a diffusionless mechanism (subdivision of the zones as a result of their being cut by moving dislocations) and a diffusion mechanism (supplementary decomposition and a redistribution of the zones with respect to the volume of the alloy and the zone size).     

冲击加载下铝中氦泡和孔洞的塑性变形特征研究

通过分子动力学模拟研究了在相同冲击加载强度下单晶铝中氦泡和孔洞的塑性变形特征,结果发现氦泡和孔洞的塌缩是由发射剪切型位错环引起的,而没有观测到棱锥型位错环发射. 氦泡和孔洞周围的位错优先成核位置基本一致,但是氦泡周围发射的位错环数目比孔洞多,位错环发射速度明显比孔洞快. 且氦泡和孔洞被冲击波先扫过部分比后扫过部分发射位错困难. 通过滑移面上的分解应力分析发现,氦泡和孔洞周围塑性特征的差别是由于氦泡内压引起最大分解应力分布改变造成的. 氦泡和孔洞被冲击波先后扫过部分塑性不对称是因为冲击波扫过时引起形状变化, 关键词： 分子动力学 冲击波 氦泡 孔洞     

A disclination model for twinning and de-twinning of nanotwinned copper

The twinning and de-twinning processes within grains of nanotwinned copper (nt-Cu) are schematically demonstrated using the concept of wedge disclination quadrupoles. The stable twin nucleus size and the equilibrium equation of the applied shear stress and twin width during twin growth are obtained. The dependence of the critical resolved shear stress for twinning on the grain size, which conforms to the classic Hall–Petch relationship, is theoretically modelled. Additionally, the disclination quadrupole model for de-twinning is used to interpret the strength softening in nt-Cu. Relative to the classic kinetic and energetic models, this novel approach is more compatible with the experiments.     

Evaluating the effects of loading parameters on single-crystal slip in tantalum using molecular mechanics

This study is aimed at developing a physics-based crystal plasticity finite element model for body-centred cubic (BCC) metals, through the introduction of atomic-level deformation information from molecular dynamics (MD) investigations of dislocation motion at the onset of plastic flow. In this study, three critical variables governing crystal plasticity mediated by dislocation motion are considered. MD simulations are first performed across a range of finite temperatures up to 600K to quantify the temperature dependence of critical stress required for slip initiation. An important feature of slip in BCC metals is that it is not solely dependent on the Schmid law measure of resolved shear stress, commonly employed in crystal plasticity models. The configuration of a screw dislocation and its subsequent motion is studied under different load orientations to quantify these non-Schmid effects. Finally, the influence of strain rates on thermal activation is studied by inducing higher stresses during activation at higher applied strain rates. Functional dependence of the critical resolved shear stress on temperature, loading orientation and strain rate is determined from the MD simulation results. The functional forms are derived from the thermal activation mechanisms that govern the plastic behaviour and quantification of relevant deformation variables. The resulting physics-based rate-dependent crystal plasticity model is implemented in a crystal plasticity finite element code. Uniaxial simulations reveal orientation-dependent tension–compression asymmetry of yield that more accurately represents single-crystal experimental results than standard models.     

Atomic simulations to evaluate effects of stacking fault energy on interactions between edge dislocation and spherical void in face-centred cubic metals

In this study, molecular dynamics simulations were performed to elucidate the effects of stacking fault energy (SFE) on the physical interactions between an edge dislocation and a spherical void in the crystal structure of face-centred cubic metals at various temperatures and for different void sizes. Four different types of interaction morphologies were observed, in which (1) two partial dislocations detached from the void separately, and the maximum stress corresponded to the detachment of the trailing partial; (2) two partial dislocations detached from the void separately, and the maximum stress corresponded to the detachment of the leading partial; (3) the partial dislocations detached from the void almost simultaneously without jog formation; and (4) the partial dislocations detached from the void almost simultaneously with jog formation. With an increase in void size or SFE, the interaction morphology changed in the above-mentioned order. It was observed that the magnitude of the critical resolved shear stress (CRSS) and its dependence on the SFE were determined by these interaction morphologies. The value of the CRSS in the case of interaction morphology (1) is almost equal to an analytical one based on the linear elasticity by employing the Burgers vector of a single partial dislocation. The maximum value of the CRSS is also obtained by the analytical model with the Burgers vector of the two partial dislocations.     

Screw dislocation in semi-infinite non-local hexagonal medium

Abstract

Nonlocal stresses of a screw dislocation near a free surface in a semi-infinite hexagonal medium are investigated by a surface dislocation model. The nonlocal image force on the screw dislocation due to the existing free surface is also obtained. All classical singularities for the stress and image force are eliminated. The maxima of the stress and image force are evaluated. A zero point of the stress is found, which predicts that different states of the shear stress exist simultaneously near the dislocation. The appearance of a zero value at the free surface and a maximum of the dislocation image force can be used to explain the existence of the dislocation free zone.     

单晶铜纳米线屈服机理的原子模拟研究

采用分子静力学方法模拟了〈100〉单晶铜纳米线的拉伸变形过程,研究了纳米线屈服的机理. 结果表明：1） 纳米线初始屈服通过部分位错随机激活的{111}〈112〉孪生实现,后继屈服通过{111}〈112〉部分位错滑移实现;2） 纳米线变形初期不同滑移面上的部分位错在两面交线处相遇形成压杆位错,变形后期部分位错在刚性边界处塞积,两者都阻碍位错滑移,引起一定的强化作用. 关键词： 纳米线 屈服 位错 分子静力学     

Micromechanistic origin of irradiation-assisted stress corrosion cracking

A multi-scale study of the micromechanics of dislocation–grain boundary interactions in proton and ion-irradiated stainless steels is presented. Interactions of dislocation channels with grain boundaries result in slip transfer, discontinuous slip without or with slip along the grain boundary. The presence of the irradiation damage enhances the importance of the magnitude of the resolved shear stress on the slip system activated by the grain boundary to transfer slip across it. However, the selected slip system is still determined by the minimization of the grain boundary strain energy density condition. These findings have implications for modelling the mechanical properties of irradiated metals as well as in establishing the mechanism for disrupting the grain boundary oxide, which is a necessary prerequisite for irradiation-assisted stress corrosion cracking.     

晶体塑性有限元方法研究辐照对多晶铜力学性能的影响

将辐照硬化理论与晶体塑性理论结合, 运用ABAQUS有限元分析软件模拟辐照后多晶铜的拉伸过程。分析辐照效应对材料屈服强度、硬化过程、晶体变形等力学性能的影响, 研究位错密度的演化及空间分布规律。数值模拟表明: 辐照效应提高多晶铜的屈服应力, 影响不同阶段的硬化和软化现象; 辐照剂量增大导致位错密度增殖总体变缓, 空间不均匀度增大; 晶体的塑性变形及晶体转动也受到辐照的影响, 在相同的应变条件下, 辐照剂量越大, 晶体塑性变形程度越小, 塑性变形分布不均匀度变大, 同时晶体转动程度及转动角离散度增大。     

Contribution from dislocation to deformation resistance in polycrystals of copper-based solid solutions

The evolution of dislocation structure in solid solutions of Cu-Al and Cu-Mn systems with different grain sizes and at different test temperatures is studied by means of transmission electron diffraction microscopy. The scalar density of dislocations is measured and its relationship to the flow stress of alloys is determined. Changes in the contribution from dislocation hardening to deformation resistance upon variations in the contributions associated with changes in grain size, solid-solution hardening, and test temperature are analyzed.     

Molecular dynamics simulation on the elastoplastic properties of copper nanowire under torsion

Influences of different factors on the torsion properties of single crystal copper nanowire are studied by molecular dynamics method. The length, torsional rate, and temperature of the nanowire are discussed at the elastic-plastic critical point. According to the average potential energy curve and shear stress curve, the elastic-plastic critical angle is determined. Also, the dislocation at elastoplastic critical points is analyzed. The simulation results show that the single crystal copper nanowire can be strengthened by lengthening the model, decreasing the torsional rate, and lowering the temperature. Moreover, atoms move violently and dislocation is more likely to occur with a higher temperature. This work mainly describes the mechanical behavior of the model under different states.