冲击条件下含纳米空洞的单晶铜的塌缩

利用分子动力学方法研究了单晶铜中不同大小的球形空洞在冲击波下的演化过程.模拟结果表明不同大小空洞的塌缩过程不同.模拟中冲击波由空洞左边扫向空洞右边.在较大尺寸的空洞塌缩过程中会产生系列的位错环.当空洞半径较小时,先在空洞的右侧形成位错环,当空洞半径增大到某一临界大小时,在空洞左右两侧同时产生位错环,当空洞半径较大时,先在空洞左侧形成位错环.当空洞左右两侧的位错环均形成以后,其右侧位错环前端的生长速度大于其左侧的.空洞半径增大,相应的位错环前端的生长速度变化不大.当空洞半径增大时,空洞中心指向位错源的矢量方 关键词： 纳米空洞 位错环 冲击波 塑性变形     

Effects of void shape and orientation on the elastoplastic properties of spheroidally voided single-crystal and nanotwinned copper

ABSTRACT

Void nucleation, growth and coalescence have been identified as the leading cause of ductile damage in metallic materials. To understand the underlying deformation and damage mechanisms, extensive theoretical, experimental and simulation efforts have been attempted on spherically voided metals. In this work, molecular dynamics simulations are performed to analyze the uniaxial straining deformation behaviours of both single-crystal and nanotwinned copper materials embedded with a preexisting spheroidal void. The coupling effects among twin boundary, spheroidal void aspect ratio and orientation on unidirectional elastoplastic behaviours are systematically examined. The dislocation-induced plastic deformation mechanism is also examined and compared with the one due to a perfectly spherical cavity. Simulation results show that elastic modulus increases with both spheroidal void aspect ratio and orientation. So do the yield stress, the first peak stress and the plasticity index. Another peak stress exists for most cases, except for a prolate void embedded in nanotwinned specimens. The slope between peak stresses decreases with both the spheroidal aspect ratio and orientation. The incorporation of a twin boundary results in lower elastic modulus, higher yield strength and smaller plasticity index. For an oblate void, the twin boundary gives rise to more severe strain softening behaviour. The dislocation extraction algorithm illustrates that the continuous nucleation, propagation and reaction of dislocations emanated from both the void front and twin boundary are responsible for the ductile damage of spheroidally voided crystals. The lower dislocation densities found in nanotwinned specimens indicate the desired suppression effects of twin boundary on dislocation activities.     

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.     

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.     

Atomic collision cascades on void evolution in vanadium

Molecular dynamics simulations were performed to study void evolution subject to unidirectional self-bombardment and radiation-induced variation of mechanical properties in single crystalline vanadium. 3D simulation cells of perfect body-centered cubic (BCC) vanadium, as well as those with one, two, four, and six voids, were investigated. For the no void case, the maximum number of defects, maximum volumetric swelling, and the number of defects left in bulk after a sufficiently long recovery period increased with higher primary recoil energy. For the cases containing voids, a primary recoil energy was carefully assigned to an atom so as to initiate a dense collision spike in the voids center, where some self-interstitial atoms gained kinetic energy by secondary replacement collision sequence traveling along the ? 111? direction. It is found that the larger or the greater the number of voids contained initially in the box, the larger the normalized void volume, and the smaller the volumetric swelling after sufficient recovery of systems. In the single void case, the void became elongated along the bombarding direction; in the multiple void cases, the voids coalesced only when the intervoid ligament distance was short. After sufficient relaxation of the irradiated specimen, a hydrostatic tension was exerted on the box, where the voids were treated as dislocation sources. It is shown that with higher primary recoil energy, the yield stress dropped in cases with smaller or fewer voids but rose in those with larger or greater number of voids. This radiation-induced softening to hardening transition with increasing dislocation density can be attributed to the combined effects of the defect-induced dislocation nucleation and the resistance of defects to dislocation motion. Moreover, as the primary recoil energy increased, the ductility of vanadium in the no void case decreased, but was only slightly changed in the cases containing void.     

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.     

Void growth in single crystal Copper-an atomistic modeling and statistical analysis study

One of the failure mechanisms in ductile materials is growth and coalescence of pre-existing voids. In view of this, we attempt to obtain atomistic insights into the prevailing mechanisms of void growth in a representative ductile material, namely Copper, using molecular dynamics simulations. In addition to shedding light on the observed length scale effects and dislocation mechanisms, we also elucidate how atomistic simulations can inform continuum-based models of failure and provide fodder for bridging different length scales. By performing a series of over 150 molecular dynamics simulations, we also try to decode the interplay between mechanical properties and void growth, and investigate the role of heterogeneity in void distribution (in terms of void size and placement) in affecting the strength of the material. Coupled with a comprehensive global sensitivity analysis technique, we explore configuration–property relationships in a subset of vast parameter space and highlight the importance of random nature of void distribution (along with some critical statistical parameters) in any successful theory of fracture.     

高应变率压缩下纳米孔洞对金属铝塑性变形的影响研究

本文利用分子动力学模拟方法研究了含纳米孔洞金属铝在[110]晶向高应变率单轴压缩下弹塑性变形的微观过程. 对比单孔洞和完整单晶的模型, 讨论了多孔金属的应力应变关系及其位错发展规律. 研究结果表明, 对于多孔模型的位错积累过程, 位错密度随应变的增加可大致分为两个线性阶段. 由同一个孔洞生成的位错在相互靠近过程中, 其滑移速度越来越小; 随着位错继续滑移, 源自不同孔洞的位错之间开始交叉相互作用导致应变硬化. 达到流变峰应力之后又由于位错密度增殖速率升高发生软化. 当应变增加到11.8%时, 所有孔洞几乎完全坍缩, 并观察到在此过程中有棱位错生成.     

Post-yielding dislocation retraction of nano-lamellar TiAl single crystals

The perfect single crystal has ultra-high strength but is often accompanied by catastrophic failures after yielding. This study reveals that nano-lamellar TiAl single crystals alleviate the catastrophic failure due to a post-yielding dislocation retraction through atomistic simulations and theoretical analyses. This dislocation retraction leads to a retained post-yielding strength of1.03 to 2.33 GPa(about 50% of the yielding strength). It is shown that this dislocation retraction is caused by local stress relaxation and interface-mediated image force. The local stress relaxation is due to successive dislocation nucleation in different slip systems, and the interface-mediated image force is caused by the heterogeneous interface. Based on dislocation theory, this study demonstrates that the size effect also plays a vital role in dislocation retraction. Theoretical modeling shows that the dislocation retraction occurs when the lamellar thickness is less than approximately 12 nm. Additionally, the post-yielding dislocation retraction is more pronounced at higher temperatures, making it more effective in alleviating catastrophic failures.These findings demonstrate a viable option for avoiding catastrophic failure of single crystals through nanoscale-lamellar design.     

沿〈111〉晶向冲击加载下铜中纳米孔洞增长的塑性机制研究

 用分子动力学方法计算模拟了沿〈111〉晶向冲击加载过程中，单晶铜中纳米孔洞（直径约1.3 nm）的演化及其周围区域发生塑性变形的过程。模拟结果表明，在沿〈111〉晶向冲击加载后，在面心立方（fcc）结构中的4族{111}晶面中有3族发生了滑移。伴随孔洞的增长，在所激活的3族{111}晶面上，观察到位错在孔洞表面附近区域成核，然后向外滑移，其中在剪切应力最大的〈112〉方向上，其位错速度超过横波声速，其它〈112〉方向的位错速度低于横波声速。模拟得到的位错阻尼系数范围与实验值基本符合。由于孔洞周围产生的滑移在空间比较对称，孔洞增长形貌接近球形。在恒定的冲击强度下，孔洞半径增长速率近似保持恒定，其速率随着冲击强度的增加而增大。     

Misfit dislocation loops in composite nanowires

A new mechanism for relaxation of misfit stresses in composite nanowires (quantum wires) is suggested and theoretically examined, namely the formation of misfit dislocation loops. The stress field of a prismatic dislocation loop in a cylinder (nanowire) is calculated. The parameters of two-phase composite nanowires at which the formation of misfit dislocation loops is energetically favourable are estimated. The effect of stress fields of dislocation loops on the formation of compositionally modulated nanowires is discussed.     

Effect of void size and Mg contents on plastic deformation behaviors of Al-Mg alloy with pre-existing void: Molecular dynamics study

The plastic deformation properties of cylindrical pre-void aluminum-magnesium (Al-Mg) alloy under uniaxial tension are explored using molecular dynamics simulations with embedded atom method (EAM) potential. The factors of Mg content, void size, and temperature are considered. The results show that the void fraction decreases with increasing Mg in the plastic deformation, and it is almost independent of Mg content when Mg is beyond 5%. Both Mg contents and stacking faults around the void affect the void growth. These phenomena are explained by the dislocation density of the sample and stacking faults distribution around the void. The variation trends of yield stress caused by void size are in good agreement with the Lubarda model. Moreover, temperature effects are explored, the yield stress and Young's modulus obviously decrease with temperature. Our results may enrich and facilitate the understanding of the plastic mechanism of Al-Mg with defects or other alloys.     

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     

面心立方铜晶体中空洞生长与贯通的尺寸效应

利用分子动力学方法模拟沿拉伸方向排布的两个空洞在单轴拉伸作用下的动力学行为.着重研究不同尺寸空洞对其拉伸贯通过程的影响.结果表明,不同尺度的空洞都是通过空洞表面发射位错环长大与贯通的.空洞在弹性阶段沿加载方向缓慢长大,在塑性阶段沿垂直方向生长后形成类八面体形状.随空洞尺寸的减小,临界屈服应力逐渐增大.当半径较大时,位错对称成核、迁移,空洞沿加载方向被拉长,演化过程相似;当半径较小时,位错不对称成核,空洞沿垂直方向被拉长.空洞生长分为弹性变形、独立长大、融合贯通和平稳生长四个阶段.独立生长阶段随尺寸的减小逐渐缩短甚至消失.     

Change in the plastic properties of ionic crystals under UV irradiation

The effect of UV irradiation on the dislocation motion in ionic crystals and relaxation growth of elastic twin in Iceland spar has been investigated. It is shown that UV radiation causes stress relaxation in crack tips, which is related to a change in the dislocation structure. It is suggested that the observed effects are based on the interaction of dislocations with low-energy excitons.     

Harnessing atomistic simulations to quantify activation parameters for dislocation nucleation from a grain boundary in Nickel

We perform atomistic simulations and nudged elastic band calculations to quantify activation energy barriers for dislocation nucleation from ∑3 grain boundary containing a pre-existing void in Ni. By changing the void size, we offer a phenomenological relationship between the activation free energy at zero stress and boundary porosity. Simulations at different temperatures are also conducted to gain some insights into the inherent intricacies of activation energy landscape. It is envisioned that the approach can be pushed forward to guide the flow rules of physics based crystal plasticity models at higher length scales that account for grain boundary effects.     

Dipole of edge misfit dislocations and critical radius conditions for buried strained cylindrical inhomogeneity

A theoretical model is proposed for elastic stress relaxation of a buried strained cylindrical inhomogeneity, which assumes the edge misfit dislocation dipole formation in the soft matrix at some distance from the interface. The critical radius of the inhomogeneity for the formation of the edge misfit dislocation dipole is given and the influence of various parameters on the critical radius is evaluated. The result indicates that the critical radius decreases with increasing misfit strain and core radius of the misfit dislocation. It is also found that, compared to the edge misfit dislocation dipole formation in the interface, the critical radius of the inhomogeneity decreases when the location of an edge misfit dislocation dipole formation is in the soft matrix at some distance from the interface.     

On the homogeneous nucleation and propagation of dislocations under shock compression

The dynamic response of crystalline materials subjected to extreme shock compression is not well understood. The interaction between the propagating shock wave and the material’s defect occurs at the sub-nanosecond timescale which makes in situ experimental measurements very challenging. Therefore, computer simulation coupled with theoretical modelling and available experimental data is useful to determine the underlying physics behind shock-induced plasticity. In this work, multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper reported at ultra-high strain rates shock loading. We compare the value of threshold stress for homogeneous nucleation obtained from elastodynamic solution and standard nucleation theory with MDDP predictions for copper single crystals oriented in the [0 0 1]. MDDP homogeneous nucleation simulations are then carried out to investigate several aspects of shock-induced deformation such as; stress profile characteristics, plastic relaxation, dislocation microstructure evolution and temperature rise behind the wave front. The computation results show that the stresses exhibit an elastic overshoot followed by rapid relaxation such that the 1D state of strain is transformed into a 3D state of strain due to plastic flow. We demonstrate that MDDP computations of the dislocation density, peak pressure, dynamics yielding and flow stress are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models. MDDP-based models for dislocation density evolution, saturation dislocation density, temperature rise due to plastic work and strain rate hardening are proposed. Additionally, we demonstrated using MDDP computations along with recent experimental reports the breakdown of the fourth power law of Swegle and Grady in the homogeneous nucleation regime.     

Molecular dynamics study of void effect on nanoimprint of single crystal aluminum

Pre-existing defects can alter mechanical behavior of materials significantly under applied load. In current study molecular dynamics (MD) simulations are performed to reveal pre-existing void effect on nanoimprint of single crystal Al thin films, such as deformation mechanism and spring back phenomenon. Current simulation results show void acts as strong barrier to dislocation motion, although plastic deformation is dominantly controlled by dislocation activities. It indicates the void volume fraction has strong influence on nanoimprint: the larger the void volume fraction, the smaller the maximum force required for initial dislocation nucleation, and the stronger the interaction between extended dislocation and void. It also demonstrates that there is a critical void volume fraction for minimum spring back, which is resulted from competition between two roles affecting dislocation annihilation.