Molecular dynamics simulation of the role of dislocations in microcrack healing

The molecular dynamics method is used to simulate microcrack healing during heating or/and under compressive stress. A centre microcrack in Cu crystal would be sealed under compressive stress or by heating. The role of compressive stress and heating in crack healing was additive. During microcrack healing, dislocation generation and motion occurred. When there were pre-existing dislocations around the microcrack, the critical temperature or compressive stress necessary for microcrack healing would decrease, and, the higher the number of dislocations, the lower the critical temperature or compressive stress. The critical temperature necessary for microcrack healing depended upon the orientation of the crack plane. For example, the critical temperature for the crack along the (001) plane was the lowest, i.e. 770K. The project supported by the Special Fund for the Major State Basic Research Projects (No. G19990650) and by the National Natural Science Foundation of China (No. 19891180, 59871010)     

纳米晶铜单向拉伸变形的分子动力学模拟

纳米材料是由尺度在1－100nm的微小颗粒组成的体系，由于它具有独特的性能而备受关注。本文简要地回顾了分子动力学在纳米材料研究中的应用，并运用它模拟了平均晶粒尺寸从1．79－5．38nm的纳米晶体的力学性质。模拟结果显示：随着晶粒尺寸的减小，系统与晶粒内部的原子平均能量升高，而晶界上则有所下降；纳米晶体的弹性模量要小于普通多晶体，并随着晶粒尺寸的减小而减小；纳米晶铜的强度随着晶粒的减小而减小，显示了反常的Hall-Petch效应；纳米晶体的塑性变形主要是通过晶界滑移与运动，以及晶粒的转动来实现的；位错运动起着次要的、有限的作用；在较大的应变下（约大于5％），位错运动开始起作用；这种作用随着晶粒尺寸的增加而愈加明显。     

Molecular dynamics study of size, temperature and rate dependent thermomechanical properties of copper nanofilms

Molecular dynamics simulations are performed to study the thermomechanical properties of copper nanofilms at different temperatures and extremely-high loading rates. The results show a drastic temperature softening effect on the film strength and modulus. The increase of strain rate could result in a much higher strength while the modulus is relatively less affected. It is shown, based on the stress results, that the observed “smaller is softer” and “smaller is stronger” behaviors of nanofilms might be due to the surface plasticity and the volumetric dislocations, respectively. It is also found that the thinner a nanofilm, the smaller the thermal expansion coefficient. The present work reveals that the quasistatic thermomechanical properties of bulk copper at room temperature might be inadequate for the continuum-based study of thermomechanical response of copper nanofilms due to ultrafast laser heating.     

Molecular dynamics studies on the dislocation gliding near a tilt boundary

The gliding behavior of edge dislocation near a grain boundary (GB) in copper under pure shear stresses is simulated by using molecular dynamics(MD) method. Many-body potential incorporating the embedded atom method (EAM) is used. The critical shear stresses for a single disocation to pass across GB surface are obtained at values of σ _c =23MPa ∼ 68 MPa and 137MPa∼274 MPa for Ω=165 small angle tilt GB at 300K and 20K, respectively. The first result agrees with the experimental yield stress σ _y (=42MPa) quite well. It suggests that there might be one of the reasons of initial plastic yielding caused by single dislocation gliding across GB. In addition, there might be possibility to obtain yield strength from microscopic analysis. Moreover, the experimental value of σ _y at low temperature is generally higher than that at room temperature. So, these results are in conformity qualitatively with experimental fact. On the other hand, the Ω=25 GB is too strong an obstacle to the dislocation. In this case, a dislocation is able to pass across GB under relatively low stress only when it is driven by other dislocations. This is taken to mean that dislocation pile-up must be built up in front of this kind of GB, if this GB may take effect on the process of plastic deformation. The project supported by KM85-33 of Academia Sinica and the National Natural Science Foundation of China     

Molecular dynamics simulation of deformation and failure of nanocrystals of bcc metals

Simulations of uniaxial and hydrostatic tension of Fe and Mo nanocrystal are made by molecular dynamics method. Stress versus strain are obtained while regularities of lattice rearrangement during nanocrystal plastic deformation are considered. Local instability of nanocrystal lattice, which is the cause for transition from elastic to plastic deformation of nanocrystal, is found. It is shown that local shear stresses is a driving force of nanocrystal lattice rearrangements under the conditions of both uniaxial and hydrostatic tension, so, local instability of nanocrystal of bcc metals should be considered as shear instability. Realization of “orthorhombic” path of deformation at 1 0 0 tension of Mo nanocrystal is specific case of above effect. It is demonstrated that unlike covalent nanocrystal, metallic nanocrystals display “heterogeneous” mechanism of crack nucleation, which essence is that cracks nucleate not in homogeneous elastically deformed lattice but in shear bands or near their boundaries, i.e., after non-homogeneous plastic deformation of nanocrystal.     

Molecular dynamics simulation of polycrystalline copper

An approach for molecular dynamics simulation of the formation of polycrystalline materials from a melt during its cooling is proposed. Atomic configurations of copper corresponding to polycrystals with the mean grain size from 2 to 16 nm are obtained. Isothermal uniaxial tension and compression of these polycrystals is studied by the molecular dynamics method. For the mean grain size of polycrystalline copper being smaller than 10 nm, it is shown that Young’s modulus and yield stress decrease as the grain size decreases. Shock adiabats for polycrystalline copper are constructed. For a material with the grain size approximately equal to 2 nm, the temperature behind the shock wave front is demonstrated to be 10% higher than that in a polycrystal with the grain size greater than 10 nm. Molecular dynamics calculations predict the presence of copper with a body-centered cubic lattice behind the shock wave front at pressures ranging from 100 to 200 GPa.     

Experimental GH singularity field around growing crack-tip

The displacement fieldsu _x ,u _y at growing crack tip of LY12-M specimens with double edge cracks are measured using moire method. The experimental singularity fields are compared with GH theoretical field [12–14]. The size and shape of the experimental GH singularity fields are obtained. The error in both the experimental and theoretical evaluations is controlled within ±10%. The experiments show that there is singularity dominant around a growing crack tip. The shape of this dominant region ranges from butterfly wing to oblate and circular. Inside GH-field, there is a 3-D deformed damage zone where no GH singularity exists. The project suppoted by National Natural Science Foundation of China     

Molecular dynamics study of deformation and fracture of Bi-segregated copper bicrystals

The microprocesses of deformation and fracture of Bi-segregated copper bicrystals Σ33 ( ) 58.99°, Σ11 ( ) 50.48° and Σ9 ( ) 38.94° have been simulated by molecular dynamics in order to study the relationship between the grain boundary embrittlement (GBE) and grain boundary (GB) structure. It is shown that GBE is related to the segregated concentration and distribution of Bi atoms, while Bi segregation is related to the GB structure. Due to their different structures, the bicrystals Σ33, Σ11 and Σ9 show an increasing propensity for Bi segregated concentration. So under the action of external force, Σ33, Σ11 and Σ9 show transgranular ductile, intergranular tearing and intergranular brittle fracture modes, respectively. The subject supported by the Chinese Academy of Sciences and National Natural Science Foundation of China     

Molecular dynamics simulations of DFZ

Dislocation emission from the crack tip in copper under mode II loading is simulated with molecular dynamics method. After 26 partial dislocations are emitted and then relaxed to reach the equilibrium under the constant displacement, the double pile-ups (including an inverse pile-up and a pile-up) are formed. i.e., the first dislocation is piled up before the obstruction, and the last dislocation is piled up ahead of the crack tip. These results conform to the TEM observations. The project supported by the National Natural Science Foundation of China     

飞秒激光辐照铝材料的分子动力学数值模拟

采用大尺度分子动力学方法对飞秒激光辐照金属铝材料的效应进行了数值模拟。利用分子动力学方法给出了飞秒激光辐照后,材料表面发生熔化和喷溅,冷凝后形成一层很薄的多孔介质的物理图像,及产生的应力波传播过程等。数值模拟结果表明分子动力学方法可以用于飞秒激光对材料辐照效应的研究。     

位错、裂纹与断裂

本文评述近些年来在位错、裂纹与断裂方面的研究进展.内容主要包括:①以屈服强度温度依赖性为例,讨论力学性能的结构敏感性,使其对位错芯结构与塑性之间的关系有一定的认识;②纯弹性断裂总是极少的,因之在加载应力作用下,裂纹顶端总是存在着或大或小的塑性区,存在着裂纹与位错的交互作用.这里以我们自己的工作为主,重点讨论裂端位错过程的研究结果.      

Experimental investigation of GH singularity field of the growing crack-tip for different ductile metals

The displacementsu _x ,u _y inx, y directions at growing crack tip of the specimens with double edge cracks for four different alumium alloys and two coppers are measured by using moire method and optical spatial filtering technique. From experimental displacement fields, the displacement singularity fields are obtained and compared with GH theoretical field. Unknown constantsA and _yo in theoretical solution are determined from experimental data. The theoretical singularity field thus compared is given for plane-stress, mode-I, strain-hardening materials. The error in both the experimental and theoretical evaluations is within ± 10%. The experiments show that there exists dominant singularity region around a growing crack-tip. In the experiments, the strain hardening indexn amounts from 3.158 to 14. The shape of this dominant region ranges from butterfly wing to oblate or circular shape. The size and shape of GH dominant region depend on the material property, the specimen geometry and loading type. Inside GH-field, there is a 3-D deformed damage zone, where no GH singularity exists. Very near to the crack-tip, there is a fracture process zone.The project is supported by the National Science Foundation of China.     

原子尺度摩擦行为的分子动力学模拟

应用大规模分子动力学方法,模拟了具有原子级光滑和原子级粗糙形貌的刚性球形探头与弹性平面基体的干摩擦行为,研究了无/有粘附条件下的载荷与摩擦力、载荷与真实接触面积,以及摩擦力与真实接触面积之间的关系,对纳米尺度下的摩擦行为规律进行了分析.几种系统的真实接触面积-载荷关系都与相应的连续力学接触模型定性的一致,它们分别是Hertz光滑表面接触模型、Greenwood-Williamson粗糙表面接触模型和Mau-gis Dugdale粘着接触模型.无论是由光滑表面还是粗糙表面构成的摩擦系统,在无粘附条件下摩擦力与载荷成正比,而摩擦力与真实接触面积之间没有一个简单的关系;在粘附条件下摩擦力与真实接触面积成正比,而摩擦力与载荷之间表现为Maugis Dugdale模型预测的亚线性关系.研究表明,当表面作用从无粘附到粘附时,控制摩擦力的决定因素从载荷转变为接触面积,摩擦行为从载荷控制摩擦转变为粘着控制摩擦.     

Molecular dynamics simulation of entropy and surface tension for grain boundary of α-Fe

The grain boundary is an interface and the surface tension is one of its important thermodynamic properties. In this paper, the surface tension of the ∑9 grain boundary for α-Fe at various temperatures and pressures is calculated by means of Computer Molecular Dynamics (CMD). The results agree satisfactorily with the experimental data. It is shown that the contribution of entropy to surface tension of grain boundary can be ignored. The project supported by the National Natural Science Foundation of China and the Science Foundation of Chinese Academy of Sciences.     

Molecular dynamical study of the energy relaxation processes after heating the vibrational degree of freedom in a diatomic molecular crystal

Energy relaxation processes initiated by sudden heating of the vibrational degree of freedom were studied with molecular dynamical method. A unit cell of bee structure containing 128 diatomic molecules with periodic boundary conditions was considered. Compound Morse potential was assumed as the atom-atom interactions. It was found that the logarithra of the equilibration time depends linearly upon a factorf ₂₁ which is proportional to the frequency ratio of the intra- and inter-molecular vibrations. The project is supported by the National Natural Science Foundation of China.     

分子动力学模拟含不同初始缺陷单晶镍的断裂行为与应力演化

断裂是一个跨尺度复杂的物理过程,对宏观尺度的断裂行为已有深入的研究和发展,然而对微观尺度的断裂行为及断裂过程中应力场的变化缺乏深入的理解。本文通过分子动力学模拟,研究了具有不同初始缺陷(尖锐裂纹、钝裂纹和孔洞)的单晶镍的断裂行为和应力分布特征。结果表明,不同的初始缺陷导致了不同的断裂机制、断裂强度和抗断裂性能。含初始孔洞的单晶镍样品有最高的断裂强度和最强的抗断裂性能,这与孔洞扩展过程中堆积层错的形成密切相关。其次是含初始钝裂纹的样品,在裂纹扩展过程中出现由[100]超位错发射引起的裂尖钝化;含尖锐裂纹的样品表现为脆性断裂,裂尖原子没有出现微结构的变化,其强度和抗断裂性能最低。此外,不同的初始缺陷也会导致断裂过程中应力分布的变化,对含有尖锐裂纹的脆性断裂试样,高应力(拉伸应力、平均应力和米塞斯应力)总是出现在扩展裂纹的裂尖。而对于含有钝裂纹或孔洞的韧性断裂试样,高应力不仅分布在裂尖,也分布在位错发射和堆积层错形成的区域,在裂纹/孔洞扩展之前,应力随着加载时间的增加而迅速增加,而一旦裂纹或孔洞开始扩展,应力增加非常缓慢或几乎不增加,但拉伸应力值始终大于平均应力和米塞斯应力值。这表明,在I型...     

On crack-tip strain energy release rate in non-principal directions of elasticity for simple layer plate of composite materials

In this paper,the fracture problem in non-principal directions of elasticity for a simple layer plate of linear-elastic orthotropic composite materials is studied.The formulae of transformation between characteristic roots,coefficients of elastic compliances in non-principal directions of elasticity and corresponding parameters in principal directions of elasticity are derived.Then,the computing formulae of strain energy release rate under skew-symmetric loading in terms of engineering parameters for principal directions of elasticity are obtained by substituting crack-tip stresses and displacements into the basic formula of the strain energy release rate.     

Molecular dynamics simulation of peeling a DNA molecule on substrate

Molecular dynamics (MD) simulations are performed to study adhesion and peeling of a short fragment of single strand DNA (ssDNA) molecule from a graphite surface. The critical peel-off force is found to depend on both the peeling angle and the elasticity of ssDNA. For the short ssDNA strand under investigation, we show that the simulation results can be explained by a continuum model of an adhesive elastic band on substrate. The analysis suggests that it is often the peak value, rather than the mean value, of adhesion energy which determines the peeling of a nanoscale material.The project supported by the Distinguished Young Scholar Fund of NSFC (10225209) and key project from the Chinese Academy of Sciences (KJCX2-SW-L2)     

纳米镍板裂纹扩展的分子动力学模拟和宏微观分析

建立了半无限弹性纳米镍板Ⅰ型裂纹扩展的二维分子动力学计算模型。采用镶嵌原子法描述原子间作用,模拟了纳观裂纹区在远场常应变率作用下变化直至起始扩展的过程。同时基于原子势函数和二维正三角形晶格常数计算材料弹性参数,进行连续介质力学断裂分析。分子动力学模拟和宏微观分析均得到裂纹起始扩展的临界时刻、裂尖应力场和原子平均能量。二者的结果比较表明本文的二维简化模型和模拟方法可以准确地描述Ⅰ型裂纹扩展的物理本质,基于原子势函数和晶格常数的连续介质力学分析也是一种可行的研究纳米材料断裂的方法。     

Molecular dynamics of dewetting of ultra-thin water films on solid substrate

Molecular dynamics simulation is applied to study the instability and rupture process of ultra-thin water films on a solid substrate.Results show the small disturbance of the film will develop linearly due to the spinodal instability,whereas the interaction between solid and liquid has less influences on the initial growth.Then the rupture occurs and the rim recedes with a dynamic contact angle.The radius of the rim varies with time as the square root of t,which is consistent with the macroscopic theory available. Stronger interaction between solid and liquid will postpone rupture time,decline the dynamic contact angle and raise the density of water near the interface between solid and liquid.