Study of mechanical properties of amorphous copper with molecular dynamics simulation

The formation and mechanical properties of amorphous copper are studied using molecular dynamics simulation. The simulations of tension and shearing show that more pronounced plasticity is found under shearing, compared to tension. Apparent strain hardening and strain rate effect are observed. Interestingly, the variations of number density of atoms during deformation indicate free volume creation, especially under higher strain rate. In particular, it is found that shear induced dilatation does appear in the amorphous metal.     

Boundary-driven colloidal crystallization in simple shear flow

Using confocal microscopy, we directly observe that simple shear flow induces transient crystallization of colloids by wall-normal propagation of crystallization fronts from each shearing surface. The initial rate of the front propagation was 1.75±0.07 colloidal layers per unit of applied strain. The rate slowed to 0.29±0.04 colloidal layers per unit of applied strain as the two fronts approached each other at the midplane. The retardation of the front propagation is caused by self-concentration of shear strain in the growing bands of the lower-viscosity crystal, an effect that leads to a progressive reduction of the shear rate in the remaining amorphous material. These findings differ significantly from previous hypotheses for flow-induced colloidal crystallization by homogeneous mechanisms.     

Molecular dynamics simulation of deformation and fracture of graphene under uniaxial tension

The paper reports on molecular dynamics simulation of deformation and fracture of graphene under uniaxial tension. Dependences of Young’s modulus, critical force and fracture strain on the strain rate, temperature and angle between the tension direction and the graphene lattice are derived. The effect of defects on fracture of graphene is studied.     

Multiscale simulation from atomistic to continuum – coupling molecular dynamics (MD) with the material point method (MPM)

A new multiscale simulation approach is introduced that couples atomistic-scale simulations using molecular dynamics (MD) with continuum-scale simulations using the recently developed material point method (MPM). In MPM, material continuum is represented by a finite collection of material points carrying all relevant physical characteristics, such as mass, acceleration, velocity, strain and stress. The use of material points at the continuum level provides a natural connection with the atoms in the lattice at the atomistic scale. A hierarchical mesh refinement technique in MPM is presented to scale down the continuum level to the atomistic level, so that material points at the fine level in MPM are allowed to directly couple with the atoms in MD. A one-to-one correspondence of MD atoms and MPM points is used in the transition region and non-local elastic theory is used to assure compatibility between MD and MPM regions, so that seamless coupling between MD and MPM can be accomplished. A silicon single crystal under uniaxial tension is used in demonstrating the viability of the technique. A Tersoff-type, three-body potential was used in the MD simulations. The coupled MD/MPM simulations show that silicon under nanometric tension experiences, with increasing elongation in elasticity, dislocation generation and plasticity by slip, void formation and propagation, formation of amorphous structure, necking, and final rupture. Results are presented in terms of stress–strain relationships at several strain rates, as well as the rate dependence of uniaxial material properties. This new multiscale computational method has potential for use in cases where a detailed atomistic-level analysis is necessary in localized spatially separated regions whereas continuum mechanics is adequate in the rest of the material.     

In-plane electronic speckle pattern shearing interferometry

An analysis of the out-of-plane shearing interferometer has been performed which shows that production of in-plane strain partial derivatives is possible, which are not affected by out-of-plane displacement function components. The in-plane data are represented as subtraction correlation fringes. This interferometer employs a single diverging illumination beam and is applicable to object plane stress and plane strain loading conditions. The interferometer was tested and compared using a compact tension crack specimen and the results are correlated with finite element software predictions of strain distributions across modelled specimens. This experimental validation was chosen because we had an existing test rig and finite element models which had been independently verified.     

Atomic mobility and strain localization in amorphous metals

Molecular dynamics simulations are employed to investigate the atomic mobility in Ni(50)Zr(50) amorphous alloys under both static conditions and shearing. Diffusion occurs under static conditions via cooperative stringlike motion involving atoms with large volumes. Atomic mobility is instead governed by rearrangements localized in shear transformation zones (STZs) under shearing. Local atomic volume plays in both cases a key role, the atomic ensembles involved in diffusion and STZ activity being strongly correlated.     

Role of the entangled amorphous network in tensile deformation of semicrystalline polymers

Being composed of crystalline lamellae and entangled amorphous polymeric chains in between, semicrystalline polymers always show a complicated deformation behavior under tensile deformation. In recent years, the process of tensile deformation was found to exhibit several regimes: intralamellar slipping of crystalline blocks occurs at small deformation whereas a stress-induced crystalline block disaggregation-recrystallization process occurs at a strain larger than the yield strain. The strain at this transition point is related to the interplay between the amorphous entanglement density and the stability of crystal blocks. We report experimental evidence from true stress-strain experiments that support this argument. It is emphasized that tie molecules, which connect adjacent lamellae, are of lesser importance with respect to the deformational behavior.     

Lorentz gas shear viscosity via nonequilibrium molecular dynamics and Boltzmann's equation

When nonequilibrium molecular dynamics is used to impose isothermal shear on a two-body periodic system of hard disks or spheres, the equations of motion reduce to those describing a Lorentz gas under shear. In this shearing Lorentz gas a single particle moves, isothermally, through a spatially periodic shearing crystal of infinitely massive scatterers. The curvilinear trajectories are calculated analytically and used to measure the dilute Lorentz gas viscosity at several strain rates. Simulations and solutions of Boltzmann's equation exhibit shear thinning resembling that found inN-body nonequilibrium simulations. For the three-dimensional Lorentz gas we obtained an exact expression for the viscosity which is valid at all strain rates. In two dimensions this is not possible due to the anisotropy of the scattering.     

平面冲击波高速撞击下Zr41Ti14Cu12.5Ni10Be22.5大块非晶合金的损伤与断裂

 利用二级轻气炮研究了在平面冲击波高速撞击下，Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5大块非晶合金的损伤与断裂行为。实验结果表明：在高速冲击压缩下，大块非晶合金断裂面的角度小于通常压缩情况的角度，这是由高速撞击下材料极高的应变速率决定的；极高的应变速率变形和局域绝热剪切加热造成局部熔化；在温度和压力的共同作用下，非晶合金发生晶化现象。     

基于逾渗理论的非晶合金屈服行为研究

从非晶合金的微观结构出发,基于处理强无序和具有随机几何结构系统常用的理论方法——逾渗理论来描述非晶合金剪切屈服时的塑性流变.为了更好地理解非晶合金剪切带萌生时的临界问题,结合已有的"自由体积(free volume)模型"和"剪切转变区(shear transformation zone)模型",建立了非晶合金剪切转变的逾渗模型.以Cu_(25)Zr_(75)二元非晶合金为例,计算了在剪切转变区内易发生塑性流动的原子团簇剪切失稳的逾渗阈值,并粗略估算了这些原子团簇的大小.研究发现,剪切失稳的逾渗阈值与临界约化自由体积浓度(x_c～2.4%)有着相似的特性,不同之处在于其值与自由体积的分散度有着密切联系.研究结果作为非晶合金的韧脆转变问题提供了新思路.     

Zn-Al共析合金的黏滞流变行为

在连续升温条件下，测量了共析Zn-Al合金的应变速率和黏度随温度的变化曲线，在共析转变温区观测到伴随相变过程的黏度极小值，研究了应力对应变速率和升温速率对黏度极小值的影响，发现在相变温区材料的力学行为遵从Newton黏滞流变定律，用相界面的数量和性质解释了相变温区Zn-Al合金的黏滞流变行为．将相变激活能和相界面的流变激活能进行比较，说明界面的流动性是影响相变的主要因素．结合过去在非晶Pd-Cu-Si合金的结构转变过程中观测到的伴随玻璃转变和晶化过程的黏度极小值，说明伴随着结构转变或相变过程，黏度出现极小 关键词：     

退火态Zr55Al10Ni5Cu30块体非晶合金在轧制过程中的自由体积演化

将Zr₅₅Al₁₀Ni₅Cu₃₀块体非晶合金在715 K等温退火30 min, 引入少量纳米晶, 然后于室温以不同的应变速率进行轧制, 用差示扫描量热仪考察不同应变量样品的热稳定性和自由体积演化. 结果表明:即使轧制到95%的最大应变量, 样品的热稳定性也几乎没有发生改变. 在各种应变速率下, 随着应变量的增加, 自由体积含量持续上升. 但随着应变速率的增加, 相同应变量下自由体积的含量先上升后降低, 该规律与单一非晶态结构合金在塑性变形过程中自由体积的变化情况截然不同.     

非晶Ti3Al合金的变形晶化机理的原子模拟

利用分子动力学方法研究了非晶Ti₃Al合金拉伸过程中的晶化行为，模拟结果表明局部塑性变形导致非晶合金晶化.从微观结构演化的角度分析了拉伸过程中的晶化机理，局部剪切导致拉伸过程中晶粒发生成核与合并，最终生成的晶粒具有面心立方结构.晶核的生长过程伴随着应力强化现象，非晶相中的纳米晶粒能提高非晶合金材料的强度. 关键词： 非晶合金 变形晶化 分子动力学     

基于缠结微结构的非晶态玻璃态高分子材料屈服行为的分子动力学研究

非晶态玻璃态高分子材料作为结构材料在工程领域应用广泛,其机械力学性能特别是屈服变形行为受到热处理、加载应变率和环境温度的影响.采用分子动力学模拟方法研究非晶态玻璃态高分子材料不同工况下的单轴拉伸变形,基于分子链缠结微结构的概念,阐明了非晶态玻璃态高分子材料屈服和应变软化过程的内在变形机制.结果表明,拓扑缠结具有较为稳定的空间结构,难以发生解缠,决定了非晶态高分子材料屈服后的软化平台.由相邻分子链的局部链段相互作用形成的次级缠结在一定外界条件下可发生破坏或重新生成,次级缠结微结构及其演化是非晶态高分子材料发生屈服及软化的内在物理原因.     

Shear influence on the structural ordering in a model metallic glass

Nonequilibrium molecular dynamics simulations of a model amorphous system are performed with the aim of studying the structural transformations induced by external shear influence. We reveal that the shear drive has both positive and negative effects on the structural ordering processes. The dependence of the phase transition rate versus the strain rate at three different temperatures is found.     

Atomistic simulations to characterize the influence of applied strain and PKA energy on radiation damage evolution in pure aluminum

Knowledge of defects generation, their mobility, growth rate, and spatial distribution is the cornerstone for understanding the surface and structural evolution of a material used under irradiation conditions. In this study, molecular dynamics simulations were used to investigate the coupled effect of primary knock-on atom (PKA) energy and applied strain (uniaxial and hydrostatic) fields on primary radiation damage evolution in pure aluminum. Cascade damage simulations were carried out for PKA energy ranging between 1 and 20 keV and for applied strain values ranging between ?2% and 2% at the fixed temperature of 300 K. Simulation results showed that as the atomic displacement cascade proceeds under uniaxial and hydrostatic strains, the peak and surviving number of Frenkel point defects increases with increasing tension; however, these increments were more prominent under larger volume changing deformations (hydrostatic strain). The percentage fraction of point defects that aggregate into clusters increases under tension conditions; compared to the reference conditions with no strain, these increases are around 13% and 7% for interstitials and vacancies, respectively (under 2% uniaxial strain), and 19% and 11% for interstitials and vacancies, respectively (under 2% hydrostatic strain). Clusters formed of vacancies and interstitials were both larger under tensile strain conditions, with increases in both the average and maximum cluster sizes. The rate of increase/decrease in the number of Frenkel pairs, their clustering, and their size distributions under expansion/compression strain conditions were higher for higher PKA energies. Overall, the present results suggest that strain effects should be considered carefully in radiation damage environments, specifically for conditions of low temperature and high radiation energy. Compressive strain conditions could be beneficial for materials used in nuclear reactor power systems.     

Characterization of the tensile behaviour of a co-woven-knitted composite in the continuous and discrete frequency domain

Laplace-transform and Z-transform theories have been applied to analyze the tensile stress–strain curves of a co-woven-knitted (CWK) composite under quasi-static (0.001/s) and high strain rates (up to 2586/s) tension. The transform results were extended to characterize the tension failure and dynamic responses of the CWK composite in the frequency domain. Specifically, the Laplace-transform theory was employed to analyze the stress–strain curves of the CWK composite along 0°, 45° and 90° directions when the composite is assumed to be a continuous system, while the Z-transform theory was used for the discrete system for the composite. From the transformed results, it was found that the stress–strain curves of the CWK composite specimen under different strain rates tension have similar stability behaviours for the Laplace- and Z-transform. For the continuous system, few pole plots are distributed on the left side of the imaginary axis, which means the system is unstable. Nevertheless, the pole-plot distribution is stable before the post-critical deformation of the CWK composite. For the discrete system, most of the poles are located inside the unit circle before post-critical deformation, indicating the system is stable. From the stiffness–time history and fracture morphology, the stability of the pole-plot distribution corresponds to the stiffness stability and fracture uniformity. From continuous and discrete system analyses, it is found that the stress–time and strain-time histories of the CWK composite can be regarded as a digital signal system. Digital signal processing (DSP) methods can be extended to the investigation of the mechanical behaviour of composites.     

Stability of fluid flow past a membrane

The stability of the flow of a fluid past a solid membrane of infinitesimal thickness is investigated using a linear stability analysis. The system consists of two fluids of thicknesses R and H R and bounded by rigid walls moving with velocities and , and separated by a membrane of infinitesimal thickness which is flat in the unperturbed state. The fluids are described by the Navier-Stokes equations, while the constitutive equation for the membrane incorporates the surface tension, and the effect of curvature elasticity is also examined for a membrane with no surface tension. The stability of the system depends on the dimensionless strain rates and in the two fluids, which are defined as and for a membrane with surface tension , and and for a membrane with zero surface tension and curvature elasticity K. In the absence of fluid inertia, the perturbations are always stable. In the limit , the decay rate of the perturbations is O(k ³ ) smaller than the frequency of the fluctuations. The effect of fluid inertia in this limit is incorporated using a small wave number asymptotic analysis, and it is found that there is a correction of smaller than the leading order frequency due to inertial effects. This correction causes long wave fluctuations to be unstable for certain values of the ratio of strain rates and ratio of thicknesses H. The stability of the system at finite Reynolds number was calculated using numerical techniques for the case where the strain rate in one of the fluids is zero. The stability depends on the Reynolds number for the fluid with the non-zero strain rate, and the parameter , where is the surface tension of the membrane. It is found that the Reynolds number for the transition from stable to unstable modes, , first increases with , undergoes a turning point and a further increase in the results in a decrease in . This indicates that there are unstable perturbations only in a finite domain in the plane, and perturbations are always stable outside this domain. Received: 29 May 1997 / Revised: 9 October 1997 / Accepted: 26 November 1997     

Strain rate induced crystallization in bulk metallic glass-forming liquid

We report on the solidification of Au49, Ag5.5, Pd2.3, Cu26.9, Si16.3 bulk metallic glass under various strain rates. Using a copper mold casting technique with a low strain rate during solidification, this alloy is capable of forming glassy rods of at least 5 mm in diameter. Surprisingly, when the liquid alloy is splat cooled at much higher cooling rates and large strain rates, the solidified alloy is no longer fully amorphous. Our finding suggests that the large strain rate during splat cooling induces crystallization. The pronounced difference in crystallization behavior cannot be explained by the previously observed strain rate effect on viscosity alone. A strain rate induced phase separation process is suggested as one of the explanations for this crystallization behavior. The strain-rate-dependent critical cooling rate must be considered in order to assess the intrinsic glass forming ability of metallic liquid.     

韧性材料的微裂纹扩展与分叉的晶体相场模拟

采用晶体相场方法研究韧性单晶材料在双轴拉伸条件下微裂纹扩展与分叉的演化过程,分析应变、温度、初始裂口形状等因素对裂纹扩展和分叉的影响.结果表明:对于简单的单向拉伸,应变需要达到一定的临界值,裂纹扩展才会启动.对于二组相互垂直的双轴拉伸作用,当应变达到临界值后,裂纹扩展过程中会发生分叉现象.温度越高,裂纹扩展越快且分叉越多.裂纹在扩展过程中,体系能量不断降低,当裂纹出现分叉时,体系能量降低得更快.在裂纹扩展过程中,有时是会在裂尖处前方出现微小的空洞,类似在裂纹尖端前方出现位错发射情况,这些微小的空洞逐渐扩大连成裂纹.本文所得结果与相关模拟结果和实验结果符合.